Talk:Big Bang

IMPORTANT: This is not the place to discuss how you think the universe began. This page is for discussing the article, which is about the Big Bang model, and about what has been presented in peer-reviewed scientific literature about it. See Wikipedia:No original research and Wikipedia:Talk page guidelines. If you wish to discuss or debate the validity of the Big Bang please do so at BAUT forum or talk.origins.

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Issue with "Features, issues and problems"

The style of the section Features, issues and problems is irritating and supercilious. It is not in accord with scientific language that states facts, not moral attitudes declaring all opponents being idiots. The style is unencyclopedic. F.ex., the second paragraph:

The core ideas of the Big Bang—the expansion, the early hot state, the formation of helium, the formation of galaxies—are derived from many independent observations including abundance of light elements, the cosmic microwave background, large scale structure and Type Ia supernovae, and can hardly be doubted as important and real features of our Universe.

Answer: just because those "facts" (or rather: strong indications) are perceived as real, they're not as strong indication of the realness of Big Bang as indicated, since the facts listed requires logics mentioned elsewhere to actually support Big Bang. F.ex. the presence of large scale structures was often mentioned as a problem for Big Bang, since it requires an older Universe or some fundamental instability that only partially but very deficiently is provided by Dark Matter.

Furthermore:

Of these features, dark energy and dark matter are considered the most secure: remaining issues, such as the cuspy halo problem and the dwarf galaxy problem of cold dark matter, are not considered to be fatal as it is anticipated that they can be solved through further refinements of the theory.

is extraordinarily weak logic. It requires citation after citation after citation. There are some indications supporting dark energy and dark matter somewhat, but there are no laboratory measurements on Earth supporting either of them. They simply are exotic physics, known only to cosmology. Such a degree of insecurity would be appropriate for the text too. Secondly:

remaining issues, such as the cuspy halo problem and the dwarf galaxy problem of cold dark matter, are not considered to be fatal as it is anticipated that they can be solved through further refinements of the theory.

signals an unfounded attitude that this will soon be solved. Actually I believe it won't. The neutrino problem will be solved, and so any standard model of physics will be replaced by a supersymmetric model, and many of the aforementioned problems will transform into new physics. The current theory won't survive, except possibly as crudely approximated special cases. Now, presenting my alternate divinations, in obvious opposition to the divinations of the text, I claim: neither the author nor me are prophets. A real scientific text shall just state the problems and point out possible solutions thereby citing some sources. ... said: Rursus (^mbork³) 20:19, 5 October 2009 (UTC)

Rursus, you say "just because those "facts" (or rather: strong indications) are perceived as real, they're not as strong indication of the realness of Big Bang as indicated, since the facts listed requires logics mentioned elsewhere to actually support Big Bang." Yes, it's true the observations require interpretation to provide evidence of the BB. But those interpretations which are favourable to the theory are overwhelminlgy popular among cosmologists, and they're generally considered very strong evidence for it. I'm not quite sure what you're referring to about "some fundamental instability that only partially but very deficiently is provided by Dark Matter" - the primordial universe is generally accepted to have had Gaussian (or near-Gaussian) fluctuations which do a pretty good job of predicting current large-scale structure, and whether the matter was dark or baryonic is not directly related to their nature. The evolution of those perturbations over cosmic time requires a significant dark component to give the correct end result, but I don't see what's deficient about that.

I agree with you that "dark energy ...[is] considered [among] the most secure" is rather too confident given the state of DE; I don't agree, though, with your assessment of dark matter. DM is more than "somewhat" supported; it's the massively dominant paradigm and a large proportion of papers on the topic take its existence for granted, so strong is the evidence considered to be. It's correct that there are "no laboratory measurements on Earth" supporting it - but we can't present that as a major problem rather than a temporary inconvenience likely to be resolved, because most cosmologists do not and we're a tertiary source.

