Why Isn’t Our Universe Perfectly Smooth?
“This seems, at first glance, to pose a tremendous problem. If inflation stretches space to be flat, uniform, and smooth, indistinguishably so from perfection, then how did we arrive at a clumpy Universe today? Both Newton’s and Einstein’s theories of gravity are unstable against imperfections, meaning that if you start with an almost-but-not-quite perfectly smooth Universe, over time, the imperfections will grow and you’ll wind up with structure. But if you start with perfect smoothness, with literally no imperfections, you’re going to remain smooth forever. Yet this doesn’t jibe with the Universe we observe at all; it had to have been born with imperfections in its matter density.”
One of the great successes of cosmic inflation is to set up the initial conditions for the Big Bang that we knew we needed, including giving us a Universe that had the same temperature and density everywhere. But this couldn’t have been a perfect smoothness, otherwise we’d never have formed stars, galaxies, and the cosmic large-scale structure we observe today in the space we inhabit. So how did we come to be clumpy? The Universe must have been born with initial imperfections in them. If you treat inflation as a classical field, you’ll never get them that way, but if you recognize that it’s a quantum field, with the associated quantum fluctuations that we know must be there, the whole story changes. Not only does inflation give you these cosmic imperfections, but it gives you the full spectrum of them that you can then go check against observations.