arXiv subjects

Seyed Naseh Sajadi

Publications and source records attributed to Seyed Naseh Sajadi.

14 recordsLinked to original sources

arXiv2026

Black holes and phase transitions in Conformal Gravity

We explore the thermodynamic properties of static black holes in conformal gravities in arbitrary, even dimensions. In the spherically symmetric case, up to local Weyl rescalings, these spacetimes are static and unique, due to a Birkhoff's theorem that applies for all conformal gravities whose Lagrangians are complete contractions of $k$ Weyl tensors in dimension $D=2k$. The solutions can be chosen to be asymptotically locally (A)dS or flat, and the horizon can be extended to be a surface of arbitrary constant curvature, $γ$. We focus on the asymptotically AdS case, for which the cosmological constant at infinity is an integration constant, and compute the mass of the spacetime in terms of the diverse hair parameters which have a purely gravitational origin. We show that the first law of thermodynamics is satisfied, and study phase transitions in the canonical ensemble, showing the existence of zeroth-order black-hole/black-hole phase transitions for arbitrary dimension.

hep-th↗

arXiv2026

Generalized Polytropic Regular Black Holes in Arbitrary Dimensions

We investigate static, spherically symmetric regular black holes with anti-de Sitter (AdS) asymptotics in arbitrary spacetime dimensions. They are solutions of Einstein gravity, sourced by an anisotropic fluid whose radial pressure corresponds to vacuum energy, while the tangential pressure satisfies a generalized polytropic equation of state. By solving the Einstein field equations, we derive a generic class of asymptotically AdS black hole solutions and determine the conditions required for spacetime regularity. We then investigate the dynamical formation of these regular black holes within the thin-shell formalism, assuming a linear barotropic equation of state for the shell matter. Next, we study the thermodynamics of the regular AdS black holes in arbitrary dimensions by verifying the first law of black hole thermodynamics and the corresponding Smarr relation. We analyze the thermodynamic stability and phase structure of solutions in four, five, and six spacetime dimensions, demonstrating the existence of dimension-dependent phase transitions.

gr-qc↗

arXiv2026

Analytically Approximate Black Hole Solution to Higher Curvature Gravity

Higher-curvature gravity usually leads to very complicated field equations, which makes it hard to find analytical solutions. In this work, we obtain analytical charged black hole solutions in higher-curvature gravity by using black hole thermodynamics and the continued fraction expansion. We study the thermodynamics of static black holes using the first law of thermodynamics and the Smarr formula. We show that our results agree with those obtained by directly solving the field equations in Einstein gravity.

gr-qc↗

arXiv2026

Chiral Quartic Massive Gravity in Three Dimensions

We study New Massive Gravity (NMG) with Chern-Simons (CS), cubic, and quartic terms under the Compère-Song-Strominger (CSS) boundary conditions. By employing a semi-product of a Virasoro and a $U(1)$ Kac-Moody current algebra as the asymptotic symmetry algebra, we calculate the entropy of BTZ black holes via the degeneracy of states belonging to a Warped-CFT. Then, we compute the linearized energy excitations using the representations of the algebra $U(1)\times SL(2, R)_{R}$ and demonstrate that the energies of excitations are non-negative at two chiral points in the parameter space.

hep-th↗

arXiv2026

More on near-horizon charges black holes with gravitational hair in three dimensions

