From what I know, the decision that the universe was expanding came from the discovery of the redshift and that other celestial objects, such as galaxies, were moving away from us; the farther they are, the faster they are moving. My question is, how was it decided that the universe is indeed expanding and not just infinite, as there is supposedly an infinite amount of space for it to expand. I'm also wondering how objects like galaxies move as a result of the expanding universe, as there are many that are currently moving away, I don't understand how it is possible for them to move.
Although this can be mathematically proven and was done so formally by Stephen Hawking and Roger Penrose it is actually intuitive to understand how the universe had a beginning in time.
Notice that through numerous observations include redshifts of galaxies we have confirmed that the universe is expanding. This means that as time moves forward the space between every point in the universe increases(also note that this spacing is finite and the expansion of the space is also now at a finite rate). Thus it is logical to conclude that if we go back or rewind time then the universe is contracting(the opposite of expansion). Thus if we know that there must’ve been a time where that spacing between points (that I talked about), were zero as both the spacing and the rate of increase was spacing is finite. Thus this condition of space where there is no space between points is the supposed singularity we often hear and sure you may call it an infinite universe but it’s not accurate in the sense we imagine(with real space between points as no time can act if there is no change possible in space).
The expansion of the universe isn’t like a border that’s being moved; the universe could be positively curved, which means that if you go in a straight line long enough, you end up at the same place you started (thus realizing it wasn’t a straight line), like a globe, but now in four dimensions. So this “globe” is expanding, and just as with a three-dimensional globe, there isn’t a border that’s being moved, the amount of area/volume (space in general) just increases between non-interacting bodies.
The universe could also be flat or negatively curved, it would be infinite from moment one, but it could still “expand”, which is rather just an upscaling of distance (as if you’re zooming in on a grid).