Talk:Observable universe/Archive 1

Archive 1

Archive 2

Archive 3

Archive 4

Edge of the Universe

Edge of the Universe redirects here. Is seems much more appropriate to be directed at shape of the universe however. Lonjers (talk) 09:38, 22 October 2009 (UTC)

Backing behind me recent edit

I recently edited the article (http://en.wiki.x.io/w/index.php?title=Observable_universe&action=historysubmit&diff=333788200&oldid=333339485). I promised backing here, because I did not believe this backing is relevant to the article, but my change may be reverted otherwise. Because the universe is expanding (IIRC at about 70 km/s/MPc right now), the light can have traveled only 13.5 billion light-years, but in the mean time, the distance to the object may have expanded behind the light, causing the object to now be much farther away than it was when the light originally set off. Therefore, a 46 billion ly edge of the observable universe is quite possible. —Preceding unsigned comment added by Lengau (talk • contribs) 10:38, 24 December 2009 (UTC)

yes merge please

yeah 2 articles should be merged 59.93.244.212 (talk) 22:54, 28 July 2009 (UTC)

References

This article is in need of serious citations, considering how much of the information is debatable. —Preceding unsigned comment added by Crbrown25 (talk • contribs) 08:37, 22 October 2009 (UTC)

Infinite Galaxies

Someone noted before that statement "The total number of galaxies may even be infinite" is questionable ("I don't think any sensible citation is possible, because it's just a statement of our ignorance. It's a rhetorical device like saying that there might be pink elephants in the Andromeda galaxy."). I've added a citation-needed tag...if there isn't one, I'll remove the sentence. It really is a jarring comment compared to the rest of the document and seems more like an author's personal speculation than established fact or even scientific community speculation.

Either cite it or drop it. Afabbro 19:13, 18 May 2007 (UTC)

...and it's now gone. If you want to add it back, cite some reference for this unlikely claim. Afabbro 02:58, 24 May 2007 (UTC)

I think a reference could probably be found but I kind of agree that the statement is sort of out of place there. However I don't see anything particularly unlikely about it. If spacetime is asymptotically flat or hyperbolic, and the simplest model consistent with that is substantially accurate, then the universe has infinite volume, and therefore presumably infinitely many galaxies (assuming uniformity). --Trovatore 04:44, 24 May 2007 (UTC)

That was the only place on the page where the word "infinite" was used, which is why it was kind of jarring. If there is serious consideration that the universe has infinite volume, then that should be discussed and then the idea that there are infinite galaxies would be more congrous with the rest of the page. Just my opionion, of course. Thanks. Afabbro 05:48, 26 May 2007 (UTC)

I'm pretty sure there is such discussion (it would follow from some of the simple models in the nonpositive curvature case) but not sure where to find refs. Peeve alert: I think you mean infinitely many galaxies. As far as I know, no one thinks any single galaxy is infinite. --Trovatore 22:39, 26 May 2007 (UTC)

Misc Old

I was under the impression that the radius of the observable universe was significantly greater than 13.7 bn ly due to the universe's accelerating expansion and the variability of the Hubble distance. Scientific American (March 2005) seems to agree with me, calling this a common misconception, because it seems intuitive. Also I believe quasars and galaxies have been observed beyond the distance of 15 bn ly (how would one calculate this?). What needs correction/clarification?

Also, is it a sphere? What about its curvature? -- Rmrfstar 02:51, 7 November 2005 (UTC)

The "border" is always the same, but there are differently defined "distances" to the border. --Pjacobi 10:50, 7 November 2005 (UTC)

Contradiction

There is contradictory information in this article and the general universe article. the number 46 billion is a new one that I have never heard before, maybe a reference could be added for this.

Universe says the diameter is 78 bn ly. I found the value of 46 bn in Scientific American (the issue cited in the aritcle). I listed both, see [1] for the source of the 78. -- Rmrfstar 16:37, 8 January 2006 (UTC)

There's no reference given in Scientific American as to where the 46bn comes from so I'm not sure how reliable it is - would it be better to stick wtih the 78bn as at least it is WMAP data?193.62.111.10 17:49, 12 December 2006 (UTC)

Requested move

The following discussion is an archived debate of the proposal. Please do not modify it. Subsequent comments should be made in a new section on the talk page. No further edits should be made to this section.

The result of the debate was: nominator conceded—jiy (talk) 01:43, 16 January 2006 (UTC)

---

Observable universe → Observable Universe – I'm only creating this section in order to oppose the move, since the nominator failed to create it. — Knowledge Seeker দ 04:35, 11 January 2006 (UTC)

• Oppose; universe should not be capitalized (see Merriam-Webster entry), for instance. — Knowledge Seeker দ 04:35, 11 January 2006 (UTC)
• Oppose, as the capitalization of "universe" is non-standard; I have lowercased it to comply with standard (and Wikipedia) usage, thus making this a non-issue. ProhibitOnions 14:13, 11 January 2006 (UTC)
• Concession -- Rmrfstar 00:50, 13 January 2006 (UTC)

The above discussion is preserved as an archive of the debate. Please do not modify it. Subsequent comments should be made in a new section on this talk page. No further edits should be made to this section.

80bln vs 140bln

The top of the contents section notes that there are 140 bln galaxies. #2 uses the number 80bln, contradiction?

An analogy to the size of the universe?

Here is a question:

If I model every galaxy in the observable universe down to the size of a grain of sand (e.g. 0.25 cm cubed). If I then distribute these grains in a manner than mimics the physical structure of the universe (i.e. clusters, super clusters) with the Milky Way ar the centre.

How big is this (reduced) universe in diameter? (As big as the our Solar System? Bigger?) How fast is the most remote galaxy from the Milky Way moving away from us?

Thanks!

Galaxies have vastly different sizes from one another, from 5,000 ly to 3,000,000 ly in some cases. Let's put the average at 300,000 ly. Sand is similarly ununiform, but let's put the average there at about 0.2 mm. Therefore our scale is about 1.4x10²⁵:1. The entire universe has an uncertain size, but the reported lower bound is 78 Gpc, which converts to about 170 meters, so the whole universe in our model must be at least 170 meters in diameter. In volume that's about 2.5 million cubic meters. If we're only talking about the observable universe, that's 28 Gpc in diameter, which converts to about 62 meters in diameter, a volume of around 125,000 cubic meters. For comparison, the Great Pyramid of Giza has about 2.6 million cubic meters in stone. I can't find a good analogy for 125,000 cubic meters, but typical medium-sized carrier ships seem to be around there (look up LNG ships). I don't think this information neccessarily belongs in the article, though.24.165.184.37 (talk) 17:29, 29 March 2008 (UTC)

Oppose merge: Particle horizon

I don't think Particle Horizon should be merged here, it's a separate concept. Although its true that the observable universe is our particle horizon, particle horizons can be measured from any point and are a key concept in large scale structure formation --Keflavich 01:23, 14 April 2006 (UTC)

I agree that Particle horizon should stay a separate article. It might need to be mentioned here, and clarified how it is different, though.

Changed the Introductory Paragraph

In the interest of quality, I changed the first paragraph completely. You can look at a history for this article but here's what it used to say: "The observable universe is a term used in cosmology to describe a ball-shaped region of space surrounding the Earth that is close enough that we might observe objects in it, i.e. there has been sufficient time for light emitted by an object to arrive at Earth. Every position has its own observable universe which may or may not overlap with the one centred around the Earth." There are several problems with this statement: first, neither of the references given in the article support this statement, the statement is awkwardly worded, and finally from the point of view of astrophysics that statement itself is incorrect. I have provided an introductory paragraph that more accurately reflects the definition of "observable universe" as is used in the context of cosmology. I left the original sources up, but I have also added peer-reviewed sources (texts and journals) to support my opening statement. Astrobayes 14:38, 20 June 2006 (UTC)

Explanation of my reverts

I have tried to assume good faith regarding the recent reverts of my original change of the introductory paragraph but those reverts have remained without citations so I changed the opening paragraph back to my original edit, which itself did have citations. To address any objections to the particular sources I cited for this original edit, I am obtaining new ones. Again, I'll stress that as I state in the paragraph above this one, my original intention in the change was to omit the vague "ball-shaped region surrounding the Earth" without a citation back in June in favor of an introduction which explicated the observable universe from a scientific standpoint, with appropriate sources cited. This was an improvement to the article and subsequent discussions have led to no new improvements over the version prior to my edit. Thus, my edit - which cites its sources to support its claim - should remain in favor of one that does not have sources cited. I have done an Internet search and I'm hard-pressed to find any reference to the observable universe as a "ball-shaped region surrounding the Earth (et. seq.)" in any peer-reviewed journal, scientific publication, or other scholarly source. I am more than willing to let such a statement stand if a scientific source for this scientific encyclopedic article can be found to support that claim. Otherwise, we're compromising the accuracy and quality of this article. Anyone's edits I've reverted should know that I am only trying to adhere to WP policy on verifiability, and that no edit or revert I've made is personal or POV in any way. I hope this elucidates my stance on this. If needed, I am more than willing to bring in an outside opinion in an RFA or an SPR. Perhaps such a discussion would improve the article further. Cheers, Astrobayes 19:02, 25 September 2006 (UTC)

Update: Link Removed from 1st Paragraph

I removed the "cosmology text" reference link from the first paragraph because it is no longer active. The two remaining reference links still more than clarify the opening statement, however if you wish to add additional links feel free. Cheers, Astrobayes 21:56, 27 June 2006 (UTC)

Per my comments in the above sub-section, I am obtaining new references for the opening section which should hopefully reflect more clearly the wording of the introduction of the article. I am doing this in response by some of the other editors that the references I have been citing in that section are a bit technical or terse. I hope the new references both suit the section better as well as adhere as well to the WP:verifiability policy as the old citations did. I will still include the old citations in the "see also" section at the end of the article. Cheers, Astrobayes 19:02, 25 September 2006 (UTC)

80bln vs 140bln -- STILL

Can someone fix this? This damages the credibility of the entire article. It's been 3.5 months since the discrepancy was first mentioned. --Scott McNay 04:32, 24 July 2006 (UTC)

First sentence

The first sentence is a bit problematic. Before my recent change, it claimed, in effect, that the observable universe was the whole universe. If that were so, the notion of "observable universe" would be unnecessary.

