arXiv subjects

H. S. Ramadhan

Publications and source records attributed to H. S. Ramadhan.

27 records · Page 2Linked to original sources

arXiv2021

Thermodynamics of EiBI-AdS black holes with global monopole

It is well-known that black hole can be endowed with topological charge coming as a result of phase transition(s) in the early universe. It has also recently been revealed that such object might exist in one of the modified theory of gravity called the Eddington-inspired-Born-Infeld (EiBI) theory. Here we shall investigate the the possibility of phase transitions, and in general thermodynamic phenomena, of EiBI-anti-de Sitter (AdS) black hole with (topologically-charged) global monopole. This is the first work that applies the full Euclidean action formalism in this model. We provide a counterterm to cancel inifinities and argue that it is the most suitable among other possibilities. Our investigation reveals that the state variables obtained are found to obey the first law of the black hole mechanics and the Smarr's law for black holes with $Λ\neq0$. Related to the second feature, we obtained the forbidden range of parameter space for EiBI AdS black hole with global monopole, which corresponds to $\frac{-(1 - Δ)}{Λ} \leq κ\leq -\frac{1}{Λ}$. The dependencies on $Λ$, the EiBI constant $κ$ and the global monopole charge $η$ of the state variables and state functions obtained manifests in a Schwarzchild AdS-like phase transition for black holes with parameters below the lower bound of the forbidden range, and could also manifest in a Schwarzschild-flat like phase behavior for black holes with parameters above the higher bound of the forbidden range.

gr-qc↗

arXiv2021

Bound orbits around charged black holes with exponential and logarithmic electrodynamics

We present exact black hole solutions endowed with magnetic charge coming from exponential and logarithmic nonlinear electrodynamics (NLED). Classically, we analyze the null and timelike geodesics, all of which contain both the bound and the scattering orbits. Using the effective geometry formalism, we found that photon can have nontrivial stable (both circular and non-circular) bound orbits. The noncircular bound orbits for the one-horizon case mostly take the form of precessed ellipse. For the extremal and three-horizon cases we find many-world orbits where photon crosses the outer horizon but bounces back without hitting the true (or second, respectively) horizon, producing the epicycloid and epitrochoid paths. Semiclassically, we investigate their Hawking temperature, stability, and phase transition. The nonlinearity enables black hole stability with smaller radius than its RN counterpart. However, for very-strong nonlinear regime, the thermodynamic behavior tends to be Schwarzschild-like.

gr-qc↗

arXiv2021

$2.6 M_\odot$ Compact Object and Neutron Stars within Eddington-Inspired Born-Infeld Theory of Gravity

In the context of whether a massive compact object recently observed in the GW190814 event is a neutron star (NS) or not, we have studied the role of the parameters $κ$ and $Λ_c$ of the Eddington-inspired Born-Infeld (EiBI) gravity theory on the NS mass-radius relation, moment of inertia, and tidal deformability. The results are compared to recent observation constraints extracted from the analysis of NS observation data. The NS core equation of state (EoS) is calculated using the relativistic mean-field model with the G3 parameter set. In the hyperon sector, the SU(3) and hyperon potential depths are used to determine the hyperon coupling constants. For the inner and outer crusts, we use the crust EoS from Miyatsu et al. (2013). We also maintain the sound speed to not exceed $c$/$\sqrt{3}$ at high densities. We have found that, in general, the NS mass significantly depends on the value of $κ$, and the radius $R$ is sensitive to the value of $Λ_c$. Moreover, as $Λ_c$ is equal to zero or less than the accepted bound of the cosmological constant, the NS within the EiBI theory is compatible with observation constraints, including $2.0 M_\odot$ mass, canonical radius $R_{1.4 M_{\odot}}$, moment of inertia, and tidal deformation. Our investigation also reveals that the $2.6 M_\odot$ mass compact object and current observational constraint of canonical radius $R_{1.4 M_{\odot}}$ can simultaneously be satisfied only when the $Λ_c$ value is unphysically too large and negative. Therefore, within the spesific EoS employed in this work, we conclude that the secondary object with $2.6 M_\odot$ observed in the GW190814 event is not likely a static (or a slow-rotating) NS within the EiBI gravity theory.

gr-qc↗

arXiv2021

An Ultra-compact Object from Semi-classical Gravity

In a recent report, Carballo-Rubio [1] utilizes the semi-classical theory of gravity to obtain a generalized Tolman-Oppenheimer-Volkoff (TOV) equation. This model has a new coupling constant $l_p$ implying two different modified TOV equation forms characterized by the sign of $p'$. The negative branch reduces to the ordinary GR-TOV in the limit of $l_p\to0$, while the positive one does not. In the positive branch, Carballo-Rubio was able to find the exact solutions using the constant-$λ$ trick. In this work, we investigate whether this theory's negative branch can also provide a different feature of the ultra-compact object compared to those obtained from the GR-TOV equation. We study ultra-compact objects with an isotropically ideal fluid matter where we use a simple but physically motivated equation of state $ρ=p/w+ρ_0$ with $w$=1 and $w$=1/3. In general, we obtain that the range of $l_p$ is very restricted and must not be equal to $r_c$. Here $r_c$ is the starting point of integration located at the center of the star. While $l_p$ should be set to be much larger than Planck length $L_\text{Pl}$. Consequently, the mass-radius curves for the various value of $l_p$ for both $w$ cases are still indistinguishable from the standard GR-TOV results. Hence from the negative branch of $p'(r)$, the additional free parameter $l_p$ does not provide a significant effect compared to the standard GR-TOV equation results, even though $l_p$ is not in the limit of $l_p\to0$ anymore. Therefore, similar to the conclusion in Ref. [3] with GR theory that the ultra-compact objects from negative branch of semi-classical gravity with a linear equation of state are unable to generate demanding gravitational echoes.

