So if the universe has a size then what do you see if you are on the edge of it? Do you see stars to the left and nothing to the right? I mean given the speed of light and the age of the universe and the rate of expansion there are regions inaccessible to us but that doesn't quite mean the universe has a finite size.
The observable universe has a size, the cosmic microwave background is what we 'see' at the 'edge' in terms of photons (~400k years after the big bang). We could see further if we could map out the gravitational wave or neutrino backgrounds (1 sec after the big bang).
But for now we can't really say if the universe in its entirety has a finite size.
For the gravitational wave background, maybe with LISA we might be able to get a glimpse, but the neutrino background seems like it'd take some truly unprecedented breakthroughs in our ability to detect neutrinos to have any chance of mapping it out.
Funny, in reading up on both, I had higher hopes for the gravitational waves.
It seems like GWB is a superposition of infinite overlapping waves that would be impossible to single out and "unwind" in order to form a map.
And big bang neutrinos are very weak, which makes them undetectable. My assumption was we'd need a breakthrough in measurement sensitivity but is there more to it?
Finite size doesn’t require an edge. Consider the surface of a balloon for a 2-D case (or the perimeter of a sphere, for a 1-D case): it has finite extent, but no edge.
It has a surface, though, which is what PP was asking about.An answer to the question is, yes, nesr the edge/face, one side is dark. But relativity and expansion makes the situation a bit more complicated.
