Talk:Observable universe/Archive 2

Formation and evolution
a. Is a Formation and evolution section required - as found in the Galaxy article? b. Do structures emerge on larger and larger scales over cosmological time? Can we expect the End of Greatness to have increased in several billion years time? --Tediouspedant (talk) 20:29, 6 February 2010 (UTC)

removed
Next paragraph in italics was removed from the main text because it seems confused. The fact that they are no nearby quasars is probably not a property of the structure of the cosmos, and as far as I know they aren't known to cluster more at large distances. Quasar statistics are *very* difficult to do.


 * Another indicator is the quasars themselves. A quasar ('quasi-stellar object') looks like a star but if estimates of their distance are correct then they are each many billions of times brighter than any star, and up to 1000 times brighter than ane entire giant galaxy. There is no quasar within 2 billion light years of earth, but they apparently cluster more thickly at greater distances, leading to speculation that they are a phenomenon of the early universe and that most of them have recently 'gone out', though in that case there ought to be some remnants of them to be discovered.

last paragraph
The last paragraph is very confusing. I tried to clean it up a little but it's not enough. I don't know what the author is trying to say so I don't want to rewrite it; which is what it needs.

QSS
Adding an unreferenced sentence about the quasi-steady state model in the middle of the article doesn't really make much sense to me. If it is the consensus to add stuff about alternative cosmologies, it needs to be done in a more structured way. (i.e. not in the middle of a paragraph about the big bang, probably in its own subsection.) –Joke137 03:06, 22 November 2005 (UTC)

Marginalizing other cosmologies that are against the one you hold, Joker, is not a NPOV. JDR (Note that you took the statement out before, without checking the varacity of the claim ... no faith at all ...)

This is truly tiresome. Do you insist on reinserting your spelling errors into the article? The authors of the article themselves admit, in calling it a toy model, that it is not a complete model of structure formation in QSS. –Joke137 16:33, 22 November 2005 (UTC)

Picture
This page could use a good picture like the red, filamental Item 8 found here or perhaps one of the large-scale dark matter growth images found here. Not sure of the copyright status of those images. --Fl e x (talk|contribs) 17:23, 1 February 2007 (UTC)

megefrom Galaxy filament

 * Oppose leave it separate, since voids are also separate, and detail can go into that article moreso than this one, as more is discovered. 132.205.44.5 (talk) 05:20, 9 January 2008 (UTC)

Added contradict tag
This article (along with others) contains a contradiction regarding the Sloan Great Wall. Is it, or is it not, a true structure? If someone can settle that debate, please clean up the article and remove the contradict tag. Angiest (talk) 17:55, 9 July 2008 (UTC)
 * From what I understand, that question is under debate by astronomers themselves. If I have a clear grasp of the issue, some argue it is a de facto structure as the configuration of the galaxies form a coherent wall-like grouping.  Others argue it should not be regarded as a structure at all, as the various galaxies in it are not actually gravitationally bound to one another.  However, it should be noted that I am not an astronomer myself, so this may be incorrect. Zmidponk (talk) 14:20, 4 August 2008 (UTC)

Cosmic Web
I have proposed that Cosmic Web be merged here. That article seems unnecessary and needs much improvement anyway. The term "cosmic web" is not the name of any theory proper and the idea of a filamentary universe is an old one. It sounds to me just a media friendly phrase similar to "God particle". There's not much at the article to merge. Jason Quinn (talk) 15:49, 25 March 2009 (UTC)
 * Agree.  Serendi pod ous  19:19, 2 June 2009 (UTC)

merge with Observable universe
These two articles feel like two parts of the same article. Also, "large-scale structure of the cosmos" is a bit neologism-ish, and at least "observable universe" is a commonly heard term in physics.  Serendi pod ous  20:31, 20 April 2010 (UTC)

How big is a quarter?
Is that what they meant by "Quarter sized"? I was a little confused by that. As for the size... 24.26 mm in diameter.

"In any case, it is interesting to note that the cosmological horizon is a maximal limit of perception and not an actual boundary" ... I'm not sure this is a safe statement: the nature of reality is always relative to an observer. for example, doesn't this in-principle boundary of visibility form an event horizon with an equivalent effect to hawking radiation?Snaxalotl (talk) 03:47, 11 June 2009 (UTC)snaxalotl

Size of the Universe
I would like to see some mention here of the size of the Universe, such as can be found on the observable universe article.

