Scalar-Tensor Theories in Gravitational Dynamics
Summary
Scalar-tensor theories generalise Einstein’s general relativity by introducing one or more scalar fields that couple nonminimally to the metric. These scalar degrees of freedom can mediate long-range forces, modify cosmological expansion and affect the structure and dynamics of compact objects. The most general four-dimensional scalar-tensor theory with second-order field equations is Horndeski gravity, while beyond-Horndeski and degenerate higher-order scalar-tensor (DHOST) theories evade the Ostrogradsky instability through hidden constraints. In weak-field regimes, scalar-tensor models reproduce the parametrised post-Newtonian phenomenology of general relativity, but in strong-field or high-curvature settings they can exhibit novel effects such as spontaneous scalarisation of neutron stars, dipolar gravitational-wave emission and screening mechanisms that restore Einstein gravity in dense environments. Disformal and conformal transformations link different frames and reveal deep interconnections between apparently distinct theories. Applications range from inflationary cosmology and late-time cosmic acceleration to precision tests of gravity with pulsar timing arrays, ground-based interferometers and space experiments. Current theoretical efforts focus on classifying healthy higher-order interactions, understanding dynamical screening processes and confronting predictions with astrophysical and cosmological observations.
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Scalar-Tensor Theories in Gravitational Dynamics publication trend
The graph below shows the total number of articles in scalar-tensor theories in gravitational dynamics across all publications each year (not limited to Nature Index journals).
Technical terms
Scalar-tensor theory: A class of gravitational theories coupling a scalar field to the curvature of spacetime, extending general relativity.
Spontaneous scalarisation: A nonperturbative phenomenon where compact objects develop a scalar field profile above a critical compactness threshold.
Disformal transformation: An invertible field redefinition mixing the metric and scalar field derivatives, altering the apparent degrees of freedom.
Ostrogradsky instability: A ghost-like instability arising in higher-derivative theories lacking sufficient constraints to eliminate extra modes.
Nonmetricity: A property of a connection where the metric is not preserved under parallel transport, introducing additional scalar degrees of freedom.
Post-Keplerian parameters: Phenomenological quantities describing relativistic corrections in pulsar binary orbits beyond classical Keplerian terms.
References
- Gravitational-wave tests of general relativity with ground-based detectors and pulsar-timing arrays. Living Reviews in Relativity (2025).
- A new pulsar timing model for scalar-tensor gravity with applications to PSR J2222-0137 and pulsar-black hole binaries. Astronomy & Astrophysics (2024).
- New dynamical degrees of freedom from invertible transformations. Journal of High Energy Physics (2023).
- Spatially covariant gravity with nonmetricity. European Physical Journal C (2024).
- Constraining Nonperturbative Strong-Field Effects in Scalar-Tensor Gravity by Combining Pulsar Timing and Laser-Interferometer Gravitational-Wave Detectors. Physical Review X (2017).
- Extended scalar-tensor theories of gravity. Journal of Cosmology and Astroparticle Physics (2016).
- Degenerate higher order scalar-tensor theories beyond Horndeski up to cubic order. Journal of High Energy Physics (2016).
- Healthy degenerate theories with higher derivatives. Journal of Cosmology and Astroparticle Physics (2016).
- Horndeski: beyond, or not beyond?. Journal of Cosmology and Astroparticle Physics (2016).
- The Confrontation between General Relativity and Experiment. Living Reviews in Relativity (2014).
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