Typically, the existence of such scalars are in tension with experimental bounds. Scalars coupled to gravity appear in many UV completions of GR such as string theory and other higher-dimensional models, but the cosmological constant problem and the nature of dark energy, two modern mysteries that GR alone cannot account for, are driving a vigorous research effort into infra-red scalar–tensor theories, with much of the effort focussing on light scalars (with cosmologically relevant masses) coupled to gravity. These scalar–tensor theories of gravity are particularly prevalent, and are natural extensions of general relativity. One of the simplest possible options is to include a new scalar degree of freedom. Several interesting and viable Lorentz-violating theories exist that may have some insight for the quantum gravity problem (Blas and Lim 2015), and, similarly, healthy theories of massive spin-2 particles have recently been constructed (de Rham 2014).Īn alternative to these approaches is to introduce new fields that couple to gravity. From a theoretical standpoint, GR is the unique low-energy theory of a Lorentz-invariant massless spin-2 particle (Weinberg 1965), and any modification must necessarily break one of these assumptions. 2016, for some compendia of popular models) but all have one thing in common: they break one of the underlying assumptions of general relativity. The zoo of modified gravity theories is both vast and diverse (see Clifton et al. To test the predictions of any theory requires alternatives with differing predictions and, for this reason, alternative theories of gravity have a history that is almost as rich and varied as that of GR itself. 2016a, b), its predictions have been perfectly consistent with our observations. From Eddington’s pioneering measurement of light bending by the Sun in 1919 to the first detection of gravitational waves by the LIGO/Virgo consortium in 2015 (Abbott et al. Since its publication in 1915, Einstein’s theory of general relativity (GR) has withstood the barrage of observational tests that have been thrown at it over the last century.
The review ends by discussing the future prospects for constraining screened modified gravity models further using upcoming and planned experiments. The simplest of these are well constrained by astrophysical probes, but there are currently few reported bounds for theories with higher powers of R. We also summarize the current bounds on f( R) models that exhibit the chameleon mechanism (Hu and Sawicki models).
The coupling of chameleons to photons is tightly constrained but the symmetron coupling has yet to be explored. Symmetron models are constrained well by astrophysical and laboratory tests, but there is a desert separating the two scales where the model is unconstrained.
Presently, commonly studied chameleon models are well-constrained but less commonly studied models have large regions of parameter space that are still viable. The purpose of this review is to summarize the present state-of-the-art searches for screened scalars coupled to matter, and to translate the current bounds into a single parametrization to survey the state of the models. The results of these searches are often presented using different parametrizations, which can make it difficult to compare constraints coming from different probes. The highly-nonlinear nature of screening mechanisms means that they evade classical fifth-force searches, and there has been an intense effort towards designing new and novel tests to probe them, both in the laboratory and using astrophysical objects, and by reinterpreting existing datasets. On other scales, the environmental nature of the screening means that such scalars may be relevant. Theories of modified gravity, where light scalars with non-trivial self-interactions and non-minimal couplings to matter-chameleon and symmetron theories-dynamically suppress deviations from general relativity in the solar system.