Dark energy, the swampland, and the equivalence principle

Carsten van de Bruck,Cameron C. Thomas

doi:10.1103/physrevd.100.023515

Carsten van de Bruck, Cameron C. Thomas

Open Access

https://doi.org/10.1103/physrevd.100.023515

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Abstract

It has recently been argued that string theory does not admit de Sitter vacua. This would imply that the current accelerated expansion of the universe is not driven by a cosmological constant (or vacuum energy) but by other means such as a quintessential scalar field. Such a scalar field is in general expected to couple to at least some matter species, such as dark matter. Cosmological observations already constrain such dark matter couplings strongly. We argue that there are a number of interesting scenarios to be explored, such as coupling functions which possess a minimum at finite field values. In these theories, the effective gravitational coupling between dark matter particles grows with time and are consistent with observations of the anisotropies in the cosmic microwave background radiation and large scale structures. We argue that such couplings might also help to alleviate the tension between the swampland conjectures and the properties of the quintessential potential. Observational signatures of violations of the equivalence principle in the dark sector are expected in the non-linear regime on intermediate or small scales.

Highlights

It is well known that the current accelerated expansion of the Universe, described very successfully by the cold dark matter (DM) model with a cosmological constant (ΛCDM), is difficult to construct from the bottom up in particle physics models beyond the standard model
These theories can be reconciled with observations of the cosmic microwave background radiation (CMB) as well as large scale structures (LSS), but they predict an equation of state different from the cosmological constant and an effective gravitational constant between DM particles considerably larger than Newton’s constant GN
Current observations of the Universe are in excellent agreement with the ΛCDM model

Summary

INTRODUCTION

It is well known that the current accelerated expansion of the Universe, described very successfully by the cold dark matter (DM) model with a cosmological constant (ΛCDM), is difficult to construct from the bottom up in particle physics models beyond the standard model. In this paper we argue that one class of models, which could be very relevant for the swampland picture, deserves further studies In these models the fifth force between DM particles switches on at late times, when the quintessence field starts to roll down its potential energy. These theories can be reconciled with observations of the cosmic microwave background radiation (CMB) as well as large scale structures (LSS), but they predict an equation of state different from the cosmological constant (and can mimic DE with an equation of state w ≲ −1) and an effective gravitational constant between DM particles considerably larger than Newton’s constant GN.

THE SIMPLEST MODELS AND AN EXTENSION

Dark energy and the least coupling principle

Attractor mechanism in the very early Universe

DERIVATIVE COUPLINGS

CONCLUSIONS