* Astronomy

Title: Graviton emission from the brane in the bulk in a model with extra dimension
Authors: Mikhail Z. Iofa
(Version v2)

In a model of 3-brane embedded in 5D space-time we calculate the graviton emission from the brane to the bulk. Matter is confined to the brane, gravitons produced in reactions of matter on the brane escape to the bulk. The Einstein equations which are modified by the terms due to graviton production are solved perturbatively, the leading order being that without the graviton production. In the period of late cosmology, in which in the generalized Friedmann equation the term linear in the energy density of matter in dominant, we calculate the spectrum of gravitons (of the tower of Kaluza-Klein states) and the collision integral in the Boltzmann equation. We find the energy-momentum tensor of the emitted gravitons in the bulk, and using it show that corrections due to graviton production to the leading-order terms in the Einstein equations are small, and the perturbative approach is justified. We calculate the difference of abundances of ^4He produced in primordial nucleosynthesis in the models with and without the graviton production, and find that the difference is a very small number, much smaller than that estimated previously.

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Neutrons could test Newton's gravity and string theory

A pioneering technique using subatomic particles known as neutrons could give microscopic hints of extra dimensions or even dark matter, researchers say.
The idea rests on probing any minuscule variations in gravity as it acts on slow-moving neutrons in a tiny cavity.
A Nature Physics report outlines how neutrons were made to hop from one gravitational quantum state to another.
These quantum jumps can test Newton's theory of gravity - and any variations from it - with unprecedented precision.
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Title: Universality in D-brane Inflation
Authors: Nishant Agarwal, Rachel Bean, Liam McAllister, Gang Xu

We study the six-field dynamics of D3-brane inflation for a general scalar potential on the conifold, finding simple, universal behaviour. We numerically evolve the equations of motion for an ensemble of more than 7 x  10^7 realisations, drawing the coefficients in the scalar potential from statistical distributions whose detailed properties have demonstrably small effects on our results. When prolonged inflation occurs, it has a characteristic form: the D3-brane initially moves rapidly in the angular directions, spirals down to an inflection point in the potential, and settles into single-field inflation. The probability of N_{e} e-folds of inflation is a power law, P(N_{e}) \propto N_{e}^{-3}, and we derive the same exponent from a simple analytical model. The success of inflation is relatively insensitive to the initial conditions: we find attractor behaviour in the angular directions, and the D3-brane can begin far above the inflection point without overshooting. In favourable regions of the parameter space, models yielding 60 e-folds of expansion arise approximately once in 10^3 trials. Realisations that are effectively single-field and give rise to a primordial spectrum of fluctuations consistent with WMAP, for which at least 120 e-folds are required, arise approximately once in 10^5 trials. The emergence of robust predictions from a six-field potential with hundreds of terms invites an analytic approach to multifield inflation.

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An introduction to M-Theory

Every decade or so, a stunning breakthrough in string theory sends shock waves racing through the theoretical physics community, generating a feverish outpouring of papers and activity. This time, the Internet lines are burning up as papers keep pouring into the Los Alamos National Laboratory's computer bulletin board, the official clearing house for superstring papers. John Schwarz of Caltech, for example, has been speaking to conferences around the world proclaiming the "second superstring revolution." Edward Witten of the Institute for Advanced Study in Princeton gave a spell-binding 3 hour lecture describing it. The after- shocks of the breakthrough are even shaking other disciplines, like mathematics.
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