
#19
Dec1911, 05:42 PM

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#20
Dec1911, 06:40 PM

Physics
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PF Gold
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#21
Dec2011, 03:15 AM

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BTW this also implies that we should be observing that slight deviation from Hubble law in the nearby universe region, but we are actually not observing it, for reference this paradox is known as the "de Vaucouleurs paradox" in honor of the brilliant cosmologist that pointed it out, and has not yet been explained. 



#22
Dec2011, 03:23 AM

P: 2,889

For the sake of the better understanding of my argument I'm ignoring the small deviation at small scales, and taking the FRW model idealization as depicting realistically our universe at our scale too, I think it is a liberty I can take, but apparently peterdonis is not catching it. 



#23
Dec2011, 03:33 AM

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#24
Dec2011, 06:19 AM

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IMO, the reason it is a very reasonable convention is because it can be applied in the identical fashion at each spatial location. 



#25
Dec2011, 07:42 AM

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#26
Dec2011, 07:55 AM

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#27
Dec2011, 08:43 AM

P: 863

Although the FRW metric is certainly a valid solution to the EFE is is not the only possible solution. Such alternative speculations are unwarranted, though we can ask what the standard solution entails observationally. Given that gravitational time dilation is absolute, though not in degree, this implies the issue Qreeus brought up. The question is how dependent is the apparent age and size of the Universe on differing gravitational depths of a pair of observers watching the Universe grow and age? If it is entirely independent, as appears likely under under relativistic symmetries, it implies that the age/size factor of the Universe is independent of our local clock rates. This implies that counting out 14 billion years on our local clocks might not properly scale to the beginning of the Universe. If the age of the Universe is dependent on local inertial clock rates, with an isotropic expansion rate of space itself, it implies that the apparent age, density, etc, is dependent on the inertial state or the observer. Trying to work through all the relativistic transforms for both general and special relativity under different initial and boundary condition assumptions is messy at best. Can anybody point at published work that explicitly compares the effects on comoving observers which do not share a common gravitational depth and/or inertial frame? 



#28
Dec2011, 09:04 AM

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PF Gold
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Edit: It looks like DaleSpam is making similar objections to mine, I agree with his posts. 



#29
Dec2011, 10:10 AM

Physics
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PF Gold
P: 5,500

http://www2.phys.canterbury.ac.nz/~d...e/general.html Here's how he describes the paradox: Wiltshire has a paper on arxiv in which he claims to be able to account for the observations currently attributed to "dark energy" by this method: http://arxiv.org/abs/0809.1183 I don't know if others on PF have seen this paper and can give any input. In particular, I don't know if his proposed model fits the other observations that have led to the current consensus LambdaCDM model, which are well described, for example, by Ned Wright in his cosmology tutorial here: http://www.astro.ucla.edu/~wright/cosmo_01.htm 



#30
Dec2011, 11:31 AM

P: 2,889

In this case the observer stressenergy can't be neglected because it is an important part of the assumption. Usually it is implied to be negligible precisely because it is understood that the FRW model is an idealization, thus galaxies and clusters are considered "dust" even if we know their stress energy is pretty great in reality. My set up is just a "cosmological rescaling" so to speak. 



#31
Dec2011, 11:48 AM

P: 863

Under GR, under a change of gravitation depth, the observational effects are essentially the same for both galaxies. Given that light speed defines both time and distance for each observer, contains the very definition of relativistic simultaneity, the isotropy remains even more generally. This doesn't conflict with the quoted paper since this does not entail a statement of how homogeneous the Universe actually is, only how boost and gravitational depths can effect an observers measure of that homogeneity. So I ask, how do you empirically justify that only a particular set of observers see the universe as homogeneous and isotropic? It seems to me that if what you claim is actually true then we should be able to measure distances just by the amount of anisotropy we can induce with a local boost. Not seeing that go anywhere. So explain? 



#32
Dec2011, 11:51 AM

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#33
Dec2011, 12:44 PM

P: 863

I would find it far easier to deal with these kinds of questions and learn far more about the science involved without all the model specific assumptions built in. Unfortunately wading through this or that model trying to explain the observations produces a large work load to try and separate out the observations from the model specific assumptions.
So, although I am not terribly interested in particular model specific description, I will expand my original question of how do you "empirically justify that only a particular set of observers see the universe as homogeneous and isotropic". Expand this to: How can you even theoretically justify that only a particular set of observers see the universe as homogeneous and isotropic, under empirical constraints. 



#34
Dec2011, 01:45 PM

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#35
Dec2011, 07:49 PM

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#36
Dec2011, 10:42 PM

Physics
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PF Gold
P: 5,500

Only observers who are at rest in the "comoving" frame used in the FRW models see the universe as homogeneous and isotropic. Observers who are not at rest in that frame don't. Earth is not at rest in that frame, because we see a dipole anisotropy in the CMBR. We don't have reports from astronomers in other galaxies, but based on what we can see of their motions, it appears that at least some of the nearby ones are not at rest in the "comoving" frame either (that is, their observed relative velocity to us is different from what it would need to be to cancel the dipole anisotropy we see in the CMBR). AFAIK it gets harder to tell as you go farther out. 


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