Your final paragraph too is fair enough as your own opinion, but Wikipedia must report what the authorities say. If cosmologists are generally optimistic about the Big Bang theory then we must report that they are. "A real scientific text shall just state the problems and point out possible solutions" - but Wikipedia is not a scientific text; it's an encyclopedia aimed at the general reader, and enough authors do say "the Big Bang is successful and robust" that we're justified, I believe, in echoing them. Olaf Davis (talk) 20:45, 5 October 2009 (UTC)

OK, I'm irritated by the text explaining Big Bang as a truth, like it was a religion. We should require no "truths", only "attested theories" or "best available model", we should require no "the majority likes this", because Alfred Wegener was in minority, yet correct as we know it today. Personally I think Big Bang have serious flaws (see Shortcomings of the Standard Cosmology), but if I have to choose between models, my honesty requires me to choose Big Bang as being the least flawed. The language of the article describes Big Bang like a majority religion, but I would like it to be written as an act of science, a model that in comparison to other kinds of modelling is very weak, but yet the best available, considering the measurement hardships. It won't become a very good article if it is written like it being truth, instead it would profit very much from holding forth all the "holes" and the qualified efforts to fill those "holes", which still makes it a pretty viable theory. ... said: Rursus (^mbork³) 15:07, 27 October 2009 (UTC)

Rursurs was right that there was some peacock terms. I tried to edit them out. The only thing I simply removed instead of addressing in the text was the "fact" tag appended to the note regarding baryogenesis and inflation since this fact is easily found within the general reference list as described (it's simply a logical truism due to causality, actually). I provided two white paper citations to demonstrate the state-of-the-field wrt dark matter and dark energy investigations. If someone could format them properly, I'd greatly appreciate it. Thanks, Rursus for pointing out some of the problems in wording. ScienceApologist (talk) 22:06, 5 October 2009 (UTC)

"Evidence?!?!"

In the section about religion, it states that theologians "reject or ignore the evidence of the Big Bang". Well, how can they reject "evidence" that doesn't exist? Was anyone around to witness it? If anyone has please step forward. This article should present the Big Bang THEORY as what it is; a THEORY. --The Great Fizack (talk) 23:11, 20 October 2009 (UTC)

You have quoted that phrase out of context - the full sentence is "Some [religious groups] accept the scientific evidence at face value, while others seek to reconcile the Big Bang with their religious tenets, and others completely reject or ignore the evidence for the Big Bang". Seems like a balanced statement to me. As the first sentence of the article says: "The Big Bang is the cosmological model of the initial conditions and subsequent development of the Universe that is supported by the most comprehensive and accurate explanations from current scientific evidence and observation". Model is just another word for theory. You are quite right to say that scientific evidence is not abslute proof - we cannot be certain about what happened 14 billion years ago, just as we cannot be certain about anything in the external world - see Descartes demon. Gandalf61 (talk) 09:48, 21 October 2009 (UTC)

Fizack, theories in science carry a lot of weight, they are not just guesses the way the term is used in popular speech. The idea that direct observation is the only way to be confident about a historical event is lunacy. Abyssal (talk) 15:12, 21 October 2009 (UTC)

Fizack, it sounds like you are confusing two very different uses of the word "theory". In colloquial use, "theory" means an uncertain guess which may or may not be true. In science, that is not called a theory, it's called a hypothesis. In science, a Theory (sometimes spelled with a capital T to denote the difference) is not just a hypothesis. It is something that has been extensively tested until it's so certain that it would be almost impossible to overturn it. It's closer to a fact than to a hypothesis, though scientists rarely use the word "fact" because there's always a teensy tiny possibility that they could learn something in the future that contradicts the current understanding. In a Theory, however, that possibility is ridiculously small. Consider that gravitation is also a Theory, but there's no way anyone would say "well, it's ONLY a theory... That means that gravity might not really exist!" --Icarus ^(Hi!) 19:12, 21 October 2009 (UTC)

Except that "string theory" is also called a "theory". <g,d&rVF!> ___A. di M. 17:13, 2 November 2009 (UTC)

True. But in every scientific discussion I've heard on the topic, one of the first things mentioned is that it really ought to be called the string hypothesis or the string model, since it is nowhere close to being an actual Theory right now. This appears to be an instance of the colloquial use bleeding over into a scientific context (it's understandable that this happens occasionally - scientists are human beings, after all, who grow up exposed to the colloquial meaning of as much as anyone else). String theory could be referred to as a "little t theory," but it is not a "bit T Theory." --Icarus ^(Hi!) 20:47, 2 November 2009 (UTC)

Fizack, of course! Except remember the saying: "theories is the best we can get". ... said: Rursus (^mbork³) 15:17, 27 October 2009 (UTC)

Related question: Why is there no page similar to Talk:Evolution/FAQ for the Big Bang? It seems that these concerns should be addressed at a centralized location.... --Eunsung (talk) 21:27, 15 November 2009 (UTC)

Copernican Principle

These parts of the article don't make sense to me:

That space is undergoing metric expansion is shown by direct observational evidence of the Cosmological Principle and the Copernican Principle, which together with Hubble's law have no other explanation. Astronomical redshifts are extremely isotropic and homogenous,[4] supporting the Cosmological Principle that the Universe looks the same in all directions, along with much other evidence. If the redshifts were the result of an explosion from a center distant from us, they would not be so similar in different directions.

