Big Bang: Discovering the Reasons Behind Its Occurrence

[finiter]

You can't do work on, "maybe it's something we haven't thought of yet," as you have to have a specific idea to at least get started. If you have a specific idea of an alternative, then great! Work on it! Flesh it out! See if it is self consistent, see if it is consistent with past observations, see if it provides testable predictions for future observations.

Cold energy, where temperature go below 0K, I think, could explain a black hole. I don't know whether anybody else have made such a suggestion earlier. Can you confirm? However, I have been working on that for some time.

 

[Chalnoth]

Cold energy, where temperature go below 0K, I think, could explain a black hole. I don't know whether anybody else have made such a suggestion earlier. Can you confirm? However, I have been working on that for some time.

Temperature can't go below zero. Temperatures that are numerically below zero turn out to act, thermodynamically, as temperatures that are hotter than positive infinity.

 

[budrap]

[

But the accusation of inflation, dark matter, and dark energy being "ad hoc" is entirely specious. Yes, they were each initially proposed as a possible explanation for a particular inadequacy in the model.

These two sentences are mutually contradictory, like claiming to be a God-fearing atheist.

You might wish to claim that maybe there is some explanation that we haven't thought of yet, but that isn't science. One of the primary driving forces of scientific work stems around simple practicality. You can't do work on, "maybe it's something we haven't thought of yet," as you have to have a specific idea to at least get started. If you have a specific idea of an alternative, then great! Work on it! Flesh it out! See if it is self consistent, see if it is consistent with past observations, see if it provides testable predictions for future observations.

But don't just stand on the sidelines and say, "Yeah, I think it's something else." That isn't science.

The objection I raised was not vague I thought but very specific. Let me restate:

1. As with the Ptolemaic model the proponents of the Modern Cosmological model only patch the existing model when it encounters a predictive failure.

2. In and of itself this is not wrong just inadequate. Whenever a model fails it's fundamental assumptions should be re-evaulated in light of that failure. If a model fails repeatedly the imperative to re-evaluate should become evermore scientifically compelling.

3. The specific underlying assumption of the MC model that I feel is most in need of re-examination is that there exists a singular "Universe" which encompasses all of the observable and unobservable cosmos. This concept of "Universe" has never been scientifically vetted, only assumed. It appears to be a vestigal cultural concept much like the the Earth centric perfect circles of Ptolemy.

If you choose not to address these points that is certainly your perogative. If you wish to argue them please do. But please, spare the strawman arguments. Thanks.

 

[Chalnoth]

These two sentences are mutually contradictory, like claiming to be a God-fearing atheist.

The key point is initially. If people had said they were true only based upon the initial evidence, yes, I might agree with you.

2. In and of itself this is not wrong just inadequate. Whenever a model fails it's fundamental assumptions should be re-evaulated in light of that failure.

The funny thing is that you think this hasn't actually been done.

3. The specific underlying assumption of the MC model that I feel is most in need of re-examination is that there exists a singular "Universe" which encompasses all of the observable and unobservable cosmos. This concept of "Universe" has never been scientifically vetted, only assumed. It appears to be a vestigal cultural concept much like the the Earth centric perfect circles of Ptolemy.

While I happen to agree with that this is often speciously assumed, it in no way places doubt upon these three models. This is, by large, a separate issue related to how we go about determining whether a particular model is likely or unlikely, but it doesn't really effect the observational evidence in favor of inflation, dark matter, and dark energy.

 

[budrap]

The funny thing is that you think this hasn't actually been done.

The funny thing is that you seem to be agreeing that it hasn't actually been done with respect to the concept of "Universe" here:

While I happen to agree with that this is often speciously assumed, it in no way places doubt upon these three models. This is, by large, a separate issue related to how we go about determining whether a particular model is likely or unlikely, but it doesn't really effect the observational evidence in favor of inflation, dark matter, and dark energy.

As far as I can see there is only one model (Modern Cosmological) with three patches (Inflation, Dark Matter, Dark Energy) required by predictive failures of the MC and there is no direct observational evidence for any of them nor is there likely to be any time soon. The best that can be said is that there are no contradictory observations.

The interesting thing about a cosmology devoid of a singular all encompassing "Universe" is that both Inflation and Dark Energy would be irrelevant in the same way that epicycles became irrelevant after Copernicus/Kepler. Dark Matter might still be an interesting hypothesis depending on whether or not the new model directly clarified the gravitational issues DE seeks to address.

