There were no exciting seminars last week here at CERN. But, since I promised to resume blogging, here I am with this brief report from the Cosmo Coffee last Wednesday. Cliff Burgess was talking about a new string inflation model that is not around yet and that is not of his making - his friends Conlon-Kallosh-Linde-Quevedo are going to fire it off soon. The inflationary model itself does not violate causality, however. It is realized in this string theory model, which is a variant of KKLT, but it assumes different value of parameters and has a different low-energy phenomenology. The model has a metastable vacuum at large volume of the internal Calabi-Yau space, which arises when stringy loop corrections are taken into account.
The new inflationary model is christened volume inflation, because the volume of the Calabi-Yau is the modulus that assumes the role of the inflaton. Inflation happens when the volume is relatively small. This means that the string scale and the Planck scale are close to each other, which is favorable to obtain large enough density fluctuations. After inflation ends, the volume rolls down to our metastable vacuum where the cosmological constant is small and where the string and Planck scales are separated by several order of magnitudes. The model incorporates a mechanism of converting the energy of the inflaton into radiaton, which helps to avoid the overshooting problem and delivers us from evil of a ten dimensional vacuum.
My problem with this string inflation model is that, as Cliff admitted himself, there is hardly any string inflation here. Inflation operates below the string and compactification scales, so that the 4D field theory approximation applies. At the end of the day, it is just standard inflation in field theory, with some building blocks motivated by currently popular string models. The string origin does not produce any smoking-gun imprint. Nor it gives any insight into the conceptual problems of inflation - the vacuum energy problem and the transplanckian problem.
Well, I know that inflation in field theory works fine, if we turn a blind eye on some conceptual problems and allow for some fine-tuning. So, if you want to impress me, try working on string inflation instead. The only attempt i know of in which the ideas of string theory are crucial is the string gas cosmology approach. But there could be more...