Main Question or Discussion Point
How does the uniformity of the universe change over time? Or maybe I should say inhomogeneity versus time.
I hope several people will respond to this. It is such a general question that it seems to me quite a challenge just to get a grip on it.How does the uniformity of the universe change over time? Or maybe I should say inhomogeneity versus time.
I thought it was just the opposite the inflation flattens any pre-existing lumps. If that were true then lumps must be initial conditions of the universe?Great answer Marcus. I think it's important to emphasize that in order to go from 1) to 2) one needs seed perturbations. In inflationary cosmology, these perturbations are generated during the primordial inflationary era.
You're quite right Phil. Inflation does smooth out initial inhomogeneities leading to a flat universe. This was initially viewed as bittersweet: it gives us a homogeneous and isotropic universe (that we observe on large scales), but seems to simultaneously preclude the formation of structure and local inhomogeneity we see around us.I thought it was just the opposite the inflation flattens any pre-existing lumps. If that were true then lumps must be initial conditions of the universe?
The key point is that inflation leads to a homogeneous universe classically. However, the inflaton field is a quantum field. As such, it is subject to inherent quantum fluctuations. As the universe inflates, these fluctuations are amplified and stretched to large scales (in fact, they are stretched to scales larger than the causal horizon since the inflating spacetime expands faster than the Hubble radius). These quantum fluctuations couple to the background spacetime and generate tiny curvature perturbations. When inflation has completed, one is left with a spectrum of seed perturbations across a continuous range of scales -- from super-horizon down to the galactic. According to inflationary cosmology galaxy clusters, galaxies, and so on, originally arose as a quantum fluctuation in the very early universe. To me, that is the most amazing scientific assertion out there.Or if lumps are not initial conditions how does inflation make lumps? And then all the questions like how many, how big, at what time, etc...
Thank you bapowell I understand the idea.To me, that is the most amazing scientific assertion out there.