 

[jtbell]

The interesting thing about a cosmology devoid of a singular all encompassing "Universe" is that both Inflation and Dark Energy would be irrelevant in the same way that epicycles became irrelevant after Copernicus/Kepler.

Can you provide a reference to a publication in a reasonably current peer-reviewed journal, or possibly on arxiv.org, that proposes or uses such a cosmology? (Note that arxiv.org carries less weight because it is not peer-reviewed.)

I'd like to remind participants in this thread about the Physics Forum rules which are accessible by clicking the "Rules" link at the top of every page here, in particular the section Overly Speculative Posts. It is not part of the mission of PF to promote challenges to currently generally-accepted theories based on pure speculation. If a viewpoint is in fact part of current scientific discussion, as reflected in professional literature, even if it is a minority viewpoint, then it's OK to discuss here. If it's an outdated viewpoint that is not supported by current evidence, then it's not appropriate to harp on it here, just as we don't support promotion of classical luminiferous-aether theories against relativity.

 

[Chalnoth]

The funny thing is that you seem to be agreeing that it hasn't actually been done with respect to the concept of "Universe" here:

Huh? No, a number of people have looked into a variety of possibilities of what may lie beyond our observable universe. But these have very little bearing on dark matter, dark energy, and inflation, which are invoked to explain observations within our own observable universe. I think that the opposition to many such ideas is a bit too vociferous myself, but that doesn't mean the investigations aren't being carried out.

What is clear, though, is that you're extremely off-base. Investigations into what may or may not occur beyond our cosmological horizon has very little bearing on inflation, dark matter, and dark energy. There are some model selection arguments that depend upon what occurs outside of our cosmological horizon, but there is no way in which what occurs beyond our cosmological horizon can possibly do away with these models.

The only somewhat reasonable alternative explanations to these models today lie in modifications to gravity. Today we've ruled out the possibility of observational artifacts accounting for dark matter/dark energy. So it just becomes a question of whether we understand the matter content of our universe, or whether we understand gravity (since gravitational measurements are the only situation in which these discrepancies arise). As of today, nobody has yet produced a compelling modification of gravity to explain either dark matter or dark energy.

As for inflation, the horizon problem essentially proves that it or something like it must necessarily have occurred. The only reasonable alternative to inflation so far is the ekpyrotic model.

 

[budrap]

Can you provide a reference to a publication in a reasonably current peer-reviewed journal, or possibly on arxiv.org, that proposes or uses such a cosmology? (Note that arxiv.org carries less weight because it is not peer-reviewed.)

Hm, this is a peculiar request. My point is precisely that the merits of the singular Universe concept that undergirds the currently accepted cosmological model have never been scientifically evaluated. Therefore it is highly unlikely that there would be a paper proposing an alternative to this unacknowledged and unexamined a priori assumption that the cosmos is comprised of a singular reference frame such that it may be considered a singular entity or Universe.

The very fact that you are obviously unaware of any such an alternative cosmology is illustrative of my point. None exists because none has ever been considered. I don't see how that disqualifies my comments under the rules you mention unless you wish to dismiss them simply on the grounds that they are novel. Hopefully the bar for Overly Speculative Posts is not quite that low.

 

[budrap]

Huh? No, a number of people have looked into a variety of possibilities of what may lie beyond our observable universe. But these have very little bearing on dark matter, dark energy, and inflation, which are invoked to explain observations within our own observable universe. I think that the opposition to many such ideas is a bit too vociferous myself, but that doesn't mean the investigations aren't being carried out.

What is clear, though, is that you're extremely off-base. Investigations into what may or may not occur beyond our cosmological horizon has very little bearing on inflation, dark matter, and dark energy. There are some model selection arguments that depend upon what occurs outside of our cosmological horizon, but there is no way in which what occurs beyond our cosmological horizon can possibly do away with these models.

I guess I am not expressing myself very clearly. I at no point meant to imply knowledge of or a theory about what lies beyond the cosmological horizon. What I am attempting to convey is the idea that the cosmos we observe may not be all or part of a singular entity commonly called the Universe. Put another way I am simply stating that the question of whether or not there exists a universal space-time reference frame needs to given a proper scientific evaluation. As of now its existence is merely an a priori feature of the MC model.

So it just becomes a question of whether we understand the matter content of our universe...

Therein lies the problem sir, you assume the existence of a "Universe" where none in fact may exist.