Why would not be the redshift same in all directions when observed from some random piece of an expanding cloud of galaxies (supposing we are far enough from the edge)? Whoever wrote this apparently meant that if you select a point in space outside the center of the explosion, the redshifts will not be symmetric. That is irrelevant, though: observers move with the stars which fly away from the center of explosion, and if you take that motion into account, you get symmetric redshifts from any point.

Explosions have a direction outward in the radial direction because they have a center. A "centerless explosion" is equivalent to a metric expansion". ScienceApologist (talk) 16:47, 25 October 2009 (UTC)

You missed my point entirely, which was that an observer co-moving with the matter flying out from the explosion sees himself as the center from which everything moves away. There is no local feature which would allow differentiating between the center and other points; assuming only objects within a finite distance are visible (thus we are unable to notice that the edge of the explosion is closer in one direction than in the other), Hubble's law fits just as well with sitting on any random object which was thrown out by the explosion than with sitting in the centre. You could say that the center is defined by the observer's frame of reference and not any inherent feature of the explosion.

Also, a centerless explosion (which is pretty much what the steady-state model assumes) might be equivalent to a metric expansion in some abstract mathematical sense, but it is not the same thing physically, if we take relativistic effects into account. With metric expansion, you can observe redshifts which correspond to much higher relative speeds than the speed of light; this would not be possible when the expansion would be nothing more than matter moving around.

--Tgr (talk) 07:56, 26 October 2009 (UTC)

an observer co-moving with the matter flying out from the explosion sees himself as the center from which everything moves away --> not at the same rate in all directions if there is a center. There is a preferred direction which corresponds to the radial direction and that is the one with the highest Hubble Constant. ScienceApologist (talk) 14:21, 26 October 2009 (UTC)

Assume C is the center of the explosion, O is the observer sitting on a planet flying away from the center, X is some random other planet flying away. Since the explosion follows Hubble's law, the velocities of O and X relative to C are $H_{0}{\overrightarrow {CO}}$ and $H_{0}{\overrightarrow {CX}}$, respectively. The velocity of X relative to O is then $H_{0}{\overrightarrow {CX}}-H_{0}{\overrightarrow {CO}}$ = $H_{0}{\overrightarrow {OX}}$, thus from the point of view of O there is a linear relation between distance and speed of X (which is exactly the same relation an observer would see from C). --Tgr (talk) 01:56, 27 October 2009 (UTC)

Since the explosion follows Hubble's law --> An explosion that follows Hubble's Law is not a normal explosion and the "center", C, is not really a "center". ScienceApologist (talk) 16:34, 29 October 2009 (UTC)

Could you please sketch out the math showing how the observations would differ? I've been lurking in this thread, and I'm having trouble seeing how observable events in the two scenarios would be distinguished in the limit of an arbitrarily distant center of the explosion (i.e. far outside the observable universe). Providing this would also go a long way towards convincing Tgr of your viewpoint (right now, you're asking him to take "no, you're wrong" on faith). --Christopher Thomas (talk) 17:34, 29 October 2009 (UTC)

Take Tgr's previous example and choose another point instead of C (call it Y, for example) and redo the analysis. You get exactly the same result if Hubble's Law applies universally. Y then acts exactly the same way as C and is therefore indistinguishable from C. For that reason, Y must either "also" be the center or there must be no center at all. There's really nothing more to it than that. A "center" implies a breaking of symmetry that is impossible in a Hubble's Law arrangement. ScienceApologist (talk) 17:08, 30 October 2009 (UTC)

You get exactly the same answer, per his math above (just swap in Y instead of O; the velocity of any X relative to Y ends up being $H_{0}{\vec {YX}}$, giving you Hubble's law with Y as the new centre). The only flaw I see in Tgr's argument is that you have to ignore relativity for it to work as-written ($|H_{0}{\vec {OX}}|$ can exceed C), and it's not obvious to me that this invalidates the argument (it might be possible to construct a deformed velocity/radius relationship such that these relations hold when using the SR velocity addition/subtraction formulae). I'm not saying the argument is _correct_; just that I don't think it's been properly refuted in this thread (important, because this should be cleaned up in the article if it's causing confusion). --Christopher Thomas (talk) 18:38, 30 October 2009 (UTC)

