Swampland Conjectures and Cosmological Expansion

TL;DR

Swampland conjectures impose constraints on cosmological models, challenging cyclic cosmology by limiting the duration of expansion, and raising questions about the validity of string theory or the conjectures themselves.

Contribution

The paper analyzes the tension between swampland conjectures and cyclic cosmology, highlighting potential modifications to conjectures or fundamental theories.

Findings

SCs restrict the duration of current expansion to about one e-folding.

Cyclic cosmology requires approximately 94 e-foldings before turnaround.

Possible resolutions include weakening the range conjecture or questioning string theory.

Abstract

Swampland conjectures (SCs) of string theory require that a constant cosmological constant $\Lambda$ be replaced by a time-dependent scalar-field quintessence with constrained parameters. The constraints limit the duration of the present expansion era because, although the SCs may be fulfilled at the present time, they will be violated at a finite time in the future allowing only an order-one number of e-foldings. In contrast, cyclic cosmology requires $\sim94$ e-foldings of the present universe before turnaround from expansion to contraction. This presents a dilemma to the original SCs. One possibility is that one of the SCs, the range conjecture, be significantly weakened. A second possibility, difficult to believe, is that cyclic cosmology vastly overestimates the number of e-foldings. A third possibility, which is the least disfavoured, is that string theory is not the correct theory of quantum gravity.