However I'm not really happy with my fix, either; there may be parts of the universe that are causally disconnected from us, but that are causally connected to regions that are causally connected to us. (It's not a transitive relation.) So "causally connected" doesn't seem exact either.

Frankly I prefer the version from before 20 July, the one that talked about a ball around the Earth. But really a cosmologist should do this (which I'm not). --Trovatore 20:40, 18 September 2006 (UTC)

I looked up the references pointed to by Astrobayes, and while they looked like fascinating papers, at least from their abstracts I was unable to see that they had any connection with the change that was made. Also the claim that the radius of the observable universe is calculated from the universe's radius of curvature, doesn't seem to make sense on the face of it: We don't even know if the global curvature of the universe is positive or negative, and yet we have a pretty good idea of the size of the observable universe, just from the Hubble constant, I think.

On the other hand I don't see anything wrong with the version Astrobayes changed. Therefore I've restored to that version. No offense intended to Astrobayes; I'll be happy to listen to his case. --Trovatore 05:48, 19 September 2006 (UTC)

Relationship of observable to unobservable universe

The article does not explain why the whole universe, back to the earliest galaxies, appears to be visible, nor what proportion of the total universe is observable. I have tried to do this in the text below, but I'm not a cosmologist. If my explanation is correct maybe we could incorporate the key points into the article.

"The observable universe is a phrase used to distinguish the extent of the universe observable to an Earth-based astronomer from the actual and unobservable current extent of the universe.

Because light travels at a finite velocity (300,000 Km/s) we observe distant objects not as they are now but as they were when the light left them.

Because the universe is expanding we observe distant galaxies as they were in a smaller universe. A stretch of space one light year wide expands at a rate of 690 kilometres per year (Hubble's Constant). In the time since the light that we see left those galaxies, the space in which they exist has expanded - but we are unable to observe this. Therefore the real unobservable universe as it is now is larger than the old universe that we can observe.

The furthest galaxies so far observed (eg Abell 1835 IR1916) appear to be 13.2 billion light years away. We see them as they were when the universe was half a billion years old, which is only one 27th of its current estimated age of 13.7 billion years. At that time the radius of the universe was about one 27th of its current radius - assuming that the rate of expansion is constant. The light from these galaxies has travelled through 13.2 billion light years of space to reach us, but that space has since expanded - so those galaxies now are much further away than they appear to be. The final stretches of space through which the light recently passed has hardly expanded at all since then, but the initial stretches have since expanded 27 fold.

Beyond the most distant galaxies we can see light from the beginning of the universe in the form of the cosmic background radiation from the big bang - so, in a sense, we can see to the 'edge' of the universe, but as it was when that edge was a point. That radiation has travelled 13.7 billion light years to reach us. Astrophysicists calculate that that distance has since expanded to about 156 billion light-years, which is therefore the radius now of the real unobservable universe.

Although we can see the entire universe as it was in the distant past, when it was much smaller, light now leaving the more distant galaxies now will never reach us. This is because although both the distant galaxy and our own are fairly stationary, the space between that galaxy and our own is now expanding faster than the speed of light. If every distance of a light year in the universe is expanding at a rate of 690 kilometres per year then the distance between us and the most distant points of the universe, which are now 156 billion light years away, is increasing at over 11 times the speed of light. Everything currently further than 13.7 billion light years away from us (one 11th of the current radius of the universe) is moving away from us faster than the speed of light and so will never be visible to future observers on the Earth."

Rjvint 17:16, 24 September 2006 (UTC)rjvint

Your initial comments regarding what proportion of the "total Universe is observable" as well as your title "Relationship of observable to unobservable universe" addresses the same basic question. And it reminds me of a recent comment by Trovatore on my talk page: "Look, it may well be the case the case that the observable universes of different observers look the same in terms of their macroscopic properties...what they are not is literally the same expanse of space, unless of course the observable universe is in fact the whole universe (which I suppose has not been ruled out). There is likely a planet somewhere whose observable universe does not even include the Earth, so it obviously can't be the same as Earth's observable universe."

What you both are getting at here is understandable - many people have raised these questions - but the physical implications of what you're both asking are exactly what science considers outside the realm of physics (i.e. science is not equipped to handle that). Any portion of space from which the electromagnetic radiation can not be detected is outside of the scope of relevance insofar as constructing a physical model of cosmology is concerned, because you can not construct a Hamiltonian from radiation or movements of celestial objects which you can not see or for which you can not obtain a statistical approximation. And you can only obtain a statistical approximation of such radiation and mass from the proportion of the sample size under consideration with respect to the size of the environment from which the sample is pulled. To do anything else is to guess what percentage of total radiation and matter we can measure from the observable universe comes from the total universe. How can you obtain equations of motion for physical systems you can not measure, and when you have no way of even obtaining an approximation of their nature based upon similar systems? Without any knowledge of the "universe" outside of that which we can observe, the statistics are meaningless and we're back to square one (how do you assign priors for such a distribution?) And say there were objects outside the observable universe. Their gravitational field would perturb either the light from and/or the motions of the very distant objects we could see (i.e. think about how the filament superstructures would look different if there were mass outside of the observable universe). Where does this lead us then? We are at that point swimming in statistics, assigning priors for a system which we can not detect. And still, such a system is by observations thus far, nonexistant. So guessing about something which observations lead us away from is not science. It would however make for a great sci-fi novel and in fact there have been some written on just this sort of premise. But until science can support claims that there is or may be a universe outside of the observable universe, such speculation remains in the realm of entertainment - the stuff of philosophy (and admittedly, I share a fascination with the idea but the physics does not support my fascination). Thus, this article should not be changed to claim so. Cheers, Astrobayes 17:43, 25 September 2006 (UTC)

It seems to me that you are taking a philosophical view -- that things which cannot be observed from Earth do not exist -- and trying to pass it off as science which it is not. JRSpriggs 05:28, 27 September 2006 (UTC)

Quite the contrary. Since I am a scientist, I would not attempt to call philosophy science. What I state above explicates that: we can discuss what may lie outside of the observable universe all we wish to but it is irrelevant since anything that may exist outside of the observable universe is forbidden from communicating with what is inside the observable universe. Relativity forbids that, since the electromagnetic radiation can not exceed c. Talk of what may or may not lie outside of the observable universe belongs in science fiction. I hope that this clarify things for you regarding my comments. I'm starting to get the feeling that most people don't read through all of the comments of others, and then actually consider them for a bit before responding. I must admit I'm a bit frustrated at this point... I am, always have been, and always will be on the side of science - testable models and experimentation. There is no experiment which has been published in a peer-reviewed article that suggests that there is, or that we could ever detect, some imaginary radiation outside of the observable universe. And I have tried - in a dozen ways - to explicate this on the talk page relevant to articles that touch upon these themes. But at this point, the whole discussion has been so muddled by individuals who wish to show how smart they are by using technical mathematical arguments that I've given up - for the time being - on trying to improve this article. I'll come back to it another time. As I've already said, now that the "Earth at the center" bit has been removed from the introductory paragraph, I'm happy for now. Cheers, Astrobayes 17:55, 27 September 2006 (UTC)

I fixed a grammar error, and requested a citation for the last sentence, of the intro paragraph. I did not revert anything from a previous version, nor do I intend to do so because I have reached my three-revert rule. If anyone reverts my edits they surely may do so because I am not going to revert that intro any more. I am, honestly, through with this article for a while. I'm going to let the embers cool. Cheers, Astrobayes 18:02, 27 September 2006 (UTC)

That's fair enough. Actually I think you've misinterpreted WP:3RR; what's forbidden, strictly speaking, is four reverts in 24 hours, though gaming the limit is seriously frowned on and subject to sanctions. Still, your version of it is in some ways preferable, and I don't intend to be the first one to violate it at this article. But there are some questions that still need to be clarified.

My current working definition is that two event points are in each other's observable universe if their future timelike cones have nonempty intersection (so that an observer starting from one of the event points could in principle travel to a position where he could be affected by something that happened at the other). However, some of the discussion I've seen seems to be working in the other direction (two event points are in each other's observable universe if their past-facing timelike cones have nonempty intersection). Clarification from the participants at WikiProject Physics would be very welcome here. Astrobayes, if you post citations, please quote the text (on the talk page) that you think supports your point. --Trovatore 18:11, 27 September 2006 (UTC)

Too small

The article says "In the sense of a comoving distance scaled to the current conditions, the observable universe is 13.7 billion light years in radius because the universe is 13.7 billion years old.". Looking at the article on comoving distance, I am fairly sure that this sentence is wrong. The observable universe is much larger (in that sense) than its age because the space traversed by the light expands after the light has passed. However, I have no idea what the correct figure is. The article also refers to a larger "physical size", but I think that that is what "comoving distance" means. 13.7 thousand million light years is the "light-travel distance", not the "comoving distance". JRSpriggs 07:56, 7 October 2006 (UTC)

Superfluous text

The article says, "However, space itself may expand faster than the speed of light making the physical size associated with this much larger." It seems to me that the conclusion isn't dependent on the "faster than the speed of light" property, but rather only on space itself expanding. I think the sentence should be "However, space itself expands, making the physical size associated with this much larger." —Preceding unsigned comment added by 131.107.0.73 (talk • contribs)

Yes. In fact, the dimensions are not even the same. The rate at which space expands is given in (Distance/Time)/Distance (see Hubble constant), i.e. 1/Time, while the speed of light is given in Distance/Time. JRSpriggs 07:58, 14 October 2006 (UTC)

Image needs fixing

The observable universe within 14 billion light years. The observable universe is thought to consist of: 10 million galactic superclusters; 25 billion Galaxy groups and clusters; 350 billion large galaxies; 3.5 trillion dwarf galaxies; and 3x10²² stars

I just deleted this image from the article because, while it's very pretty, the length scale it shows is wrong by a factor of three. The diameter of the visible universe is 93 Gly, not 27 Gly. There is a notion of "light travel distance" in which the diameter is 27 Gly, but (a) it shouldn't be used and (b) it's nonlinearly related to metric distance, so even if you do use it the image is still wrong: 1 Gly near the middle of the image would be very different from 1 Gly near the edge.