I think you just proved my point. There can only be one center (by definition). You just proved that there exists another point (Y) indistiguishable from the proposed center (C). Ergo, there is no center. ScienceApologist (talk) 19:46, 30 October 2009 (UTC)

You miss my point (and apparently Tgr's point, if I understand it correctly): that an "explosion" (expansion of some cloud of pointlike objects through space) is observationally indistinguishable from the metric expansion of space, provided that the edge of the cloud of expanding objects is outside the observation horizon. The appearance of Hubble's law recession does not itself demonstrate that the metric expansion exists, at least without arguments beyond those sketched above. Do you see what I'm getting at, here? --Christopher Thomas (talk) 21:38, 30 October 2009 (UTC)

If the edge is unobservable then the center is unobservable and the explosion is indistinguishable from a metric expansion. Therefore it is a metric expansion. ScienceApologist (talk) 16:35, 31 October 2009 (UTC)

I'm pretty sure the therefore it is a metric expansion doesn't follow. At minimum, you could construct an "explosion" scenario where there were the edge would eventually come into view. In an explosion scenario, you'd also be able to prove that an event horizon forms (roughly at the edge of the observable universe, if our density estimates are correct), whereas in a metric expansion scenario you have an observation horizon, but no event horizon in the black hole sense of the term. --Christopher Thomas (talk) 16:47, 31 October 2009 (UTC)

An edge eventually coming into view is a meaningless thought-experiment in regards to the base assumptions we are considering here. The Copernican and Cosmological Principles are only applied to the universe in a space-like slice of spacetime (and that's why, for example, we avoid the perfect cosmological principle). Event horizons and future light cones are speculative based on universality. They do not affect cosmological models based on testable observational assumptions. ScienceApologist (talk) 16:55, 31 October 2009 (UTC)

(deindent) I'm having a lot of trouble understanding this subthread, largely because I can't understand half the stuff that you (ScienceApologist) say. I'm not even sure if you disagree with Christopher Thomas or if this whole thread is just the result of a miscommunication.

All experimental evidence is consistent with (for example) our being somewhere inside a very large spherical homogeneous ball of matter expanding into an infinite Schwarzschild vacuum. That setup is consistent with GR and it would look homogeneous and isotropic from any point not too close to the edge. It has a geometric center which needn't coincide with our location. One can argue against this on Occam's razor grounds, but it's not directly ruled out by the evidence. I dislike the term "metric expansion" because it suggests that the physics of the expansion of the universe is fundamentally different from the physics of an ordinary explosion, which is not the case, and also because I don't understand what it's supposed to be saying about the nature of the world. The FLRW metric, ds² = dt² − a(t)²dΣ², does "undergo metric expansion" in the sense that equal coordinate distances represent larger metric distances at later coordinate times, but that's merely a statement about the coordinates involved and not about the physical world. -- BenRG (talk) 18:35, 31 October 2009 (UTC)

I agree with all you are saying, BenRG. I don't see why this is in contradiction to the article text. ScienceApologist (talk) 19:29, 31 October 2009 (UTC)

BenRG: My understanding was that our experimental evidence was not consistent with our being inside a large spherical homogeneous ball of expanding matter, because any such model of the universe would run into the same problem as static universe models: no matter what density you assume, any sufficiently large volume ends up inside an event horizon, contradicting the assumptions made (i.e., that that volume is not undergoing collapse to a singularity). In virtually all descriptions of the big bang model that I've seen, metric expansion (or alternatively, a spacetime upon which drawing a coordinate system like that makes sense) is presented as being fundamentally different from an explosion through "non-expanding" space. This is consistent with the lede at metric expansion of space, among other places.

If I'm wrong, fine, but I'm getting inconsistent messages here. --Christopher Thomas (talk) 21:38, 31 October 2009 (UTC)

An event horizon outside our particle horizon does not affect the observations inside the particle horizon. You can have both metric expansion observed and an "exterior" event horizon and be perfectly consistent. ScienceApologist (talk) 14:26, 1 November 2009 (UTC)

Measurements of the effects of the cosmic microwave background radiation on the dynamics of distant astrophysical systems in 2000 proved the Copernican Principle, that the Earth is not in a central position, on a cosmological scale.[notes 5] Radiation from the Big Bang was demonstrably warmer at earlier times throughout the Universe. Uniform cooling of the cosmic microwave background over billions of years is explainable only if the Universe is experiencing a metric expansion, and excludes the possibility that we are near the unique center of an explosion.

