A lot of cosmology rests on far less stable ground than most might realize. So for instance the big bang was hypothesized by observing than everything is expand

A lot of cosmology rests on far less stable ground than most might realize. So for instance the big bang was hypothesized by observing than everything is expanding. So if you roll back the tape then it seems obvious that everything must have been closer. Roll it way back and you get the basic foundation of the idea. So one natural consequence of this explosion would be the cosmic microwave background radiation, the radiant heat left after the explosion. And we do observe that! Kind of... The thing is, it's nothing like what we expected. When we look at the CMB it's extremely homogeneous, everywhere. The reason this is a problem is because areas that are not causally connected (could not have reached each other at the speed of light, since the beginning of the universe) actually seem to be causally connected. We would expect to see a much more heterogeneous universe after a big bang. So we resolved this by inventing cosmic inflation. [1] The idea is that somehow, shortly after the big bang, everything just sped up dramatically - well past the speed of light. And then it slowed down. And then started accelerating again in a fashion similar to what we currently observe. If you assume this, then we can get the big bang to match what we observe. But there's no causal mechanism or anything of the sort proposed. It's literally just retrofitting a hypothesis to avoid having it falsified, an ad hoc hypothesis. [2] Inflation does fit what we see quite well, but it's untestable, unfalsifiable (especially given the ad-hoc nature of its own existence), and the predictions its offered to date were not particularly unexpected, given the homogeneity of the CMB. [1] - https://en.wikipedia.org/wiki/Inflation_(cosmology) https://en.wikipedia.org/wiki/Inflation_(cosmology) [2] - https://en.wikipedia.org/wiki/Ad_hoc_hypothesis https://en.wikipedia.org/wiki/Ad_hoc_hypothesis