Mainstream physics doesn’t say there is anything the universe is expanding into. Rather, “expansion” is just that the distance between all objects in space grows over time, there is no claim that there is anything “outside” of space that space is “expanding into”, indeed that claim doesn’t make a whole lot of sense (“outside” is a spatial concept, so “outside of space” is somewhat of an oxymoron)
(While space is expanding everywhere, we don’t observe it within galaxies, only between groups of galaxies-because within galaxies, and between nearby galaxies, the pull of gravity outweighs the expansion.)
It is just saying that distances between all (non-gravitationally-bound) objects grow over time.
Mathematically, the space is infinite, and the distance between all (non-gravitationally bound) points in that infinite space is growing. However, while mathematically space is infinite, whether physically it is infinite is considered unknown–and quite possibly unknowable.
Interesting. I was under the impression that new space with finite zero point energy was being created, and this was one possible mechanism for photons to redshift and lose energy.
Is the idea then that the big bang was the creation of the infinite mathematical space for things to expand into?
> I was under the impression that new space with finite zero point energy was being created, and this was one possible mechanism for photons to redshift and lose energy.
Not familiar with that idea. Which physicist(s) have proposed it?
> Is the idea then that the big bang was the creation of the infinite mathematical space for things to expand into?
Big Bang theory says that, as you go back in time, distances shrink, and density and temperature increases, until at a finite time in the past, density and temperature become infinite. "Creation" is a philosophical or theological concept, not part of physics, so physics doesn't really have anything to say about it.
The ontological relationship between physical space, and mathematical space(s), is also something which physics itself doesn't have anything to say about–that's a question for philosophy of physics and philosophy of mathematics. Max Tegmark claims the two are identical ("radical Platonism", "ultimate ensemble theory", "mathematical universe hypothesis", etc), but his view is far from mainstream (I don't think there really is a mainstream view per se, just that few would bet on his highly speculative theories being right). And although he is a physicist, in proposing that theory, he is arguably acting as a philosopher of physics rather than as a physicist proper.
(While space is expanding everywhere, we don’t observe it within galaxies, only between groups of galaxies-because within galaxies, and between nearby galaxies, the pull of gravity outweighs the expansion.)