How does that have anything to do with the Copernican Principle? Sure, the cooling of the CMB (or the CMB itself in the first place) cannot be explained by an explosion, but that's irrespective of any symmetry considerations. Supposing we are in the center of an explosion does not give us any extra explaining power over supposing we are at some random point of an explosion. --Tgr (talk) 02:01, 25 October 2009 (UTC)

The Copernican Principle demands that every observer on a space-like slice of spacetime see the same cosmologically relevant phenomenon. However, it is not possible to actually observe what a distant observer sees since we are stuck in a very small part of spacetime. These observations enable us to "see" what a distant cosmologist sees and confirm the fact that there is nothing preferred about either observation. ScienceApologist (talk) 16:47, 25 October 2009 (UTC)

I don't see how that is relevant to what I said. Maybe the CMB cooling differentiates between Copernican and non-Copernican models of the Big Bang, and it certainly differentiates between Big Bang and the explosion model (which has nothing to do with the Copernican principle since the explosion model is not necessarily non-Copernican), but it makes no sense to say that it differentiates between non-Copernican (sitting in the center) and non-Copernican (sitting in some random point) versions of the explosion model, since that model cannot explain the CMB in the first place. Thus, the claim of the article the the CMB cooling "excludes the possibility that we are near the unique center of an explosion" is wrong. --Tgr (talk) 07:56, 26 October 2009 (UTC)

The source clearly indicates that the CMB has to be universal as opposed to local. ScienceApologist (talk) 14:21, 26 October 2009 (UTC)

I don't think it's technically wrong, but it is misleading: the CMB does exclude our being near the 'unique centre' of an explosion because it excludes any type of explosion. However, phrasing it the current way does make it sound like it rules that out while leaving in the 'edge of an explosion' model, which as you say it doesn't, so I agree the wording should be changed. Olaf Davis (talk) 10:37, 26 October 2009 (UTC)

I think this section of the article should be rewritten, because there really is no difference between "metric expansion" and a sufficiently symmetric explosion. In the zero-density case, when spacetime is flat, the FLRW metric is $ds^{2}=d\tau ^{2}-\tau ^{2}\left({\frac {d\rho ^{2}}{1+\rho ^{2}}}+\rho ^{2}d\Omega ^{2}\right)$, which is literally just Minkowski space in different coordinates: substitute $\tau ={\sqrt {t^{2}-r^{2}}}$ and $\rho =r/{\sqrt {t^{2}-r^{2}}}$ and you get $ds^{2}=dt^{2}-dr^{2}-r^{2}d\Omega ^{2}\!$. The "metric expansion" in FLRW coordinates is recessional velocity in Minkowski coordinates, the cosmological redshift in FLRW coordinates is special relativistic redshift in Minkowski coordinates, the larger-than-c comoving speeds in FLRW coordinates are lower-than-c special relativistic speeds in Minkowski coordinates, and so on. When you introduce enough matter that spacetime is noticeably curved then it gets harder to see what's going on, but nothing has fundamentally changed. The big bang was an explosion; it was far more uniform than any other explosion in nature, but that's only a difference of degree, not of kind. The article repeatedly assumes that the astronomical data is only consistent with an explosion if we're at the center of it, which as Tgr points out isn't true: all that you can measure is relative distances and speeds, and those look the same whether or not you're at the center. -- BenRG (talk) 13:36, 26 October 2009 (UTC)

The center of the explosion argument is fine for the null hypothesis associated with the Copernican Principle. As a general rule, all one needs to do to test the Copernican Principle is show that there is nothing preferred about our frame of reference. One could assume that Hubble's Law provided that we inhabit a preferred reference frame but the observations of higher temperature CMB at distant locations is evidence contrary to this assertion. ScienceApologist (talk) 14:21, 26 October 2009 (UTC)

The reason we refer to the Copernican Principle is that, strictly, it is necessary to assume it to derive anything from cosmological observations. As noted elsewhere in the article, we can observe isotropy but we need CP to get to homogeneity. Specifically, we directly observe systematic changes with distance (e.g. lower clustering amplitude, higher CMB temperature, more quasars & star formation at high redshift than nearby). By assuming CP we infer that these are actually due to change with time rather than to our being located at the centre of the cosmic spheres. Of course, modern cosmology has been able to predict some of these changes (notable the CMB temperature) ahead of discovery, and such confirmed instances provide supporting evidence for the whole structure of cosmological theory, including the CP on which it rests, but (despite ESO press release) it is not a test of the CP specifically; for that we would also need direct evidence that the T_CMB was higher in the past in our local region of the universe, and of course this would only confirm one small aspect of what the CP asserts.

