String theory, cosmology and varying constants

TL;DR

This paper reviews string-inspired models predicting correlated variations of fundamental constants, discusses their compatibility with experimental constraints, and emphasizes high-precision free-fall tests as the best probes of such variations.

Contribution

It introduces string-inspired models that incorporate spacetime variations of coupling constants consistent with current phenomenological constraints.

Findings

Large variations in the fine-structure constant are incompatible with existing constraints.

High-precision free-fall tests are the most effective way to detect varying constants.

String theory models can naturally incorporate varying constants without fine-tuning.

Abstract

In string theory the coupling ``constants'' appearing in the low-energy effective Lagrangian are determined by the vacuum expectation values of some (a priori) massless scalar fields (dilaton, moduli). This naturally leads one to expect a correlated variation of all the coupling constants, and an associated violation of the equivalence principle. We review some string-inspired theoretical models which incorporate such a spacetime variation of coupling constants while remaining naturally compatible both with phenomenological constraints coming from geochemical data (Oklo; Rhenium decay) and with present equivalence principle tests. Barring a very unnatural fine-tuning of parameters, a variation of the fine-structure constant as large as that recently ``observed'' by Webb et al. in quasar absorption spectra appears to be incompatible with these phenomenological constraints. Independently of any model, it is emphasized that the best experimental probe of varying constants are high-precision tests of the universality of free fall, such as MICROSCOPE and STEP.