 

[AC130Nav]

I guess I am not expressing myself very clearly...
Therein lies the problem sir, you assume the existence of a "Universe" where none in fact may exist.

Haven't got a clue what you're saying.

At first I thought you were a multiverse type; now I think you're a nihilist. That last doesn't belong here but might explain why you make no sense.

 

[DaveC426913]

Therein lies the problem sir, you assume the existence of a "Universe" where none in fact may exist.

AC130Nav is correct. This is the Cosmology Forum; the universe exists here by definition.

Philosophy Forum is two doors over. You may deny existence there.

 

[budrap]

AC130Nav is correct. This is the Cosmology Forum; the universe exists here by definition.

Philosophy Forum is two doors over. You may deny existence there.

Yes, this is the Cosmology Forum but it is not the existence of the cosmos (or anything else that can be empirically demonstrated) that I am questioning. What I do question is the assumption that the cosmos is possessed of a universal space-time reference frame which consequently renders it a "Universe" or singular entity. It is my contention that said assumption is unsupported by any empirical evidence.

If you wish disagree then produce the empirical evidence that supports the assumption of a universal space-time reference frame. Scientifically speaking you can't just insert it "by definition" except as hypothesis and if it's only a hypothesis then it must be subject to review or discussion.

If the distinction I am drawing between the cosmos and its characterization as a singular "Universe" is lost on you perhaps you should avoid discussions of matters that tax your ability to think and stick to comedy. But just to be clear here, I don't object to your disagreeing with me, only please offer scientific objections not quasi-religious arguments from authority. That is not the way of science.

 

[AC130Nav]

Yes, this is the Cosmology Forum but it is not the existence of the cosmos (or anything else that can be empirically demonstrated) that I am questioning. What I do question is the assumption that the cosmos is possessed of a universal space-time reference frame which consequently renders it a "Universe" or singular entity. It is my contention that said assumption is unsupported by any empirical evidence.

If you wish disagree then produce the empirical evidence that supports the assumption of a universal space-time reference frame. Scientifically speaking you can't just insert it "by definition" except as hypothesis and if it's only a hypothesis then it must be subject to review or discussion.

If the distinction I am drawing between the cosmos and its characterization as a singular "Universe" is lost on you perhaps you should avoid discussions of matters that tax your ability to think and stick to comedy. But just to be clear here, I don't object to your disagreeing with me, only please offer scientific objections not quasi-religious arguments from authority. That is not the way of science.

Actually there are good arguments against "quasi-religious arguments from authority," but your nonsense is giving them a bad name.

Let me be specific in one regard, "produce the empirical evidence that supports the assumption of a universal space-time reference frame." I'm not sure that's part of the current theories (I like it though, as long as 4 dimensions is not the limit). But it has nothing to do with science, which is restricted to what can be demonstrated. It is not that ONLY four dimensions must be demonstrated, but that AT LEAST four dimensions must be demonstrated (more to be proven by experiment).

 

[FizixFreak]

Now if you use for instance a Fermi normal coordinate chart in curved spacetime or simply a rest frame in Cartesian coordinates in flat space you can use time (which is then proper time) on one axis so it looks like it is a separate dimension. But just by using such a charts does not make it a dimension.

There is a distinction between the manifold and a choordinate chart and it is a mistake to assume that any of the dimensions of the manifold is time.

according to the einstine picture of space and time any thing having a mass can curve the space time fabric but since time is not a spatial dimension how can it be curved by any body??

 

[DaveC426913]

according to the einstine picture of space and time any thing having a mass can curve the space time fabric but since time is not a spatial dimension how can it be curved by any body??

You answered your own question.

As you said, mass curves spacetime (not just space). A massive object curves all 4 dimensions.

 

[finiter]

It is not part of the mission of PF to promote challenges to currently generally-accepted theories based on pure speculation. If a viewpoint is in fact part of current scientific discussion, as reflected in professional literature, even if it is a minority viewpoint, then it's OK to discuss here.

I think that PF does allow questions that may challenge the validity of the generally-accepted theories? Questioning the existing concepts is different from arguing on the basis of an un-accepted theory.

 

[AC130Nav]

You answered your own question.

As you said, mass curves spacetime (not just space). A massive object curves all 4 dimensions.

It might have been better to say (or at least mention) that time slows in the vicinity of a mass and then have given an example (from experimental data).

