Cosmological Solutions of Horava-Witten Theory

TL;DR

This paper explores inhomogeneous cosmological solutions within Horava-Witten theory, revealing new BPS domain wall configurations that depend on both time and the orbifold coordinate, advancing understanding of strongly coupled heterotic string cosmology.

Contribution

It introduces novel inhomogeneous cosmological solutions in Horava-Witten theory, including a generalized solution with a nontrivial Ramond-Ramond scalar, derived via separation of variables.

Findings

Presented two new inhomogeneous solutions involving BPS domain walls.

Extended solutions to include a nontrivial Ramond-Ramond scalar.

Demonstrated solutions depend on both time and orbifold coordinate.

Abstract

We discuss simple cosmological solutions of Horava-Witten theory describing the strongly coupled heterotic string. At energies below the grand-unified scale, the effective theory is five- not four-dimensional, where the additional coordinate parameterizes a S^1/Z_2 orbifold. Furthermore, it admits no homogeneous solutions. Rather, the vacuum state, appropriate for a reduction to four-dimensional supersymmetric models, is a BPS domain wall. Relevant cosmological solutions are those associated with this BPS state. In particular, such solutions must be inhomogeneous, depending on the orbifold coordinate as well as on time. We present two examples of this new type of cosmological solution, obtained by separation of variables rather that by exchange of time and radius coordinate applied to a brane solution, as in previous work. The first example represents the analog of a rolling radii solution with the radii specifying the geometry of the domain wall. This is generalized in the second example to include a nontrivial ``Ramond-Ramond'' scalar.