15 research outputs found
A new cosmological model with quadratic expansion
We present a new cosmological model capable of reproducing late-time
acceleration, i.e. by supporting certain parametrization of the Hubble parameter. By using
observational data from Hubble, Pantheon, and Baryonic Acoustic Oscillations
(BAO) dataset, we investigate the constraints on the proposed quadratic Hubble
parameter . This proposal caused the Universe to transition from its
decelerated phase to its accelerated phase. Further, the current constrained
value of the deceleration parameter from the combined Hubble+Pantheon+BAO
dataset is , which indicates that the Universe is
accelerating. We also analyze the evolution of energy density, pressure, and
EoS parameters to infer the Universe's accelerating behavior. Finally, we use a
stability analysis with linear perturbations to assure the model's stability.Comment: Physics of the Dark Universe published versio
Quasinormal Modes of Black holes in gravity
In this work, we have studied the quasinormal modes of a black hole in a
model of the type in
gravity by using a recently introduced method known as Bernstein
spectral method and confirmed the validity of the method with the help of well
known Pad\'e averaged higher order WKB approximation method. Here we have
considered scalar perturbation and electromagnetic perturbation in the black
hole spacetime and obtained the corresponding quasinormal modes. We see that
for a non-vanishing nonmetricity scalar , quasinormal frequencies in
scalar perturbation are greater than those in electromagnetic perturbation
scenarios. On the other hand, the damping rate of gravitational waves is higher
for electromagnetic perturbation. To confirm the quasinormal mode behaviour, we
have also investigated the time domain profiles for both types of
perturbations.Comment: 15 pages, 6 figures. Published versio
A New Diagnostic of Dark Energy in General Relativity Theory
In this paper, we propose a new parametrization of dark energy based on the
diagnostic tool behavior. For this purpose, we investigate a functional
form of the that predicts the popular dark energy dynamical models,
namely phantom and quintessence. We also found the famous cosmological constant
for specified values of the model's parameters. We employed the Markov Chain
Monte Carlo approach to constrain the cosmological model using Hubble, Pantheon
samples, and BAO datasets. Finally, we used observational constraints to
investigate the characteristics of dark energy evolution and compare our
findings to cosmological predictions.Comment: The European Physical Journal C accepted versio
Quasinormal modes and greybody factors of symmergent black hole
Symmergent gravity is an emergent gravity framework in which gravity emerges
guided by gauge invariance, accompanied by new particles, and reconciled with
quantum fields. In this paper, we perform a detailed study of the quasinormal
modes and greybody factors of the black holes in symmergent gravity. Its
relevant parameters are the quadratic curvature term and the vacuum
energy parameter . In our analyses, effects of the both parameters are
investigated. Our findings suggest that, in both positive and negative
direction, large values of the parameter on the quasinormal modes
parallel the Schwarzschild black hole. Moreover, the quasinormal model spectrum
is found to be sensitive to the symmergent parameter . We contrast the
asymptotic iteration and WKB methods in regard to their predictions for the
quasinormal frequencies, and find that they differ (agree) slightly at small
(large) multipole moments. We analyze time-domain profiles of the
perturbations, and determine the greybody factor of the symmergent black hole
in the WKB regime. The symmergent parameter and the quadratic
curvature term are shown to impact the greybody factors
significantly. We provide also rigorous limits on greybody factors for scalar
perturbations, and reaffirm the impact of model parameters.Comment: 23 pages, 16 figures. Published versio
Quasinormal Modes and Optical Properties of 4-D black holes in Einstein Power-Yang-Mills Gravity
This paper explores the impact of the Yang-Mills charge parameter and the
exponent term on a D black hole solution in the Einstein Power-Yang-Mills
theory. Through an investigation of the massless scalar quasinormal mode
spectrum, black hole shadow, and emission rate, we have determined that the
effects of these two parameters are opposite. Specifically, the Yang-Mills
charge parameter causes an increase in the real quasinormal frequencies with a
correspondingly smaller damping rate. It also results in a smaller black hole
shadow and a lower evaporation rate.Comment: 13 pages, 9 figure
Impact of energy-momentum conservation violation on the configuration of compact stars and their GW echoes
This work investigates the impacts of energy-momentum conservation violation
on the configuration of strange stars constraint with gravitational wave (GW)
event GW190814 as well as eight recent observations of compact objects. The
gravitational wave echoes from these interesting classes of compact objects are