 

[jackmell]

What I do question is the assumption that the cosmos is possessed of a universal space-time reference frame which consequently renders it a "Universe" or singular entity. It is my contention that said assumption is unsupported by any empirical evidence.

May I ask what are the consequences if the cosmos does not have a universal space-time reference? I would guess from your dialog you suggest this would imply the cosmos then would not be a single entity, a "Universe", but would rather be somehow "disjointed" or separated in some way. If so, could you explain what that would mean.

 

[budrap]

May I ask what are the consequences if the cosmos does not have a universal space-time reference? I would guess from your dialog you suggest this would imply the cosmos then would not be a single entity, a "Universe", but would rather be somehow "disjointed" or separated in some way. If so, could you explain what that would mean.

What it would mean, I think, is that the early astronomers were correct in their first impression that galaxies are Island Universes. Meaning that galaxies each have their own unique reference frames because they each have unique origins relative to other galaxies. If that is true of course, we should expect to see evidence of galactic formation and evolution as we survey the cosmos. The observations of the astronomer Halton Arp support this view though they are widely ignored since they also cast doubt on the redshift as recessional velocity assumption that is the fundamental basis of the Big Bang model.

There is really nothing that novel in idea that there is no universal spacetime reference frame; it arises directly within the context of Special Relativity. A universal reference frame reappears in discussions of General Relativity, however not as a direct consequence of the theory but is simply imposed as a matter of convenience. If you apply the GR equations to the "Universe" you are inherently invoking a universal reference frame regardless of whether or not one actually exists. That, I think, was Einstein's greatest mistake.

 

[Chalnoth]

What it would mean, I think, is that the early astronomers were correct in their first impression that galaxies are Island Universes. Meaning that galaxies each have their own unique reference frames because they each have unique origins relative to other galaxies. If that is true of course, we should expect to see evidence of galactic formation and evolution as we survey the cosmos. The observations of the astronomer Halton Arp support this view though they are widely ignored since they also cast doubt on the redshift as recessional velocity assumption that is the fundamental basis of the Big Bang model.

Halton Arp's views are flatly contradicted by the existence of the CMB. Full stop. Nothing more needs to be said about his views unless he (or somebody else) can demonstrate a thermal distribution for such CMB light stems directly from his model.

Because the best he's been able to do so far is claim that his model predicts a "nearly uniform background light", which I consider a very spotty claim, but in any case there is no expectation of anything like a thermal distribution.

There is really nothing that novel in idea that there is no universal spacetime reference frame; it arises directly within the context of Special Relativity. A universal reference frame reappears in discussions of General Relativity, however not as a direct consequence of the theory but is simply imposed as a matter of convenience. If you apply the GR equations to the "Universe" you are inherently invoking a universal reference frame regardless of whether or not one actually exists. That, I think, was Einstein's greatest mistake.

Huh? This is flatly wrong. General Relativity contains even more freedom in reference frame than special relativity did. One can even do a number of calculations in GR without using any reference frame at all. This is useful, for instance, in demonstrating whether or not any weird features you see in your current equations are just a result of a bad choice of coordinates, or whether they actually exist in the model.

Of course, when it comes to understanding cosmology, it turns out that there exists a very convenient coordinate system (comoving coordinates). This is most certainly not a preferred reference frame, just a convenient one when talking about the expansion of the universe and its effects. In other areas, different coordinates are preferable.

 

[budrap]

Halton Arp's views are flatly contradicted by the existence of the CMB. Full stop. Nothing more needs to be said about his views unless he (or somebody else) can demonstrate a thermal distribution for such CMB light stems directly from his model.

I said nothing about Halton Arp's views. It is his observations that I explicitly referred to and his observations clearly contradict the assumption that the cosmological redshift is due to a recessional velocity. In science observations carry more weight than theory.

Huh? This is flatly wrong. General Relativity contains even more freedom in reference frame than special relativity did. One can even do a number of calculations in GR without using any reference frame at all. This is useful, for instance, in demonstrating whether or not any weird features you see in your current equations are just a result of a bad choice of coordinates, or whether they actually exist in the model.

I clearly said that a universal reference frame does not arise as a consequence of GR theory so what are you complaining about here?

Of course, when it comes to understanding cosmology, it turns out that there exists a very convenient coordinate system (comoving coordinates). This is most certainly not a preferred reference frame, just a convenient one when talking about the expansion of the universe and its effects. In other areas, different coordinates are preferable.