I would also like to know what the difference is between the/a Cosmological Horizon and the/a Particle horizon.

I copied some text over myself from the OU article, but it didn't quite fit so I undid it. Nagualdesign (talk) 04:16, 16 February 2009 (UTC)

Particle horizon ?
Isn't the particle horizon the distance at which are now objects from which we receive the light now, that's to say roughly 50 billion light years, not to confuse particle horizon with Hubble's horizon at 13.7 Gly ? (imagine an object emitting on the Hubble's horizon, the time its light comes to us, it would have moved (faster than light)) —Preceding unsigned comment added by 129.175.69.214 (talk) 08:56, 31 March 2010 (UTC)

Method for computing the size of the universe
Objects on our horizon are now 3 times farther away than they currently appear. The consensus value of 13.7 Gy for the age of the universe translates into 13.7 x 3 = 41.1 GLy for the radius of the observable universe, 41.1 x 2 = 82.2 GLy for its diameter, and 4/3 x π x 41.13 = 2.91 x 1032 cubic light years (1.5 × 1059 cubic meters). The numbers stated in the article lack internal consistency.

The section labeled Size is a jumble of units. The first value it states was in parsecs — a unit that was never used before in this article and never appears again. Stick with light years and (occasionally) meters. It also uses the short scale number naming convention for an unusually large number — again for the first time and then never again. Short scale number beyond billion should be avoided.

I made these changes on 20 August 2010 and they were removed the next UTC day. That's not very nice. —Preceding unsigned comment added by 66.108.69.201 (talk) 19:01, 21 August 2010 (UTC)


 * Where are you getting the idea that the calculation for the radius of the observable universe is just based on multiplying the age of the universe by 3? Multiple sources say both that the age of the universe is 13.7 billion years and that the radius is 14 billion parsecs (about 46-47 billion light years), so it seems unlikely that this is the official method. I would guess that the method cosmologists use to calculate the radius is based on estimates of how the expansion rate has been varying with time (based on redshift surveys of huge numbers of galaxies which show evidence the expansion rate has been accelerating). Unless you have a source saying that your method is the one used by cosmologists, this would seem to be original research. Hypnosifl (talk) 23:43, 24 August 2010 (UTC)


 * I just found a broad discussion of how both the radius and the age of the universe are calculated, go to this page and start with the paragraph that begins "The concept of inflation..." about 1/3 of the way down the page, a little below the diagram that's titled "era of inflation". Apparently it involves evaluating a calculus integral, so clearly the simple multiplication of the age by 3 you suggest is not correct. Even more of the detailed math can be found on p. 47-48 of this pdf. Hypnosifl (talk) 06:38, 25 August 2010 (UTC)

The limit of that integral is probably 3r. I don't remember where I learned that technique. I thought it was generally well known. I'll look into this later. —Preceding unsigned comment added by 66.108.69.201 (talk) 08:22, 2 September 2010 (UTC)


 * OK, the page you told me to visit gives a value of 42 GLy in the section you told me to read, which is 3 times 14 GLy. A brief explanation of why you should multiply by 3 is presented here. The relevant entry ends with this statement. "The current best fit model which has an accelerating expansion gives a maximum distance we can see of 47 billion light years." So apparently multiplying by 3 is a simplification. I do not understand the method presented in the pdf you pointed me to, but it does indeed use 13.7 GLy to give a radius 46 GLy using a very fancy formula. 46-47 GLy is probably the best answer out there. —Preceding unsigned comment added by 66.108.69.201 (talk) 09:17, 2 September 2010 (UTC)


 * It looks like the pdf settled the matter for you, but on the subject of the other link, but I just wanted to respond to your comment that "the page you told me to visit gives a value of 42 GLy in the section you told me to read, which is 3 times 14 GLy". Actually, 42 GLy is only mentioned when the author talks about estimates he's found through random internet searches, but then he goes on to talk about the detailed calculations he saw in "in Lecture 8 of the Cosmology DVD, presented by Dr. Mark Whittle, and cited at the beginning of this page". After discussing some of the equations presented in the lecture, the author goes on to say "For a second example, take the earliest expression of the Cosmic Background Radiation as the source of the photon(s) whose distances are to be determined. Using the same approach, and specifying the Redshift Stretch Factor as rsf = 1000, dLT comes out as 13.7 billion light years, dnow is calculated to be 46 billion light years, and demit is found to be 26 million light years (a reasonable estimate for the very early Universe's limits)." So, the author is saying that when you actually do the calculations using the correct equations, you do find that the most distant light we can see today was emitted 13.7 billion years ago by matter that would now be 46 billion light years away from us (even though at the time the light was emitted, the matter that emitted it was only 26 million light years from our present position). Hypnosifl (talk) 01:23, 6 September 2010 (UTC)

