Why isn’t the universe homogenous?

Let me preface this by saying that my knowledge of astrophysics is extremely limited. Basically wondering why if the universe all started at one point and exploded into a huge cloud of energy which then formed into mass, you would end up with planets, stars, empty space, etc rather than one homogenous universe? Wouldn’t the conditions that make one star form be about the same throughout and you would have a universe filled with identical stars equally space out?

I am not a physicist, but I believe that the inflationary epoch took the random quantum fluctuations that always exist in small things and made them much, much bigger.

The universe is commonly thought to be approximately homogeneous on a sufficiently large scale; however, that scale does seem to be expanding over time. That is, we seem to keep finding larger and larger structures, galaxies, clusters of galaxies, super clusters, etc. It is our knowledge rather than the actual scale that is changing.

However, any small deviations from uniform will tend to get emphasized. A small clump here, has more mass than surrounding areas and gravitationally attracts other stuff in making stars, galaxies, etc.

Where do the small deviations come from. Current thought, I believe, is that small fluctuations didn’t have a chance to get smoothed out in the early universe before inflation took over and expanded the universe so large that parts of it became separated further than any information could be sent between them. That is why we’re still seeing cosmic background radiation for the very early stages of the universe. This is the first chance that anything traveling from those areas (even at the speed of light) has had to reach here. Since no information could be sent there cold be no smoothing out. Note I’m not implying here at all that there was any purpose to this information.

That the Universe is homogeneous on large enough scales is a central assumption of big bang theory and one that is borne out by observation.

Why the Universe isn’t completely homogeneous, nobody knows, but it’s quite widely believed the smaller scale inhomogeneity is result of small fluctuations of a quantum nature very early in the Universe’s history. Inflationary theory is actually used to explain why the Universe appears more homogeneous than one might expect in classic big bang theory.

It eventually will be…but don’t hold your breath.

See “Heat Death of the Universe.” http://en.wikipedia.org/wiki/Heat_death_of_the_universe

This is exactly what I wanted to say. It’s still “mixing”.

One important factor is that gravity tends to amplify any slight inhomogeneity that may be present.

In small scale - say, the air in a room - gas pressure tends to equalize the density, and you end up with constant density everywhere inside an enclosed space. But that’s because gravity is negligible at that small scale. At larger scales - solar system size and larger - if you have a region of gas that has slightly higher density than the area around it, the gravity from that slightly denser region attract even more matter. So a slightly dense gas cloud becomes a much denser cloud, which further collapses into stars and planets.

Oscar Wilde was homogeneous, though.

Recently a structure that’s 4 billion light-years across has been found. This is so large that it shows that there’s something wrong with current theories about what the largest such structures should be: