Thread by @seanmcarroll

A good overview of the status of inflationary cosmology in the wake of the @Planck satellite observations, by Chowdhury, Martin, Ringeval, and Vennin. I do have one nontrivial objection... 1/n
arxiv.org/abs/1902.03951

They discuss whether early inhomogeneities could prevent inflation, and conclude that the issue is open. It's not! It's perfectly clear that most inhomogeneous initial conditions, by any sensible measure, do not lead to inflation. 2/n

There are many arguments for this, the simplest of which is that you wouldn't expect a collapsing universe to anti-inflate. It's a reflection of the fact that the early universe had low entropy, which is automatically "unlikely". 3/n

Roger Penrose has of course been banging on about this for years. A couple of other references:
arxiv.org/abs/gr-qc/9811…
arxiv.org/abs/1007.1417
And I discuss the overall situation here:
arxiv.org/abs/1406.3057
4/n

That doesn't mean that inflation didn't happen. Just that it's not automatic or natural. Time for cosmologists to admit that inflation requires a theory for why appropriate proto-inflationary initial conditions happened in the first place. 5/5