Funnily enough, I was just writing a bit of code to do this calculation when I saw this comment. Hypnosifl is correct that it involves an integral. If the universe were expanding at a constant rate you could just multiply that rate by the age. But since the rate has been changing it's more complicated. Suppose you split the history of the universe into lots of little chunks of time, and you can work out the expansion rate in each. Then you know how much expansion there was in each chunk, and adding them all together gives the total size. Calculating the rate at each time requires you to know what the universe was made of at that time - how much of it was filled with matter, how much with radiation, and how much with dark energy. That gives you the Hubble rate, the rate of expansion. When you do the integral in turns out that the answer is a little over three times what you would have got if you'd assumed no expansion, but that's a complete coincidence - there's nothing special about the number 3. Olaf Davis (talk) 16:01, 2 September 2010 (UTC)


 * The number 3 is special. According to the source I mentioned, "the scale factor for the Universe goes like the 2/3 power of the time". The number 3 pops out of the resulting integral when it's evaluated. It is exactly 3 only for the critical density case, however.66.108.69.201 (talk)

The comoving universe versus the observable universe
No trouble understanding a universe that is larger that the observable universe, but I still believe that we cannot observe objects farther away than 13.8 Gly. I don't believe that the comoving universe actually is the observable universe. I don't believe that we can observe the limits of the comoving universe, because the limit of the comoving universe is the farthest objects as they would look today, objects that we actually observe as they were ~13.5 Ga ago (including recombinations, darknesses and such). We cannot see how they look today, because of the limited speed of light. Am I wrong? ... said: Rursus ( m bork³ ) 11:50, 15 September 2009 (UTC)


 * Another suspicion: is it that the comoving limits of the universe presume a certain metric and expansion model, such as ΛCDM? Then if that is true, all numbers in the article will change as soon as the expansion model is changed, and can only be considered conventional, not really measured distances, a convention designed to compensate for the expansion rate and avoid artifacts from using a non-euclidic space.


 * If this is the case, then the whole section of Misconceptions aren't misconceptions, but instead truths reached by using different metric conventions as those presented as "truths" in the article. The article then confuses a model for a "truth" and confused other models as "misconceptions", so that the article misconceives models for truths. Which is loony indeed ... ! ... said: Rursus ( m bork³ ) 12:08, 15 September 2009 (UTC)


 * Hi Rursus. To answer your points:
 * "we cannot observe objects farther away than 13.8 Gly" would be true if the universe weren't expanding. But if I look at a galaxy which was, say, 10 Gly away when the light I'm seeing now left it, it's had 10 Gyr to move away from me since then. So the current distance to the galaxy could well be over 13.8 Gly. It's important to think carefully about what the distance to an object means, because it can become quite non-obvious in cosmological contexts.
 * I'm not quite sure what you mean by 'the comoving universe'. Comoving distance refers to a set of coordinates which can be used to measure the size of the observable universe or other distances. Olaf Davis (talk) 12:52, 15 September 2009 (UTC)
 * I mean a dimensioning (a model) of the universe based on one theory of comoving distances, or so I believe. ... said: Rursus ( m bork³ ) 13:57, 15 September 2009 (UTC)
 * "We cannot see how they look today, because of the limited speed of light." That's right - for my hypothetical galaxy above we have no way of knowing what's happened to it in the last 10 Gyr.
 * "all numbers in the article will change as soon as the expansion model is changed, and can only be considered conventional, not really measured distances" I agree with the first half of this but not the second. It's true that some new development might change our estimates of the comoving distance to the edge of the observable universe. But that doesn't make the distances 'not really measured'. The same is true for any scientific measurement: it's possible that tomorrow we'll discover that our model of radioactive decay is wrong, and certain rocks on Earth are a much different age than had been thought. But until that happens the current measurement can be considered a good one, and the age is gives is just as much a fact as anything in science is a 'fact' rather than a 'model'. Similarly the level of evidence and scientific consensus on the size of the observable universe is sufficient that it's fair to portray it as a fact.Olaf Davis (talk) 12:52, 15 September 2009 (UTC)
 * But the radioactive decay example is not really good, since it doesn't presume a certain half life metrics during the life time of the universe except the simplest possible: a linear one. ... said: Rursus ( m bork³ ) 13:57, 15 September 2009 (UTC)
 * Perhaps I could have thought of a better example. But the point is that the measurements are based on the current model which best fits the observations. The model and hence measurements could change, but for now we can consider them 'true'. Olaf Davis (talk) 17:58, 15 September 2009 (UTC)


