Rotating black holes with primary scalar hair in beyond Horndeski gravity produce shadows whose diameter increases for negative Q and whose distortion increases for positive Q, with EHT bounds on M87* restricting but not ruling out the (a, Q) parameter space.
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f(T) teleparallel gravity and cosmology
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abstract
Over the past decades, the role of torsion in gravity has been extensively investigated along the main direction of bringing gravity closer to its gauge formulation and incorporating spin in a geometric description. Here we review various torsional constructions, from teleparallel, to Einstein-Cartan, and metric-affine gauge theories, resulting in extending torsional gravity in the paradigm of f(T) gravity, where f(T) is an arbitrary function of the torsion scalar. Based on this theory, we further review the corresponding cosmological and astrophysical applications. In particular, we study cosmological solutions arising from f(T) gravity, both at the background and perturbation levels, in different eras along the cosmic expansion. The f(T) gravity construction can provide a theoretical interpretation of the late-time universe acceleration, and it can easily accommodate with the regular thermal expanding history including the radiation and cold dark matter dominated phases. Furthermore, if one traces back to very early times, a sufficiently long period of inflation can be achieved and hence can be investigated by cosmic microwave background observations, or alternatively, the Big Bang singularity can be avoided due to the appearance of non-singular bounces. Various observational constraints, especially the bounds coming from the large-scale structure data in the case of f(T) cosmology, as well as the behavior of gravitational waves, are described in detail. Moreover, the spherically symmetric and black hole solutions of the theory are reviewed. Additionally, we discuss various extensions of the f(T) paradigm. Finally, we consider the relation with other modified gravitational theories, such as those based on curvature, like f(R) gravity, trying to enlighten the subject of which formulation might be more suitable for quantization ventures and cosmological applications.
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New exact charged black hole solutions in (2+1)D f(Q) gravity with cubic form yield a novel AdS solution without GR counterpart, with multiple horizons, stable thermodynamics, and stable photon orbits.
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.
In 5D f(T, T_G) gravity, thick branes develop splitting and internal structure controlled by the coupling, while supporting a normalizable chiral fermion zero mode and modified resonant Kaluza-Klein states due to the torsional Gauss-Bonnet term.
Quintessence potential decreases monotonically with redshift while kinetic energy crosses zero near z=1, with negative values at intermediate redshifts being statistical artifacts from derivative reconstruction.
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.
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.
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.
Derives gauge-invariant perturbation equations for F(T, T_G) cosmology and provides physical interpretations for new contributions in each mode.
In f(Q) symmetric teleparallel gravity, accelerating expansion is geometric; dynamical analysis of f(Q)=Q+αQ² yields five critical points with stable de Sitter (P4) and matter-dominated (P5) attractors.
Symmetric teleparallel gravity has the same number of degrees of freedom as general relativity, confirmed via its Hamiltonian formulation after deriving generalized extrinsic geometry relations.
In a thick braneworld model with f(T) = T + α T², the parameter α induces brane splitting and alters the decay rates of quasinormal modes, with two numerical methods agreeing on the low-overtone spectrum.
All viable NGR models, including TEGR and 1P-H&S, exhibit divergences in torsion scalars at local horizons, obstructing black hole interpretations.
Power-law and logarithmic coupling models in covariant f(Q) gravity reproduce radiation, matter, and dark energy eras through dynamical systems analysis of critical points and their stability.
This review traces the history of dynamical dark energy, presents the no-go theorem against single-field crossing of w = -1, and surveys viable Quintom constructions including multi-field models and modified gravity in light of DESI DR2 hints.
citing papers explorer
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Rotating Black Holes with Primary Scalar Hair: Shadow Signatures in Beyond Horndeski Gravity
Rotating black holes with primary scalar hair in beyond Horndeski gravity produce shadows whose diameter increases for negative Q and whose distortion increases for positive Q, with EHT bounds on M87* restricting but not ruling out the (a, Q) parameter space.
-
3-dimensional charged black holes in $f({Q})$ gravity
New exact charged black hole solutions in (2+1)D f(Q) gravity with cubic form yield a novel AdS solution without GR counterpart, with multiple horizons, stable thermodynamics, and stable photon orbits.
-
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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Thick branes and fermion localization in five-dimensional $f(T,T_G)$ gravity
In 5D f(T, T_G) gravity, thick branes develop splitting and internal structure controlled by the coupling, while supporting a normalizable chiral fermion zero mode and modified resonant Kaluza-Klein states due to the torsional Gauss-Bonnet term.
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Model-Independent Reconstruction of Quintessence Potential and Kinetic Energy from DESI DR2 and Pantheon+ Supernovae
Quintessence potential decreases monotonically with redshift while kinetic energy crosses zero near z=1, with negative values at intermediate redshifts being statistical artifacts from derivative reconstruction.
-
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.
-
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.
-
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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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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Cosmology in symmetric teleparallel gravity and its dynamical system
In f(Q) symmetric teleparallel gravity, accelerating expansion is geometric; dynamical analysis of f(Q)=Q+αQ² yields five critical points with stable de Sitter (P4) and matter-dominated (P5) attractors.
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Extrinsic geometry and Hamiltonian analysis of symmetric teleparallel gravity
Symmetric teleparallel gravity has the same number of degrees of freedom as general relativity, confirmed via its Hamiltonian formulation after deriving generalized extrinsic geometry relations.
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Quasinormal modes of the thick braneworld in $f(T)$ gravity
In a thick braneworld model with f(T) = T + α T², the parameter α induces brane splitting and alters the decay rates of quasinormal modes, with two numerical methods agreeing on the low-overtone spectrum.
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On black holes in new general relativity
All viable NGR models, including TEGR and 1P-H&S, exhibit divergences in torsion scalars at local horizons, obstructing black hole interpretations.
-
Dynamical analysis of the covariant $f(Q)$ gravity models
Power-law and logarithmic coupling models in covariant f(Q) gravity reproduce radiation, matter, and dark energy eras through dynamical systems analysis of critical points and their stability.
-
The Quintom theory of dark energy after DESI DR2
This review traces the history of dynamical dark energy, presents the no-go theorem against single-field crossing of w = -1, and surveys viable Quintom constructions including multi-field models and modified gravity in light of DESI DR2 hints.