An entropy functional yields the Brown-York tensor via conjugate momentum projection, unifying null and timelike hypersurfaces and reproducing equations in scalar-tensor gravity.
Introduction to Modified Gravity and Gravitational Alternative for Dark Energy
9 Pith papers cite this work. Polarity classification is still indexing.
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abstract
We review various modified gravities considered as gravitational alternative for dark energy. Specifically, we consider the versions of $f(R)$, $f(G)$ or $f(R,G)$ gravity, model with non-linear gravitational coupling or string-inspired model with Gauss-Bonnet-dilaton coupling in the late universe where they lead to cosmic speed-up. It is shown that some of such theories may pass the Solar System tests. On the same time, it is demonstrated that they have quite rich cosmological structure: they may naturally describe the effective (cosmological constant, quintessence or phantom) late-time era with a possible transition from decceleration to acceleration thanks to gravitational terms which increase with scalar curvature decrease. The possibility to explain the coincidence problem as the manifestation of the universe expansion in such models is mentioned. The late (phantom or quintessence) universe filled with dark fluid with inhomogeneous equation of state (where inhomogeneous terms are originated from the modified gravity) is also described.
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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.
Models of interacting bosonic dark energy and fermionic dark matter in Einstein-scalar-Gauss-Bonnet gravity with exponential and power-law potentials are dynamically analyzed and constrained by observational data, showing consistency with LambdaCDM.
Interacting scalar fields coupled to Gauss-Bonnet gravity yield viable dark energy and dark matter models that match Pantheon+ and DES supernova data while preferring over LambdaCDM at high redshifts with Roman mocks.
Thesis summarizing an upper limit of 0.12 eV on the neutrino mass sum, bias calibration via CMB lensing cross-correlations, and tighter limits plus stronger normal-ordering preference in non-phantom dynamical dark energy models.
In f(R,T²) gravity, the growth index decreases with redshift and approaches standard matter-dominated values at early times, with fσ8 predictions staying within ±2σ observational bounds for viable coupling parameters α and n=1/2 or 1/4.
Ghost-free non-local gravity fits Pantheon+, DESI, and H(z) data but fails with added CMB, while generalized exponential F(R) gravity outperforms Lambda CDM across all datasets including CMB.
Classical solutions for F(R) gravity in Bianchi type I cosmology with barotropic matter are derived via the Hamiltonian formalism in two gauges.
Modified gravity theories supply viable mathematical frameworks for inflation, bounces, and dark energy eras that match observational data.
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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.