 * "the whole section of Misconceptions aren't misconceptions, but instead truths reached by using different metric conventions" They aren't really using different metrics, though: they are genuine failures. The 13.7 Gyr figure for example assumes a non-expanding universe (so my 10 Gly galaxy remained at 10 Gly for ever): that's not a different convention, it's just an assumption which has been disproved. Olaf Davis (talk) 12:52, 15 September 2009 (UTC)
 * Maybe so. Shouldn't it then be explained that the "meter" that have occurred during the life time of the light quantum have since expanded, so that the earlier meters are longer today, and the later meters are more like our current meters. Then shouldn't it also be stressed that any distance measure claimed depends on a certain expansion model, expanding a "raw distance", like one based on a static universe, to a model based distance? ... said: Rursus ( m bork³ ) 13:57, 15 September 2009 (UTC)
 * We convert a time into a distance (which requires a form of the scale factor a(t) over time as you say) but we're not in any meaningful sense converting the distance for some hypothetical static universe to the distance for our own. Thus there's no sense in which the 13.7 Gly distance is more "raw" than the actual given value.
 * As for pointing out that the measurement depends on the form of a(t): maybe we could do that as a "how is this measured" section, but I think we should be careful not to give the impression that the currently accepted model of the expansion is particularly tentative because it's not. Olaf Davis (talk) 17:58, 15 September 2009 (UTC)


 * So, I don't believe there's anything wrong with the current article's portrayal of this. Olaf Davis (talk) 12:52, 15 September 2009 (UTC)
 * Maybe, your right in the essentials, but I'll take a further look to see if the article might profit from clarifications. ... said: Rursus ( m bork³ ) 13:57, 15 September 2009 (UTC)


 * Again, is the article claiming that the "observable universe" follows the orange lines in the attached figure, or the red ones? I would intuitively have said the red one, since we cannot know anything about the further development of the object. Then is the red distance longer than the travel time of c:a 13 Gy? ... said: Rursus ( m bork³ ) 14:42, 15 September 2009 (UTC)


 * The description here indicates that the circles in the diagram are lines of constant time, and the radial lines are of constant position. The centre of the diagram is the Big Bang, with the present day near the edge. The red line is meant to be a photon's worldline. Clearly the observable universe should be bounded on one side by the semi-circular orange line (assuming that's at today). You're asking whether the other boundary should be the red line or the radial orangey line. The latter I think clearly isn't the case since it would include events outside our light-cone, which I think is incompatible with general useage of the term. So the answer should either be the former, or that the observable universe is just the orange semi-circle and has no extent in time. Which of those, though, is irrelevant to the current point.
 * You then ask about the length of the red line. Well, the red line is the worldline of a photon and thus a null geodesic, which actually has length zero! The radial line has length 13 Gyr (yr not ly, since it's a time-like curve) and the semi-circle has a length of 78 Gly. This latter is the diameter of the observable universe. Olaf Davis (talk) 18:22, 15 September 2009 (UTC)
 * (The diameter is ~95 Gly, not 78 Gly.) -- BenRG (talk) 20:19, 15 September 2009 (UTC)


 * The observable universe is a region of space, not spacetime, where "space" is basically defined by the Hubble flow. The stuff with worldlines crossing the orange line on that diagram also exists at earlier and later times, and over all times it covers a region of spacetime whose edges are the brown and yellow lines. There's some diffusion across the edges but not too much. We can see all of the matter in that region (in principle), though we can only see one particular time in the history of each bit of it, namely the time that it crosses the red line. The orange line on that diagram is not actually the radius of the visible universe, it's a line from present-era Earth to the quasar CFHQS J2329-0301 (or whatever's become of it), with a length of about 28 billion light years, and the middle of the diagram is about 12–13 Gyr ago, not quite all the way to the big bang, but the principle is the same. -- BenRG (talk) 20:19, 15 September 2009 (UTC)