This is a distinction without a difference. Your comoving coordinates which consist of a universal origin at t=0 and a series of universal epochs (inflation, decoupling, galaxy formation and the universal simultaneity of now) constitutes a universal spacetime reference frame whether you want to call it that or not.

 

[Chalnoth]

I said nothing about Halton Arp's views. It is his observations that I explicitly referred to and his observations clearly contradict the assumption that the cosmological redshift is due to a recessional velocity. In science observations carry more weight than theory.

No, these are just his views. His observations say nothing of the sort (they were suggestive once upon a time, but we've much better observations today). It is only his opinion (and those of a few of his followers) that they do. Halton Arp may have done some good science in decades past, but for some time now has been nothing but a pseudoscientific crackpot, continually making wild claims that are completely disconnected from reality.

I clearly said that a universal reference frame does not arise as a consequence of GR theory so what are you complaining about here?

That you are complaining about it in the first place, that you think that this is a "blunder" at all and not simply a matter of convenience.

This is a distinction without a difference. Your comoving coordinates which consist of a universal origin at t=0 and a series of universal epochs (inflation, decoupling, galaxy formation and the universal simultaneity of now) constitutes a universal spacetime reference frame whether you want to call it that or not.

1. The origin at t=0 is not considered to be valid.
2. These other physical processes in no way require a universal space-time reference frame, but describing them is definitely more convenient in such a frame.

 

[budrap]

No, these are just his views. His observations say nothing of the sort (they were suggestive once upon a time, but we've much better observations today). It is only his opinion (and those of a few of his followers) that they do. Halton Arp may have done some good science in decades past, but for some time now has been nothing but a pseudoscientific crackpot, continually making wild claims that are completely disconnected from reality.

I am citing only Arp's observations of high redshift quasars in close proximity to and in some cases even apparent interaction with low redshift galaxies. I am not interested in discussing Arp's theories about the same. It is his theories that have been shown to be in error not the underlying observations.

That you are complaining about it in the first place, that you think that this is a "blunder" at all and not simply a matter of convenience.

OK, let's take it from the top and try again. It falls out (meaning it is a consequence not an a priori assumption) of the theory of Special Relativity that there is no universal spacetime reference frame. The further elaboration of SR into GR did not alter this state.

However, assuming the existence of a "Universe" and consequently a universal reference frame that contains it and then applying the equations of GR to said "Universe" was indeed a "blunder" as you would have it. If you want claim this is a valid scientific approach then cite the empirical evidence supporting the assumption of this "Universality". Cite evidence that this issue was ever scientifically vetted. Show me where it was scientifically proven and not merely assumed that the cosmos constitutes a singular entity that you like to think of as the "Universe". If you are saying that the assumption was "simply a matter of convenience" I would have to agree.

1. The origin at t=0 is not considered to be valid.
2. These other physical processes in no way require a universal space-time reference frame, but describing them is definitely more convenient in such a frame.

1. If by not valid you mean it doesn't make any sense we are in complete agreement. It is however a logical consequence of the Big Bang model and ducking that inconvenient fact doesn't change the necessary conclusion that the BB model leads straight back to an illogical absurdity.

2. I'm not saying that they require a universal spacetime reference frame. I'm saying that they constitute a universal spacetime reference frame. Calling them comoving coordinates is simply a semantic dodge.

 

[Chalnoth]

I am citing only Arp's observations of high redshift quasars in close proximity to and in some cases even apparent interaction with low redshift galaxies. I am not interested in discussing Arp's theories about the same. It is his theories that have been shown to be in error not the underlying observations.

Higher-resolution observations (e.g. from the HST) show that there is no reason to believe these are anything but chance correlations, and that there isn't actually any interaction. The information is available on the Internet if you're willing to look for it. Just pick a specific observation and go hunting.

However, assuming the existence of a "Universe" and consequently a universal reference frame that contains it and then applying the equations of GR to said "Universe" was indeed a "blunder" as you would have it.

That's just plain false, though. First, the results only become inconsistent between different reference frames when you start run into irregularities in the coordinate system (typically singularities). Thus taking, as a tentative hypothesis, the proposal that there exist reference frames for which the universe appears homogeneous and isotropic is a perfectly reasonable thing to do. You can't trust the behavior of the result in the vicinity of any singularities in the coordinate system (which would be at t=0), but other than that it doesn't mess anything up.