Can anyone produce an equally pretty image with the correct length scale in it? It just needs to be 1/93 of the diameter instead of 1/27. -- BenRG 23:11, 17 November 2006 (UTC)

Some changes reverted

I reverted some recent edits; here's a summary.

Edits by Paul venter: Virtually everything in this article is specific to the Big Bang model. In the Big Bang model, the observable universe is not defined as what we can actually see, and it does not stop at the surface of last scattering (CMBR). I replaced "cosmology" with "Big Bang cosmology" in the first paragraph to make this clearer.

Request to "present a citation about infinite galaxies": I don't think any sensible citation is possible, because it's just a statement of our ignorance. It's a rhetorical device like saying that there might be pink elephants in the Andromeda galaxy.

"in the shape of a circle, following a hypothetical curvature of space": Even if the universe is spatially spherical or elliptical, a geodesic that circumnavigates the universe is not really shaped like a circle. And a closed universe would not necessarily be spherical! In fact, if the universe is as small as 24 gigaparsecs across, it's definitely not spherical, because we'd easily be able to detect such a large positive curvature (I think). A more plausible option is the Poincare dodecahedral space, which is closed and negatively curved. It may be a mistake to use words beginning "circum-" here, but I can't think of any better alternatives. -- BenRG 15:04, 19 November 2006 (UTC)

Observable universe being entire universe

I don't really follow this passage:

No one believes, however, that the observable universe is precisely the entire universe; that would imply that the Earth is exactly at the center of the universe, violating a fundamental assumption of astronomy (and indeed all of science).

Why would it imply that the Earth were exactly at the center? Wouldn't it just mean that all event points are causally connected (i.e. that their future-facing timelike cones meet, or maybe it's past-facing, I'm not too clear on that point)? That would look the same from any location, not just the Earth. --Trovatore 04:40, 20 November 2006 (UTC)

To User talk:BenRG: If you look at the remarks of User talk:Astrobayes above, you will see that he is saying almost exactly the opposite of what you said. For example, he said "And still, such a system is by observations thus far, nonexistant. So guessing about something which observations lead us away from is not science.". Recasting his position in my words -- observations are the basis of science, and thus, it is unscientific to suppose that anything which is not observable exists. Needless to say, I disagree with him. JRSpriggs 05:21, 20 November 2006 (UTC)

I don't understand what JRSpriggs is saying, but I agree completely with Trovatore and move to remove the offending passage. -- Rmrfstar 01:12, 22 November 2006 (UTC)

I think JRSpriggs's point is just that "no one believes" is wrong, since someone does believe it. I'm not sure I understand Trovatore. Is he talking about the case where the universe is small enough that we can see the whole thing? That's a fair point. I don't like the sentence in question much myself. I feel like something should be there, but I'm not sure what exactly. What I was trying to say is that identifying the universe with the visible universe violates the cosmological principle. -- BenRG 02:26, 22 November 2006 (UTC)

I think you mean the Copernican Principle, but who's counting? --ScienceApologist 02:37, 22 November 2006 (UTC)

I was bothered by something else, the statement that an earth centered universe violates a fundemental assumption of science. Assuming a heliocentric view violates a fundemental assumption. But AFAIK the fundemental assumptions of science simply imply that it is vanishingly unlikely that the Earth happens to be at the center of the universe. IOW, I assume all scientists would agree that the Earth can be assumed not to be at the center of the Universe (and I am sidestepping the issue of defining "center") but if it turned out it was, it wouldn't violate any fundemental principles. --Israeld 16:41, 5 June 2007 (UTC)

I agree that this passage is too strongly worded:

No one believes, however, that the observable universe is precisely the entire universe; that would imply that the Earth is exactly at the center of the universe, violating a fundamental assumption of astronomy (and indeed all of science).

Occam's razor has always lead me to hope that the observable universe equals the entire universe. No fudge factors thrown in just to make the latest theory work. This would probably be via the universe being finite but unbounded. Every point in the universe would look like the center. cliffsjunk TA earthlink TOD net (checked infrequently) --[[User:|User:]] 15:39, 4 July 2007 (UTC)

Merge from The Hubble Limit

Since Jan/06 The Hubble Limit has had a merge proposal to be integrated into this article. I noticed the item while attempting to clear backlogged maintenance. I am adding the merge from template to this page to promote the discussion to resolve this proposal. Thank you. Alan.ca 08:22, 21 January 2007 (UTC)

I don't see a problem with the merge. The hubble page is a stub that isn't linked to by many pages, and could be easily integrated into this article. Radagast83 16:10, 25 January 2007 (UTC)

The Hubble Limit now redirects to Observable universe. I guess the merge happened already. SheffieldSteel 00:59, 7 March 2007 (UTC)

I think the merge should be undone. The observable universe article still links to the hubble limit, and I think it was fine the way it was. The Hubble limit defines the boundary of the observable universe: they are different concepts. --Michael C. Price ^talk 20:54, 19 September 2007 (UTC)

Actually the Hubble limit is not even the boundary of the observable universe. The radius of the Hubble sphere is defined to be c/H_o, which is about 14 billion light years, much smaller than the radius of the observable universe (and somewhat larger than c times the age of the universe). It definitely needs its own article. -- BenRG 23:40, 28 September 2007 (UTC)

I wrote a new Hubble volume article. The observable universe article still needs to be fixed. I don't really know much about this topic. In particular, I don't know why the Hubble volume was defined in the first place; it doesn't have any interesting physical properties that I'm aware of. -- BenRG 00:16, 29 September 2007 (UTC)

You're right, of course, that the radius of the Hubble volume is c/H₀ and that it is unrelated to the size of the observable universe. However, this is not the same as the Hubble sphere, which is bounded by the Hubble limit: the radius at which the apparent relativistic recession velocity is c (i.e. the redshift is infinite) and radiation of any kind emitted from that point now can never reach us. In other words it is the region inside the (current) cosmological event horizon. This is one definition of the observable universe (albeit not the one given in the observable universe article). Cosmo0 11:48, 1 October 2007 (UTC)

Actually I withdraw that comment. I wrote it down before I'd thought it through properly. The Hubble limit is, as you said, the boundary of the Hubble volume. The relationship between redshift and velocity has nothing to do with it. Cosmo0 18:02, 1 October 2007 (UTC)

Is it flat or not?

First, we say this (emphasis mine):

Since the visible universe is a perfect sphere and space is roughly flat

Then, in the next section, we say this (emphasis mine):

in the real universe spacetime is highly curved at cosmological scales (general relativity), and light does not move rectilinearly

These are contradictory. Can someone who understands the issue resolve this? --Doradus 12:22, 4 May 2007 (UTC)

Anyone? Bueller? --Doradus 18:05, 14 June 2007 (UTC)

I'm sorry. I can't answer your (seemingly) valid concern with any authority. I tagged the article for having this contradiction, though. Soon some physicist will see it. -- Rmrfstar 18:09, 14 June 2007 (UTC)

Good find! And even attributing this nonsense to poor Ned Wright who runs a very nice cosmology website. The correct word is "static" not flat. The "13.7 billion light-years"-misunderstanding would be a static, unchanged space time and the big-bang occuring somewhere in this space time. This concept is not only theoretically strange, but also outruled by observations. --Pjacobi 18:20, 14 June 2007 (UTC)

It wouldn't necessarily be true in a static spacetime either; there might be some gravitational effects affecting the size of the Universe. It would however be valid in a _flat_ (Minkowski) spacetime. But our spacetime is curved; only the 3d cross-sections known as "space" are flat. Ben Standeven 17:48, 3 July 2007 (UTC)

These were not contradictory statements, and both were correct. One of them is about space (at a given cosmological time), while the other is about spacetime. The spacetime curvature is zero if the energy density is zero (which it isn't), while the spatial curvature is zero if the energy density is equal to the critical density (which it very nearly is). When I originally wrote the paragraph it called attention to the difference, but that got deleted at some point. I'm reverting to my original version. -- BenRG 06:44, 6 July 2007 (UTC)

statement about Earth not being the centre

I'd like to propose removing this line: "No findings suggest, however, that the observable universe is precisely the entire universe, which is exceedingly unlikely in that it would imply that the Earth is exactly at the center of the universe."

The first part is acceptable, if a little vacuous: no one's done an experiment to show we're at the centre. That said, a variety of proposed cosmologies involving unbounded but compact universes suggest that any point in the Universe can be considered the centre. It is not at all clear why being able to observe the entire Universe would imply that Earth was at the centre. If the Universe were a small torus, for instance, you might be able to see around it twice, but in that case there is no centre (or every point can equivalently be called the centre). These topologies are even mentioned later in the article.

Also, the word "unlikely" here is being used as an opinion, rather than in a scientific sense -- we have no way of gauging the likelihood of most proposed topologies.

I haven't done much editing here, so I don't really know what's appropriate/how it's done. Just thought I'd point this out on this page because I feel that the statement is incorrect, and I'll leave it to someone who knows what to do. —Preceding unsigned comment added by 205.250.225.55 (talk) 08:43, 26 September 2007 (UTC)

I agree. Especially since the next paragraph states that it is “also possible that the universe is smaller than the observable universe”, which would imply that the observable universe is the entire universe. Perhaps the sentence in question is attempting to say that the boundary of the observable universe is unlikely to be identical to the boundary of the whole universe, but that's just statistics and goes without saying.

