Non-BPS Brane Cosmology

TL;DR

This paper explores cosmology on a non-BPS D3-brane within a 10D supergravity background, demonstrating that such branes can naturally inflate and estimating the string scale from observational normalization.

Contribution

It introduces a bi-metric effective theory for non-BPS brane cosmology, showing inflation occurs naturally and connecting decay dynamics to observable string scale estimates.

Findings

Non-BPS branes can inflate without self-gravity.

The effective theory below compactification is bi-metric.

String scale estimated at around 10^14 GeV.

Abstract

We study cosmology on a BPS D3-brane evolving in the 10D SUGRA background describing a non-BPS brane. Initially the BPS brane is taken to be a probe whose dynamics we determine in the non-compact non-BPS background. The cosmology observed on the brane is of the FRW type with a scale factor $S(\tau)$. In this mirage cosmology approach, there is no self-gravity on the brane which cannot inflate. Self-gravity is then included by compactifying the background space-time. The low energy effective theory below the compactification scale is shown to be bi-metric, with matter coupling to a different metric than the geometrically induced metric on the brane. The geometrical scale factor on the brane is now $S(\tau) a(\tau)$ where $a(\tau)$ arises from brane self-gravity. In this non-BPS scenario the brane generically inflates. We study the resulting inflationary scenario taking into account the fact that the non-BPS brane eventually decays on a time-scale much larger than the typical inflationary time-scale. After the decay, the theory ceases to be bi-metric and COBE normalization is used to estimate the string scale which is found to be of order $10^{14}$ GeV.