Why is matter in the universe uniformly distributed?

Question

The Big Bang would have thrown debris in every direction. Is the debris (planets, stars, etc) that is furtherest away travelling at a much faster speed?

If so, then why does the density of the matter in the universe remain uniform throughout?

Isn't it like a long distance race? Initially the athletes keep pace, prefering to cluster together, to conserve their energy (high matter density). But only when the finishing line is in sight, do the best runners break free from the rest of the group. The "density" of athletes is normally lower towards the end of the race as the gap between the athletes increases. Doesn't the same apply in cosmology? Or is space a factor here?

Cosmological Principle states that on a large scale the universe is inotrophic and homogenous. (matter is evenly distributed and the universe looks the same in every direction providing you are not at some place special)

Answer 1

Runners don't cluster for matter density. They do it for psychological reasons. The comsological principle is simply a grandiose name for an observation. To the limits of observations, we see no preferred direction. It's a useful observation, but don't read too much into it. But here's what the observation led to.

If the universe is expanding uniformly, then there's no measurement you can make that will tell you where the origin was. Obviously, if you go far enough back in time, you could tell. You can plot the paths of all objects you observe and trace the paths back to a single intersection. If there was a big bang, and we had enough data, we could do that now. But if the universe 'began' as a fairly uniform distribution of matter and is uniformly expanding, there is no center and no uniqueness by position.

I can't even imagine such a geometry. Uniform expansion in all directions over all of space seems a topological impossibility. But I can imagine nonuniformity so small that we can't measure it.

Answer 2

I'm not sure that the density of matter in the universe is uniform throughout.
Certainly the density of visible matter is not uniform as stars form galaxies and galaxies tend to exist in clusters with rather large gaps between these super clusters.
Current theory would seem to suggest that visible matter comprises somewhere between just 1% to 5% of the mass of the universe, the rest being dark matter.
Again current theory suggests that there is insufficient mass within the universe for gravity to slow down the accelerating rate at which the universe is expanding.
As everything is accelerating away from everything else then, broadly speaking, the density of matter within the universe could conceivably be uniform as the clusters of visible matter mentioned above are so insignificant a mass as to make little difference. As the universe expands, the average density decreases (the common analogy is to think of the universe as an inflating balloon - as it expands each point on the surface becomes more distant from its neighbour).
In 100 billion years time there will be total darkness in the sky as there will be no bodies within the event horizon of the Earth (not that the Earth will survive the death of the Sun anyway).

Answer 3

where do you get the idea that matter is uniformly distributed? if it was, we would not have planets and space, we would have a constant mist of dust throughout