Just go ahead and change it in whatever way you think will improve the article. You've explained you're reasoning here for anyone to see and challenge. -- Cosmo0 10:18, 26 September 2007 (UTC)

My 2 cents: The fact is that moat (perhaps all) cosmologists do consider that the universe extends beyond the edge of the observable universe; one reason being a definite prediction of cosmic inflation. Speculations about whether the entire universe is bounded or not are besides the point, since the Hubble volume, in these models, is considered to be much smaller than the bounded volume. Also it needs pointing out that the universe being exactly in the centre of the Hubble volume -- if that's all the universe is -- would contradict the cosmological principle. Although that doesn't mean it is impossible.--Michael C. Price ^talk 11:39, 26 September 2007 (UTC)

I agree with your points, but it seems odd to mention both the possibility of the universe being smaller than, and bigger than, the size of the observable universe in the article, while seeming to rule out the possibility of it being the same size, which is statistically unlikely but doesn't break the cosmological principle if the universe is periodic, which is also required in the first case. I think pains need to be taken to distinguish between comparisons of size and comparisons of objects: while the idea that the observable universe and the actual universe are identical would break the cosmological principle, it's possible (but still unlikely) for the size of the observable universe and the size of the actual universe to be the same. I'll have a think about how this could be better explained. -- Cosmo0 12:22, 26 September 2007 (UTC)

I don't understand what you mean by "doesn't break the cosmological principle if the universe is periodic". How does time effect the cosmological principle? --Michael C. Price ^talk 14:11, 26 September 2007 (UTC)

Periodic in space. The next paragraph of the article states:

It is also possible that the universe is smaller than the observable universe. In this case, what we take to be very distant galaxies may actually be duplicate images of nearby galaxies, formed by light that has circumnavigated the universe.

I'm simply saying that in such a model, the cosmological principle is obeyed regardless of whether the whole universe is larger, smaller or the same size as the observable universe, because there is no 'center'. It may be a minor point, but it might be confusing to some readers that the article seems to suggest that the universe can be bigger or smaller than the observable universe, but not the same size, as though that is a special case, which it isn't. I think that was the argument that the original poster was making. -- Cosmo0 19:12, 26 September 2007 (UTC)

It's a little bit bizarre to talk about the observable universe being bigger than the universe. The observable universe (in the sense that it seems natural to think of it) is part of the universe, and the part can't be bigger than the whole; at most it can be the whole. Is the article making the point that you might be able to see more than one image of some star, the light in one image having made a complete loop of the universe before getting to you? That could well happen, but it's the same star; it would be odd to count it twice as part of the observable universe.

Still, I can figure out what is meant here (I think), and I have no objection if it's standard usage. But is it, really? I'd like someone more familiar with the literature than I am to address that. ---- Trovatore (talk) 19:39, 16 November 2007 (UTC)

I agree this is badly-worded. What we really want to say is that the distance to the horizon can (in principle) be larger than the actual size of the Universe, if the Universe is periodic. But without using the word 'horizon', which might add to the confusion. Maybe 'the edge of the observable Universe can be...'. If that were the case, it would be more sensible to say that the entire Universe was observable. -- Cosmo0 (talk) 21:42, 16 November 2007 (UTC)

Proposed Merger

I'm proposing to merge Mass of the observable universe into this article because it seems to be just a rather confusing version of the Matter content section of this article, which doesn't make a clear distinction between estimates of stellar mass and total mass. Cosmo0 20:13, 19 September 2007 (UTC)

I concur with this proposal. Unless more detail and proofs are added to the Mass of the observable universe article, that content would make a good section for this article. --Parsifal _Hello 07:50, 28 September 2007 (UTC)

No comments since Sept 19, so I've completed the merge to a new section in this article. The information fits well; this article is deeper and the other one was just a stub so the solution seems good. --Parsifal _Hello 07:39, 1 October 2007 (UTC)

Size source

The statement that the diameter of the universe is 92 billion light years cites a Scientific American article that states that the farthest one can see is 46 billion light years. Logically, if the radius of the universe is 46 billion light years, then the diameter must be 92, but the article doesn't say that 46 billion is a radius in the first place. Also, I could not confirm that fact that the universe has a diameter of 92 billion light years on any other website. The math formula stated in the section does not have a source, and the section states that "it is sometimes quoted as a diameter of 92.94 billion light-years". Who, other then Scientific American, has said this? Latitude0116 05:34, 17 October 2007 (UTC)

Well, the 92.94 was wrong; it was supposed to be the range 92–94. We're talking about the observable universe here, i.e. the part we can see. If the farthest we can see is about 46 billion light years in any direction (as the Scientific American article claims) then the observable universe is a sphere with that radius.

There are lots of sources that give totally different numbers for the size of the observable universe. Many of those numbers are dead wrong; others are less-accurate earlier estimates and/or estimates expressed in terms of a different distance scale. The scale that's being used here is comoving distance.

Unfortunately misunderstanding of cosmology is very common even among professional physicists. Appendix B of astro-ph/0310808 has some examples of misstatements by famous physicists on this subject. This Physics Factbook page on the diameter of the observable universe cites 12 sources, every single one of which is wrong. Most popular-press articles on the subject get all the numbers wrong. The Scientific American article is a rare exception in print, and Ned Wright's cosmology pages are a rare exception on the web. You can use Ned Wright's cosmology calculator to reproduce the Scientific American figures. For the comoving radius enter a large number (at least 1089) for z and click "Flat". The answer depends on H_o, which isn't known very accurately. -- BenRG 13:37, 17 October 2007 (UTC)

So is this resolved now? Can we get rid of the {{citecheck}} tag? If not, what still needs to be checked? Tags like this aren't supposed to remain indefnitely -- if the issue has been resolved to everyone's satisfaction then we should dump the tag; otherwise we need to figure out the resolution. ---- Trovatore (talk) 19:26, 16 November 2007 (UTC)

I've moved the size reference to belong to the value of 46.5 billion light years, since the article quotes the radius, not the diameter as implied by the previous placement. Converting the radius to a diameter is trivial. I think this ought to address the issue raised above. -- Cosmo0 (talk) 21:54, 16 November 2007 (UTC)

Question on idea of universe being larger than observable universe

The CMB is the oldest, most distant, and most redshifted signal we can see, at a redshift of 1100 and an age of 13.7 billion years. It pre-dates any star or galaxy formation.

So it seems to me that all of the matter that has formed or has ever formed into stars and galaxies is less redshifted, less old and less distant than the CMB and therefore lies within the observable universe. There may be some of it we cannot ever see due to opaqueness in the early universe, but it does not seem correct to claim that there is star/galaxy material we cannot see because it is too distant; since from our observational point it can't be as old/distant as the CMB.

208.48.21.145 02:09, 16 November 2007 (UTC)SteveP

The only part of the primordial fireball that we can see (via the CMB) lies on a sphere at a constant distance away from us. We can't see nearer parts because the light from them already passed the earth eons ago and is now moving away from us, and we can't see farther parts because the light hasn't reached us yet. Given the amazing homogeneity of the part of the fireball we can see, there's every reason to believe that it was equally homogeneous inside the sphere and for a good distance outside. It's highly implausible that the homogeneous region happened to end just beyond the spherical part that we happen to be able to see right now. If it was finite, it was probably much larger than that, which implies the existence of galaxies outside the sphere (that we can't see yet). -- -- BenRG (talk) 16:47, 16 November 2007 (UTC)

Imagine, for the sake of argument, that the Universe is infinitely large and filled with galaxies that all formed at the exact same time. Light from more distant galaxies takes longer to reach us, so we observe them as they were at an earlier time. As you look at more and more distant galaxies, there comes a point at which the light we observe left the galaxies at the moment of their formation. Beyond that point, light from galaxies at even greater distances has not had time to reach us since they formed, but this does not mean that they don't exist (remember, we're assuming that the Universe is infinite) or that they would necessarily have to be older than galaxies nearer to us (we're also assuming that all galaxies formed at the same time). All it means is that, in order for us to be able to observe them, they would need to be older (possibly older than the age of the Universe, or the age of the CMB, if you take into account the fact that the very early Universe was opaque to light). This is why we can't observe them.

Of course, I'm not implying that the Universe is necessarily infinite, just that the finite extent of the observable Universe doesn't place an upper limit on the actual size of the Universe.

-- Cosmo0 (talk) 17:44, 16 November 2007 (UTC)

Observable universe image

I've removed the image purporting to show the observable universe because it further confuses an already confusing subject. What the image actually shows is the Hubble volume, with a size of ~14 billion light years, while the article goes to lengths to explain that the observable universe is much larger. For now, I've restored the original image - although it's also not a very satisfactory illustration of the subject because it only shows galaxies in the local universe (smaller even than the Hubble volume), at least it's potentially less confusing. Cosmo0 (talk) 10:52, 7 January 2008 (UTC)

Observable Universe vs Visible Universe

Numerous references made to this distintion, but none really explains the distinction very well. Can someone elaborate ? Nkingsland (talk) 20:04, 1 April 2008 (UTC)

Totally agree. I had to reread some of the stuff above that part but was able to comprehend it, whereas the visible vs. observable bit came out of the blue and wasn't any clearer after several rereads. Specifically, it says (emphasis mine), "although as noted in the introduction, it's expected that the visible universe is somewhat smaller than the observable universe", yet the introduction only discusses the observable universe vs. the (probable/possible/whatever) entirety of the universe. (The notion of a visible universe isn't even brought up until the Size section.) So, IS there a true distinction between visible and observable, or is this just confusing gobbledygook that needs to be cleaned up?

Why isn't 156 billion light years the correct diameter?

As the article currently states in the "misconceptions" section, 156 billion light years has been repeatedly reported as the diameter of the universe, based on two papers published by Cornish. Since Cornish is cited in all of the articles claiming the 156 b.l.y. diameter, I am inclined to trust that number. Someone has claimed that 78 b.l.y. was already the diameter and was incorrectly interpreted as the radius, but cites no sources or reason validating this. It seems to be an unfounded interpretation of the paper, and somehow I doubt Cornish would have made such a simple mistake. Unless someone can show that the numerous articles citing 156 b.l.y. were mistaken (I cannot find any corrections or retractions online), then it must be assumed that 78 b.l.y. is the radius and 156 b.l.y. the correct diameter.