The question, then, is whether or not there actually is a reference frame for which our universe is approximately homogeneous and isotropic. The second part to that is, today, trivial to answer, just by looking at the CMB. The CMB is uniform to about one part in one thousand in each direction. Once we take out the dipole of the CMB (presumably due to our own motion with respect to it), the CMB is uniform to about one part in one hundred thousand.

That is pretty darned isotropic.

So, the only question remains, is the assumption that there exists a reference frame for which our universe is also homogeneous valid? First, the default answer to this would most definitely be yes, for the simple reason that a universe that appears isotropic, but isn't actually homogeneous, would indicate that we are extremely near the center of an extremely big universe. And that is something that is rather ridiculous on its face. However, can we test it?

Indeed we can!

You see, for a while some cosmologists thought that it was possible to explain the acceleration of our universe due to our universe being isotropic but not homogeneous. This would indicate that we live near the center of a very large, underdense region (a void). Well, this hypothesis does provide some definite predictions that don't line up with observation, as seen here:
http://arxiv.org/abs/1007.3725

Thus, with all of the other observations that do make sense when we keep the assumption of homogeneity, we can be pretty darned confident that this assumption is accurate. And since there are no singularities in the coordinate system far from t=0, we don't have to worry about it giving us incorrect results due to picking a bad coordinate system.

We should obviously be careful not to extrapolate it too far beyond our cosmological horizon, or too close to t=0. And we certainly wouldn't want to use these coordinates to attempt to describe behavior too close to overdense/underdense regions. But other than that it isn't a concern.

1. If by not valid you mean it doesn't make any sense we are in complete agreement. It is however a logical consequence of the Big Bang model and ducking that inconvenient fact doesn't change the necessary conclusion that the BB model leads straight back to an illogical absurdity.

The big bang model is not expected to be complete. General Relativity itself is the problem here: GR predicts that there will be a singularity in the finite past, almost no matter what sort of physical model we use. We expect that a correct theory of quantum gravity will correct this flaw in GR.

2. I'm not saying that they require a universal spacetime reference frame. I'm saying that they constitute a universal spacetime reference frame. Calling them comoving coordinates is simply a semantic dodge.

Except they don't. You're mixing different terms here. The very idea of a universal reference frame is one that if you are within a perfectly-insulated, closed container, you can tell how fast you are moving. Picking a particular coordinate system within which to do calculations doesn't change the fact that we can't do this. In the FRW universe, we would still have to look outside to see the CMB, for instance. There would be no way to determine our motion without looking outside.

 

[AWA]

So, the only question remains, is the assumption that there exists a reference frame for which our universe is also homogeneous valid? First, the default answer to this would most definitely be yes, for the simple reason that a universe that appears isotropic, but isn't actually homogeneous, would indicate that we are extremely near the center of an extremely big universe. And that is something that is rather ridiculous on its face.

That is simply false.There is a growing bibliography(Sylos-Labini, Pietronero,Mittal,Barrett) with very good observational support that points to a fractal structure of the universe on large scales. And a fractal dispositon of matter may indeed be isotropic and not homogenous. Ever heard of Mandelbrot "conditional cosmological principle?
Not only that, there is a whole family of spacetimes (Stephani) that includes the FRW universes that also allows inhomogenous isotropic solutions. You call yourself "science advisor"?. Why do yo make such categorical assertions when they are not backed up by sound science? That shows either ignorance if you don't know or dishonesty if you choose to ignore those facts that disprove your arguments.

Thus, with all of the other observations that do make sense when we keep the assumption of homogeneity, we can be pretty darned confident that this assumption is accurate. And since there are no singularities in the coordinate system far from t=0, we don't have to worry about it giving us incorrect results due to picking a bad coordinate system.

Yeah, nice and easy.

 

[Chalnoth]

That is simply false.There is a growing bibliography(Sylos-Labini, Pietronero,Mittal,Barrett) with very good observational support that points to a fractal structure of the universe on large scales. And a fractal dispositon of matter may indeed be isotropic and not homogenous. Ever heard of Mandelbrot "conditional cosmological principle?

This isn't actually a disagreement. Everybody agrees that our universe is inhomogeneous on small scales. The very existence of planet Earth is proof positive of that. The question isn't that, rather, but whether we can accurately describe our universe as homogeneous on large scales. And that certainly seems to be what all of our observations have shown us to date. The CMB is nearly anisotropic. Galaxies are, on large enough scales, distributed homogeneously (though are obviously very inhomogeneous on smaller scales). The details of the expansion rate and other observations rule out any major deviations from homogeneity with distance.

