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How do we know that the expansion of the universe is radially symmetric? 
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#1
May1006, 06:23 PM

P: 108

Forgive me if I have used the wrong phrase to characterize the phenomenon. If my understanding is flawed, someone please correct me.
From what I understand, theory postulates that all points in space will measure a red shift. How has this been tested? It seems to me such an effect would be noticable by comparing observations by terrestial telescopes and those in orbit. 


#2
May1106, 02:08 AM

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PF Gold
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Assuming the universe originated from an extremely dense, hot and homogenous state [e.g., big bang], matter would not be homogenously distributed today if inflation/expansion was not uniform. Surveys such as SDSS show the large scale distribution of matter in the universe is extremely homogenous.



#3
May1106, 11:33 AM

P: 108

Indeed, and I understand this.
What I don't understand is that, for all observers in our universe, will telescopes reveal that all galaxies are red shifted (barring those in their local neighborhood that are on collision courses due to gravitational attraction)? obviously terrestrial telescopes have measured this, but I'd like to know what experimental justification there is for us to say that the universe is expanding uniformly everywhere. For if that were not the case, one could make the argument there is a center, no? 


#4
May1106, 11:17 PM

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How do we know that the expansion of the universe is radially symmetric?
When we see that the universe appears to be homogeneous and isotropic from the Earth, we assume that it appears to be homogneous and isotropic from everywhere in the universe.
To not assume this would be to assume that the Earth was "special" or "priveleged". 


#5
May1206, 02:02 AM

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PF Gold
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#6
May1206, 02:03 AM

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PF Gold
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Garth 


#7
May1206, 03:41 AM

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#8
May1206, 07:15 AM

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#9
May1206, 09:44 AM

P: 108




#10
May1206, 11:14 PM

P: 1,011

Do the rates of galactic evolution (first 3 billion years) agree in all parts of the observed sky? Or does certain part of evolution vary within the universe by 1 billion years? Could one part of the sky at redshift z=2 be 500 million years ahead in evolution than another part of the sky at redshift 2?



#11
May1206, 11:29 PM

P: 1,011

For now we assume the cosmological priniciple, we impose our 3D perspective of the universe onto other places (even billions of light years away)  the antithesis saying of we have a unique view of our home  the universe. Given such a extrapolation, the answer is yes. 


#12
May1406, 12:46 PM

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My position is this: we can observe the universe from the Earth, and find that it is (almost) homogeneous and isotropic. There are some small natural variances due to "clumpiness" of the universe which average out (i.e. they are random) and some other small variances that are associated with the "peculiar" velocity of the Earth with respect to the CMB which are systemic. The random variations are of little concern  the systemic variances are associated with our velocity with respect to the "Hubble flow". We basically expect the universe to appear H&I only from someone at rest with respect to the "Hubble flow". We can't directly observe the universe from other points of view, but there is no reason to believe that the Earth is in the "center of the universe", i.e. we expect that anyone at rest with respect to the Hubble flow will also see the universe as homogeneous and isotropic no matter where they are. We can support this POV of the lack of "specialness" of the Earth with modelling efforts (i.e. with our model of the geometry of the universe, we can compute what we would expect observers at different locations to see), but without interstellar (preferably intergalactic) travel, we can't actually perform the experiments. 


#13
May1406, 04:17 PM

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Garth 


#14
May1406, 04:37 PM

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Garth, why do you say that the Copernican Principle was generalized into the Cosmological Principle? It seems to me that the Cosmological Principle is a specific application of the Copernican Principle concerning the homogeneity and and isotropy of the universe.



#15
May1506, 01:24 AM

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PF Gold
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i.e. The Copernican Principle is applied in FRW cosmology to mean: "We are not living at a special place in the universe, because in FRW cosmology there are no special places" and then in the Steady State Cosmology to mean: "We are not living at a special time in the universe, because in SS cosmology there are no special times". As we know the universe is evolving we know the latter application of the prinicple is false. Garth 


#16
May1506, 02:52 AM

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PF Gold
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That sounded a bit hostile. I merely meant to point out the 'illusion of simultaneity' is a very real obstacle in the course of understanding GR. Most graduate students struggle with the concept. Einstein was a thinker, and the subtleties run deep. It still has me pretty much baffled. But, tensors never lie. 


#17
May1506, 03:00 AM

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#18
May1506, 04:25 AM

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PF Gold
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So we do have to exist at the 'special' time between the birth of Pop I stars complete with organic elements, and before the future 'dark age' when the universe slides towards either a heat death or a cosmological 'big crunch'. In the Perfect Cosmological Principle of the Steady state Theory the Copernican principle was applied to the universe at all epochs, so that it would 'look the same' to any observers no matter when they existed as well as where they existed. Garth 


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