Boogaborg (talk) 19:47, 13 April 2008 (UTC)

Cornish et al didn't make a mistake. They never said anything about 156 billion light years or 48 gigaparsecs. That was some reporter's mistake. Nor did they claim to have established the size of the universe, or the size of the visible universe. That was again a reporter's misunderstanding. There's no way anyone could establish a lower bound of 156 billion light years on the size of the universe by looking for correlated circles in the WMAP data. The intersection of two spheres of radius R with centers separated by d is a circle of angular radius cos⁻¹ d/2R. Therefore if you can rule out correlated circles with an angular radius larger than α in the WMAP data then you can rule out a universe with a diameter smaller than 2R cos α, where R is the distance to the last scattering surface (i.e. the radius of the visible universe). If you rule out all circles then α=0 and your lower bound is about 95 billion light years. That's the best you could ever hope to do with this approach (and probably any approach). Their bound was worse than that because they only ruled out circles larger than 25°. If you don't believe me, read page 4 of the paper, where they use the formula I just gave to calculate the value of 24 gigaparsecs (bottom of the first column) and explicitly say that it is a diameter (bottom of the second column). -- BenRG (talk) 00:43, 20 April 2008 (UTC)

Actually, you are incorrect, at least partially. In response to their article entitled "Universe Measured: We're 156 Billion Light-years Wide!" ( http://www.space.com/scienceastronomy/mystery_monday_040524.html ), Space.com received several inquiries about how that could be with the Cosmos being only 13.73 billion years old. They referred the inquiries to Cornish, whose answer is appended to the bottom of the article. Were your suppositions about Cornish's findings correct, he undoubtedly would have corrected that in his reply. Chuck Hamilton (talk) 05:36, 13 December 2009 (UTC)

Mass of the observable universe

Wouldn't it simply be most accurate to say that we don't know the mass of it (disregarding the fact that it's rather pointless to weigh anyway, and rather impossible, seeing as space does not 'rest' on anything, it's just space), because we haven't seen all the planets, and thus cannot possibly make an accurate assessment? —Preceding unsigned comment added by 70.52.67.223 (talk) 19:39, 22 April 2008 (UTC)

It's not sensible to talk about the weight of the observable universe, for the reasons you give, but the mass of the observable universe is something that is frequently discussed by astronomers and can, at least in principle, be measured. Planets, by the way, make up a negligible fraction of the total mass: most of the directly observable mass is in stars and various gas phases, while the majority of the total mass is (probably) in the form of dark matter. Cosmo0 (talk) 21:56, 22 April 2008 (UTC)

Fraction

Are there any estimations on which fraction of the entire universe made up by the observable one is? I read that it is a 10100th part of it, but that number was just blared out without actually explaining it. —Preceding unsigned comment added by 91.11.180.190 (talk) 21:37, 13 June 2008 (UTC)

The entire universe may be infinite, so this is simply not known. --Trovatore (talk) 21:39, 13 June 2008 (UTC)

The fraction quoted (which should read 10¹⁰⁰th) appears in this New Scientist article. It seems to come from a model of inflation intended to explain asymmetries in the CMB, although I don't know the details of how the number is calculated. Anyway, the point is that it's a theoretical prediction (and a somewhat speculative one at the moment, I think). Cosmo0 (talk) 23:20, 13 June 2008 (UTC)

New findings say that the Universe is 15.8 billion years old and 180 billion light years in diameter

Here are my sources for NASA's discovery. Please let me know if I can use this information in the article.

http://www.space.com/scienceastronomy/060807_mm_huble_revise.html

http://astrogeology.usgs.gov/HotTopics/index.php?/archives/197-Universe-Might-be-Bigger-and-Older-than-Expected.html

http://science.slashdot.org/article.pl?no_d2=1&sid=06/08/07/1222245

http://www.space.com/scienceastronomy/060807_mm_huble_revise.html

http://www.physicsforums.com/archive/index.php/t-128263.html

http://www.politicsandcurrentaffairs.co.uk/Forum/general-chat/26001-universe-might-bigger-older-print.html

http://astrogeology.usgs.gov/HotTopics/index.php?/archives/2006/08/10.html Maldek (talk) 08:32, 21 June 2008 (UTC)

They do not say this. The articles say "might" and it is a 2 year old story and it is already incorporated into the Wikipedia article !. Did you read the articles or even this article first before posting your latest link farm ? Ttiotsw (talk) 09:14, 21 June 2008 (UTC)

Original research?

It appears to me there are only a couple of highly debatable sources given in the article, and a lot of speculation based upon them. Other articles do not appear to necessarily agree with the content of this WP article. Take, for instance, this_article, which describes the observable universe as finite with a radius of about 10²⁸ cm, which is, if my equivalence calculation is correct, roughly 10 billion (thousand million) light years. Double that to get a diameter, and you end up with about 20 billion years in diameter. Offhand, I do not see wide agreement among cosmologists that these massive numbers such as 72 billion LY or even greater are very credible. What additional reliable sources point to consensus on such inflated numbers? ... Kenosis (talk) 02:16, 18 August 2008 (UTC)

There is a consensus among experts that these numbers are correct (with error bars, of course). Ned Wright's online cosmology calculator will work it out for you. Set the parameters H₀, Omega_M and z from H₀, Ω_m and z_dec in the WMAP five-year results here, then click "Flat". The output labeled "comoving radial distance" is the one you want. The math is shown here (the comoving radial distance is there called D_now).

The reason you often see smaller numbers is that there are many people out there who don't understand the math but do know that the universe is about 13.7 billion years old and that light travels at c, so they multiply the two together and get a visible distance of 13.7 billion light years. They're so sure they've done this calculation correctly that they publish it in textbooks and papers without double checking, and it often gets past peer review because so many physicists labor under the same misconception. So we have the unfortunate situation that there are very many "reliable sources" in cosmology that confidently state misinformation as fact. Other common mistakes are assuming that the whole universe can't be any larger than 13.7 billion light years (because it can't expand faster than c) and that faraway objects are retreating from us at nearly the speed of light (based on the special-relativistic relationship between redshift and speed). A couple of decades ago the age of the universe wasn't know very accurately, but was known to be on the order of 10 billion years, so you'll also see older sources claiming a radius of around 10 billion light years. The figure you quoted, 10²⁸ cm, is just an order-of-magnitude estimate and isn't far off (the actual radius is around 10^28.6 cm).

I'm not sure how Wikipedia normally deals with unreliable reliable sources. I can dig up reliable sources supporting what I just said, but given that there are also reliable sources contradicting what I said, would my choice of sources itself constitute OR? Who would you bet on in a dispute between Ned Wright and Richard Feynman? I'm hoping we never have to face these awful questions head on. -- BenRG (talk) 16:39, 18 August 2008 (UTC)

Actually, the just-linked paper is by Davis and Lineweaver, mentions Feynman only once. The Davis-Lineweaver paper asserts that Feynman's statement is one of 25 widely misunderstood or misquoted statements, but without directly addressing the question of what Feynman's assertion is purported to mean in the minds of Davis and Lineweaver, or what its purported misunderstandings are and where the rest of us "mere mortals" can find them, or why those putative "misunderstandings" are indeed misunderstandings of what Feynman may have meant. It's a shotgun-style list of things the authors of this 2004 paper think is subject to misunderstanding but without any explanation why they think so. That paper by Davis and Lineweaver is not widely drawn upon by other cosmologists with scholarly credentials, AFAIK. I think it's fair to say that a major part of the content of this WP article is built on the David-Lineweaver paper. But the more important issue appears to me to be that this article relies primarily on the Davis-Lineweaver paper for its content, asserting more or less the same "subject to widespread misunderstanding" numbers, without giving the reader the opportunity to know anywhere near how divergent the prevailing opinions are about the topic.

In Wikipedia, we need to do our best to report what the sources say about the topic at hand, and to the best extent possible give readers a perspective on what the reliable sources say. What constitutes a reliable source is, of course, always a fair topic of debate. But as far as I can tell, the various sources about this topic in general have very wide significant disagreement about what is meant by "observable" and also wide discrepancies about the size of the observable universe.

Thus I think it's time to put, at the top of this WP article, a caveat to the reader that there may be original research or unverified claims in this article, at least until it can be brought into line with the range of opinions on the topic. Since it's been in its present state for awhile now without a whole lot of visible complaint, I should suppose there's no desperate hurry, as the bulk of the current content was introduced some time ago. But on a topic of this kind of importance, which asserts among other things that the observable universe may be of greater physical magnitude, or even equal physical magnitude, than the range of credible (reliable) estimates of the magnitude of the, might we say, "entire universe", I should think the process of bringing the article more into compliance with WP's content policies should start. Therefore, lacking come compelling reason otherwise, I'm inclined to put up an "original research" template to warn readers that this stuff is highly debatable even among experts, that it leans overly heavily on one source, and is substantially the original research of a WP editor. ... Kenosis (talk) 02:53, 19 August 2008 (UTC)

Another apparent issue I noticed in my very brief research is this. Davis and Lineweaver state in their paper: "Currently observable light that has been travelling towards us since the beginning of the universe, was emitted from comoving positions that are now 46 Glyr from us." Robert Roy Britt makes a similar point when he says: "The light has not traveled that far, but 'the starting point of a photon reaching us today after travelling for 13.7 billion years is now 78 billion light-years away,' Cornish said. That would be the radius of the universe, and twice that -- 156 billion light-years -- is the diameter. That's based on a view going 90 percent of the way back in time, so it might be slightly larger." This appears a reasonable way of estimating the size of the universe, and there appears to me to be fairly broad consensus on this basic issue.
..... But an image that has been traveling for, say, 10 billion years is the image of the object as it was 10 billion years ago, not as it is now, so it would seem to me the present state of the object would [not] be observable. Hence the estimate of the objects current distance ought not be termed "observable" I would think the reader of the WP article ought know about such issues and the various range of views in defining what's meant by "observable universe" and what the various views indeed are. Alternately, does anyone who's more familiar with this topic know if there are reliable sources that indicate a broad consensus among cosmologists that agree with what Davis and Lineweaver, and this WP article, are currently asserting? ... Kenosis (talk) 18:31, 19 August 2008 (UTC)

First of all, I'm very familiar with this topic and when I said there's consensus I meant it; I'm not guessing on the basis of a few papers I found with a web search. Those other numbers you found on the web are wrong. But I think some of your criticisms are reasonable. The article could do a better job of explaining what the "observable universe" is, and the notion of the "size" of the observable universe is inherently problematic given the combination of continuing expansion and light-travel delays. Let me try to explain it more carefully here.