 * I thank you all for taking the effort to explain. I think I've gotten everything pretty correctly: we're speaking about the current size of the universe we can see, except we see it as it was, not as it is. Actually I believe how to define the observable universe is a pretty tough philosophical question rather than a hard to comprehend universe. As per logical positivism and a general physics culture, we "shall not" speak about what is not detectable. Except that logical positivism is essentially obsolete, unless used as a set of heuristics for physics that don't regard quantum mechanics. What is the "observable universe" seems to be a matter of how to define the concept. My initial reaction was what the "observable universe" is the universe that we actually observe, but this need not be the case, by the example of the supersonical aircraft explained elsewhere. Thanks. Rursus dixit. ( m bork3 !) 22:16, 5 February 2010 (UTC)

See Also links
I'm confused by the link to a song called [Nine Million Bicycles]. The subtext says it is about a physicist challenging the scientific accuracy of the lyrics, but the page referenced doesn't mention anything about any physicist or challenge. I'm removing the link right now because it doesn't belong here. Preceding easy (talk) 07:09, 20 January 2010 (UTC)

Extraneous Citation Needed Tag
"Every location in the Universe has its own observable universe which may or may not overlap with the one centered around the Earth.[citation needed]"

This sentence doesn't need a citation - it directly follows from the preceding paragraph (which explains that the observable universe is a sphere around the observer of a radius equal to the maximum distance at which events can be observed, given the age of the universe). Indeed, that sentence doesn't even assert anything new or different from the previous paragraph. It's just another way of saying the same thing, in order to clarify or help the reader imagine what is being described.Tzepish (talk) 23:36, 2 March 2010 (UTC)


 * Last time I checked, there was controversy regarding whether the initial state was infinitesimal. In this case the observable universe is the same from all locations if expansion does not exceed the speed of light, since this tiny initial volume eventually expands to create the present day universe. Different locations see different parts of the universe at different times, but these parts all descend from the same tiny volume so there is only one "observable" universe. Would be nice to actually have a citation since it confuses me that this issue can be disregarded. (Someday I will get back to my GR studies since I don't think any of this expanding universe stuff really makes sense without understanding how space and the universe can be geometrically curved.) —Preceding unsigned comment added by 69.231.150.254 (talk) 09:38, 30 January 2011 (UTC)


 * Why do you think the idea that if different parts "all descend from the same tiny volume" that would imply the whole universe can't be larger than the observable universe? Keep in mind that in general relativity the proper distance between us and a distant galaxy can grow much faster than c due to the expansion of space (see Comoving_distance for more info on this). Hypnosifl (talk) 22:16, 8 February 2011 (UTC)

Newer "Size comparison" pic
The newer pic of the hubble ultra deep field image with the superimposed image of the moon on "for scale" doesn't make sense to me and seems to detract from the overall quality of the image. It makes the moon appear enormous compared to the rich spacescape of galaxies behind it, and has a small red square which does not at all accurately compare the moon to this image. I advocate the return of the original image. Other opinions are appreciated.--206.28.43.16 (talk) 00:15, 18 March 2010 (UTC)

merge with Large-scale structure of the cosmos
These two articles feel like two parts of the same article. Also, "large-scale structure of the cosmos" is a bit neologism-ish, and at least "observable universe" is a commonly heard term in physics.  Serendi pod ous  20:32, 20 April 2010 (UTC)

Oppose merge The Large-scale structure of the cosmos article is necessary in order to define the structure of the universe in the orders of magnitude between the Virgo Supercluster and the observable universe as a whole. Also, merging the articles would make the article too long. Keraunos (talk) 19:59, 17 June 2010 (UTC)
 * The article didn't make any distinction between the observable universe and any "lower orders of magnitude". Any attempt to fix such orders would be arbitrary. If you want a structure between Virgo cluster and Observable universe than Pisces-Cetus Supercluster Complex fits the bill. And anyway, I already merged "Large scale structure..." after waiting a month, and the result is only 37K; hardly "too long"; it's average length at most.  Serendi pod ous  20:11, 17 June 2010 (UTC)

Diameter of the entire universe
note 1: Will it ever be possible (or is there studies) on the maximum, or estimated size of the universe?

note 2: —Preceding unsigned comment added by 82.31.55.148 (talk) 01:32, 16 July 2010 (UTC) I don't see the point of this paragraph. It follows logically from the above-mentioned inflation theory (by Alan Guth) and constitutes a lower bound anyway.

