Status of Superstring and M-theory
If someone is interested in getting an idea of the status of string theory today, in a readable form for non specialists, I seriously recommend this paper by John Schwarz Status of Superstring and M-theory.
It begins with an introduction to the very subject of string theory, explaining the basic of string theory. It explains in more detail than usual the connection between the former "hadronic" string and the actual unified field theory string (and gives the link to another paper by the author devoted to the beginnings of string theory which is worth reading also). Later, in the paper, it explains the second string revolution and the dualities among the five string theories. It talks about the D-branes revolution, the flux compactifications, the warped compactifications sceneries and the possible role of string theory in cosmology. Obviously such a lot of material compressed in 13 pages means that it doesn’t go nearly deep in any of the subjects. It can be readed as an almost divulgative article (a divulgative article suited for theoretical physicists not specialist in string theory, if such thing still exists at all xD).
The second part of the article is devoted to the actual subject, the review of the status of sting theory. Or, to be more concrete, the status of string theory phenomenology. It discuss the most studied sceneries of "compactification" (understood in a general sense). It briefly mentions the early attempts of compactifitcation in perturbative string theory, specially in the heterotic string.
Later he talks about heterotic M-theory, the Horawa-Witten model (you can read additional info about it in this entry of my blog). Later he briefly discuses compactifications of (non heterotic, i.e. type-IIA strong limit coupling) M-theory in G2 Manifolds. Up to here most of the material of the article is written in form that closely resembles the introductions of the corresponding chapters in his recent book (Becker-Becker-Schwartz) about string theory. The remaining part is devoted to subjects not covered in the book.
For example, it follows with an intro to intersecting D-branes in type II string theory. He explains very clearly the relevant points and later he gives bibliography with appropriate reviews (as, in fact, makes for all the subjects). The next subject is type II in calaby-yaus with fluxes. In this kind of sceneries string theory admits a lot of vacuums. This enormous amount of vacuums have been used to implement an old idea of Weinberg to explain the existence of a positive and small cosmological constant and have given rise to the string landscape and, even worst, the resurgence anthropic principle. Schwarz, as many others, doesn’t support too much the idea of the landscape.
Perhaps the most interesting chapter is the last one. It is devoted to a review of the recent papers by Vafa et all about F-theory based phenomenology. This is a very recent subject and excepting for the corresponding entries in the blogs of Motl and Distler (well, Woit did his try, but he lacks of enough knowledge in string theory to be able to extract usfull info from the papers, as he admits in the post)there has not been, as far as I know, too much discussion on this papers (why the hell is the string coffee clossed?). I have not still readed the original papers of Vafa (I am waiting to conclude a doctorate courses I am assisting about algebraic geometry before doing that) so this chapter is the first non blog info I have about it. I must say that Schwarz exposition of the subject is very, very clear. He introduces very well what F-theory is. He explains that these models are based in a novel idea, the decoupling between the GUT scale and the Planck scale. Models with this feature are named local models to distinguish them from the usual models, named global models. I´ll not try to describe here the details of how local models of F-theory are constructed and I remit the readers to the actual paper.
Quintessence from string theory
Another interesting paper, at least for me, is arXiv:0810.5346v1 entitled "Where in the String Landscape is Quintessence". When the expanded acceleration was firs observed there were many proposals. Nowadays the most commonly accepted one is that the universe is de-Sitter, that is, there is a cosmological constant. Apparently one of the reasons for this broad acceptance stems from string theory and the works of Bousso, Polchinsky and others who explain this cosmological constant in the scenario of the string theory landscape. I guess that the logical chain is something like: String theory must be true->string theory can explain an small positive constant->we have an small positive constant. Well, this paper uses similar ideas to that of Bousso and Polchinsky to support quintessence.
Some readers of the blog may be don’t know what quintessence is. Let’s remember first that a cosmological constant may be seen as due to the vacuum expectation value of a quantum field. That field, usually a scalar one, must have constant value in space and time. Quintessece also should be due to a scalar field. But this would vary in space and time. It must also fit some additional features related to its equation of state (the equation of state that represent the field in the cosmological models based on general relativity. In particular it must have pq = wρq. Here p is the pressure, rho the density and w is the quotient of bothe (w=p/rho). The actual value of w must equal to 1/3 during radiation domination and 0 during matter domination) until w undergoes a transition to less than -1/3 which initiates the accelerated expansion of the universe. For more details I suggest to read the relevant wikipedia entries: http://en.wikipedia.org/wiki/Physical_cosmology , http://en.wikipedia.org/wiki/Dark_energy, http://en.wikipedia.org/wiki/Quintessence_(physics) and http://en.wikipedia.org/wiki/Equation_of_state_(cosmology). Or, if they have time and are very interested in cosmology they could try to read the recent book of Steven Weinberg on the subject: http://www.amazon.com/Cosmology-Steven-Weinberg/dp/0198526822
The reason I find this paper particularly interesting is because many solutions which allow the existence of wormhole solutions, a topic that I find particularly interesting. Maybe people not so wormholmy wouldn’t find so interesting this paper.
Astrophysical Probes of Unification
The last paper that I want to mention is this. It is very recent, it has appeared just today in arxiv, and I guess that soon Sean Carrol and/or Lubos Motl (or Distler if he decides to reapear from it's retire) and possibly others will make extenses comments on it so I will only copy here the abstract:
Traditional ideas for testing unication involve searching for the decay of the proton and its branching
modes. We point out that several astrophysical experiments are now reaching sensitivities that allow
them to explore supersymmetric unied theories. In these theories the electroweak-mass DM particle can
decay, just like the proton, through dimension six operators with lifetime 1026 sec. Interestingly, this
timescale is now being investigated in several experiments including ATIC, PAMELA, HESS, and Fermi.
Positive evidence for such decays may be opening our rst direct window to physics at the supersymmetric
unication scale of MGUT 1016 GeV, as well as the TeV scale. Moreover, in the same supersymmetric
unied theories, dimension ve operators can lead a weak-scale superparticle to decay with a lifetime of
100 sec. Such decays are recorded by a change in the primordial light element abundances and may well
explain the present discord between the measured Li abundances and standard big bang nucleosynthesis,
opening another window to unication. These theories make concrete predictions for the spectrum and
signatures at the LHC as well as Fermi.