The Confining Heterotic Brane Gas: A Non-Inflationary Solution to the Entropy and Horizon Problems of Standard Cosmology

TL;DR

The paper tackles the horizon and entropy problems without inflation by proposing a brane-gas–driven cosmology in which six extra dimensions are orbifolded and a weak inter-brane confinement governs a two-phase evolution. Phase 1 yields isotropic bulk expansion while the projected energy density on the orbifold planes remains constant; Phase 2 contracts the extra dimensions and, near the self-dual radius, string-momentum modes stabilize the radion, after which the familiar 3D expansion resumes with energy density set by the string scale. This sequence enlarges the spatial volume without requiring inflation and can implicitly address the entropy problem, though it leaves flatness and perturbation generation as open issues requiring further mechanisms. The framework also predicts potential observational signatures, such as cosmic strings, and hints at a possible link between the late-time cosmological constant and string-scale physics.

Abstract

We propose a mechanism for solving the horizon and entropy problems of standard cosmology which does not make use of cosmological inflation. Crucial ingredients of our scenario are brane gases, extra dimensions, and a confining potential due to string gas effects which becomes dominant at string-scale brane separations. The initial conditions are taken to be a statistically homogeneous and isotropic hot brane gas in a space in which all spatial dimensions are of string scale. The extra dimensions which end up as the internal ones are orbifolded. The hot brane gas leads to an initial phase (Phase 1) of isotropic expansion. Once the bulk energy density has decreased sufficiently, a weak confining potential between the two orbifold fixed planes begins to dominate, leading to a contraction of the extra spatial dimensions (Phase 2). String modes which contain momentum about the dimensions perpendicular to the orbifold fixed planes provide a repulsive potential which prevents the two orbifold fixed planes from colliding. The radii of the extra dimensions stabilize, and thereafter our three spatial dimensions expand as in standard cosmology. The energy density after the stabilization of the extra dimensions is of string scale, whereas the spatial volume has greatly increased during Phases 1 and 2, thus leading to a non-inflationary solution of the horizon and entropy problems.