Furthermore, the first of these two paras is almost meaningless. In what sense are observed redshifts supposed to be "isotropic and homogeneous"? Strictly speaking they definitely are not, as this would mean that redshift (or at least statistical distribution of redshifts) was independent of distance! One could make a case that the Hubble expansion is isotropic (but not using Hubble's original data, as cited!), but this says nothing about homogeneity which is the crux for the CP. One could claim that the 3-D distribution of galaxies revealed by redshift surveys is homogeneous, but this claim is seriously disputed in the published literature: the large-scale structure extends to scales up to at least 100 Mpc, and some argue that fractal structure continues to arbitrarily large scales (in which case the CP is wrong as are all Friedman-based universe models).

By the way, these two paras were inserted (after the last FA review) by banned sockpuppet user:Publicola, if that's relevant to anything. PaddyLeahy (talk) 19:13, 8 November 2009 (UTC)

Please, take a stab at rewriting it. I don't know if we really need those paragraphs at all, and will not object to their removal. ScienceApologist (talk) 21:11, 9 November 2009 (UTC)

Not a matter of faith

The first sentence of Features, issues and problems, tells us:

While very few professional researchers now doubt the Big Bang occurred

is formulated like Big Bang was a matter of faith, where this or that supernatural power declared a "truth" and the true and only true adherents hailed the rightness of the supernatural. The real issues, religion aside, is whether Big Bang is tentatively accepted as a valid model, or not — not whether it is "true". What if there in fact is a lot of model agnostics that think: "Big Bang is posed with so many troubles, that I'll rather concentrate on observational astronomy...", and what if there are a lot of Big Bang professionals that think: "OK, I'm working with this flawed model in preparation for novel LHC to occur and solve most of our problems!".

I can understand if the text tries to educate that this is the real creation, not that Creationist pseudoscience, but the scientific philosophies is not comparable to pseudoreligious pseudoscience by far, and so we shouldn't label science as "truth" but instead of "attested theory", as opposed to "weird ad-hoc-dogma". ... said: Rursus (^mbork³) 14:34, 27 October 2009 (UTC)

Please forgive me the querulant tone, the article is pretty OK according to my taste. Many improvement since last time. ... said: Rursus (^mbork³) 15:32, 27 October 2009 (UTC)

I tried to fix it myself, pardon all inconveniences. ... said: Rursus (^mbork³) 15:38, 27 October 2009 (UTC)

I made a little tweak to the wording to avoid special pleading. Hope it's okay. ScienceApologist (talk) 15:41, 27 October 2009 (UTC)

For what it's worth, Rursus, I can say with pretty high confidence from my own experience that the membership of those two groups you posit within cosmologists is essentially zero. Olaf Davis (talk) 17:40, 27 October 2009 (UTC)

temperature of around space before bigbang

It is stated in the article that the Universe has originated from a primordial hot and dense initial condition at some finite time.

I can’t comprehend how the universe cooled down. Was the cold medium (temperature) present around before big bang?68.147.38.24 (talk) 04:31, 8 November 2009 (UTC) khattak

This talk page is for improvements to the article; not for tutorials on cosmology. But for what it is worth, space itself doesn't actually have a temperature, and the article does not speak of a temperature for space. The temperature of the background radiation that fills the universe is falling as the universe expands. This is described in the article. —Duae Quartunciae (talk · cont) 04:39, 8 November 2009 (UTC)

The temperature given is the average temperature of the material within the universe. As the universe is a self-contained system, changing the volume of the universe (and density of the material within it) changes the average temperature (this is an "adiabatic expansion", described at adiabatic process#Ideal gas (reversible case only).

The early universe was filled with a large number of photons and with a plasma of particles and antiparticles that were continually created and destroyed (the temperature being high enough for pair production. As it expanded, it cooled, with a number of effects occurring (as described in the article).

If parts of the article are unclear, these can be improved. Were there any specific sentences or paragraphs that you felt should be changed to better describe the material? --Christopher Thomas (talk) 04:54, 8 November 2009 (UTC)

Were there weird "atoms" around 10-13 seconds?