Moreover, it contains circularity, particularly the sentence:

"Using the rounded off figure of 1011 light years for the diameter of the observable cosmos and the lower figure of 1034 light years for the diameter of the entire cosmos means that there is a difference of at least 23 orders of magnitude between the size of the observable cosmos and the size of the entire cosmos".

This is laughable: the 23 orders of magnitude difference have just been used as a starting point assumption for the calculations in this paragraph. Circular reasoning.

I would suggest to completely delete this paragraph. Also in view of the proposed merging with "Large-scale structure of the universe". Ronald12, 06 May 2010


 * I deleted it (edit). -- BenRG (talk) 23:50, 8 May 2010 (UTC)


 * Let me get my ruler. Chuck Hamilton (talk) 00:00, 10 January 2011 (UTC)

Matter Content
The article states there are almost 80 Billion galaxies in the Universe. The citations given -- #23 and #25 -- do not support this claim. One of the citations states that the Hubble is *capable of detecting* 80 Billion galaxies. The actual number of galaxies is estimated by many sources to be much higher: 100B to 200B.

See: http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/021127a.html which estimates 125 billion galaxies in the universe.

http://spacemath.gsfc.nasa.gov/weekly/6Page14.pdf estimates 165 Billion galaxies in the universe. —Preceding unsigned comment added by Gerntrash (talk • contribs) 18:41, 19 June 2010 (UTC)


 * I agree, that number 80 billion, and all the rest of the calculations in that section (and the madsci reference) are old and not very helpful and don't correspond to the phrase "the observable universe contains....". And even those citations you give are still all discussing what could be actually distinguished as a galaxy by Hubble or some other instrument.  I'd like to see a number that corresponds to how many galaxies etc we expect there to be inside the horizon based on the density we observe "nearby". --NealMcB (talk) 01:26, 6 September 2010 (UTC)


 * On this page there's an estimate based on the number of galaxies known to be present in our Local Group vs. the number that would be visible at the distance seen in Hubble deep field, the author concludes the number of galaxies in the visible universe might be around 240 billion. Of course this isn't any sort of peer-reviewed publication, but it is written by a NASA astronomer, Dr. Sten Odenwald. Hypnosifl (talk) 01:53, 6 September 2010 (UTC)

End of greatness
100 Mpc seems a remarkably short distance for the homogeneity of the universe to become apparent. Our local galaxy filament is 300 Mpc across, after all. I think maybe an extra zero may have been dropped?  Serendi pod ous  22:15, 13 August 2010 (UTC)
 * If by the local filament you mean the Local Supercluster, it is ~30 Mpc in size. To the question of homogeneity. The First acoustic peak has the scale of 120 Mpc/h. The h (Hubble constant) is about 0.75. So scale is 160 Mpc. This corresponds to the highest amplitude of CMB fluctuations. The largest coherent structures should have size of an order of 160 Mpc. So, 100 Mpc is probably too low a number. The reasonable number is closer to 300 Mpc or even to 1000 Mpc. Ruslik_ Zero 14:12, 14 August 2010 (UTC)
 * I meant the Pisces-Cetus Supercluster complex  Serendi pod ous  14:18, 14 August 2010 (UTC)

Decreasing Hubble constant
I removed a couple sentences here as uncited and looks wrong. On second thought, though, the Hubble distance is increasing even while the acceleration of the expansion of the universe is increasing. Am I just confusing myself here? - 2/0 (cont.) 07:29, 26 November 2010 (UTC)


 * I added some more detailed discussion of the issue along with supporting references, hopefully this clears it up. Hypnosifl (talk) 03:53, 28 November 2010 (UTC)

Why is "Universe" capitalized everywhere in article?
I don't think this is a usual convention in scientific papers or textbooks, for example. Would anyone object to de-capitalizing it? Hypnosifl (talk) 03:52, 28 November 2010 (UTC)
 * Also, it looks like it was decided to de-capitalize the word in the Universe article, see this revision. Hypnosifl (talk) 14:59, 4 December 2010 (UTC)

Misconceptions section: 24 Gpc a diameter or radius?
Recently in this edit Natty4Bumpo suggested there was some ambiguity about whether 24 Gpc is a radius or diameter, writing (and using this article as a reference):


 * Some have suggested this figure is incorrect; however, Dr. Cornish when answering questions about how the Universe could be 156 billion light years across stemming from the 2004 Space.com article, which first published the figure, through the website's commentary section made no attempt to correct the figure, leaving some ambiguity on the question.

