* Astronomy

Title: D-branes and cosmic structure
Authors: Danielle Wills, Konstantinos Dimopoulos, Ivonne Zavala

We outline the embedding of the vector curvaton scenario as a promising mechanism to generate statistical anisotropy within Type IIB string theory, where the vector field on a single D-brane plays the role of the vector curvaton. We first consider a toy model in the context of open string inflation, and then begin to construct a concrete model in the context of closed string inflation.

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Title: Topological Ghosts: the Teeming of the Shrews
Authors: Nemanja Kaloper, McCullen Sandora

We consider dynamics of spacetime volume-filling form fields with "wrong sign" kinetic terms, such as in so-called Type-II^* string theories. Locally, these form fields are just additive renormalisations of the cosmological constant. They have no fluctuating degrees of freedom. However, once the fields are coupled to membranes charged under them, their configurations are unstable: by a process analogous to Schwinger pair production the field space-filling flux increases. This reduces the cosmological constant, and preserves the null energy condition, since the processes that can violate it by reducing the form flux are very suppressed. The increase of the form flux implies that as time goes on the probability for further membrane increases, in contrast to the usual case where the field approaches its vacuum value and ceases to induce further transitions. Thus, in such models spaces with tiny positive vacuum energy are ultimately unstable, but the instability may be slow and localized. In a cosmological setting, this instability can enhance black hole rate formation, by locally making the vacuum energy negative at late times, which constrains the scales controlling membrane dynamics, and may even collapse a large region of the visible universe.

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 Do we need String Theory for Quantum Gravity? - Lance Dixon (SETI Talks)



The strong, weak, and electromagnetic interactions all have consistent, relativistic and quantum mechanical descriptions in terms of pointlike particles, but Einstein's theory of gravitation has long resisted a similar treatment, because of severe ultraviolet divergences. String theory solves these problems, but it introduces a new length scale, perhaps 16 orders of magnitude below what can be tested experimentally. Dr. Dixon will describe recent theoretical progress in showing that a particular pointlike theory of gravity, called N=8 supergravity, might also be quantum mechanically consistent. In particular, N=8 supergravity has been shown explicitly to have no ultraviolet divergences in perturbation theory through the four-loop order. Dr. Dixon will also discuss the possible implications of these results.

The mechanism that explains why our universe was born with 3 dimensions

Superstring theory predicts a space with 9 dimensions, which poses the big puzzle of how this can be consistent with the 3-dimensional space that we live in.
A group of 3 researchers, Jun Nishimura (associate professor at KEK), Asato Tsuchiya (associate professor at Shizuoka University) and Sang-Woo Kim (project researcher at Osaka University) has succeeded in simulating the birth of the universe, using a supercomputer for calculations based on superstring theory. This showed that the universe had 9 spatial dimensions at the beginning, but only 3 of these underwent expansion at some point in time.
This work will be published soon in Physical Review Letters.
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