There may be some corrections we should apply to our equations for expansion that assume perfect homogeneity due to the fact that it's not really homogeneous on smaller scales, but there really isn't any question that the picture is approximately accurate.

Not only that, there is a whole family of spacetimes (Stephani) that includes the FRW universes that also allows inhomogenous isotropic solutions. You call yourself "science advisor"?. Why do yo make such categorical assertions when they are not backed up by sound science? That shows either ignorance if you don't know or dishonesty if you choose to ignore those facts that disprove your arguments.

I don't call myself anything. The staff here at PF were kind enough to place this label on my account. I neither requested it nor sought it out, though I do thank them. And I am making this sort of assertion because it is, to my knowledge, backed up by solid evidence. The possibility of galaxies distributed in a fractal pattern is potentially interesting, as it would be telling us something specific about the nature of gravity, but doesn't undercut this view at all.

Unfortunately I don't feel like taking the time to search out the evidence for every forum post I make, but if we get into a solid disagreement I am willing to do so (though I will note: you also presented bald assertions without evidence, so please don't act self-righteous here).

 

[finiter]

And that certainly seems to be what all of our observations have shown us to date.

If the universe is homogeneous, on a large scale, at every instant, can we regard it as a system in which the members interact gravitationally? Or is it just an absurd collection of galaxies and that the actual reason why it remains uniform remains unknown?

 

[Chalnoth]

If the universe is homogeneous, on a large scale, at every instant, can we regard it as a system in which the members interact gravitationally? Or is it just an absurd collection of galaxies and that the actual reason why it remains uniform remains unknown?

Well, if the uniformity were perfect, you can still calculate the gravitational interaction. That's precisely what the Friedmann equations are.

When you look into the system in a bit more detail, and properly consider the fact that it isn't actually uniform, you end up with some interesting behavior: on very large scales, you get what is called "linear evolution of structure". This can be rather simply calculated, and you get that small inhomogeneities to start with become small inhomogeneities later on: you don't get, on large scales, very huge deviations from a homogeneous universe.

But on smaller scales the picture is entirely different. Once the matter in a given region goes above a certain density relative to the surroundings, it starts to collapse in on itself. This is non-linear evolution, and it can't be so easily computed, but must be simulated. From this you end up with the more dense places in the universe collapsing and forming galaxies, galaxy clusters, and superclusters, complete with interesting-looking structures visible on larger scales, but the statistical properties on even larger scales left undisturbed.

The way this sort of thing works in a bit more detail is that they divide the matter in the universe up into small particles, and start with a slightly inhomogeneous distribution of said particles (based upon, for example, CMB data). They then run the simulation forward, calculating the gravitational attraction between the different particles at each step. Here's a video flythrough of the end result of one such simulation for how matter tends to clump:

 

[finiter]

Well, if the uniformity were perfect, you can still calculate the gravitational interaction. That's precisely what the Friedmann equations are.

Then, can the expansion be regarded as three dimensional, the shape of the universe remaining either spherical or as a spherical surface?

But on smaller scales the picture is entirely different. Once the matter in a given region goes above a certain density relative to the surroundings, it starts to collapse in on itself. From this you end up with the more dense places in the universe collapsing and forming galaxies, galaxy clusters, and superclusters, complete with interesting-looking structures visible on larger scales, but the statistical properties on even larger scales left undisturbed.

Then can we simplify the whole thing as: once matter particles were formed, matter started contracting due to gravity while the universe continued expanding.

 

[Chalnoth]

Then, can the expansion be regarded as three dimensional, the shape of the universe remaining either spherical or as a spherical surface?

The expansion was in three dimensions, yes. But we don't know the overall shape.

Then can we simplify the whole thing as: once matter particles were formed, matter started contracting due to gravity while the universe continued expanding.

Unfortunately, it's not quite that simple. When the dark matter first condensed, yes, it started to clump. But, at the time the normal matter condensed, our universe was still a plasma, which meant that the protons and electrons were separated from one another, and interacted very strongly with the photons around them. This meant that they felt pressure, so they might start to fall into a gravitational potential well, but then they'd bounce right back out again. It wasn't until the protons and electrons became neutral atoms that the normal matter started to become clumpy.

 

[finiter]

When the dark matter first condensed, yes, it started to clump.
It wasn't until the protons and electrons became neutral atoms that the normal matter started to become clumpy.

