The big bang

finiter

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

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

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

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

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.

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

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.

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

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

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.

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.

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

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.

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. 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

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.

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.

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.

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

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.

Yeah, nice and easy.

Chalnoth

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.

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

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

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:
http://www.youtube.com/watch?v=y0ToCreO9fU

finiter

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


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

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

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

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

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.