gr-qc↗

arXiv2020

Geodesic of nonlinear electrodynamics and stable photon orbits

We study the geodesics of charged black holes in polynomial Maxwell Lagrangians, a subclass of models within the nonlinear electrodynamics (NLED). Specifically, we consider black holes in Kruglov, power-law, and Ayon-Beato-Garcia models. Our exploration on the corresponding null bound states reveals that photon can orbit the extremal black holes in stable radii outside the corresponding horizon. The reason behind this is the well-known theorem that a photon in a NLED background propagates along its own {\it effective} geometry. This nonlinearity is able to shift the local minimum of the effective potential away from its corresponding outer horizon. For the null scattering states we obtain corrections to the weak deflection angle off the black holes. We rule out the power-law model to be physical since its deflection angle does not reduce to the Schwarzschild in the limit of the vanishing charge.

gr-qc↗

arXiv2019

Coleman-de Luccia Tunneling Wave Function

We revisit the famous Coleman-de Luccia formalism for decay of false vacuum in gravitational theory. Since the corresponding wave function is time-independent we argue that its instanton's interpretation as the decay rate probability is problematic. We instead propose that such phenomenon can better be described by the Wheeler-de Witt's wave function. To do so, the Hamilton-Jacobi formalism is employed in the WKB approximation. The scalar and gravitational fields can then be treated as a two-dimensional effective metric. For a particular case of dS-to-dS tunneling, we calculated the wave function and found that it depends only on the potential of the false, and not on the true, vacuum; reminiscent of, though in totally different formalism with, the Hawking-Moss result. In general, this alternative approach might have significant impact on the study of very early universe and quantum cosmology.

gr-qc↗

arXiv2018

Compactness, masses and radii of compact stars within the Eddington-inspired Born-Infeld theory

We investigate the compactness, masses and radii of realistic neutron stars (NSs) and quark stars (QSs) within the Eddington-inspired Born-Infeld (EiBI) theory of gravity, and the energy conditions of their apparent equation of states (EOSs). We show that the maximum compactness and maximum masses constraints extracted from the recent pulsars masses and radii observations can provide the upper and lower limits of $κ$ value of EiBI theory. By using BSP parameter set of relativistic mean field (RMF) model to describe NS core EOS including hyperons, it can be estimated that $ (2.7 \le κ_g \le 7.9) \times$ $\rm 10^{-2} m^5 kg^{-1} s^{-2}$. If we use confined-isospin-density-dependent-mass (CIDDM) model with additional scalar Coulomb term with QSK046 parameter set to describe QS EOS, we can obtain lesser value i.e., $ (1.6 \le κ_g\le 2.2) \times$ $\rm 10^{-2} m^5 kg^{-1} s^{-2}$. We have also observed that for large $κ$ values, mass-radius relations of NSs and QSs do not exceed causality restrictions because the compactness of NS and QS are saturated after passing certain large critical value. This observation is in agreement with the results obtained in Ref.\cite{Delsate12} for the case of pressure-less stars. We have also found that if $κ$ larger than certain non-zero value, the CIDDM with vector Coulomb model prediction of QS can reach the maximum mass $\gtrsim$ 2 $M_\odot$. We have found also that if the NS or QS EOS becomes stiffer, the upper limit of $κ$ from compactness constraint becomes smaller and lower limit of $κ$ from maximum masses constraint becomes larger. We also observe that the uncertainty of the systematic in canonical mass compact stars radii measurements data can affect the $κ$ range. It is also shown that the non-physical apparent EOS of NSs and QSs can satisfy the energy conditions.

gr-qc↗

arXiv2017

Neutron stars in the braneworld within the Eddington-inspired Born-Infeld gravity

We propose the disappearance of "the hyperon puzzle" in neutron star (NS) by invoking two new-physics prescriptions: modified gravity theory and braneworld scenario. By assuming that NS lives on a $3$-brane within a $5d$ empty AdS bulk, gravitationally governed by Eddington-inspired Born-Infeld (EiBI) theory, the field equations can be effectively cast into the usual Einstein's with "apparent" anisotropic energy-momentum tensor. Solving the corresponding brane-TOV equations numerically, we study its mass-radius relation. It is known that the appearance of finite brane tension $λ$ reduces the compactness of the star. The compatibility of the braneworld results with observational constraints of NS mass and radius can be restored in our model by varying the EiBI's coupling constant, $κ$. We found that within the astrophysically-accepted range of parameters ($0<κ<6\times10^6\text{m}^2$ and $λ\gg1~ \text{MeV}^4$) the NS can have mass $\sim2.1~ \text{M}_\odot$ and radius $\sim10$ km.

astro-ph.CO↗

arXiv2016

Hyperons in neutron stars within Eddington-inspired Born-Infeld theory of gravity

We investigate the mass-radius relation of neutron star (NS) with hyperons inside its core by using the Eddington-inspired Born-Infeld (EiBI) theory of gravity. The equation of state of the star is calculated by using the relativistic mean field model under which the standard SU(6) prescription and hyperons potential depths are used to determine the hyperon coupling constants. We found that, for $4\times 10^{6}~\rm{m^2}~\lesssim~κ\lesssim~6\times 10^{6}~\rm{m^2}$, the corresponding NS mass and radius predicted by the EiBI theory of gravity is compatible with observational constraints of maximum NS mass and radius. The corresponding $κ$ value is also compatible with the $κ$ range predicted by the astrophysical-cosmological constraints. We also found that the parameter $κ$ could control the size and the compactness of a neutron star.

astro-ph.SR↗