Types of Multiverses Can anyone recommend me books or other online resources on the concept of the Multiverse, it types, and about higher mathematical dimensions of space-time. I have been interested in such topics for a while but have not found any accessible book that could explain them in a way that's comprehensible to me. (I'm new to the subject of astrophysics, just getting started).
What is the numerical value of $c$, the SR does not say that. It is a global parameter of our Universe, we need to measure it and substitute it into the SR formulas.

There are also more trivial global parameters. Like the dimension of the spacetime. With (space) dimensions more or less than 3, the formulas of the classical mechanics would be the same.

All the essays talking about "multiverses" or "alternate Universes", are playing with the consequences of the current theories, but substituted with different global parameters than we have now. We could imagine, they are playing with alternative Universes.

However, here we have two major problems.


The logical problem. What does "Universe" mean? The word has latin root, "uni versum" means "to contain everything". The "Universe" is the thing what contains everything. How can multiple be existing then?


For example, the sci-fi topos on which a parallel 3D spacetime referring as "another Universe". No, it is not "another Universe". If there would be proof that such a thing exists, then "our Universe" and this "alternate Universe" would be the "Universe" on its original meaning. We could refer the alternative as an "other spacetime" or "parallel space" or so.


We have also a major problem, what is science. If we can access the parallel 3D space, i.e. if we can interact with it, then it is not "another Universe", it is just a newly found region of ours. But what is no interaction is possible between them and us? Then they are not existing. They might be existing like that our thoughts exist. In any case, it is not physics/astronomy any more, it is phylosophy in the best case.


Professional scientists generally get itchy if they need to answer not science questions. It is because you are assuming rhat they know it, while they know that it is not even science. For example, how hot is a 10 solar masses supernova, that is their field. Thus, your question will be likely closed as unclear, because it is.

What you can find:


You can find popular science texts about playing the current theories, but with different global constants. For example, "what if speed of light would be different" will give you likely useful google hits.
General relativity has some interesting topological extensions. For example, the Kerr-solution looks as if rotating black holes would bind different Minkowski-spacetimes together and so.
There might be interesting hits for googling for "string theory landscape".


In any case, make it more clear, what do you understand on "multiverse" or so, use science terminologies for that. You can find these terminologies by googling forums.

Probably the most popular google keywords will change some decade later.