Interaction rates in string gas cosmology

Abstract

We study string interaction rates in the Brandenberger-Vafa scenario, the very early universe cosmology of a gas of strings. This cosmology starts with the assumption that all spatial dimensions are compact and initially have string scale radii; some dimensions grow due to some thermal or quantum fluctuation which acts as an initial expansion velocity. Based on simple arguments from the low energy equations of motion and string thermodynamics, we demonstrate that the interaction rates of strings are negligible, so the common assumption of thermal equilibrium cannot apply. We also present a new analysis of the cosmological evolution of strings on compact manifolds of large radius. Then we discuss modifications that should be considered to the usual Brandenberger-Vafa scenario. To confirm our simple arguments, we give a numerical calculation of the annihilation rate of winding strings. In calculating the rate, we also show that the quantum mechanics of strings in small spaces is important.

The phantom divide in string gas cosmology

“…It will formally resemble that of a "bounce" cosmology, in the sense that there are components of conformal infinity both at the bottom and at the top. However, the cosmology in fact does not bounce: time increases away from the dashed line in both directions 7 .…”

String Gas Cosmology and Structure Formation

“…For a mode with fixed comoving wavenumber k, we compute the correlation function of the off-diagonal spatial elements of the string gas energy-momentum tensor at the time t i (k) when the mode exits the Hubble radius and use (13) to infer the amplitude of the power spectrum of gravitational waves at that time. We then follow the evolution of the gravitational wave power spectrum on super-Hubble scales for t > t i (k) using the equations of general relativistic perturbation theory.…”

“…For a mechanism to achieve the separation into three large and six small compact dimensions in the context of SGC see [1] (see however [12,13] for caveats), and for a natural dynamical mechanism arising from SGC to stabilize all of the moduli associated with the extra spatial dimensions see [18,19,20,21,22,23] (see also [24]). …”

S-duality in string gas cosmology

“…The stringy T-duality invariance is the key symmetry which may resolve the initial big-bang singularity of standard cosmology. Moreover, the naive dependence of the tree level annihilation cross section of winding strings on the number of dimensions might explain the hierarchy between the observed three and internal directions [1] (however it seems there are some caveats in that argument [4][5][6][7] and a more complete understanding of string thermodynamics in an expanding universe is needed). In late times, SGC offers a natural mechanism to stabilize extra dimensions as a result of a balance between negative winding and positive momentum pressures [8].…”