> What if the universe doesn't expand at all?Not possible. Redshift is not the only observation we have. The totality of all the observations we have cannot be explained in any other way than an expanding universe.
> How can we be so sure it's space that's expanding, not time?
Our best current model does not say "it's space that's expanding, not time". It says that in one particular frame (the comoving frame), the overall spacetime geometry can be described using a "time" that always corresponds to the time of comoving observers and a "space" whose scale factor increases with that time.
> The more I read about this, the more it feels like phlogiston theory
This is an extremely unjustified comparison. Phlogiston theory never accounted well for actual observations.
> as more observations are made, some contradict the theory
None of the observations being discussed contradict the general model of an expanding universe. They only pose problems for the indirect methods we use to convert our direct observations into model parameters.
Not possible. Redshift is not the only observation we have. The totality of all the observations we have cannot be explained in any other way than an expanding universe.
> How can we be so sure it's space that's expanding, not time?
Our best current model does not say "it's space that's expanding, not time". It says that in one particular frame (the comoving frame), the overall spacetime geometry can be described using a "time" that always corresponds to the time of comoving observers and a "space" whose scale factor increases with that time.
> The more I read about this, the more it feels like phlogiston theory
This is an extremely unjustified comparison. Phlogiston theory never accounted well for actual observations.
> as more observations are made, some contradict the theory
None of the observations being discussed contradict the general model of an expanding universe. They only pose problems for the indirect methods we use to convert our direct observations into model parameters.