• I strongly encourage you to read Ned Wright's cosmology tutorial and Metric expansion of space for background.
• Spacetime is four-dimensional. The portion of spacetime that we can see at this point in history is three-dimensional. It has the shape of a hollow cone (but see below), with the here-and-now at the apex and the sides sloped at c. It extends back in time to the last scattering surface, which is the limit of what we can see. The last scattering surface is a 3D surface in 4D spacetime, but it's probably better thought of as an "era" than a "surface"; it's the time at which the early universe cooled off enough to become transparent. Aside from its shape in spacetime it's very much like the surface of the Sun. It's made of glowing plasma and it has an optical depth. The Sun's optical depth has units of distance and the last scattering surface's has units of time, but we can't see into the early universe for the same reason we can't see into the Sun. The image of the last scattering surface in the sky is also known as the cosmic microwave background.
• The cone is not actually cone-shaped (to the extent that "shape" makes sense in a curved manifold). It's more accurate to say that you get it by starting with a cone in our local tangent space and geodesically extending it into the past. Because the curvature of spacetime isn't too large in the present era the extended cone continues to be cone-shaped for a while (on the order of a billion (light) years), but eventually it starts being more "teardrop-shaped". This image from Ned Wright's tutorial gives the general idea, though with a couple of caveats: it doesn't show our universe (the Lambda-CDM parameters are wrong), and it's a flat projection of a curved geometry which inevitably introduces distortion (similar to a Mollweide projection).
• Most objects in the universe move roughly with the Hubble flow. You can think of the Hubble flow as motion on "lines of longitude" in spacetime. The black lines in the teardrop picture I just linked are Hubble flow worldlines. Our past light "cone" intersects some lines of longitude and not others. The collection of all the lines of longitude intersecting our past light cone is a four-dimensional "cylindrical" region which you can think of as the collected worldlines of everything that we can currently see at some era in its history.
• The "size of the observable universe" is the size of the cylinder. The universe is expanding and the cylinder along with it, so its size is time dependent, but in the present era it has a radius of about 46–47 Gly.
• One could argue that this is arbitrary, and it is, but it's hard to come up with any other measure of the size that makes sense.
  • You can't use the size of the cone because it doesn't have one: it's a null surface.
  • You could use the size of the cylinder in another era. The only other non-arbitrary era we could use would be the last scattering era. The cylinder's radius in that era is about 40 million (not billion) light years, about 1100 times smaller than its radius now. The problem is that this isn't consistent with local astronomy. The Local Supercluster is 200 million light years in diameter, and it certainly doesn't extend outside the observable universe. In order for local distance measures to extend smoothly to cosmological distances we have to use the radius in the present era. Your suggestion to quote the distance to observed objects in the era when they emitted the light, instead of the distance in the present era, has the same problem. We would wind up saying that the most distant observed objects are closer than the edge of our local supercluster.
  • There's no distance in this geometry that's equal to 13.7 billion light years. There's a time interval of 13.7 billion years (the distance from the here-and-now to the last scattering surface along a Hubble flow line of longitude), but no corresponding distance.
• There is some ambiguity in the radius because there's some ambiguity in how far you extend back the light cone. You can't extend it "all the way" because nobody knows what the geometry of the very early universe looked like; it's certainly not correctly modeled by Lambda-CDM. But there's still a choice of extending it back to the last scattering surface or beyond the last scattering surface to the earliest era where we're reasonably certain of the geometry. Fortunately the resulting difference in size is only a couple of percent.

If you're still suspicious of my claim that those other numbers are wrong, I offer the following:

• I assume you accept that the Lambda-CDM model has broad support among cosmologists. The parameters of the model are known fairly accurately thanks to WMAP and other astronomical experiments. I hope you find it at least plausible that Lambda-CDM would predict a specific value for the radius of the observable universe (as defined above).
• Not only does it predict a specific value, but I linked the math. If you have the appropriate software handy, you can evaluate the integral and verify that D_now ≈ 46-47 Gly, with some variation because of uncertainty in the cosmological parameters and in how far you extend the light cone. I also linked Ned Wright's cosmology calculator which will evaluate the integral for you. You'll never find the mathematical theory behind the other numbers, because there isn't any.
• If there were controversy over the size of the observable universe then there would be back-and-forth debate in the literature, but there isn't. There are papers (like Davis-Lineweaver) saying that 13.7 Gly is wrong and 46 Gly is correct, but none the other way around. The papers that claim 13.7 Gly is correct show no awareness that anyone might think otherwise.
• The other numbers are easily explainable as resulting from specific misconceptions about cosmology. Those explanations can be found in the Davis-Lineweaver paper and this Wikipedia article. Since no other justifications for those numbers exist in the literature, it seems reasonable to assume that they do in fact result from those misconceptions.
• The Davis-Lineweaver paper is pedagogical. It's not drawn upon by professional cosmologists because it's not a contribution to the field, nor intended to be.

I should mention that Neil Cornish is enough of an expert to know that "the starting point of a photon reaching us today after travelling for 13.7 billion years is now 78 billion light-years away" is false (a correct figure would have to be 47 Gly or less). Either he was misquoted or he misspoke under interview pressure. My guess is the former—78 occurs in Cornish's work (in a different context) and Britt probably couldn't figure out why he said 47, and "fixed" it. -- BenRG (talk) 00:00, 20 August 2008 (UTC)

Ben, I'm going to place the "OR" template for now, just to warn readers that there are still issues with this article. I should probably also place a WP:NPOV template, but will refrain for now because it's not clear there are opposing camps but instead just seems like a lot of confusion among the many secondary sources about how relativity and other factors such as the recently ascertained apparent acceleration of the Hubble constant affect the estimates of the magnitude of the universe. The very term "observable universe" as differentiated from "universe" is also problematic, and this article presently does not deal with this issue, opting instead for an approach that makes a set of assertions in support of one point of view, then asserts to the reader that all other points of view about what methods of assessment of the concept of what should be termed "observable" and magnitudes (sizes and/or distances) of what's "observable" are all "misconceptions" except for the POV of the article. The main concern at present: lacking a reasonably clear identification of what is meant by "observable" along with a reasonable identification of what sources define it in what way, along with a near-total lack of description and identification of what sources take which of the various positions on the relevant issues, it requires at minimum an "OR" template. Over time, I trust this will result in a more accurate and useful article. ... Kenosis (talk) 03:46, 21 August 2008 (UTC)

Is this all that the OR tag is for? The ~90 Gly diameter is accurately described and completely uncontroversial. Can we change it out for a {{Refimprove}} tag on the section, or better yet, inline {{citation needed}} tags for specific statements? - Eldereft (cont.) 16:40, 21 September 2008 (UTC)

Topic ordering

There are 3 sections of the article: (1) a comparison of Observable Universe vs Universe and a Whole (UAW) (2) Size and (3) Mass

It seems confusing to put the comparison first, mostly becuase the 2nd paragraph discusses a number in relation to the size, which has not yet been covered in the article. The first "size" you read is not the generally accepted on- but one from a single paper. It would make more sense to me to put the order Size, Mass, Comparison to UAW instead of the current order.

Thoughts? 75.36.198.68 (talk) 20:54, 18 August 2008 (UTC)

Now you mention it, I agree - the ordering does seem wrong. The article should start with the basic facts and discussion of the universe versus the observable universe should probably go towards the end. That said, the comparison between the two is important enough that it should probably be briefly mentioned in the introduction as well. I'll give it some more thought and maybe make some changes. Cosmo0 (talk) 20:24, 21 August 2008 (UTC)

do we test forward or backward?

I'd like to hear BenRG or some other expert explain this point that I'm a bit unclear on (and, I think, so is the corpus of WP cosmology articles). Namely, which (if either) of the following definitions is correct:

1. Two events are in each other's observable universe if their backwards-facing timelike cones have nonempty intersection
2. Two events are in each other's observable universe if their forwards-facing timelike cones have nonempty intersection

The reason I think this needs to be clarified, other than for my own personal edification, is that this articles seems to be using the "backwards" definition (though it's certainly possible I've misread it), whereas the notion that the Big Rip involves a shrinking observable uinverse seems to be referring to the "forwards" one. --Trovatore (talk) 00:26, 20 August 2008 (UTC)

I think you've hit on a key problem of definition here, because I've seen the term observable universe defined in both those ways. What is described in this article is the first definition: the region of the Universe that can be observed - in principle - right now. That region is always expanding over time. The second definition corresponds to what is described in the Hubble volume article: the region of the Universe that will be able - again in principle - to be observed by us as it exists now at some time in the future. This is a much smaller volume and, as you say, it can shrink over time if the expansion of the Universe is accelerating. I guess we need to make the distinction much clearer in both articles and cross-link to each other to avoid confusion. Cosmo0 (talk) 20:16, 21 August 2008 (UTC)

Forward from zero

I thought That the Hubble radius was what the special relativity laws told us we could see from the illuminating but red shifted light from the matter of the universe during it's estimated 13.7 x10E9 year time of existence and neglected any concept of space expansion. And then the expansion was inferred and calculated from the GR mathematics involving a cosmological principal permitting either a steady state or expansion or contraction, depending upon the hypothesis. And The Friedmann hypothesis, which only proposed an initial expansion followed by a reducing rate was later replaced by the Lemaitre hypothesis, which proposed an initial variable but now increasing rate was substituted, but has problems with the assumption of any rationality of physical rule as to why it should occur. But anyway the rate of space inflation is generated by that theory as being 3.38 times the hubble rate, thus generating a universe universe radius (for now) of 46 MPC. WFPMWFPM (talk) 21:25, 27 October 2008 (UTC)

Dark flow

This page may need to be updated soon if the dark flow is found to be correct and found to be from things outside the observable universe. We'll have to wait and see. YouthoNation (talk) 14:35, 28 September 2008 (UTC)

Dead Citation Link

The third citation link (Lineweaver, Charles; Tamara M. Davis (2005). "Misconceptions about the Big Bang". Scientific American. Retrieved on 2007-03-05.) fetches a 'You have reached this page due to an error' message.

