What are the Different Types of Multiverse Theories in Cosmology?

[Schnellmann]

My understanding is that there are two types of multiverse discussed in cosmology: one where universes can form or break off from existing universes such that our universe sprouted from another. Each new universe could have different initial conditions and with an almost infinite number of universes some like ours are suitable for complex life to evolve. Seems a reasonable idea although requires some evidence to support it.

The other type of multiverse I understand comes from quantum mechanics with the idea that every time you have a quantum state with more than one possibility then every possibility is actually played out by the creating of a new universe. That seems to give an ever expanding "infinity" of universes because even a piece of "empty" space has virtual particles popping in and out of existence (based on quantum probabilities) so generating new universes.

The latter multiverse sounds absurd to me. That doesn't mean it is wrong as lots of physics is counter intuitive!

Is the second form of multiverse a mainstream belief in cosmology?

 

[Chronos]

The mathematically inclined tend to favor versions where every possible quantum state is realized, whereas the causally inclined tend to favor branching versions where new universes are the consequence of specific events - e.g., inflation or black holes. All such models depend more heavily upon imagination than evidence.

 

[Chalnoth]

My understanding is that there are two types of multiverse discussed in cosmology: one where universes can form or break off from existing universes such that our universe sprouted from another. Each new universe could have different initial conditions and with an almost infinite number of universes some like ours are suitable for complex life to evolve. Seems a reasonable idea although requires some evidence to support it.

There are a number of different multiverse ideas. I really like how Max Tegmark categorized them:
http://arxiv.org/abs/0905.1283

The other type of multiverse I understand comes from quantum mechanics with the idea that every time you have a quantum state with more than one possibility then every possibility is actually played out by the creating of a new universe. That seems to give an ever expanding "infinity" of universes because even a piece of "empty" space has virtual particles popping in and out of existence (based on quantum probabilities) so generating new universes.

The latter multiverse sounds absurd to me. That doesn't mean it is wrong as lots of physics is counter intuitive!

This is the many worlds interpretation of quantum mechanics, and it's probably the correct one.

The disagreement within quantum mechanics here is with regard to what happens with wavefunction collapse. The basic formulation of quantum mechanics states:
1. Wavefunctions evolve via Schroedinger's equation.
2. When a measurement is performed, the wavefunction collapses so that only one branch of the wavefunction is observed.

The many worlds interpretation does nothing but remove the second piece: it just says that the wavefunctions evolve via Schroedinger's equation. It turns out that if you follow that line of thought through to completion, the appearance of collapse happens anyway, as the natural evolution of the wavefunction results in multiple branches which have very little interaction with one another.

Is the second form of multiverse a mainstream belief in cosmology?

I don't know how common the many worlds interpretation is. Most cosmologists I've spoken to tend to lean towards that or another mostly equivalent interpretation (such as the consistent histories approach). The main issue is that the interpretation of quantum mechanics really doesn't have an impact on much of anything that cosmologists actually do. It's a much more relevant field of study in areas where understanding wavefunction collapse is important, such as quantum computing (where it is critically important to avoid wavefunction collapse for as long as possible).

 

[windy miller]

There have been surveys done on which interpretation of QM is favoured by physicists. Its important to remember of course that physics isn't decided by surveys but by experimental evidence. However in the absence of empirical tests for different interpretations , a survey at least gives you a feel for where the community is at. Alas there hasn't been a comprehensive survey and most of the surveys done are small scale and informal. they tend to give a pretty mixed bags of answers. the many worlds does seem a consistently popular view but i think its fair to say that no interpretation dominates.

for the cosmological multiverse i think the situation is slightly different. This arises from the process of inflation. We do actually have observers like NASA and ESa telling us they have evidence for inflation. Theorists that work in inflation tell us that inflation leads to a multiverse. So here there does seem to be data. of course you can challenge each step here but you can't say there is no evidence there at all.

 

[Chalnoth]

for the cosmological multiverse i think the situation is slightly different. This arises from the process of inflation. We do actually have observers like NASA and ESa telling us they have evidence for inflation. Theorists that work in inflation tell us that inflation leads to a multiverse. So here there does seem to be data. of course you can challenge each step here but you can't say there is no evidence there at all.

That really depends upon what kind of multiverse you're talking about.

What inflation implies is that the universe is much, much larger than our observable region. It doesn't necessarily imply some of the other features of various multiverse ideas, such as different physical laws in different regions.

I think that the strongest reason to think that there are different low-energy physical laws in some regions of the universe comes not from cosmology at all, but instead from high-energy physics. The standard model of particle physics includes some spontaneous symmetry breaking, and proposed more fundamental theories have far more. Spontaneous symmetry breaking means that some features of the standard model of particle physics are a direct result of random events in the past. Which means, for example, that the strength of the weak nuclear force could have been very different.

So if we have some fundamental laws that include this feature, then the natural expectation is that there should be far-away parts of the universe, beyond our horizon, which have different low-energy physical laws. If inflation is also true, then this is guaranteed. But if the symmetries in the past weren't broken spontaneously, but were instead the result of some deterministic process (e.g. some process that minimized energy), then inflation won't produce different physical laws in different regions.