Exacerbating the cosmological constant problem with interacting dark energy

TL;DR

This paper demonstrates that interacting dark energy models can worsen the cosmological constant problem and suggests that future observations could rule out certain anthropic solutions based on string theory vacua.

Contribution

It shows that interacting dark energy models exacerbate the cosmological constant problem and argues that current string theory vacua counts are insufficient to support these models as solutions.

Findings

Interacting dark energy models can significantly worsen the cosmological constant problem.

Current string theory vacua counts are too small to support these models as solutions.

Future gamma-ray observations could potentially rule out these models and the anthropic solution.

Abstract

Future cosmological surveys will probe the expansion history of the universe and constrain phenomenological models of dark energy. Such models do not address the fine-tuning problem of the vacuum energy, i.e. the cosmological constant problem (c.c.p.), but can make it spectacularly worse. We show that this is the case for 'interacting dark energy' models in which the masses of the dark matter states depend on the dark energy sector. If realised in nature, these models have far-reaching implications for proposed solutions to the c.c.p. that require the number of vacua to exceed the fine-tuning of the vacuum energy density. We show that current estimates of the number of flux vacua in string theory, $N_{\rm vac} \sim {\cal O}(10^{272,000})$, is far too small to realise certain simple models of interacting dark energy \emph{and} solve the cosmological constant problem anthropically. These models admit distinctive observational signatures that can be targeted by future gamma-ray observatories, hence making it possible to observationally rule out the anthropic solution to the cosmological constant problem in theories with a finite number of vacua.