Can someone supply another link to support the Size sub-topic claim that the universe is 46 billion light-years in diameter. Otherwise, it is unsupported and may need to be deleted. Makuabob (talk) 23:27, 2 November 2008 (UTC)

I fixed the link. Unfortunately it looks like the images from the article are no longer available for free. Fortunately the text still is. -- BenRG (talk) 23:41, 6 November 2008 (UTC)

Got it! Thanks! :-)
Makuabob (talk) 02:15, 12 November 2008 (UTC)

Removed paragraph about Scott and Zibin paper

I removed this paragraph:

On the other hand, another supposition (ref: Douglas Scott, and J. P. Zibin, How many Universes do there need to be?. [2]), using data from the WMAP and Hubble Space Telescope Key Project, is that the entire universe contains at least 21 observable-universe-sized patches. The caveats are: the multiverse is spatially closed (consistent with WMAP findings) and globally the metric is homogeneous and isotropic FRW spacetime.

Scott and Zibin don't actually make that claim. They do make the following point: if the universe has the spatial geometry of a hypersphere of radius R, then its total volume is 2π²R³ (which is the surface area of a 3-sphere, just as 4πR² is the surface area of a 2-sphere). The current astronomical data is consistent with this model provided R is at least (some value), which implies that 2π²R³/V_obs is at least 21 or so. But they're not arguing that the universe is at least that size, they're arguing that it could be as small as that size. In fact it could be much smaller than that: it could be elliptic (volume π²R³) or it could be a Poincaré homology sphere (volume π²R³/60) or it could be a Seifert-Weber space (volume about 11R³), or it could be a flat torus (any volume at all). They partially acknowledge this with the comment

In addition, non-trivial topologies can render a spatially flat universe finite in volume, but that possibility only strengthens our contention that we cannot conclude that the Universe is spatially infinite.

So they aren't trying to establish a lower bound on the size, they're just giving an example (and not the smallest possible one) of a finite universe that's consistent with the data. None of these small models has any theoretical motivation, though. The best idea we have for why the universe is so flat and uniform in the first place also predicts (as far as I know) that it's much larger than these toy models suggest. -- BenRG (talk) 23:34, 6 November 2008 (UTC)

Edge of the visible universe

The following statement appears incorrect:

The comoving distance from Earth to the edge of the visible universe (also called particle horizon) is about 14 billion parsecs (46.5 billion light-years) in any direction.

46.5 billion light years requires light 46.5 billion years to traverse, assuming c doesn't change. This would put the age of the universe out to at least 46.5 Gyr, which is much larger than the currently accepted 13.7 Gyr. Hence I think it should say "13.7 billion light years". The distance in parsecs would then be 4.2 billion.—RJH (talk) 23:32, 22 December 2008 (UTC)

Except that the universe is expanding - the distance now is not the distance the light has traveled. This is treated at Observable universe#Size and at Comoving distance. The surface of last scattering I believe defines the visible universe, whereas the observable universe includes all volumes of space whose observation in principle is not precluded by c. - Eldereft (cont.) 08:18, 23 December 2008 (UTC)

Comoving distance is approximately 3 times the time the light has traveled. If would be exactly 3 times if the dark energy were absent (assuming density equals critical density). Ruslik (talk) 17:05, 23 December 2008 (UTC)

I see. Thank you for explaining it.—RJH (talk) 17:46, 24 December 2008 (UTC)

Contradiction (2)

Two questions, firstly the article dismisses the notion that the age of the universe (regression local expasion to a singularity) of 13.7billion years can not be the size of the whole universe. Yet surely nothing, not even space can exceed the speed of light (ignore the fraction of a second of inflation which did not add very much in macro-scale). Sure Hubble constant indicates expandion rate, but are we really stating the universe must be expanding at some 3c in each direct (ie diameter increasing at nearly 6c)?

With this is current explanation in the article of "78 billion light-years. This is a lower bound for the size of the whole universe, based on the estimated current distance between points that we can see on opposite sides of the cosmic microwave background radiation (CMBR), so this figure represents the diameter of the sphere formed by the CMBR. If the whole universe is smaller than this sphere, then light has had time to circumnavigate it since the big bang" - yet this is unexplained drivel, CMBR needs only a distance of 13.8billion light years not to be able to reach us in the 13.7billion years of the universe (light need only travel in the quickest possible route which is as a radius to us, accepting no central position for earth; arcing circumferentially through the space is an alternative longer route but not the quickest and so not the fastest means of information exchange)... either I'm being daft, or the current explanation ain't that clear for us dim wits. David Ruben ^Talk 05:26, 12 January 2009 (UTC)

Yes, we really are; er sort of - see Comoving distance and Metric expansion of space. If any of that inspires you with language which is more clear without sacrificing precision or too much conciseness, please add it to the article. - Eldereft (cont.) 16:36, 12 January 2009 (UTC)

Oooh, ahh. Ok I can get that expanding universe means that whilst light been travelling for last 13.7 billion years, the original starting point has receeded away and so now is at a distance further than 13.7billion light years ... this is both given by at least by the c expansion of universe on simple "big bang" idea and added to by Hubble constant - link to Metric expansion of space perhaps most helpful and in particular its reference to http://arxiv.org/abs/astro-ph/0310808 but even that is going to need a few read throughs (various light curves et al) and as that article shows, whole topic been poorly described in the literature previously.

Description though in this article is just confusing, indeed on re-reading the lead in and first section I still do not see this in the description given. I rather suspect that this article for the lay reader is more confusing than helpful, if so then I would tentatively propose its total deletion (AfD) ! A rewrite is needed: "region of space bounded by a sphere" is just an awful phrasing for a lay reader to see in the first sentance of a lead in. The 1st section "This can be justified on the grounds that we can never know anything by direct experimentation about any part of the universe that is causally disconnected from us, although many credible theories, such as cosmic inflation, require a universe much larger than the observable universe" is (whilst technically correct) frankly just both poor English and reads (to a general reader) as just jibberish in its current phrasing. David Ruben ^Talk 09:48, 13 January 2009 (UTC)

I just took a stab at clarifying the lead (and adding some information so that it more accurately summarizes the article). Did that help at all? - Eldereft (cont.) 23:26, 13 January 2009 (UTC)

Your edit to the first paragraph completely changed the definition of "observable universe". I reverted it to the old version which, I think, reflects what cosmologists mean by the term. The original definition refers to a region of spacetime like this:

                       +--------+-- comoving boundaries
                       v        v
                        ________
                   now |        |
                       |        |
                       |        |
              big bang |________|

but your revised definition would refer to a region of spacetime like this:

                   now     /\
                          /  \
                         /    \
              big bang  /______\

Writing about this stuff is extremely difficult because of the inadequacy of English vocabulary. I go crazy trying to answer questions as simple-sounding as "is the observable universe expanding?". -- BenRG (talk) 09:57, 14 January 2009 (UTC)

Ah, that would explain why I kept wanting to write light cone. Also, I think what I wrote has problems with black holes. Thank you. - Eldereft (cont.) 13:53, 14 January 2009 (UTC)

Agree difficult to write about this clearly as we talk about "light-years" which of course is a distance, but this makes the English awkward as we refer to 13.7b years and yet light having traveled 46b light-years. What we actually need is a concept akin to "distance light travels in a year in a vaccuum from a static source to a static observer, as opposed to the current separation which has been expanding during this time".

Also I would suggest above square diagram is part of problem why people don't understand the terms - a square is visually interpreted to be a fixed size rigid shape & size and labelling a baseline as "comoving boundaries" does not give the sense of dynamic expantion (were the image to be animated with enlarging baseline then that might get it). The universe has been expanding since the light we now see, so what is really needed is a expanded greyed-out trapezium overlying the square showing where the expanded universe has now reached (ie the baseline of the square is the intuitive Time x c), whereas the base of the trapezium is more akin to Time x 2c^(hubble expansion) for expansion of universe and its acceleration (the maths formula obviously crude, i.e. wrong, but illustrative)

Trying to think of a better way to diagram:

Firstly the idea of a trapezium to cover time, distance of 13.7b ly, scale of actual expanded observable universe. Time is vertical, inner 'Λ' what we assume is a light cone, the outer slopes of the 'W' (ideally in grey) the sense of what we observe outwards has expanded away from current visual position (as the current image is 13.7b years out of date)

                        1) Big bang point
                                ↓
                                ↓
                         \      /\      / ←←3)Observable universe expanding 
                          \    /  \    /      since light started out to us,
                           \  /    \  /       and so now is much larger than 
2)Now 13.7 billion years →→ \+------+/        just distance light travels 
  later light reaching us                     in 13.7 billion years.   

Secondly attempt not to flip back on ourselves but show difference in light-distance, against universe expanding outwards in all directions (i.e. after inflation, starting at 2c) and with attempt to show outer edges as a bell curve as it then accelerates outwards (the inner light curve should be in grey to help clarity)

                           _/\_                     ←←Big bang
     TIME                _/ || \_                
       ↓               _/  /  \  \_                 
       ↓            __/   |    |   \__           
                 __/     /      \     \__        
              __/       |        |       \__        
          ___/          /        \           \___    
         /             |          |              \   
                                                    ←←Now 13.7 billion years later
                       |←←(LC)→ →|
         |←← ←← ←← ←← ←← (OU) → → → →→ →→ →→ →→|
                      ← ← DISTANCE → →
.
Caption: The universe has expanded since the Big Bang and so has therefore the source
of light we now see at the limit of the Observable Universe (OU).  The universe's size
is not given by just the size of an imaginary Light Cone (LC) travelling from a
stationary source over a period of the 13.7 billion years, but this distance plus 
that by which the universe has expanded by, and doing so at an accelerating rate.

