Cosmological Chameleons, String Theory and the Swampland

TL;DR

This paper explores transient cosmological acceleration mechanisms inspired by string theory, demonstrating limitations on the duration of acceleration and highlighting challenges in embedding these models within string theory frameworks.

Contribution

It introduces a novel string-inspired chameleon mechanism for transient acceleration and analyzes its limitations and constraints within effective field theories.

Findings

Transient acceleration lasts at most one e-fold without transplanckian fields.

Heavy state density effects weaken over cosmological expansion.

Embedding these models in string theory faces significant theoretical challenges.

Abstract

We study a scenario with a transient phase of cosmological acceleration that could potentially be realized in asymptotic corners of String Theory moduli space. A very steep scalar potential is temporarily stabilized by the effect of a nonzero density of heavy states, leading to acceleration, in what amounts to a cosmological version of the Chameleon mechanism. The density of heavy states is diluted by cosmological expansion, weakening their effect. After roughly one $e$-fold their effect can no longer stabilize the potential, and the accelerating phase ends. We also study a scenario where there is no potential and the transient acceleration is achieved by the counterbalancing effects of light and heavy towers of states. In both cases we show that it is not possible to obtain more than $\mathcal{O}(1)$ $e$-folds without transplanckian field excursions. We also discuss the general EFT constraints on these models and explore a number of first attempts at concrete embeddings of the scenario in String Theory. These all turn out to face significant challenges.