## Wednesday, July 01, 2009

### Predictive landscape of G2 vacua in M-theory

Konstantin Bobkov - who has previously written papers e.g. with Bobby Acharya and Gordon Kane (besides his own) - posted a very interesting paper,
Scanning the fluxless $G_2$ landscape.
He argues that the degeneracy of M-theory compactifications on G_2 holonomy manifolds can be comparable to a googol, depending on the topology of the manifold. Recall that so far, the counting only led to googol-like results in the type II or F-theory compactifications, not the heterotic and M-theoretical ones which were assumed to have "modest" populations. Bobkov argues that the M-theoretical vacua have a large number of vacua, too.

Such a large degeneracy - one that is obtained by choosing fluxes in the F-theory KKLT context - emerges because of discrete Wilson lines and various other topological data (I found it pretty hard to find a clear description what these choices are in the paper). Bobkov says that the dependence of the cosmological constant on these things is nontrivial and pretty random - so their large number can be responsible for the smallness of the cosmological constant via "coarse tuning".

On the other hand, he also claims that these new discrete choices basically don't affect the "bulk" of particle physics (n-leg graphs for "n" positive) much, so except for the cosmological constant, the framework remains predictive in practice. The superpartner scale etc. is only influenced by an effective parameter P_{eff}, a logarithm of a ratio of parameters that must be between 60 and 65 or so for realistic values of the vacuum energy.

At any rate, Bobkov proposes a completely new philosophy about the randomness of the cosmological constant and other parameters - and their correlations. There aren't almost any, he says. The cosmological constant is a decoupled problem. Of course, that was the answer many of us have wanted to see for quite some time - but if his paper is right, Bubkov gives a possible realization how it could occur.

A heterotic paper

There's another paper with the word "landscape" in it today. Jonas Schmidt, in his
Local grand unification in the heterotic landscape,
does the first steps to construct GUT models where the GUT scale is kept at 10^{16} GeV but the string scale is moved higher, close to the conventional Planck scale 10^{19} GeV. Conventionally, the string scale is assumed to be closer to the GUT scale. Well, the first excited states of a string are as heavy as the first black hole microstates in his models, so I don't think that it would be a "weakly coupled" string theory in any sense. That doesn't imply that the scenario is wrong.

Also, in his setup, there are discrete choices, like in the previous paper. But this time, they (especially the unbroken discrete groups) influence the phenomenology significantly.

A new Hořava-Lifshitz no-go paper

Archil Kobakhidze offers a new proof, using lots of Poisson brackets and Dirac constraints, that Hořava's diff-breaking theory of gravity has one new, unwanted, and non-decoupled excitation besides the two transverse polarizations of the graviton in the infrared, and is therefore ruled out.