Is our galaxy in the centre of the universe?

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  • #1
wolram
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Main Question or Discussion Point

Is our galaxy in the centre of the universe?
If it is not, how can the age of galaxies be determined?
how can data from the CMBR be correct?
IF our position in the universe is unknown, (we could be near the event horizon)
how would this effect our knowledge of cosmology?
 

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  • #2
Garth
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The fact that the CMB is isotropic to 1 part in 105 is very good evidence that the universe is not only isotropic but also homogeneous, everywhere, the Cosmological Principle. That being so verifies the Copernican Principle: there is nothing special about our position in the universe. On the largest scales the universe would look the same from most other observation places, although GR effects would affect the view from within a powerful local gravitational field.

Therefore there is no centre to the universe, for such a centre would be a preferred location within it, instead the universe must be unbounded, either infinite, or finite like the surface of a sphere.
Garth
 
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On the CMBR:

We were discussing this on another thread. The question there was 'If I have a frame of reference that is at rest wrt the CMBR, doesn't that imply a universal rest frame?' Your question is different, but I think the reasoning behind it is the same. I dug up this quote from http://www.cosmologymodels.com/general.html [Broken] website:

Ed Barlow said:
While in special relativity there is no standard of rest and no preferred reference frame, in general relativity the situation is somewhat different. A rest frame for any particular region of space can be defined as the frame which is not rotating compared to the background of distant stars and for which the cosmic microwave background radiation, CMBR, has the same spectrum in all directions and so the concept of local motion in that frame is meaningful. In another region of space, there would also be a rest frame for which the CMBR is the same in all directions, but the two rest frames are moving with respect to each other so there is no preferred frame of that type for the universe as a whole and no center of the universe. We can however, define a comoving rest frame for the universe as a whole for which the requirement of the CMBR being the same in all directions and there being a standard of rotation based on the distant stars is met at all locations.
As for the age of galaxies, I believe this is determined largely by spectral analysis. Our models of the early universe show it consisting of primarily hydrogen and helium. By determining the abundance of the heavier elements, we can determine how many generations of stars the galaxy has had and (using our estimate of stellar life cycles) determine how old the galaxy is.
 
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  • #4
marcus
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wolram said:
(we could be near the event horizon)
this doesnt make sense to me, wolram.
one's horizon is defined outwards from where one is
so it is hard to see how one could get close to it

the simple universe that cosmologists assume is roughly the same in all directions and the distance to horizon is same in all directions
like being in a flat calm out at sea
the distance to horizon is same to east and north and west etc.


imagine the spherical surface of the planet is just one big calm ocean
(with maybe a sprinkling of small islands here and there)
so then, nomatter where you are it is always the same distance to your horizon and that is the same in all directions.

maybe you should make the question more precise
or say what I am not getting.

"Is our galaxy in the centre of the universe?"

No, it is not in the centre.
And there is no centre as far as we know.

"If it is not, how can the age of galaxies be determined?"

what's the problem? by their redshift and the FRW model (whose parameters they try to determine as accurately as they can from the best data available)

"how can data from the CMBR be correct?"
I dont understand. aside from the usual measurment error one gets in any batch of data, what could be wrong?
 
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  • #5
Chronos
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Permit me to add that the question infers the universe has a boundary and there is a location in space that is equidistant from that boundary in all directions, i.e., is the center of the universe. This perception hinges on the notion the universe is embedded in a larger, external space - like a sphere suspended in a fish tank. But that is not the case. There is no external space, the universe is space. Regardless of where you happen to be in this universe, you are both at the center and the edge at the same time!

Think of it this way. Earth is the most ancient object in our observable universe - hence we are at the edge of time in the universe. When we look at the surrounding universe, everything is as it appeared in our past: the sun is 8 minutes younger, alpha centauri is 4 years younger, the andromeda galaxy is 2 million years younger. The rub is, no matter what direction we look, we are uniformly surrounded by objects progressively younger... all the way back to the CMB photons, which are ~13.6 billion years younger and bombard us from every direction.
 
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