On the Cosmological Implications of the String Swampland

TL;DR

This paper examines how two proposed string Swampland criteria constrain cosmological models, finding tensions with inflation but compatibility with certain dark energy models, and predicts a future phase transition of the universe.

Contribution

It applies the string Swampland criteria to cosmology, analyzing their implications for inflation and dark energy, and predicts a universe phase transition if these criteria hold.

Findings

Inflation models are generally in tension with the criteria.

Certain quintessence dark energy models satisfy the criteria and current observations.

A universe phase transition is predicted within a few Hubble times if criteria are valid.

Abstract

We study constraints imposed by two proposed string Swampland criteria on cosmology. These criteria involve an upper bound on the range traversed by scalar fields as well as a lower bound on $|\nabla_{\phi} V|/V$ when $V >0$. We find that inflationary models are generically in tension with these two criteria. Applying these same criteria to dark energy in the present epoch, we find that specific quintessence models can satisfy these bounds and, at the same time, satisfy current observational constraints. Assuming the two Swampland criteria are valid, we argue that the universe will undergo a phase transition within a few Hubble times. These criteria sharpen the motivation for future measurements of the tensor-to-scalar ratio $r$ and the dark energy equation of state $w$, and for tests of the equivalence principle for dark matter.