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View submission: Ask Anything Wednesday - Physics, Astronomy, Earth and Planetary Science
What is the commonly understood meaning of the term 'Universe' amongst physicists?
'all that is, has been and can be' is a broad summary that I have used. The broadest of definitions to describe everything that is known to exist. It's important to note -the scope of the definition expands to include anything new that is discovered.
Terms like 'multi-verse' and 'parallel universe' are used alot but are contradictory if the classical definition is used.
Comment by prappleizer at 15/09/2022 at 05:16 UTC
3 upvotes, 0 direct replies
We’re usually more specific. There’s the observable universe, for example, which is all thing which emitted light at a point when the universe was small enough that with its rate of expansion that light could reach us within the universe’s current ~13-14 Gyr age. Things further than that are moving away faster than the speed of light due to space expansion and their light will never reach us. But we also have estimates for what the actual size of the universe should be based on extrapolating expansion from the Big Bang and local measurements, and that is closer to 40 billion light years. And then you get into concepts of whether that region fell out of the inflation field early on and there are other universes with different physical constants, etc. that’s all mostly theoretical work.
In short I’d say an observational astronomer almost always uses the observable universe (since they prove by observation), and a few theorists might make use of the larger universe we know to exist but can’t see/measure. (But even then, not usually a useful exercise, as the laws of physics should be the same there and by the principle of cosmological equivalence it shouldn’t provide any new info that our observable universe doesn’t).