Galileon models must obey a void-depth limit tied to expansion history to avoid force breakdowns, excluding ~60% of a linear parameterization's space by z less than or equal to 10.
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Extended Theories of Gravity
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abstract
Extended Theories of Gravity can be considered a new paradigm to cure shortcomings of General Relativity at infrared and ultraviolet scales. They are an approach that, by preserving the undoubtedly positive results of Einstein's Theory, is aimed to address conceptual and experimental problems recently emerged in Astrophysics, Cosmology and High Energy Physics. In particular, the goal is to encompass, in a self-consistent scheme, problems like Inflation, Dark Energy, Dark Matter, Large Scale Structure and, first of all, to give at least an effective description of Quantum Gravity. We review the basic principles that any gravitational theory has to follow. The geometrical interpretation is discussed in a broad perspective in order to highlight the basic assumptions of General Relativity and its possible extensions in the general framework of gauge theories. Principles of such modifications are presented, focusing on specific classes of theories like f (R)-gravity and scalar-tensor gravity in the metric and Palatini approaches. The special role of torsion is also discussed. The conceptual features of these theories are fully explored and attention is payed to the issues of dynamical and conformal equivalence between them considering also the initial value problem. A number of viability criteria are presented considering the post-Newtonian and the post-Minkowskian limits. In particular, we discuss the problems of neutrino oscillations and gravitational waves in Extended Gravity. Finally, future perspectives of Extended Gravity are considered with possibility to go beyond a trial and error approach.
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representative citing papers
Scalar-tensor gravity admits a frame-invariant perfect-fluid description with zero temperature, so that general relativity corresponds to diffusive equilibrium for both minimal and nonminimal theories.
Non-polynomial quasi-topological gravity models reproduce the standard thermal history, generate dynamical dark energy of geometric origin, and fit supernova, cosmic chronometer, and BAO data competitively with ΛCDM.
Regular black holes in the bulk of quasi-topological gravity drive a de Sitter inflationary phase on the brane at small scales, with e-fold number set by the ratio of black hole radius to higher-curvature scale.
First background-level constraints on Luciano-Saridakis entropic cosmology using CC, Pantheon+ with SH0ES, DESI DR2 BAO and compressed Planck data show robust fit, 2sigma exclusion of LambdaCDM, and potential Hubble tension alleviation.
In f(R,T) = R + F(T) gravity, nonlinear F makes the averaged modified term differ from F at averaged T, invalidating the common unity-ratio assumption and giving dust nonzero proper pressure.
Sign-switching dark energy with a transition at z_† fits recent DESI DR2, Planck CMB, and Pantheon+ data better than ΛCDM while raising the inferred Hubble constant and easing the Hubble tension.
Black bounce geometries exist in 2+1D f(R) gravity with scalar-nonlinear electrodynamics matter, including vanishing scalar curvature solutions whose viability is checked via scalaron mass and energy conditions.
f(Q) gravity yields Taub-de Sitter-like plane symmetric vacuum solutions, and quadratic models support isotropic slabs where maximum pressure is offset from the center with thickness and pressure increasing for negative α.
Fisher-matrix forecasts for LSST- and CMB-S4-like surveys show kSZ tomography tightens constraints on dark energy parameters w0 and wa by 15% and 32% while assessing detectability of perturbations for different sound speeds.
Authors add induced gravity and a polynomial potential to an F(R) model, transform to a two-field chiral cosmology, and find parameter choices that match ACT inflation data while yielding PBH masses compatible with dark matter.
Derives gauge-invariant perturbation equations for F(T, T_G) cosmology and provides physical interpretations for new contributions in each mode.
A non-canonical generalized Brans-Dicke theory admits background cosmological solutions matching Lambda CDM characteristics for constant, power-law, and exponential potentials, with dynamics distinct from other scalar-tensor models.
Quantum deformation of projective phase-space geometry induces a conformally deformed FLRW metric whose time-dependent corrections modify inflationary background equations, slow-roll parameters, and perturbations in a covariant manner.
Kaniadakis entropic cosmology modifies early-universe dynamics and is constrained by its predictions for Starobinsky inflation and the primordial tensor spectrum using current CMB and gravitational-wave observations.
Bouncing solutions in quadratic curvature gravity with a scalar field satisfy null, weak, and dominant energy conditions but violate the strong one when using the scalar-field energy-momentum tensor, while all four conditions are violated near the bounce in the effective tensor formulation.
Derives cosmological field equations and preliminary scalar perturbation equations for a non-minimally coupled Weyl-connection gravity model that introduces non-metricity to mimic dark sectors.
Off-diagonal Einstein solutions constructed via the anholonomic frame and connection deformation method can mimic f(R) gravity and dynamical dark energy effects inside GR and Lambda CDM cosmology.
Proposes gravitational redshift measurements with atomic clocks in varying density environments to constrain the parameter space of the environment-dependent dilaton model, focusing on regions with weak couplings complementary to existing tests.
Applies the f(R) = R - μ Rc (R/Rc)^p model and shape function b(r) = r log(r+1)/log(r0+1) to Morris-Thorne wormholes and analyzes energy conditions for non-exotic matter support.
Modified gravity theories supply viable mathematical frameworks for inflation, bounces, and dark energy eras that match observational data.
A review summarizing modified theories of gravity, their effects on compact objects, existing bounds from astrophysical observations, and the promise of future gravitational wave tests for strong-field gravity.
citing papers explorer
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How deep can a cosmic void be? Voids-informed theoretical bounds in Galileon gravity
Galileon models must obey a void-depth limit tied to expansion history to avoid force breakdowns, excluding ~60% of a linear parameterization's space by z less than or equal to 10.
