The status of varying constants: a review of the physics, searches and implications

TL;DR

This review discusses recent efforts to test the stability of fundamental constants, highlighting their potential to reveal new physics, current constraints, and future prospects with advanced observational facilities.

Contribution

It provides a comprehensive overview of recent observational tests of varying fundamental constants and discusses future experimental plans to explore new physics.

Findings

Preference for variations of α and μ at about two-sigma level

Inconsistencies between different data subsets suggest possible systematics

Strong constraints on violations of the Weak Equivalence Principle

Abstract

The observational evidence for the recent acceleration of the universe demonstrates that canonical theories of cosmology and particle physics are incomplete---if not incorrect---and that new physics is out there, waiting to be discovered. A key task for the next generation of laboratory and astrophysical facilities is to search for, identify and ultimately characterize this new physics. Here we highlight recent developments in tests of the stability of nature's fundamental couplings, which provide a direct handle on new physics: a detection of variations will be revolutionary, but even improved null results provide competitive constraints on a range of cosmological and particle physics paradigms. A joint analysis of all currently available data shows a preference for variations of $\alpha$ and $\mu$ at about the two-sigma level, but inconsistencies between different sub-sets (likely due to hidden systematics) suggest that these statistical preferences need to be taken with caution. On the other hand, these measurements strongly constrain Weak Equivalence Principle violations. Plans and forecasts for forthcoming studies with facilities such as ALMA, ESPRESSO and the ELT, which should clarify these issues, are also discussed, and synergies with other probes are briefly highlighted. The goal is to show how a new generation of precision consistency tests of the standard paradigm will soon become possible.