New solutions with accelerated expansion in string theory

TL;DR

This work constructs explicit, UV-complete string-theoretic backgrounds that realize accelerated expansion by combining a small set of stress-energy sources with approximate scaling symmetry. It develops two complementary routes: (i) scaling FRW solutions in which multiple moduli and axions roll under exponential potentials to yield discrete sequences of equations of state $w=-1+\frac{2}{3K}$, including accumulations near $-1$ and near $-\tfrac{1}{3}$, and (ii) finite-density driven acceleration from networks of domain walls or branes, generating $1<K<2$ while remaining perturbatively controlled. A key technical advance is the flux-averaging construction that yields an effective rank-$p_{\rm eff}$ flux, allowing consistent scaling in toroidal orientifolds, and the lifting of many RR axions via $H_3$ flux to maintain control at large $D$. The results have potential implications for dark energy phenomenology and for holographic descriptions of cosmology, offering a tractable set of explicit models to study observable signatures and dual interpretations in a UV-complete setting.

Abstract

We present concrete solutions with accelerated expansion in string theory, requiring a small, tractable list of stress energy sources. We explain how this construction (and others in progress) evades previous no go theorems for simple accelerating solutions. Our solutions respect an approximate scaling symmetry and realize discrete sequences of values for the equation of state, including one with an accumulation point at w=-1 and another accumulating near w=-1/3 from below. In another class of models, a density of defects generates scaling solutions with accelerated expansion. We briefly discuss potential applications to dark energy phenomenology, and to holography for cosmology.

Figures (1)

• Figure 1: In order to preserve sufficient fluxes to lift our axions, we consider a geometry which generalizes the toroidal model discussed earlier. Here, the orientifold planes (purple) act freely on the homology cycles, leaving invariant fluxes on odd or even combinations of them.