Around 10-13 seconds after the Big Bang, things like the bottom quark and the tauon were stable particles (surviving as long as the Universe itself). Given their larger mass and the density of particles in the universe, could they have formed atom-like composites or produced interesting astronomic structures? Wnt (talk) 07:12, 21 November 2009 (UTC)

At 10^-13 seconds ABB, you are approaching the end of the electroweak epoch and the start of the quark epoch. At this stage the average energy of particle interactions is still too high to allow quarks to combine into mesons or baryons. The universe is filled with a sea of high-energy quarks, leptons and their antiparticles, with the quarks interacting via the strong force by exchanging gluons in a quark–gluon plasma. So the short answer is no, no composite structure could have survived at this time. Gandalf61 (talk) 14:17, 21 November 2009 (UTC)

Thanks for responding! But let's look into some details, if I ask the question for a point solidly within the quark epoch, perhaps 10^-9 seconds ABB. If the average energy of particle interactions is too high to allow "quarks" to combine into "mesons or baryons", does that definitely mean that it is too high for bottom, charmed, and strange quarks to combine into exotic mesons or baryons? For example quark stars apparently can contain strange matter even though they consist of a quark-gluon plasma where regular quarks are concerned.

I tried to look into this directly for a bit, but it is tough going for the uninitiated. Apparently bottom quarks have energy around 5 GeV, yet mesons with bottom-antibottom can have energies 26 keV to 110 MeV (????) [1] In the unlikely instance that I'm not misinterpreting this, that would seem to mean that almost the entire 5 GeV mass of a bottom quark can be released as energy when it binds another quark, and would need to be returned in random collisions to recreate it. Since it has a mass 3000 times that of an up quark, I'd expect it to need 3000 times hotter a temperature and to become stable in mesons/baryons roughly 3000 times sooner than regular quarks i.e. at 10^-9 seconds ABB. I'm sure there are fallacies in what I just said, but it might be enlightening to see them corrected. ;) Wnt (talk) 10:54, 22 November 2009 (UTC)

A singularity?

The article refers to the big bang as a singularity but it's not entirely clear that there is a consensus on this issue:

"So in the end our work became generally accepted and nowadays nearly everyone assumes that the universe started with a big bang singularity. It is perhaps ironic that, having changed my mind, I am now trying to convince other physicists that there was in fact no singularity at the beginning of the universe -- as we shall see later, it can disappear once quantum effects are taken into account." - Hawking, A Brief History of Time p. 50 —Preceding unsigned comment added by Craig Pemberton (talk • contribs)

This is mostly a case of the answer depending on what model we choose to apply when extrapolating back to the instant of the Big Bang. Using general relativity alone, it produces a gravitational singularity (using the same mathematical proof that shows that a black hole collapses to a gravitational singularity). However, most scientists expect that a theory of quantum gravity would produce different predictions (for both black holes and the Big Bang). We have no satisfactory theory of quantum gravity (or of unification of the fundamental forces, which is also expected to happen at those temperatures). Instead, different people have tried to use various different approximations to estimate how the system in question (universe or black hole, depending on who's doing it) would behave. Whether any of these answers is correct is debatable (and indeed, is being vigorously debated within the scientific community). There is general agreement, if I understand correctly, that you'd at least be starting with a plasma at or near the Planck temperature and with correspondingly high density, in the case of the early universe.

Does this address your question? --Christopher Thomas (talk) 04:32, 3 December 2009 (UTC)

Is the "singularity" real or only mathematical? We know that there are only 14 million years' worth of vibrations of a cesium atom possible in our past, but the point that makes is only that if you go back far enough in time, cesium atoms become increasingly unwieldy, until another physics takes over. We know that all sorts of cosmic things "happened" in miniscule fractions of a second at the beginning by this definition of time, but we could just as easily use the logarithm of time, or perhaps some other unit of spacetime defined by the assumption that a photon of light remains always the same color and always traverses the same number of wavelengths in a unit time (in which case space is exactly?? the same size at any time). Can we rule out that vast galactic empires played out in attoseconds of quark-gluon plasma, or that once 10^100 years is a tick of the clock our neutrinos and WIMPs will turn out to form interesting new patterns of life? Wnt (talk) 16:23, 6 December 2009 (UTC)

Matter at relativistic speeds

The article says that "Temperatures were so high that the random motions of particles were at relativistic speeds, and particle–antiparticle pairs of all kinds were being continuously created and destroyed in collisions." But isn't the first part true for most of the matter in the universe today, with neutrinos and dark matter moving so quickly they can only barely (if at all) stick to one galaxy? I'm not sure where those things stand regarding the antiparticle pairs. Wnt (talk) 16:03, 7 December 2009 (UTC)

The vast majority of dark matter is believed to be cold dark matter (CDM), with the 'cold' meaning that its motion is non-relativistic. It tends to stay bound to dark matter 'haloes' which are permanent structures often containing one or more galaxies, with a relatively small interchange of mass between haloes. Relativistic DM is called hot dark matter. You're right that massive neutrinos would qualify and they're a prime candidate for HDM, but their density in the universe is far lower than that of CDM.

