Current and future constraints on Bekenstein-type models for varying couplings

Abstract

Astrophysical tests of the stability of dimensionless fundamental couplings, such as the finestructure constant α and the proton-to-electron mass ratio µ, are an optimal probe of new physics. There is a growing interest in these tests, following indications of possible spacetime variations at the few parts per million level. Here we make use of the latest astrophysical measurements, combined with background cosmological observations, to obtain improved constraints on Bekensteintype models for the evolution of both couplings. These are arguably the simplest models allowing for α and µ variations, and are characterized by a single free dimensionless parameter, ζ, describing the coupling of the underlying dynamical degree of freedom to the electromagnetic sector. In the former case we find that this parameter is constrained to be |ζα| < 4.8 × 10 −6 (improving previous constraints by a factor of 6), while in the latter (which we quantitatively compare to astrophysical measurements for the first time) we find ζµ = (2.7 ± 3.1) × 10 −7 ; both of these are at the 99.7% confidence level. For ζα this constraint is about 20 times stronger than the one obtained from local Weak Equivalence Principle tests, while for ζµ it is about 2 orders of magnitude weaker. We also discuss the improvements on these constraints to be expected from the forthcoming ESPRESSO and ELT-HIRES spectrographs, conservatively finding a factor around 5 for the former and around 50 for the latter.

Current and future constraints on extended Bekenstein-type models for a varying fine-structure constant

“…found for the case of the standard Bekenstein model [8,9], the Webb et al data set has a mild (less than two standard deviations) statistical preference for non-zero couplings, but the other sub-sets and the combined data set are compatible with the null result. This is further illustrated in Fig.…”

“…The cosmological implications of these models were first explored by Sandvik, Barrow and Magueijo [4], who also obtained some qualitative constraints on the model. Stronger constraints, benefiting both from additional data and from a more detailed statistical analysis, were later obtained in [8,9].…”

Fisher matrix forecasts for astrophysical tests of the stability of the fine-structure constant

“…This can be quantified through the dimensionless Eötvös parameter η, which describes the level of violation of the WEP [1]. One can show that for the class of models we are considering the Eötvös parameter and the dimensionless coupling ζ are simply related by [1,13,14] η ≈ 10 −3 ζ 2 ; (8) we note that while this relation is correct for the simplest canonical scalar field models we will consider in what follows, it is somewhat model-dependent (for example, it is linear rather than quadratic in ζ for Bekenstein-type models [7]).…”

“…Moreover, the results of these tests-whether they are detections of variations or null results-have a range of additional cosmological implications. They provide competitive constraints on Weak Equivalence Principle (WEP) violations [1,6,7] and, in the more natural scenarios where the same dynamical degree of freedom is responsible both for the dark energy and the α variation, can also be used in combination with standard cosmological observables to constrain the dark energy equation of state [8,9] and indeed to reconstruct its redshift dependence [10,11].…”

Varying fundamental constants and dark energy in the ESPRESSO era

“…Typically they do not directly constrain cosmological parameters, but they may still indirectly help constrain them by breaking degeneracies in the overall parameter space (which will include cosmological and particle physics parameters). The simplest example of this class are the Bekenstein type models 22,23 . In some cases, such as the Dirac-Born-Infeld type models, astrophysical measurements of α may be the only possible observational probe that, given already available constraints, can distinguish these models from ΛCDM 24 .…”