Scalar-Tensor Gravity and Cosmological Models

Scalar-tensor gravity represents a class of extensions to Einstein’s theory of general relativity in which the gravitational interaction is mediated not only by the spacetime metric but also by one or more scalar fields. These additional degrees of freedom allow the effective gravitational “constant” to vary in space and time, offering a natural framework to address cosmological puzzles such as the acceleration of the universe, the nature of dark energy and the early-time inflationary epoch. In the Jordan frame, the scalar field couples directly to the Ricci curvature, whereas in the Einstein frame it is recast as a minimally coupled field with an effective potential and non-standard matter couplings. This dual description helps to clarify phenomena such as screening mechanisms—chameleon, symmetron and Vainshtein effects—that reconcile modified gravity with stringent local tests. The most general class of scalar-tensor theories with second-order field equations is encapsulated by Horndeski gravity, which encompasses many popular models of dark energy and inflation while avoiding instabilities. Cosmological studies typically adopt the homogeneous and isotropic Friedmann–Lemaître–Robertson–Walker metric to derive dynamical equations for the scale factor and scalar field. Perturbation theory within this framework yields predictions for the growth of large-scale structure and gravitational-wave propagation, providing critical tests of alternative gravity theories. Scalar-tensor models also admit rich phenomenology in strong-field regimes, influencing black-hole hair theorems, neutron-star structure and the propagation speed of gravitational waves. By combining observational probes across cosmic, astrophysical and laboratory scales, researchers continue to refine the allowed parameter space of these theories and to explore their implications for the ultimate unification of gravity with the other fundamental forces.

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Scalar-Tensor Gravity and Cosmological Models publication trend

The graph below shows the total number of articles in scalar-tensor gravity and cosmological models across all publications each year (not limited to Nature Index journals).

Technical terms

Scalar-tensor gravity: A gravitational theory in which the metric tensor and one or more scalar fields jointly mediate gravitational interactions.

Jordan frame: A formulation of scalar-tensor theories where the scalar field multiplies the Ricci curvature in the action, leading to variable effective gravitational strength.

Einstein frame: A conformally related representation in which the scalar is minimally coupled to curvature but non-minimally coupled to matter, with an explicit scalar potential.

Horndeski gravity: The most general scalar-tensor theory yielding second-order field equations, avoiding Ostrogradsky instabilities and unifying many dark-energy models.

Parametrized Post-Newtonian formalism: A systematic expansion of the metric and field equations in weak-field regimes, characterised by parameters that quantify deviations from general relativity.

Friedmann–Lemaître–Robertson–Walker (FLRW) cosmology: A class of homogeneous and isotropic spacetimes used to describe the large-scale expansion history of the universe.

References

1. Multi-scalar theories of gravity with direct matter couplings and their parametrized post-Newtonian parameters. Journal of Cosmology and Astroparticle Physics (2023).
2. First-order thermodynamics of scalar-tensor cosmology. Journal of Cosmology and Astroparticle Physics (2022).
3. Fluid nature constrains Horndeski gravity. General Relativity and Gravitation (2023).