Astrophysical and local constraints on string theory: runaway dilaton models

TL;DR

This paper investigates the runaway dilaton models inspired by string theory, using cosmological, astrophysical, and laboratory data to constrain the model parameters and test their consistency with standard cosmology.

Contribution

It provides updated constraints on the dilaton coupling to matter and dark sector, improving previous bounds and testing the viability of runaway dilaton models against current data.

Findings

Constraints on dilaton coupling to baryonic matter improved sixfold.

Constraints on dilaton coupling to dark sector improved twofold.

Dark sector couplings of order unity are excluded at one sigma.

Abstract

One of the clear predictions of string theory is the presence of a dynamical scalar partner of the spin-2 graviton, known as the dilaton. This will violate the Einstein Equivalence Principle, leading to a plethora of possibly observable consequences which is a cosmological context include dynamical dark energy and spacetime variations of nature's fundamental constants. The runaway dilaton scenario of Damour, Piazza and Veneziano is a particularly interesting class of string theory inspired models which can in principle reconcile a massless dilaton with experimental data. Here we use the latest background cosmology observations, astrophysical and laboratory tests of the stability of the fine-structure constant and local tests of the Weak Equivalence Principle to provide updated constraints on this scenario, under various simplifying assumptions. Overall we find consistency with the standard $\Lambda$CDM paradigm, and we improve the existing constraints on the coupling of the dilaton to baryonic matter by a factor of six, and to the dark sector by a factor of two. At the one sigma level the current data already excludes dark sector couplings of order unity, which would be their natural value.