## Saturday, October 17, 2009

### Universe or multiverse?

Universe or multiverse?

Recently there has been some peak of comments on the blogosphere about the multiverse, partially because of a new article by Linde and Vanchury titled How many universe are in the multiverse?.

But the battle against the multiverse, and it's buddy's, the anthropic principle and the string landscape are not new at all. Peter Woit is a championship of that cause. I have always considered P.W. as innocuous, and a source of information about string theory, even if he doesn't like it. Being he a mathematician, or at most a mathematical physicist it is not a lack of respect to his position in an university to not take seriously their objections to a branch of physics that he mostly doesn't understand.

But recently I am beginning to think that in fact he can be causing some damage. The problem I see is that he is so intended to criticise string theory that he only search the part that is good for his purposes without worrying of understanding the whole picture. Worse still, he can make believe that his biased view is the whole view. And that's very bad because it gives a very wrong perspective of what is being done in string theory, and in cosmology and high energy physics in general.

In particular what he says about multiverse, string landscape and similar topics is totally misleading. I am not saying that these are not conflictive areas. Only that what Woit says about them is not representative. To begin with one may realize that the existence of multiverses is, in some cases, inflation, mostly a consequence of already proved physicist with the only assumption of some special issues on he potential of the inflaton. Also, if one trust string theory, the multiverses would rise as a consequence of a saltatory cosmological constant. In fact that two scenarios are similar in spirit, although very different in the details.

If a reader of this blog would want to get a much better idea I would recommend him the reading of the book presented at the beginning of this entry. IT is edited by Bernard Card, who also makes a presentation chapter and thematic one, "the anthropic principle revisited". The firs chapter gives an overview of the rest of the book and explains the many meanings of the term "multiverse" that are treated.

The book itself is bases in a series of conferences partially supported by the templeton foundation. The list of participants includes a list of very prominent physicist such as S. Weinberg, S. Hawkings, L. Susskind,A. Linde, P. Davies, R. Kallosh and a long etc. The list of topics is also very broad, covering many of the variants of the multiverse idea and why it raises in nowadays physics research.

I only have read a few (six) part of the articles and I am alternating them with other articles about inflation in string theory, supersymmetry breaking and general literature about the cosmological constant. My idea is that probably there are better alternatives for the apparent existence of an accelerated universe (and in general, possibly, for some fine tunning problems) but that it would be stupid not to read (at least a part of) what appears in that book if ones is concerned about it.

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## 3 comments:

The approach of Peter Woit tends to induce "multiverse-bad" highly emotional associations. It also seems to me that Peter Woit is not interested in new ideas but more on the role of a hero who demonstrated that string theory was not even wrong.

One should clearly distinguish between notions of multiverse, landscape, and anthropic principle. The concept of multiverse in some sense is unavoidable in any geometric quantum theory of gravitation and introduced already by Wheeler as super space. Multiverse need not imply either landscape or anthropic principle or different laws of physics in different corners of universe.

The notion of multiverse should be clearly defined. To my view here is the basic weakness of M-theory. If one accepts landscape, one should begin by formulating precisely what it means mathematically. Does one have infinite-D space of 11-D geometries for instance? And what about the space of all brane configurations: should one try to geometrize also it?

To outsider this looks to me very very messy. There is just too much stuff reflecting the fact that first Kaluza-Klein approach was thought to work and let to Calabi-Yays, and when it became clear that it does not branes were introduced and led to 11-D M-theory. Later F-theory emerged. M-theory is like a recycle bin full of all kind of garbage after 25 years use. Just an attempt to give precise mathematical meaning to this stuff as infinite-D geometry -following Einstein's program- could make this clear also for the proponents of M-theory.

In TGD framework multiverse - or (classical spinor fields in the) world of classical worlds (WCW) as I call it- has now amazingly precise number theory based identification. WCW is the space of associative sub-algebras of local octonionic Clifford algebra of M^8 (I am of course simplifying somewhat, see this and this). This implies all the basic structures of quantum TGD. This notion is mathematically precisely defined and mathematicians could start immediately work with it.

Hi Matti.

Things are a little bit more sensible in string theory that what P.W. seems to promote.

The multiverse in string theory is a way to populate the landscape. The idea goes something like this. One, at least after supersymmetry breaking, has a universe with too much vacuum energy, that is inflating very fast. Some way (inflation, instanton transitions to vacua with less energy) one gets bubbles writhing the generally inflating universe that are causally disconnected from the rest of the outer (and faster) expanding universe. That bubbles themselves can generate it's own sub-bubbles and so on.Not necessarily all bubbles have the same spacetime dimension and (low energy) physical laws.

But, mathematically, they would belong to the same manifold/space-time. It is only the c speed limit which forbids going from one into the other. In fact a tachyon culd go from one universe to another. Also one could open (lorentzian) wormhole from one to other. They are different universes only because of the light speed limit (well, and possibly because they, possibly, have different low energy physic laws).

What you say, about some bizarre mixture of different vacua, that ism, that the actual vacua is some superposition/mixture/whatever, of all the allowed vacua of string theory is an image that I had got from reading to P.W., but it doesn't too much correspond to what one can read in most papers.

Said slightly differently. One could introduce the word "megaverse" to mean what one would usually think as the universe. In every diferent parts of the megaverse there would be a different vacua of the ones allowed by the string theory landscape. The anthropic reasoning says that only the parts where complex live evolves and creates astrophysics is susceptible of being observed and studied.

About branes, flux vacua, etc, you wouldn't see them as contrary to the Kaluza-Klein idea. They are important for many things, for example, moduli stabilization. An easy and intuitive way to understand it is as follows. As you know in pure Kaluza-Klein the compactified dimensions have cycles of a given size. The problem is that nothing fix that sizes, it takes no energy to change that sizes. Given that the low energy physics (for example coupling constants) depends on that sizes they could vary freely (technically there are no potential for the moduli fields). But if one has flux, or branes, filling that cycles if one wants to modify that sizes one must face the change of the flux, or the tension of the brane, that is, fluxes and branes create a potential for the moduli.

The relation between this flux + branes and the landscape is that their existence is what lead to the existence of so many vacua. But I still don' think that the picture is complete. The deSitter vacua appearing in KKLT like papers are totally unrealisitc from the viewpoint of particle physics phenomenology. They are toy models that show that one can get deSitter models, and use them to get models with an small cosmological constant. I guess that the idea is that even in realistic models regarding particle physics one still can have a large number of variants of them with deSitter cosmology and the instantons connecting them to release to a tiny cosmological constant. But as far as I know that is whisefull thinking.

About what you say of Wheller-de Witt and function of the universe I still haven't read almost nothing. I think that the idea is that some people pruchase is that the many universes of that equation correspond to the many parts of the "megaverse" in the picture I have explained. In that case, maybe that as you say, one would have to face that there are different string vacua involved and see how to make sense of it. But as I said I still haven't read too much and can't say you for sure.

That's the picture that some people in string theory support, if I understand correctly. I need to learn more thing to even get a midly firm position about the particular but what certainly I don't think emotional associations are a good way to deal these topics. I also think that some string theorists are doing a publicity of the topic that remembers me the buzz about chaos + complexity. Too much emphasis in ideology and new paradigms but the actual results are much more down to earth (not saying that unimportant).

Regards.

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