[fc417fc802]

> Our established 3 dimensions are actually compact, i.e. loop around

Do they not loop? What other option is there? I assume you can't sail off the edge of the disk, so to speak.

[AnimalMuppet]

Option 1: They loop.

Option 2: They go on forever without looping.

Option 3: They end - there is some kind of boundary to spacetime.

[trhway]

The expansion of the space is the feature which prevents any physical process inside to distinguish between those options. Kind of a hack - make compact Universe, add expansion and it would inside look and feel indistinguishable from non-compact.

[fc417fc802]

How does option 2 fit with the big bang? The obvious issue (at minimum) being accounting for the CMB.