Talk:Observable universe

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Contents 1 Infinite Galaxies 2 Misc Old 3 Contradiction 4 Requested move 5 80bln vs 140bln 6 An analogy to the size of the universe? 7 Oppose merge: Particle horizon 8 Changed the Introductory Paragraph 8.1 Explanation of my reverts 8.2 Update: Link Removed from 1st Paragraph 9 80bln vs 140bln -- STILL 10 First sentence 11 Relationship of observable to unobservable universe 12 Too small 13 Superfluous text 14 Image needs fixing 15 Some changes reverted 16 Observable universe being entire universe 17 Merge from The Hubble Limit 18 Is it flat or not? 19 statement about Earth not being the centre 20 Proposed Merger 21 Size source 22 Question on idea of universe being larger than observable universe 23 Observable universe image 24 Observable Universe vs Visible Universe 25 Why isn't 156 billion light years the correct diameter? 26 Mass of the observable universe

[edit] 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)

[edit] 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)

[edit] 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)

[edit] Requested move

[edit] 80bln vs 140bln

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

[edit] 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)

[edit] 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.

[edit] 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)

[edit] 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)

[edit] 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)

[edit] 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)

[edit] 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)

[edit] 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)

[edit] 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)

[edit] 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)

[edit] Image needs fixing

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)

[edit] 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)

[edit] 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)

[edit] 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)

[edit] 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? --P3d0 12:22, 4 May 2007 (UTC)

Anyone? Bueller? --P3d0 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)

[edit] 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)

[edit] 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)

[edit] 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)

[edit] 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)

[edit] 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)

[edit] 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)

[edit] 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)

[edit] 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)