This is basically the same question as “Are there parallel universes?” (a multiverse is a set of parallel universes), so I’ll copy my answer from there:
We don’t know. Almost by definition, we can’t know: universes are closed systems, so information does no tpass from one to another. Evidence for parallel universes would most likely come indirectly – if a theory makes lots of predictions which are testable and pass the tests, and it also predicts parallel universes, then you would be likely to accept that this is indirect evidence for parallel universes (because it is direct evidence for the correctness of a theory that predicts parallel universes).
There are a couple of theories that would predict parallel universes. One is the Many Worlds Interpretation of quantum mechanics. In QM, you cannot preduct the actual result of a measurement, only the probability of obtaining a given result. The MWI states that all possible results of a measurement “exist” in some multi-dimensional space, but you are only aware of one. The other results effectively generate parallel universes (hence the “many worlds” of the title). However, there are other solutions to what’s called the Quantum Measurement Problem (i.e. how is the result you actually get generated from the wavefunction, which gives multipke results wuth defined probabilities), and I do not know of any way in which you could prove that the MWI is correct. (But then, I’m not a quantum theorist.)
Another theory which leads to parallel universes is the cosmological theory of inflation, which states that the very early universe underwent a period of exceptionally rapid (exponential) expansion. In some versions of inflation, the end of inflation, when the universe settles back into slower expansion, creates a “bubble” of non-inflation in an infinite sea of continuing inflation, and there will be infinite other bubbles, each representing another universe. Again, I don’t see how one would prove this, though I think that different versions of inflation do predict different properties of primordal gravitational waves, so detecting those (probably by their impact on the cosmic microwave background – they are too faint for the likes of LIGO) would be a good start.
So, in summary, the answer is, “We don’t know, but some theories give us reason to think so.”
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Susan commented on :
This is basically the same question as “Are there parallel universes?” (a multiverse is a set of parallel universes), so I’ll copy my answer from there:
We don’t know. Almost by definition, we can’t know: universes are closed systems, so information does no tpass from one to another. Evidence for parallel universes would most likely come indirectly – if a theory makes lots of predictions which are testable and pass the tests, and it also predicts parallel universes, then you would be likely to accept that this is indirect evidence for parallel universes (because it is direct evidence for the correctness of a theory that predicts parallel universes).
There are a couple of theories that would predict parallel universes. One is the Many Worlds Interpretation of quantum mechanics. In QM, you cannot preduct the actual result of a measurement, only the probability of obtaining a given result. The MWI states that all possible results of a measurement “exist” in some multi-dimensional space, but you are only aware of one. The other results effectively generate parallel universes (hence the “many worlds” of the title). However, there are other solutions to what’s called the Quantum Measurement Problem (i.e. how is the result you actually get generated from the wavefunction, which gives multipke results wuth defined probabilities), and I do not know of any way in which you could prove that the MWI is correct. (But then, I’m not a quantum theorist.)
Another theory which leads to parallel universes is the cosmological theory of inflation, which states that the very early universe underwent a period of exceptionally rapid (exponential) expansion. In some versions of inflation, the end of inflation, when the universe settles back into slower expansion, creates a “bubble” of non-inflation in an infinite sea of continuing inflation, and there will be infinite other bubbles, each representing another universe. Again, I don’t see how one would prove this, though I think that different versions of inflation do predict different properties of primordal gravitational waves, so detecting those (probably by their impact on the cosmic microwave background – they are too faint for the likes of LIGO) would be a good start.
So, in summary, the answer is, “We don’t know, but some theories give us reason to think so.”