The big bang


by FizixFreak
Tags: big bang
budrap
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#91
Aug31-10, 12:24 PM
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Quote Quote by Chalnoth View Post
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.

Quote Quote by Chalnoth View Post
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?

Quote Quote by Chalnoth View Post
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
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#92
Aug31-10, 12:30 PM
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Quote Quote by budrap View Post
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.

Quote Quote by budrap View Post
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.

Quote Quote by budrap View Post
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
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#93
Aug31-10, 02:52 PM
P: 40
Quote Quote by Chalnoth View Post
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.

Quote Quote by Chalnoth View Post
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.


Quote Quote by Chalnoth View Post
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
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#94
Aug31-10, 09:32 PM
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Quote Quote by budrap View Post
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.

Quote Quote by budrap View Post
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.

Quote Quote by budrap View Post
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.

Quote Quote by budrap View Post
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
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#95
Sep1-10, 02:46 PM
P: 134
Quote Quote by Chalnoth View Post
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.

Quote Quote by Chalnoth View Post
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
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#96
Sep1-10, 04:14 PM
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Quote Quote by AWA View Post
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.

Quote Quote by AWA View Post
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
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#97
Sep2-10, 05:52 AM
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Quote Quote by Chalnoth View Post
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
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#98
Sep2-10, 09:23 AM
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Quote Quote by finiter View Post
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:
http://www.youtube.com/watch?v=y0ToCreO9fU
finiter
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#99
Sep4-10, 05:17 AM
P: 26
Quote Quote by Chalnoth View Post
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?


Quote Quote by Chalnoth View Post
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
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#100
Sep4-10, 05:51 AM
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Quote Quote by finiter View Post
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.

Quote Quote by finiter View Post
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
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#101
Sep6-10, 08:39 AM
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Quote Quote by Chalnoth View Post
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
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#102
Sep6-10, 08:45 AM
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Quote Quote by finiter View Post
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
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#103
Sep7-10, 12:58 AM
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Quote Quote by Chalnoth View Post
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
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#104
Sep7-10, 01:05 AM
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Quote Quote by finiter View Post
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
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#105
Sep8-10, 11:19 AM
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Quote Quote by Chalnoth View Post
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.

Quote Quote by Chalnoth View Post
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.

Quote Quote by Chalnoth View Post
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.

Quote Quote by Chalnoth View Post
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.
Chalnoth
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#106
Sep8-10, 11:35 AM
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Quote Quote by budrap View Post
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.
I wasn't talking about beliefs. I was talking about observations. I'm saying that the observations that Arp made where he claimed there was some interaction between a low redshift galaxy and a high-redshift quasar were shown to be misleading: higher-resolution observations by Hubble of these same galaxies show no interaction whatsoever.


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