With the aim of continuing the exploration of near-horizon charges in higher-curvature gravity, searching for sectors leading to universal behaviors, we first provide a thorough revision and formulae of the covariant phase-space method applied to arbitrary gravitational theories containing up to quartic terms in the Riemann tensor in arbitrary dimension. These results can be applied in diverse setups, in particular in the context of $α'$ corrections to String Theory, where it is known that in Type II theories, the first correction to the Einstein-Hilbert Lagrangian goes as $α'^3 \mathcal{R}^4$. Then, we test these formulae for near horizon asymptotic symmetries of the rotating BTZ spacetime where the first law of black hole thermodynamics is consistently recovered. It was recently realized that a subset of these higher curvature gravities do admit black holes with gravitational hair, whose entropy can be microscopically accounted for, as is the case of New Massive Gravity. In this case, the four maximally symmetric vacua of the theory coincide, and the theory acquires an extra gauge symmetry when linearized around such a vacuum. We study the near-horizon asymptotic symmetries and compute the associated charges, both in the static and rotating hairy black holes, extending up to $\mathcal{R}^4$, a work that was previously done only up to a quadratic term. In order to allow for a continuous lecture on the work, we report the explicit expressions of the general Lagrangians in the appendices.

gr-qc↗

arXiv2025

Superradiance of Charged Static Black Hole in Cubic Gravity

Higher curvature gravity usually has complicated field equations, and solving them analytically is strenuous. In this work, we obtain an analytical charged black hole (BH) solution in higher curvature gravity using the thermodynamics of black holes and employing the continued fraction expansion. We investigate the thermodynamics of static black holes using the first law of thermodynamics and the Smarr formula in their proper form for Einstein-cubic gravity (ECG). Next, we obtain the thermodynamic quantities and show that our results are similar to those from solving the field equations. Then, we study the superradiance of the black hole using massless charged-scalar perturbations. We derive the superradiant conditions and compute the amplification factor through direct integration. We demonstrate how the amplification factor will change as a function of the black hole charge and frequency of the incident wave. We also show that a black hole mass and charge are decreasing in the superradiance region. Finally, we discuss superradiance as a consequence of black hole thermodynamics in ECG.

gr-qc↗

arXiv2025

Slowly rotating black hole solution to Einstein-Bel-Robinson gravity

We study slowly rotating black hole solutions in the Einstein-Bel-Robinson gravity (EBR) in four dimensions. At the leading order in the rotation parameter, the only modification with respect to the static case is the appearance of a non-vanishing $g_{tϕ}$ component. We construct approximate solutions to these equations and study how physical properties of the solutions, such as the angular velocity, photon sphere, black hole shadow, and innermost stable circular orbit, are modified, working to leading order in the coupling constant and the rotation parameter. Finally, we study the superradiance of a massive scalar wave scattering off slowly rotating black holes. Using direct integration, we derive the superradiant conditions and compute the energy flux through the event horizon and amplification factor. We demonstrate how the flux and amplification factor will change as a function of the black hole rotation and frequency of the incident wave.

gr-qc↗

arXiv2025

Physical Properties of Black Hole Solution in Einstein-Bel-Robinson Gravity

In this paper, we study the different properties of static spherically symmetric black hole solutions of Einstein-Bel-Robinson gravity (EBR), a modified four-dimensional theory of gravity quartic in curvature. We look at the orbit of massless and massive test bodies near a black hole, specifically computing the innermost stable circular orbit and photon sphere and finding them smaller than their Einstein counterparts in general relativity. Next, we obtain the deflection angle and shadow by an EBR black hole and find that both decreased compared to a non-rotating black hole in general relativity. We obtain a bound value for the coupling constant using the Shapiro time delay. Then, we explore the EBR black hole's lifetime and find that it decreases to Einstein's gravity. Quasinormal modes (QNMs) are computed using Padé averaged 6th order WKB method showing that increasing the coupling constant lowers the damping rate of ring-down gravitational waves (GWs). The oscillation frequency of scalar QNMs decreases with the coupling constant, whereas it increases for electromagnetic QNMs. We also provide analytical rigorous bound of greybody factor. We show that the coupling constant has a small effect on the greybody factor. Finally, correspondence between greybody factor and quasinormal modes is also considered.