I was pretty sure 24 Gpc was a diameter, for two reasons: 1) multiple other sources say the radius of the observable universe is around 14 Gpc, and it doesn't make sense that looking for repeating patterns in the CMBR would allow you to rule out finite universes significantly larger than the observable universe, and 2) the draft of the Cornish et al. paper on arxiv.org says (as noted in the article) "By extending the search to all possible orientations, we will be able to exclude the possibility that we live in a universe smaller than 24 Gpc in diameter". But just to be sure, I emailed Dr. Cornish to confirm, and he wrote back:


 * The 24 Gpc is a diameter. Unfortunately our friends in the press read it as a radius, doubled the result, converted to light years and quoted 156 billion light years. I think I was misquoted once, then it got repeated.

I realize a personal communication like this can't be cited in a wikipedia article, but I think the quote from the draft on arxiv.org paper was already pretty definitive, and Natty4Bumpo's argument above seems to violate the WP:No original research rule, specifically the part about coming up with original arguments based on inferences from published sources in WP:No original research (the cited source doesn't have Cornish specifically commenting on the 156 billion ly figure, so taking his failure to correct it as evidence that the arxiv.org paper might be wrong seems to go beyond anything explicitly stated by Cornish). So based on the above, hopefully Natty4Bumpo won't object if I revert this edit. Hypnosifl (talk) 19:00, 8 February 2011 (UTC)

Approximate value?
I feel that the '=' should be replaced by '≈' in the following equation. $$\frac{5 \times 10^{21}\ \textrm{stars}}{4 \times 10^{30} \ \textrm{ly}^3} = 10^{-9} \ \textrm{stars}/\textrm{ly}^3 = 1\ \textrm{star} \ \textrm{per}\ \textrm{billion}\ \textrm{ly}^3 $$


 * While this is technically correct comment for the first equality, I think it is pretty clear and don't see need to change. However I think the level of precision in the section on Matter Content and on Critical Density is rather high in parts and most numbers should be reduced to 1 or at most 2 significant digits. Gierszep (talk) 20:19, 17 April 2011 (UTC)

Distance in KM
$$9,460,800,000,000 * 93,000,000,000 =

879,854,400,000,000,000,000,000$$

Eight hundred and seventy-nine Sextillion, eight hundred and fifty-four Quintillion and Four Hundred Quadrillion km to the edge of the visible universe..

Is this right? Neobenedict (talk) —Preceding undated comment added 11:56, 9 May 2011 (UTC).


 * A light year is exactly 9,460,730,472,580.8 km, so if you want to round it off to five significant figures it should be 9,460,700,000,000 km. And 93 billion light years is the diameter of the observable universe (not the visible universe, see the distinction in the second paragraph of the intro), the distance from us to the edge of the observable universe would only be half that (14.3 billion parsecs works out to 46.6 billion light years). So, 9,460,700,000,000 * 46,600,000,000 = about 4.41 times 1023 km, or 441,000,000,000,000,000,000,000 km (Four-hundred and forty-one sextillion km). Hypnosifl (talk) 00:10, 19 May 2011 (UTC)

Shrinking Observable Universe???
The statement "in practice an increasing number of galaxies will become extremely redshifted due to ongoing expansion, so much so that they will seem to disappear from view and become invisible." is wrong in spite of the multiple references. This is due to a misinterpretation of the theory. The theory states that we will never be able to see objects that currently exist (but are not yet visible) because the light from them will never get to us - true, but we will always be able to see something at any place in the universe that we can currently observe. It will just appear to be aging much more slowly due to the red shift. We will simply never get to see that part of the universe when it gets to be 13.7 Billion years old as it actually is today. (Which is what the reverences mean.)

