The accelerated expansion of the universe is usually attributed to a mysterious dark energy, but there’s another conceivable explanation: modified gravity. Unmodified gravity—that is, Einstein’s general relativity— satisfactorily accounts for the dynamics of the solar system, where precision measurements can be made without the confounding influence of dark matter. Nor have any violations been detected in one of general relativity’s principal ingredients, the strong equivalence principle, which posits that inertial mass and gravitational mass are identical.
But those observational constraints are not ineluctable. In particular, a class of gravitational theories called Galileon models can also pass them. In 2012 Lam Hui and Alberto Nicolis of Columbia University devised a cosmic test that could refute or confirm the models. Their test hinges on the models’ central feature: an additional scalar field that couples to mass. The coupling can be characterized by a charge-like parameter, Q. For most cosmic objects, Q...