Can becoming clumpy be regarded as becoming cold? In that case, the entropy should decrease; so it would appear that the approved model of the black hole goes against what is expected.

 

[Chalnoth]

Can becoming clumpy be regarded as becoming cold? In that case, the entropy should decrease; so it would appear that the approved model of the black hole goes against what is expected.

Well, it gets a bit complicated there. Becoming clumpy does relate to a loss of energy, but the way that gravity works, things that become more clumpy tend to have higher temperatures. This is a statement that the specific heat of gravitational systems is negative. So, for instance, as a cloud of gas collapses into a star, it loses total energy, but ends up getting hotter as the gas falls lower and lower into the potential well.

One thing to bear in mind is that the way entropy interacts with gravitating systems is not simple, and cannot be directly related to the usual thermodynamic concepts we're used to. In fact, except in very special circumstances, we don't even know how to calculate the entropy of a gravitating system.

 

[finiter]

In fact, except in very special circumstances, we don't even know how to calculate the entropy of a gravitating system.

That means the 'entropy of a gravitating system' is a grey area, and one can try some unexplored ideas to relate heat, gravity and entropy.

 

[Chalnoth]

That means the 'entropy of a gravitating system' is a grey area, and one can try some unexplored ideas to relate heat, gravity and entropy.

Well, it's not terribly difficult to at least get a handle of when a gravitational system increases in entropy. If you take some gravitational system, and let it be, then whatever happens will be an increase in entropy. This typically means an emission of particles such as photons which leads to a reduction in energy of the system, which causes it to collapse inward, which causes the temperature to increase.

We don't currently know how to precisely define the value of the entropy in such a situation, but we can be very confident it increases.

 

[budrap]

Higher-resolution observations (e.g. from the HST) show that there is no reason to believe these are anything but chance correlations, and that there isn't actually any interaction. The information is available on the Internet if you're willing to look for it. Just pick a specific observation and go hunting.

I am not interested in beliefs - yours or mine, I prefer empirical data. The chance correlations argument is statistical in nature and disingenuous when applied to individual observations. For any statistical argument to have merit it needs to be applied to a statistically significant set of high redshift/low redshift pairs. The one man who bothered to make a survey of such pairs was Halton Arp and he was denied telescope time for the attempt.

You can't trust the behavior of the result in the vicinity of any singularities in the coordinate system (which would be at t=0), but other than that it doesn't mess anything up...

We should obviously be careful not to extrapolate it too far beyond our cosmological horizon, or too close to t=0. And we certainly wouldn't want to use these coordinates to attempt to describe behavior too close to overdense/underdense regions. But other than that it isn't a concern.

So the argument seems to be that the Big Bang model gets the right answers (after proper adjustments for predictive failures) except for those areas where it yields illogically absurd results which we are to ignore as inconvenient and thus we can consider the model a great and scientifically sound success. "We get the right answers except when we don't" is nothing but a scientifically unjustifiable evasion.

The nature of my criticism can be summarized as follows:

1) The observed cosmos either comprises a singular entity or it does not.

2) Scientists have assumed the first option without ever properly vetting either.

The problem with your posts is that you seem incapable of even grasping the conceptual distinction between the two possibilities. All of your responses consist of retreating into the shelter of your preferred model, pointing out its successes and discounting its failures and inconsistencies. But discounting inconvenient results is a mathematical strategy only, one that should have no place in science.

The big bang model is not expected to be complete. General Relativity itself is the problem here: GR predicts that there will be a singularity in the finite past, almost no matter what sort of physical model we use. We expect that a correct theory of quantum gravity will correct this flaw in GR.

No, GR is not the problem here it is the imposition on the cosmos of a conceptual "Universe" that causes GR to spit out absurd results - the very concept itself being antithetical to GR.

The very idea of a universal reference frame is one that if you are within a perfectly-insulated, closed container, you can tell how fast you are moving. Picking a particular coordinate system within which to do calculations doesn't change the fact that we can't do this. In the FRW universe, we would still have to look outside to see the CMB, for instance. There would be no way to determine our motion without looking outside.

You are conflating a Universal SpaceTime Reference Frame with the long discarded concept of the Aether, which is a USTRF with a pervasive physical component. They are not one and the same. Relativity theory dispensed with the need for any form of USTRF not just the Aether. The Big Bang model however, by treating the "universe" as a singular entity, inherently assumes the existence of a USTRF albeit one without a pervasive physical component.