The background radiation is redshifted about 1098 times. That means that what we currently see only appears to be aging about 1 day in every three of our years. We will always be able to observe this background radiation. No mater how much the universe expands or how much faster the expansion rate gets, the background radiation will fill every part of the universe. We will always be seeing some part of the universe just transitioning into a neutral gas where the background radiation appears and it will always be 300 Million years old as timed from the Big Bang. But it will be even more redshifted and it will appear to us to be aging even more slowly than what we now see. Even if inflation stopped today, in order to see the part of the universe that we currently see as background radiation age to 13.7 Billion year would take 15 Trillion more years. Since inflation is not stopping today, we will never see that part of the universe at age 13.7 Billion year. (That is what the theory means.)

Allyn — Preceding unsigned comment added by 97.65.82.66 (talk) 15:20, 5 July 2011 (UTC)


 * It's not wrong, because the statement is not saying the light from these galaxies will stop arriving, just that it'll be too redshifted to observe in practice by equipment resembling what we use today (there are plenty of possible wavelengths that are much too large for equipment available today, or in the forseeable future, to detect). Perhaps the statement does need to be clarified to explain more what is meant by "in practice", though. I found this thread discussing the issue in which "caveman1917" posted a number of useful references, like this paper which says on p. 2: "Finally, their apparent brightness declines exponentially, so that the distance of the objects inferred by an observer increases exponentially. While it strictly takes an infinite amount of time for the observer to completely lose causal contact with these receding objects, distant stars, galaxies, and all radiation backgrounds from the big bang will effectively "blink" out of existence in a finite time; as their signals redshift, the timescale for detecting these signals becomes comparable to the age of the universe, as we describe below." Hypnosifl (talk) 16:30, 30 July 2011 (UTC)

Units for star quantity
In section 4, Matter Content, the number of stars in the observable universe is estimated at about 3 to 100 × 1022 stars (30 sextillion to a septillion stars). Given that the SI unit for quantity is the mole and that the range of numbers reported here is on the same scale as a mole, would it be useful to also report this quantity in units of moles? It would come out to .05 to 1.666... moles of stars. 74.134.129.136 (talk) 02:23, 27 August 2011 (UTC)
 * The unit "mole" is a chemistry-specific term meant to count the number of "elementary entities" (usually molecules, but sometimes also other basic chemical units like atoms or electrons) in a substance...see section 2.1.1.6 on p. 22 here. I've never heard it used for large numbers of other entities that aren't basic elements in chemistry, like stars or cells or grains of sand. Hypnosifl (talk) 20:05, 5 September 2011 (UTC)

4% atoms, 22% cold dark matter, & 74% dark energy
Latest measurments: 4% atoms, 22% cold dark matter, & 74% dark energy. http://hubblesite.org/hubble_discoveries/dark_energy/de-what_is_dark_energy.php http://www.sciencemeetsreligion.org/physics/big-bang.php - Brad Watson, Miami 71.196.11.183 (talk) 04:49, 21 December 2011 (UTC)

Danwitty Neutrino Reuptake Correction.
The article fails to mention or even to accommodate Danwitty's Neutrino Reuptake Correction, insofar as conformal topological mappings of Govenwile eruptics have led to revisions of the arc secant functors of temporal cohomotophy, as for instance as has been observed in nolinear compact neighborhoods of gravitational masses near the Dimwitty Limit. Be that as it may, however, and noting how quark wake turbulence is now expressly supposed to NOT perturb alpha-pneumatic deBrollie strains (and this without qualification of the clitsine hydrogen spectra as depicted by Samuelson and Drovebit), it seems likely that Devonshire-Radish velocities in excess of 1.0x10^54 megaparsecs/nanosecond would need to be explained, or perhaps drafted as a regular semi-calonic cacaphore. And perhaps an even more egregious lapse is the failure to mention Largassey's unbounded tensor products and their implication at distances beyond Bloombers's Castle Rail Shore-Jamb plethorae. These are the most serious flaws, ignoring for the sake of argument the author's inability to form non-psychotic Chomsky aural-olfactory chemical emissions. I would strongly recommend that Edith's Head of the Great Dome in plenstissimal or exhubearential tonal sacs be deviated by malagamathy shirks. — Preceding unsigned comment added by Iwanturkitty (talk • contribs) 13:14, 26 December 2011 (UTC)