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Frame invariant diffusive formulation of scalar-tensor gravity
Scalar-tensor gravity admits a frame-invariant perfect-fluid description with zero temperature, so that general relativity corresponds to diffusive equilibrium for both minimal and nonminimal theories.
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Cosmologically viable non-polynomial quasi-topological gravity: explicit models, $\Lambda$CDM limit and observational constraints
Non-polynomial quasi-topological gravity models reproduce the standard thermal history, generate dynamical dark energy of geometric origin, and fit supernova, cosmic chronometer, and BAO data competitively with ΛCDM.
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Cosmic Inflation From Regular Black Holes
Regular black holes in the bulk of quasi-topological gravity drive a de Sitter inflationary phase on the brane at small scales, with e-fold number set by the ratio of black hole radius to higher-curvature scale.
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Observational constraints on Luciano-Saridakis entropic cosmology
First background-level constraints on Luciano-Saridakis entropic cosmology using CC, Pantheon+ with SH0ES, DESI DR2 BAO and compressed Planck data show robust fit, 2sigma exclusion of LambdaCDM, and potential Hubble tension alleviation.
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Cosmological Averaging in Nonminimally Coupled Gravity
In f(R,T) = R + F(T) gravity, nonlinear F makes the averaged modified term differ from F at averaged T, invalidating the common unity-ratio assumption and giving dust nonzero proper pressure.
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Sign-Switching Dark Energy: Smooth Transitions with Recent DESI DR2 Observations
Sign-switching dark energy with a transition at z_† fits recent DESI DR2, Planck CMB, and Pantheon+ data better than ΛCDM while raising the inferred Hubble constant and easing the Hubble tension.
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Three dimensional black bounces in $f(R)$ gravity
Black bounce geometries exist in 2+1D f(R) gravity with scalar-nonlinear electrodynamics matter, including vanishing scalar curvature solutions whose viability is checked via scalaron mass and energy conditions.
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Static plane symmetric solutions in $f(Q)$ gravity
f(Q) gravity yields Taub-de Sitter-like plane symmetric vacuum solutions, and quadratic models support isotropic slabs where maximum pressure is offset from the center with thickness and pressure increasing for negative α.
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Probing Dark Energy Microphysics with kSZ Tomography
Fisher-matrix forecasts for LSST- and CMB-S4-like surveys show kSZ tomography tightens constraints on dark energy parameters w0 and wa by 15% and 32% while assessing detectability of perturbations for different sound speeds.
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Primordial black holes formation in inflationary $F(R)$ models with scalar fields
Authors add induced gravity and a polynomial potential to an F(R) model, transform to a two-field chiral cosmology, and find parameter choices that match ACT inflation data while yielding PBH masses compatible with dark matter.
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Gauge invariant perturbations of $F(T,T_G)$ Cosmology
Derives gauge-invariant perturbation equations for F(T, T_G) cosmology and provides physical interpretations for new contributions in each mode.
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Cosmological Dynamics of a Non-Canonical Generalised Brans-Dicke Theory
A non-canonical generalized Brans-Dicke theory admits background cosmological solutions matching Lambda CDM characteristics for constant, power-law, and exponential potentials, with dynamics distinct from other scalar-tensor models.
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Quantum-Deformed Phase-Space Geometry and Emergent Inflation in Effective Four-Dimensional Spacetime
Quantum deformation of projective phase-space geometry induces a conformally deformed FLRW metric whose time-dependent corrections modify inflationary background equations, slow-roll parameters, and perturbations in a covariant manner.
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Constraints on Kaniadakis Cosmology from Starobinsky Inflation and Primordial Tensor Perturbations
Kaniadakis entropic cosmology modifies early-universe dynamics and is constrained by its predictions for Starobinsky inflation and the primordial tensor spectrum using current CMB and gravitational-wave observations.
-
Energy conditions of bouncing solutions in quadratic curvature gravity coupled with a scalar field
Bouncing solutions in quadratic curvature gravity with a scalar field satisfy null, weak, and dominant energy conditions but violate the strong one when using the scalar-field energy-momentum tensor, while all four conditions are violated near the bounce in the effective tensor formulation.
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On Scalar Cosmological Perturbations in Non-Minimally Coupled Weyl Connection Gravity
Derives cosmological field equations and preliminary scalar perturbation equations for a non-minimally coupled Weyl-connection gravity model that introduces non-metricity to mimic dark sectors.
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Off-diagonal solutions in Einsteingravity modeling f(R) gravity and dynamical darkenergy vs Lambda CDM cosmology
Off-diagonal Einstein solutions constructed via the anholonomic frame and connection deformation method can mimic f(R) gravity and dynamical dark energy effects inside GR and Lambda CDM cosmology.
-
Prospect on constraining environment-dependent dilaton model from gravitational redshift measurements
Proposes gravitational redshift measurements with atomic clocks in varying density environments to constrain the parameter space of the environment-dependent dilaton model, focusing on regions with weak couplings complementary to existing tests.
-
Wormhole Modeling Supported by Non-Exotic Matter
Applies the f(R) = R - μ Rc (R/Rc)^p model and shape function b(r) = r log(r+1)/log(r0+1) to Morris-Thorne wormholes and analyzes energy conditions for non-exotic matter support.
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Modified Gravity Theories on a Nutshell: Inflation, Bounce and Late-time Evolution
Modified gravity theories supply viable mathematical frameworks for inflation, bounces, and dark energy eras that match observational data.
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Testing General Relativity with Present and Future Astrophysical Observations
A review summarizing modified theories of gravity, their effects on compact objects, existing bounds from astrophysical observations, and the promise of future gravitational wave tests for strong-field gravity.
- Photon Sphere and Shadow of a Perturbative Black Hole in $f(R,\mathcal{G})$ Gravity