It's not that there is a shifting boundary where the universe ends, but what is meant with universe expansion is (per my understanding) that objects grow further apart over time without experiencing acceleration or any force acting upon them. It's like drawing a map on grid paper and then increasing the grid size without adjusting the scale: all distances become larger
This stretching includes any photons traveling across the grid, and the stretching causes them to expand. Longer wavelengths are more red, giving us the word redshift for stretched light rays. That's one of the ways we measure the expansion: see how much redder the light is as you look at more and more distant objects
From my understanding, The infinitely large theory relies on the Universe being incredibly complex in terms of dimensions. The best way to describe it is like a balloon Take a deflated balloon and draw two dots next to each other on it. Then blow up the balloon. The dots are further away from each other (universe expansion). And if you "walk" across the balloon, you can make a full circumference so you never run into an edge (infinite).
I'm not sure if we know that the universe is infinitely large.
I do know[1] that:
- it's larger than we will ever be able to know or observe (ie.: the observable universe is smaller than "all of the universe"... although that feels a bit hand-wavy to say)
- it's expanding.
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[1] I am a scientist, but not a physicist. Everything I said could be wrong.
This stretching includes any photons traveling across the grid, and the stretching causes them to expand. Longer wavelengths are more red, giving us the word redshift for stretched light rays. That's one of the ways we measure the expansion: see how much redder the light is as you look at more and more distant objects