A Resolution of the Cosmological Singularity with Orientifolds

TL;DR

Motivates and addresses the cosmological initial singularity by proposing a horizon-based resolution using negative-tension orientifold planes. Embedding this scenario in Type II string theory, the authors identify a de Sitter orientifold singularity and construct a three-region spacetime (contracting, orientifold boundary, expanding) connected across a cosmological horizon, enabling a non-singular transition. They validate the orientifold interpretation with D-brane probes and compute a past-to-future vacuum amplitude that yields a Hawking-like thermal spectrum in the far future. The work offers a concrete, string-theory–consistent mechanism for singularity-free cosmology with horizon thermodynamics and potential observational implications.

Abstract

We propose a new cosmological scenario which resolves the conventional initial singularity problem. The space-time geometry has an unconventional time-like singularity on a lower dimensional hypersurface, with localized energy density. The natural interpretation of this singularity in string theory is that of negative tension branes, for example the orientifolds of type II string theory. Space-time ends at the orientifolds, and it is divided in three regions: a contracting region with a future cosmological horizon; an intermediate region which ends at the orientifols; and an expanding region separated from the intermediate region by a past cosmological horizon. We study the geometry near the singularity of the proposed cosmological scenario in a specific string model. Using D-brane probes we confirm the interpretation of the brane singularity as an orientifold. The boundary conditions on the orientifolds and the past/future transition amplitudes are well defined. Assuming the trivial vacuum in the past, we derive a thermal spectrum in the future.

Figures (3)

• Figure 1: De Sitter orientifold as the boundary of space--time. We shall see that the associated geometry develops a cosmological horizon that allows the smooth transition form a contracting to an expanding cosmological phase.
• Figure 2: The potential $\mathcal{V}_D$ seen by a $D$--brane probe in the geometry created by the $O-\overline{O}$ system, for $d=1$. The potential repels the $D$--brane from both the $O$ and the $\overline{O}$--plane. The vertical dashed line represents the location of the cosmological horizon.
• Figure 3: The effective temperature $2\pi{\mathcal{T}}$ as a function of $\omega$.