“Viewed on a sufficiently large scale, the properties of the universe are the same for all observers”
Why is this?
The universe is isotropic. It looks pretty much the same whichever direction we look at it. Unless we assume that we are in the centre of the universe, and that the galaxies form concentric rings around is, this must mean that the universe is homogeneous.
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Imagine the universe as a homogeneous gas; galaxies are equivalent to particles. Instead of interacting via electrostatics, galaxies interact with each other using gravity. Gravity is really the only important force on that scale.
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Measuring distance and movement in space
Let’s divide space into a grid with co-ordinates, with the points always falling on the same galaxies.
Galaxies move coherently, frozen on the grid (rather than moving in random directions). The co-ordinates are not defined by distance.
How do you figure out the distance?
Dab = a(t)Δxab
Where D is the distance moved and a is the scale parameter.
So, how do you figure out the velocity at which the galaxies are moving apart?
Vab = a’ Δxab
Where a’ = da/dt
The Hubble Constant H(t): Vab/Dab = a’/a : the Δx cancels out; it doesn’t matter which two galaxies we’re talking about.
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I hope you’re a tiny bit less in the dark about cosmology after reading this – if you want a better explanation, watch this: http://www.youtube.com/watch?v=P-medYaqVak He explains it really well, and basically the first 20 minutes or so is what I’ve gone over here.
This Statement Is Not True 