also calculated. To describe the matter of strange stars, we have used two
different equations of state (EoSs): first an ad-hoc exotic EoS, the stiffer
MIT Bag model and next realistic CFL phase of quark matter EoS. We choose
Rastall gravity as a simple model with energy-momentum conservation violation
with a set of model parameter values. Our results show that this gravity theory
permits stable solutions of strange stars and the resulting structures can
foster GW echoes. We illustrate the implication of the gravity theory and found
that the negative values of the Rastall parameter result in more compact
stellar configurations and lower GW echo frequency. With an increase in the
Rastall parameter, both the compactness of the stellar configurations and echo
time decrease. It is worth mentioning here that with the chosen set of some
probable strange star candidates from observational data and also in light of
GW 190814, we have evaluated the radii of stellar models. Also, the GW echo
frequencies associated with strange stars are found to be in the range of
kHz for both cases.Comment: 8 figures and 4 table
Deflection angle and quasinormal modes of a de Sitter black hole in gravity
This work is intended to investigate the influence of the boundary term on
the bending angle of light for a static spherically symmetric black hole in
modified gravity. To this end, we use the
Ishihara et al. method which allows us to derive the deflection angle of light
for an observer and source at finite distances from a lens object in a
non-asymptotically flat spacetime. This method interprets the viewpoint of the
receiver. The obtained deflection angle becomes divergent at far distances from
the lens object, which is due to the non-asymptotically flat spacetime.
However, the divergence of the deflection angle can be controlled with the
boundary term parameter . For small values of the parameter this
divergence can be minimized within the finite range of the source and observer.
We also calculate the quasinormal modes of axial gravitational perturbations in
the background of the black hole using the Pad\'e averaged sixth order WKB
approximation method. We observed that the boundary term of the model has
notable influence on the quasinormal modes of the black hole. It is seen that
for the physically perceptible quasinormal mode frequencies from the black
hole, the value of the boundary term parameter should be less than
. This result in fact supports the outcome of our deflection angle
analysis.Comment: 12 pages, 4 figure
A new
In this paper, we have introduced a new f(R) gravity model as an attempt to have a model with more parametric control, so that the model can be used to explain the existing problems as well as to explore new directions in physics of gravity, by properly constraining it with recent observational data. Here basic aim is to study the properties of Gravitational Waves (GWs) in this new model. In f(R) gravity metric formalism, the model shows the existence of scalar degree of freedom as like other f(R) gravity models. Due to this reason, there is a scalar mode of polarization of GWs present in the theory. This polarization mode exists in a mixed state, of which one is transverse massless breathing mode with non-vanishing trace and the other is massive longitudinal mode. The longitudinal mode being massive, travels at speed less than the usual tensor modes found in General Relativity (GR). Moreover, for a better understanding of the model, we have studied the potential and mass of scalar graviton in both Jordan frame and Einstein frame. This model can pass the solar system tests and can explain primordial and present dark energy. Also, we have put constraints on the model. It is found that the correlation function for the third mode of polarization under certain mass scale predicted by the model agrees well with the recent data of Pulsar Timing Arrays. It seems that this new model would be useful in dealing with different existing issues in the areas of astrophysics and cosmology
Weak gravitational lensing and shadow cast by rotating black holes in axionic Chern-Simons theory
We investigate the impact of the axionic coupling parameter on the bending
angle of light and the shadow cast by slowly rotating black holes in
Chern-Simons modified gravity. We utilize the Ishihara \etal method to derive
the deflection angle of light for an observer and source located at finite
distances from a lens object in an asymptotically flat spacetime, using the
Gauss-Bonnet theorem. The deflection angle exhibits an increasing trend up to a
certain point, followed by a decrease as a function of the impact parameter,
with the presence of the axion matter field causing the observed increase.
Additionally, we calculate the Einstein ring radius as a direct application of
the weak deflection angle. We also investigate the effect of the axion matter
field on the time delay of light and analyze its impact on the shadow cast by
slowly rotating black holes. Our findings reveal a significant effect of the
axionic coupling parameter on the black hole's shadow.Comment: 18 pages and 9 figures. arXiv admin note: text overlap with
arXiv:2211.0241