# How do we know that the expansion of the universe is radially symmetric?

by ptabor
 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.
 Sci Advisor PF Gold P: 9,172 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.
 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?
Emeritus
P: 7,433

## 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".
PF Gold
P: 9,172
 Quote by ptabor 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?
We cannot. It is impossible for us to observe what is beyond our observable universe. What is within our observable universe suggests it is highly improbable any remote observer would draw different conclusions. It is highly improbable [emphasis on highly] that any other observer within our observable universe would observe any remarkable differences in their observable universe. Were that the case we would, indeed, occupy a priveleged position in the 'real universe'.
PF Gold
P: 3,273
 Quote by pervect 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".
Quite the opposite in fact - this is the "Copernican Principle" that has been generalised into the "Cosmological Principle".

Garth
P: 1,047
 Quote by ptabor 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?
There are indeed experimental ways to test the Copernican Principle. Since the CMB is considered to be one of the most striking manifestations of homogeneity, one way is, for example, to observe the spectra of some distant clouds or plasmas (e.g. cyanogen clouds or electrons in jets) and, assuming a certain interaction with the photons of the CMB, infer about the temperature of the CMB at that remote point and verify that this temperature is the same than the temperature measured by our satellites here. I am not aware of the latest tests, but I think that the current possibilities to test this are still primitive.
P: 3,172
 Quote by pervect To not assume this would be to assume that the Earth was "special" or "priveleged".
but that way also other places would be considered "special"/"privliged".
P: 108
 Quote by Chronos We cannot. It is impossible for us to observe what is beyond our observable universe. What is within our observable universe suggests it is highly improbable any remote observer would draw different conclusions. It is highly improbable [emphasis on highly] that any other observer within our observable universe would observe any remarkable differences in their observable universe. Were that the case we would, indeed, occupy a priveleged position in the 'real universe'.
If I were an observer at the "edge" (I doubt the universe has an edge in the strictest sense of the word. A balloon certainly doesn't. What I mean is our horizon, past which we cannot see) of the observable universe, would I still see the universe expanding uniformly in all directions? (including the region "behind" me)?
 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?
P: 1,011
 Quote by ptabor If I were an observer at the "edge" (I doubt the universe has an edge in the strictest sense of the word. A balloon certainly doesn't. What I mean is our horizon, past which we cannot see) of the observable universe, would I still see the universe expanding uniformly in all directions? (including the region "behind" me)?
In that region, we only have opinion, not certain knowledge.

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.
Emeritus
P: 7,433
 Quote by Garth Quite the opposite in fact - this is the "Copernican Principle" that has been generalised into the "Cosmological Principle". Garth
Sorry, you lost me.

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.
PF Gold
P: 3,273
 Quote by pervect Sorry, you lost me..
You misunderstood me, I was agreeing with you!
 Quote by pervect To not assume this would be to assume that the Earth was "special" or "priveleged".
 Quote by Garth Quite the opposite in fact - this is the "Copernican Principle" that has been generalised into the "Cosmological Principle".
I was agreeing with your first statement:
 Quote by pervect 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.
I agree my post wasn't the clearest -sorry!

Garth
 Emeritus Sci Advisor PF Gold P: 2,977 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.
PF Gold
P: 3,273
 Quote by SpaceTiger 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.
That is what I meant, it was probably my fault again in not using a more tightly defined word; I was using "generalised" in the sense that it was applied in Friedmann Robertson-Walker models to the whole spatial universe, and then we note it was generalised again in the Steady State theory to apply to the universe in time as well.

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
PF Gold
P: 9,172
 Quote by ptabor If I were an observer at the "edge" (I doubt the universe has an edge in the strictest sense of the word. A balloon certainly doesn't. What I mean is our horizon, past which we cannot see) of the observable universe, would I still see the universe expanding uniformly in all directions? (including the region "behind" me)?
All observable regions of this universe are 'behind' us in time. By definition, we occupy the most ancient region in our observable universe. It appears you are stuck on the illusion of simultaneity.

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.
Emeritus
PF Gold
P: 2,977
 Quote by Garth 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.
I don't understand what you're trying to say here. An evolving universe does not necessarily mean that we're living at a special place in time, only that there is a difference from one time to the next. Rather, the Copernican Principle is usually applied to Big Bang cosmologies to cast to doubt on theories that put us near a transitional era. This is why we talk about a "cosmic coincidence problem" for cosmologies that put us near the lambda transition (like vanilla $\Lambda CDM$).
PF Gold
P: 3,273
Quote by SpaceTiger
 Quote by Garth 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.
I don't understand what you're trying to say here. An evolving universe does not necessarily mean that we're living at a special place in time, only that there is a difference from one time to the next.
Well, a universe with the evolving history of our own means that we cannot be existing in the Inflation era, for example, because it was too hot, or the epoch of Last Scattering, because the universe was too homogeneous so there were no stars or planets, or in the pre-, or early, galactic epoch when the heavier elements had not yet been created in stellar nucleo-synthesis, or in the much later epoch when all stars have consumed their nuclear fuels and exist in a state of burnt out hulks or BH's.

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.
 Rather, the Copernican Principle is usually applied to Big Bang cosmologies to cast to doubt on theories that put us near a transitional era. This is why we talk about a "cosmic coincidence problem" for cosmologies that put us near the lambda transition (like vanilla $\Lambda CDM$).
Agreed - thank you for another clear example of what I was saying.

Garth

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