-1
$\begingroup$

In this paper (https://arxiv.org/abs/hep-th/0211048) by physicist Andrei Linde, the multiverse concept, the anthropic principle, quantum cosmology and inflationary cosmology are analyzed.

In the paper, he analyzes a multiverse with a large landscape of universes (described by different spaces and dimensions, different lagrangians, different actions, different types of evolution…)

He mentions and analyzes various models of theoretical physics (like inflation cosmology, quantum cosmology, Wheeler-DeWitt equation…) but does not clarify whether any of these models could be applied to the vast multiverse he is describing. He neither does specify which of these models could produce such variety of hypothetical universes nor if any of them could produce it.

So my question is:

Is it there any theory/model/equation that could produce such a vast multiverse? Any model that Linde mentions? Could Wheeler-DeWitt equation be an option for that?

Linde suggested that Wheeler's "it from bit" could contain such universe. Since there are loop quantum gravity models which are based on this idea, this could be an option...But apart from this, is it there any other model/theory?

In summary, is it there any theory/model/equation (apart from loop quantum gravity models) that could produce all the universes studied by Linde and proposed by certain versions of the anthropic principle (https://en.wikipedia.org/wiki/Anthropic_principle)?

$\endgroup$
1
$\begingroup$

The idea of a multiverse first arose from combining eternal inflation with the string theory “landscape”.

Briefly...

The idea of eternal inflation says most of the multiverse undergoes continuous exponential expansion, but in a huge number of places this inflation stops at some point. Each of these places becomes an essentially-independent universe within the larger multiverse.

The idea of the string theory landscape is that there are a huge number of different quantum vacuum states in string theory, based on the vast number of ways that the extra dimensions can compactify. In each vacuum state, string theory at below-Planck-scale energies reduces to various “effective” quantum fields and interactions between them.

Combining these two ideas, one can easily imagine that when a region of inflating multiverse stops inflating, it settles into a random vacuum state, and the new universe appears to be populated with a somewhat random set of particles and forces. (But what’s really going on is just string theory throughout the entire multiverse.)

As for the anthropic principle, the idea is that we evolved in a particular universe whose particles and forces happened by chance to be compatible with life.

In the eyes of multiverse aficionados, if you accept cosmic inflation as likely, then eternal inflation seems natural; if you accept string theory as likely, then a landscape of vacua seems natural; and the combination of the two to get a multiverse with some universes fine-tuned for life seems natural.

I will confess to being someone to whom all this speculation seems quite plausible and satisfying. But I would guess that in a hundred years we will have moved on to other wild ideas, and hopefully ones that we can test.

$\endgroup$
  • $\begingroup$ I think this is a superb resume of the main multiverse idea. I also find this multiverse as a very satisfying concept, even if, unfortunately, it stands outside of Science as we usually define it. So +1 for this answer. $\endgroup$ – Cham May 30 at 2:38
  • $\begingroup$ @G.Smith thank you for your answer, but what I did actually ask was whether any theory/model/equation in particular could produce all the universes that Linde considers in his article? I mean, surely string theory and inflation could produce a vast multiverse, but they could not produce universes described by different fundamental laws... Linde discusses a multiverse described by different spaces and dimensions, different lagrangians, different actions, different types of evolution...He even includes in his multiverse Tegmark's Mathematical Universe and Wheeler's "It from Bit"-based universes $\endgroup$ – user233281 May 30 at 21:09
  • $\begingroup$ @G.Smith so, basically, what I was trying to ask was: is it there any single theory/model/equation that could contain all these universes? All universes proposed by the anthropic principle and Linde's multiverse? I reccomend you to take a look to the paper to get a clearer view of what I am asking. (Section 6 I think is the most relevant) Also, Tegmark cited the paper in one of his works (arxiv.org/abs/0704.0646), and there suggested that "Linde's multiverse" could be contained in modified versions of Everett's Many Worlds Interpretation. $\endgroup$ – user233281 May 30 at 21:09
  • $\begingroup$ @G.Smith But apart from Tegmark's mathematical universe hypothesis, Wheeler's "It from bit" and Everett's interpretation (or quantun mechanics in general), is it there any other model/theory that you can think of that could reproduce all these universes proposed by Linde? $\endgroup$ – user233281 May 30 at 21:10
  • $\begingroup$ I personally don’t find the kind of multiverse described in Section 6, where the different universes have nothing to do with each other, to be interesting. Linde isn’t suggesting that there any unifying model to this kind of multiverse. The various universes have whatever laws can be imagined. In the kind of multiverse that I described, there is a common model, which is why I find it interesting. $\endgroup$ – G. Smith May 30 at 21:29