Any of this helpful ? David Ruben ^Talk 13:19, 15 January 2009 (UTC)

If I'm understanding your W-shaped diagram correctly, it does show a true fact about cosmology that people sometimes overlook, namely that the stuff we see has moved farther away from us since it emitted the light. But this just makes people double their estimate of the maximum possible radius of the observable universe, from 13.7 billion ly to 27.4 billion ly (13.7 billion of initial distance, plus at most another 13.7 billion of further expansion). That's still wrong—wrong because it's numerically too small, but also because this approach is wrong from the get-go. In fact the CMBR that we now see was emitted from about 40 million (not billion) light years away, and the distance to that matter has increased by 46 billion light years in the mean time, and its total distance is 40 million ly + 46 billion ly = 46 billion ly. It's correct to take the sum, but that's small comfort when people don't understand the distances that go into the sum.

Your second diagram seems incorrect. The size of the observable universe is the size of a light cone originating at the Big Bang. Your (LC) and (OU) lines should coincide. The observable universe isn't defined by that light cone—it's defined like this:

                 <-------- comoving position -------->

                     here-and-now ->               | <- edge of observable universe
                                  _/\_             |
                               __/    \__          |
      past light cone ---> ___/          \___      |
                      ____/                  \____ |
                     /____________________________\|
                      Big Bang / end of inflation

—but because of homogeneity that gives the same distance as this:

                     here-and-now ->               | <- edge of observable universe
                     \_                          _/|
                       \__                    __/  |
   future light cone ---> \___            ___/     |
                              \____  ____/         |
                     ______________\/______________|
                      Big Bang / end of inflation

Incidentally, I rewrote the first paragraph of the article—what do people think? -- BenRG (talk) 00:12, 16 January 2009 (UTC)

Yes like your second diagram ! (the first less easy to grasp as intuitively feels wrong (although I can understand it is correct) given "now" gets placed at the apex looking out into the width of the universe). The curve shape though seems curving to a slower rate of expansion (a "U" shape) rather than accelerating ever fast outwards as a trumpet-bell shape (not that ascii is best way of drawing). PS My diagram was not meant to have a light cone of the universe size (which I agree is one & same line as observable universe size), rather it was meant to be the misunderstanding of what a 13.7b l-y wide cone is, and how it is clearly not nearly the same size as the observable universe. :-) David Ruben ^Talk 03:26, 16 January 2009 (UTC)

Lack of sources in the opening section

The entire opening section to this article, although well written, contains not a single source. Especially the claim that: "The edge of the observable universe is now located about 46.5 billion light-years away." needs to be sourced. Where did this information come from? No source appears to be given. (I know the figure to be correct, but that doesn't mean it shouldn't be sourced. It is a crucial part of the article).

Antarctic-adventurer (talk) 17:02, 30 June 2009 (UTC)

Hi Antarctic-adventurer. Although the statement is unsourced in the lede, it appears again with a citation in the "Size of the observable universe" section. WP:LEDE says "Because the lead will usually repeat information also in the body, editors should balance the desire to avoid redundant citations in the lead with the desire to aid readers in locating sources for challengeable material." I suppose there may be an argument for duplicating the citation since "the radius of the observable universe can't be bigger than its age times c" is such a common misconception, but equally we could leave people to find it in the relevant section. I'm somewhat inclined towards the latter option - what do you think?

Ahh indeed, I missed the reference to this given below the lead. Thanks for pointing it out. If you say that standard procedure in wikipedia is to avoid redundant citations then I bow to that, but in this case I would see value in sourcing this one number in the lead as well. It is the most important value in the whole article. Antarctic-adventurer (talk) 14:08, 7 July 2009 (UTC)

Orange and Earth comparisons

equals
???
Rursus dixit. (mbork3!) 22:02, 5 February 2010 (UTC)

There is currently the following statement:

For comparison: assuming a factor of 10²⁴, then the observable universe in comparison with the entire universe would be about the same as the earth compared with a sphere with a diameter of just over a lightyear, or about the same as an orange in comparison with the earth.

This comparison seems to be based on volume: A light year is about 9.5e15 meters; Earth is about 8e6 meters, and an orange is about 0.05 to 0.1 meters. So, light year divided by Earth is a ratio of about 10⁹, and Earth to orange is about 10⁸. To make the comparison work you need to raise both ratios to the 3rd power.

However, if you trace the references to the source, (which is here: [3]) you see a diagram that clearly shows the "inflationary period" increasing the radius of the universe by a factor of about 10⁵⁰. So the 10²³ or 10²⁶ figure refers to the radius, not the volume. —Preceding unsigned comment added by Mrob27 (talk • contribs) 22:51, 21 July 2009 (UTC)

Hi Sinebot; I made both comparisons, of a 1 ly diameter sphere with earth and earth with an orange, based on a volume increase during inflation of 10^24, corresponding with a radius/diameter increase of 10^8. I could not find clear literature references unambiguously stating whether the quoted increase is indeed volume or radius/diameter. In the latter case this increase should still be raised to the 3rd power to get to volumetric increase; Ronald12, 14:40, 10 August 2009.

The number quoted is a change in the cosmic scale factor which corresponds to a "radius", not a "volume" (though such things are not well-defined for most conceptions of the universe). Please adjust the article appropriately. ScienceApologist (talk) 21:00, 10 August 2009 (UTC)

Could you mention (a) literature source(s) for this? In that case I will adjust the article correspondingly, also quoting the reference.Ronald12 (talk) 17:58, 12 August 2009 (UTC)

[4] Reference for the scale factor increasing by 10^50. ScienceApologist (talk) 23:06, 12 August 2009 (UTC)

I don't think this comparison is a good idea because nobody knows how big the universe is or how much inflation might have increased its size. The figure of 60−70 e-folds (e⁶⁰ ≈ 10²⁶, e⁷⁰ ≈ 10³⁰) is a lower bound (not an estimate) obtained from the requirement that the universe be at least as flat as we observe it to be. It's not a prediction, and in fact actual predictions from theoretical considerations tend to give absurdly larger expansion factors, like 10^10something. Also, the inflationary expansion ratio isn't the same thing as the ratio of the present-day observable universe to the "entire universe" (i.e. the whole flat region created by inflation). I'm not sure the size of the flat region is necessarily even finite. -- BenRG (talk) 20:34, 13 August 2009 (UTC)

Mass of the observable universe - inconsistent estimates

In that sections the mass is estimated based on the volume of the observable universe which is wrongly given as

${\frac {4}{3}}\pi {S_{\textrm {horizon}}}^{3}=9\times 10^{30}\ {\textrm {ly}}^{3}$

because it is the result when using $S_{\textrm {horizon}}$=13 billion ly. With the "correct" value of $S_{\textrm {horizon}}$=93/2 ly the volume becomes $4.2\times 10^{32}{\textrm {ly}}^{3}$.

The universe versus the observable universe

It is described that the observable universe is a solid sphere (a ball) centered on the observer. In terms of observation, the section of spacetime that can be observed is the backward light cone (points within the cosmic light horizon, given time to reach a given observer), also the past horizon, though in practice our view is also limited by the opacity of the universe at early times. The past horizon is not objects moving away faster than the speed of light but the beginning of time, and the inside surface of the sphere is the early opaque universe, which is reaching us in the form of red-shifted cosmic microwave background.

Billions of years ago, the observable universe should be smaller in size. The inside surface that observable universe should also be the early opaque universe only closer and with higher temperature. At about 400,000 year of age, the observable universe should only have a few light years in diameter.

Just like a galaxy observed billions of years ago would look billions of years older when we observe today, the early opaque universe observed billion of years ago should not just moved further away, more red-shifted and cooler. That vast area of the early universe should also become older and have developed into galaxies. The surface of our current observable universe should be something new which replaced the old surface. Therefore, the surface of our observable universe should always be some new matter/energy that constantly comes out from the opaque early universe.

The opaque early universe beyond the inner surface of the observable universe is like the shell of the sphere. Although cannot be observed, the further back into the shell the further we move back in time – until near the beginning of time where there are infinite density and temperature at a finite time. For the observer in the observable universe, the rest of the none-observable universe should all exist in this shell.

When we look out into space, we also look back in time. In large scale, the universe in the past and closer to the surface of the observable universe will eventually grow older and be developed into what we have around the Earth – near the center of the observable universe.

In the observable universe, the closer to the surface of the observable universe, the higher the density of energy/matter is. Space will have to be created at a faster paste for the density to decrease toward the density near the center of the observable universe. The space expansion or the speed of moving apart between objects would have to be faster near the surface of the observable universe than the center of the observable universe.

As “Every location in the universe has its own observable universe which may or may not overlap with the one centered around the Earth.” at any moment there should be infinite possible observing locations, and there should be infinite number of observable universes. A horizontal slice of the spacetime of that moment would be a set of observable universes at the same age. That infinite number of observable universes would be relatively the same size. A few billions of years earlier that set of observable universes should all be a few billions of light years smaller. Near the beginning of time, the horizontal slice of the universe should be infinite number of not yet observable universe or infinite number of possible observing locations in the infinite density and temperature at a finite space.

As we consider the universe as the universe is defined as everything that exists, has existed, and will exist, Observing from any location at any time is merely a self centered and limited view of this total reality. Imiloli (talk) 20:00, 2 September 2009 (UTC)Imiloli (talk) 21:34, 9 September 2009 (UTC)

Article talk pages are not really the place for philosophical essays; they're for discussing how the article can be improved. Would you care to summarize, in as little text as you can manage while making your meaning clear, just what your proposal is for the article? --Trovatore (talk) 21:37, 9 September 2009 (UTC)