– How do we know the universe is expanding, rather than the stuff in it shrinking?

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All the science stuff says the universe is expanding at an ever increasing rate. However, when reading about the possibilities of creating micro-universes (like in colliders), they say that those universes would appear to decay quickly from our outside perspective. Wouldn’t it make more sense that our universe is a micro-universe, which ought to be common for the same reason simulates universes are common, and that it’s decaying? What’s the difference between space expanding and the stuff within space all shrinking? Would the distinction even matter from an “inside” perspective?

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It’s not that we see it getting smaller, it’s that the light the stuff emits is getting *redshifted*

It’s similar to how a siren from an ambulance sounds lower pitched as it moves away from you as opposed to moving towards you. Light emitted from things moving away from us is shifted to longer wavelengths/lower frequencies. Almost everything out of our near galactic neighborhood has had the light they emit shifted to this lower frequency, meeting theyre moving away from us, and the distance they’re moving away is directly related to how fast they’re moving.

> However, when reading about the possibilities of creating micro-universes (like in colliders)

This stuff is all extremely speculative. Unfortunately, pop physics writers don’t tend to make very clear distinctions between accepted results and wild guesses, so unless you manage to find a particularly careful one, it’s better to take everything they say with a pinch of salt. Rule of thumb: if they say anything about multiple universes, wormholes, faster-than-light communication/travel, singularities (regarding either black holes or the big bang), the Planck scale, string theory, or anything that happened before the big bang, they are talking about guesses.

Anyway, if you think about it, the only way we have to define distances is by assuming that *something* has a fixed size. The sizes of the stuff around us – our bodies, our computer screens, the Sun, water molecules, etc. – all stay roughly fixed relative to each other. So it seems pretty reasonable for us to use them to define our notion of size. If you wanted, you could redefine distance so that the universe doesn’t expand and everything inside it shrinks. But I think that would be a huge violation of Occam’s razor, and it wouldn’t really achieve anything. For us to get a definitive answer to which of these two views is correct (or if they’re both wrong), we would need to be able to measure the size of something outside the universe for comparison, and clearly we can’t do that, because we don’t even know if there is anything “outside” the universe, let alone something that has a comparable notion of “size” to things inside the universe.