gr-qc↗

arXiv2025

Constructing regular black holes from multi-polytropic equations of state

Regular black holes are imagined as solutions to Einstein's field equations, with no singularities, albeit characterized by the presence of an internal structure. With the intention not to use non-linear electrodynamics, we here propose to obtain new classes of solutions that can also satisfy the Tolman-Oppenheimer-Volkoff (TOV) equations, plus adding a non-zero core. Thus, we present regular black holes as solutions to the TOV equations using multipolytropic equations of state and investigate whether these solutions behave, tuning the underlying free parameters. Our analysis demonstrates that, within specific parameter ranges, repulsive gravity effects may occur in precise regions. Accordingly, black hole remnants are also investigated, showing that, under certain circumstances, they may turn into dark energy sources in view of the corresponding repulsive gravity effects, located outside the horizons. Moreover, quite remarkably, critical sets of parameters imply that solutions may exhibit transitions to regular repulsive relativistic compact objects from black hole behaviors. Finally, we explore the interpretation of these regular black hole solutions in terms of topological thermodynamic defects.

gr-qc↗

arXiv2025

Anisotropic Generalized Polytropic Spheres: Regular 3D Black Holes

We model gravitating relativistic 3D spheres composed of an anisotropic fluid in which the radial and transverse components of the pressure correspond to the vacuum energy and a generalized polytropic equation-of-state, respectively. By using the generalized Tolman-Oppenheimer-Volkoff (TOV) equation, and solving the complete system of equations for these anisotropic generalized polytropic spheres, for a given range of model parameters, we find three novel classes of asymptotically AdS black hole solutions with regular core. We show the regularity of the solutions using curvature scalars and the formalism of geodesic completeness. Then, using the eigenvalues of the Riemann curvature tensor, we consider the effects of repulsive gravity in the three static 3D regular black holes, concluding that their regular behavior can be explained as due to the presence of repulsive gravity near the center of the objects. Finally, we study the stability of the regular black holes under the flow of the energy through the Cauchy horizon.

gr-qc↗

arXiv2023

Analytically Approximation Solution to $R^{2}$ Gravity

In this paper, we obtain analytical approximate black hole solutions in the framework of $f(R)$ gravity and the absence of a cosmological constant. In this area, we apply the equations of motion of the theory to a spherically symmetric spacetime with one unknown function and derive black hole solutions without any constraints on the Ricci scalar. To do so, first, we obtain the near horizon and asymptotic solutions and then use both of them to obtain a complete solution by utilizing a continued-fraction expansion. Finally, we investigate the stability of the solutions by employing the thermodynamics and quasi-normal modes.

gr-qc↗

arXiv2022

More on Boundary Conditions for Warped AdS3 in GMG

In this paper, we study the Aggrawal, Ciambelli, Detournay, and Somerhausen (ACDS) boundary conditions ref[42] for Warped AdS$_3$ (WAdS$_3$) in the framework of General Massive Gravity (GMG) in the quadratic ensemble. We construct the phase space, the asymptotic structure, and the asymptotic symmetry algebra. We know that the global surface charges are finite, but not integrable, and also we find the conditions to make them integrable. In addition, to confirm that the phase space has the same symmetry as that of Warped Conformal Field Theory (WCFT), we compare the bulk entropy of Warped BTZ (WBTZ) black holes with the number of states belonging to a WCFT.

hep-th↗

arXiv2021

New Chiral Generalized Minimal Massive Gravity

We study the generalized minimal massive gravity (GMMG) in Compere, Song and Strominger boundary conditions employing a semi-product of a Virasoro and a $\hat{u}(1)$ Kac-Moody current algebras as the asymptotic symmetry algebra. We calculate the entropy of BTZ black holes via the degeneracy of states belonging to a Warped-CFT. We compute the linearized energy excitations by using the representations of the algebra $\hat{u}(1) \times SL(2,R)_R$ and show that energies of excitations are non-negative at (two) chiral points in the parameter space. At these special points, the charge algebra is described by either Virasoro algebra or Kac-Moody algebra. We also consider some special limits of the GMMG theory which correspond to $2+1$-dimensional massive gravity theories such as new massive and minimal massive gravity theories.

hep-th↗