Talk:Observable universe

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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!

[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." —The preceding unsigned comment was 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

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 ^[1]

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)