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.
Constant spatial torsion induces spin-dependent effective masses for neutrinos that modify both frequencies and amplitudes of flavor oscillations in the QFT description, with largest effects at low momentum when torsion scale matches neutrino masses.
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.
Boson stars in teleparallel gravity with nonminimal coupling show negative energy density and energy-condition violation in excited states, with EMRI waveforms potentially detectable by LISA.
In symmetric teleparallel f(Q) gravity with nonminimal EM-nonmetricity coupling, the distance duality relation is dynamically violated, yielding a generalized formula relating observational distances to the Hubble rate.
Rotating traversable wormholes in f(R,T) gravity are supported by anisotropic fluid satisfying null and strong energy conditions in the slow-rotation approximation, with particle dynamics and gravitational lensing analyzed.
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.
In ghost-free two-scalar f(R) thick branes, internal structure produces no narrow real-axis tensor resonances; quasinormal modes are broad with quality factors 0.9-1.9.
Three-form dark energy with Gaussian potential is fitted to multi-probe cosmological data and shows mild statistical preference over ΛCDM only in heavily tensioned dataset combinations.
Observational constraints on teleparallel cubic Galileon cosmologies with quadratic and exponential potentials show viability for late-time acceleration, with the fixed b1 quadratic case competitive under AIC but not BIC.
Axion EDE model fitted to Planck/ACT/SPT CMB, DESI BAO, and JWST UV luminosity function data yields H0 = 71.58 ± 1.05 km s^{-1} Mpc^{-1}, reduces H0 tension to 1.0 sigma, and improves Δχ^{2}_tot = -18.26 over Λ CDM.
Quadratic f(Q) gravity adds an H^4 term to the Friedmann equation and introduces a time-dependent G_eff that suppresses linear growth and halo abundance, offering a modified-gravity route to easing the S8 tension.
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.
The running vacuum model derives dynamical vacuum energy from QFT in curved spacetime, using H^4 terms for inflation and H^2 terms for dark energy while G evolves logarithmically.
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Late-Time Cosmology and Structure Formation in Quadratic $f(Q)$ Gravity
Quadratic f(Q) gravity adds an H^4 term to the Friedmann equation and introduces a time-dependent G_eff that suppresses linear growth and halo abundance, offering a modified-gravity route to easing the S8 tension.