## Saturday, October 31, 2009

### The dark side of the landscape

In the previous post I had presented the multiverse in a way that made it look almost innocuous. As I have said a few times in this blog I had heard about how the landscape (existence of a large number of vacua) in string theory made it unavailable to make predictions.

Despite it the actual articles I had read didn't give to me that impression, so I suspected that I was missing something, the problem is that I didn't know what. Reading a recent entry in Lubos blog titled A small Hodge three-generation Calabi-Yau I faced again that problem of missing information. So I reared once again the KKTLT paper and I searched for bibliography that would give me some cloud.

At last I was lead to the correct paper, The statistics of string/M theory vacua by Michael R. Douglas (he is not the actor, of course). The abstract of the paper says all it:

We discuss systematic approaches to the classification of string/M theory vacua, and physical
questions this might help us resolve. To this end, we initiate the study of ensembles of
effective Lagrangians, which can be used to precisely study the predictive power of string
theory, and in simple examples can lead to universality results. Using these ideas, we outline
an approach to estimating the number of vacua of string/M theory which can realize
the Standard Model.

I still haven't finished to read the paper, but the image is clear. Yes, one can have a chaotic/eternal inflation scenario that creates an infinite of universes, or one can go from one to another thought some kind of CDL (Coleman de Luccia) or Hawkings instantons among deSitter vacua or whatever mechanism to create an universe for whatever vacua of the one available in string theory. An yes, every new universe would have an smaller cosmological constant that the previous one. In that way one has an universe with the small cosmological constant (cc) observed in ours. The anthropic principle (or ideology as prefer to name it Lubos) says that in universes with large cc there are no observers so it is not that bizarre that we observe such an small cc, despite the fact that naturally theories with broken supersymmetry would have a big one to begin with.

The real problem is that in that paper is argued that even with the restrictions of an small cc and the observed gauge content (the standard model one) one still has a large number of solutions with the values of the coupling constants, masses of the particles and etc. in the observed margin of the standard model. I have made quick search in the paper to see if it was here where it appeared the famous $10^{500}$ but I couldn't find it (the search feature of acrobat seems to not work with math expressions) but in the text appear ofthem $10^{100 }~ 10^{400}$ so it is in the right order of magnitude. I have intention of reading this paper soon, as well as another by Kallosh and Linde, Landscape, the scale of SUSY breaking, and inlation

It is not that I like the idea of the landscape, I don't, and that's why I hadn't found sooner this papers and I had searched other lines of investigations, such as the ones mentioned in this blog. But like it seems that cosmology is a such a hot topic nowadays, mainly because the large amount of data available, I think it is a good idea to know this kind of things in some detail.

As I had said previously, in other blog entries, I was aware that there were some concrete approaches that tried to disprove the landscape, understood in the sense presented here-that is, too many vacuums compatible with the standard model, not just too many vacuums compatible with an small cosmological constant-. Some of that papers are The String Landscape and the Swampland discussed by Lubos here and also discussed by Distler in his entry YOU CAN’T ALWAYS GET WHAT YOU WANT. More entries in Lubos blog discussing papers against the (SM)landscape are Ooguri and Vafa's swampland conjectures. He also has a paper with C. Vafa and Nima Arkani-Hamed titled The String Landscape, Black Holes and Gravity as the Weakest Force. I think that I have seen a blog entry of him about that paper, but it doesn't appear in the trackback for some reason.

Well, I leave this entry as a loosely discussed bibliography of the real problems of the landscape ideology. As I said there are possibly good reasons to expect a good "vacuum selection method" as M. Douglas calls it and so one wouldn't care too much about it. Possible the LHC could give a cloud of it. It is good to know that beams are beginning to circulate in it again, at least partially, and thatvry soon-if everything goes ok-it will be giving data.

## 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.