As for particle-antiparticle pairs, both neutrinos and CDM particles are thought to be extremely weakly interacting, so although they may be producing pairs (this is the mechanism through which people are searching for a direct DM detection), the interaction rate is far lower than it would have been for relativistic baryons. Olaf Davis (talk) 17:32, 7 December 2009 (UTC)

Simple revision requested

Since this is a theoretical model it should be noted in the first paragraph that, by definition, it is not fact and lacks certainty. Toneron2 (talk) 08:03, 12 December 2009 (UTC)

In a scientific context, being theoretical and being a fact are not mutually contradictory. Similar discussions have been had here many times before - you might want to look at Icarus's first post in this section above which is a good short summary. Olaf Davis (talk) 19:09, 12 December 2009 (UTC)

As the first sentence of the article says, the Big Bang is a model that is consistent with the known facts, as opposed to models that are inconsistent with the known facts, such as the Steady State theory. Gandalf61 (talk) 23:35, 12 December 2009 (UTC)

Current best estimate for age of the universe?

A recent edit on Dec 14 emphasizes "measurement" for inference of the age of the universe since the Big Bang. I think this is potentially misleading.

The age itself is not a measurement; but an estimate based on best fit of parameters to measurements of supernovae, CMBR, and so on; along with some theoretical assumptions about the model (six parameter ΛCDM). Hence measurements are used; but the age itself really is an estimate, given by a best fit of parameters.

Furthermore, the value given (13.7 Gy) is already out of date, I think, given improved measurements of the Hubble constant by Reiss et al (2009) A Redetermination of the Hubble Constant with the Hubble Space Telescope from a Differential Distance Ladder, arXiv:0905.0695v1. The current reference in the article is to Komatsu et al (2008) Five-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Cosmological Interpretation arXiv:0803.0547v2, which proposes a significantly smaller Hubble constant. Note that the new value for H₀ is based more directly on measurement, whereas H₀ from Komatsu et al is a secondary inference from other parameters. Although age of the universe has a lot of popular interest, it isn't actually one of the significant parameters of most interest to cosmologists, so it is often not given explicitly as part of recent results.

With more recent measurements, I think 13.2 Gy is probably closer; except that this number is often not given explicitly. I'll hunt around; in the mean time other suitable references would be good. However, I do think that "measurement" in the context of age of the universe is misleading. I appreciate that the current wording speaks of an age based on measurements; but it is still a very indirect process; going from measurements, to cosmological parameters, and then to derived parameters like age via reasonable assumptions about the model. —Duae Quartunciae (talk · cont) 07:17, 14 December 2009 (UTC)

Update. The value I suggested of 13.2 is not a good estimate; merely my own off the cuff guess based on revisions to H₀. I would suggest saying around 13.5 to 14 Gy (to be consistent with [Age of the Universe]) or 13.6 +/- 0.3 based on Menegoni, Eloisa; et al. (2009), "New constraints on variations of the fine structure constant from CMB anisotropies", Physical Review D, 80 (8), doi:10.1103/PhysRevD.80.087302{{citation}}: CS1 maint: numeric names: authors list (link)

An extract from Menegoni et al (2009) reads:

We indeed found that if one allows for variations in α, the WMAP five year data bounds the age of the universe to t0 = 13.9 ± 1.1 Gyrs (at 68% c.l.), with an increase in the error of a factor ~3 respect to the quoted standard constraint (see [2]). Including all CMB datasets improves the constraint to t0 = 14.3 ± 0.6 while combining with the HST prior yields t₀ = 13.6 ± 0.3 Gyrs (all at 68% c.l..).

The HST prior in this case is the work by Reiss et al I cited above.

I propose that the phrase current reading

• (currently best measurements place initial conditions at a time approximately 13.7 billion years ago)

be replaced with

• (best available measurements in 2009 suggest that the initial conditions occurred around 13.3 to 13.9 billion years ago)

with the reference going to Menegoni et al (2009). —Duae Quartunciae (talk · cont) 08:05, 14 December 2009 (UTC)