nonGR

NonGR, short for non-general relativity, is an umbrella term for theoretical frameworks that extend, modify, or depart from Einstein's theory of general relativity (GR). Researchers use nonGR models to address open questions such as the nature of cosmic acceleration, the behavior of gravity in strong-field regimes, and the pursuit of a quantum theory of gravity. While GR remains the standard description of gravitation in most regimes, nonGR theories explore alternative dynamics and couplings that could manifest under certain conditions or scales.

Theoretical approaches in the nonGR landscape often involve changes to the gravitational action, inclusion of additional fields, higher-order derivatives, or extra dimensions. Common families include scalar-tensor theories (where a scalar field couples to gravity), f(R) gravity (nonlinear functions of the Ricci scalar), vector-tensor and tensor-tensor models, massive gravity, and certain braneworld or Horava-Lifshitz constructions. Many models recover GR in a suitable limit through screening mechanisms such as Chameleon, Vainshtein, or Symmetron effects, preserving agreement with solar-system tests while allowing deviations on cosmological scales.

Observationally, nonGR predictions are tested through solar-system experiments, binary pulsars, gravitational wave observations, and cosmological surveys. The parameterized post-Newtonian formalism provides a framework for comparing weak-field predictions, while gravitational wave data constrain propagation speed, dispersion, and additional polarizations. The detection of GW170817 and its electromagnetic counterpart, for example, placed strong constraints on the speed of gravity, ruling out broad classes of nonGR models.

Status and challenges remain: nonGR theories must be theoretically consistent and empirically viable, explaining new phenomena without conflicting with precision tests. The term is routinely used to describe deviations from GR rather than a single, unified theory.