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 A common question in cosmology is "why are all the galaxies receding from each other?" In
other words, the cosmological principle requires that we not be at a special place in the
Universe. Since all the galaxies are moving away from us, then they must all be moving away
from each other. This is explained if the Universe, as a whole, is expanding.
In a real sense, Hubble's law, the recession  velocity of galaxies, is an illusion. The
galaxies are not moving, the space between them is literally expanded. To see how this
produces a Doppler effect, consider a simply Universe that is a circle. To the observers in
this type of Universe, they believe they live in a 1D structure. But, in fact, they live in a
2D structure, a circle. The position of the galaxies can be measured by the distance between
them (S, see diagram below) or what are called the co-moving coordinates, an angle q between the galaxies.
- the Universe must be describe with a geometry that includes a description of the curvature of the Universe
oustide of the coordinates we measure
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The radius of the Universe is given by R, notice that R is a quantity only seen in 2D space,
not measured directly by the inhabitants of the 1D circle unless they measure 2pR by walking around the Universe. Now, we let the Universe expand by a
factor of 2, R becomes 2R. The distance between the galaxies becomes 2S, but the co-moving
coordinate, angle q remains unchanged. Since the distance between
the galaxies has increased, then the galaxies will appear to have moved apart by S/time of
expansion. When, in fact, the galaxies have not moved at all, the space between them has
increased.
Expanding spacetime also explains the redshift of galaxies, which is interpreted as Doppler
motion. Since space expands, any photons traveling through that space (from distant galaxies
to us) must also expand, i.e. the photons are `stretched' as they travel across the
Universe. |
- galaxy redshift is the effect of photons being stretched, not motion
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So the redshift we see for distant galaxies is really an effect of spacetime expanding, not real motion. This is
good because some of the redshifts for the most distant galaxies have recessional velocities in excess to the
speed of light. But this is not a contradiction for special relativity since the space is expanding, not true
motion. We will also see that photons created as gamma rays in the early Universe are now redshifted to the
microwave region of the spectrum to make up what is called the cosmic microwave background (CMB). |
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