28 research outputs found
Shadow of black hole surrounded by magnetized plasma: Axion-plasmon cloud
By exploiting the extreme environment of the black hole (BH) as a potential
place for axion-photon interaction, we use an axion-producing model of the
magnetized plasma to study the shadow of an asymptotically flat rotating BH
immersed into an axion-plasmon cloud. By aiming to reveal footprints of axion
in the dark shadow of BH, we in this paper explore the influence of the fixed
axion-plasmon background on the motion of incident photons around the rotating
BH. Under some free parameter settings, we find that axion-plasmon cloud around
rotating BH affects the shape and size of the shadow in such a way that its
role is distinguishable from non-magnetized plasma and standard vacuum
solutions. By being limited to high rotation BH, we show that the size of the
BH shadow increases as the axion-plasmon coupling gets strong. Interestingly,
our analysis indicates that as the mass of axion gets heavier, it can leave a
subtle imprint of itself on the shadow. Conversely, in the non-rotating limit
(Schwarzschild), by recovering the spherical symmetry of the shadow shape of
BH, its size decreases. In coordination with the trend of change in shadow
size, the investigation of the energy emission from the BH surrounded by the
magnetized plasma shows that the maximal energy emission rate from the rotating
BH in the presence of axion-plasmon cloud increases compared to the
non-magnetized plasma and the vacuum solutions. Subsequently, by relaxing the
rotation, the axion-plasmon cloud causes a decrease in the energy emission rate
from the BH.Comment: 23 pages, 15 figures, for publication in " Nuclear Physics B
Probing the Lorentz Symmetry Violation Using the First Image of Sagittarius A*: Constraints on Standard-Model Extension Coefficients
Thanks to unparalleled near-horizon images of the shadows of Messier 87*
(M87*) and Sagittarius A* (Sgr A*) delivered by the Event Horizon Telescope
(EHT), two amazing windows opened up to us for the strong-field test of the
gravity theories as well as fundamental physics. Information recently published
from EHT about the Sgr A*'s shadow lets us have a novel possibility of
exploration of Lorentz symmetry violation (LSV) within the Standard-Model
Extension (SME) framework. Despite the agreement between the shadow image of
Sgr A* and the prediction of the general theory of relativity, there is still a
slight difference which is expected to be fixed by taking some fundamental
corrections into account. We bring up the idea that the recent inferred shadow
image of Sgr A* is explicable by a minimal SME-inspired Schwarzschild metric
containing the Lorentz violating (LV) terms obtained from the post-Newtonian
approximation. The LV terms embedded in Schwarzschild metric are dimensionless
spatial coefficients associated with the field responsible for
LSV in the gravitational sector of the minimal SME theory. In this way, one can
control Lorentz invariance violation in the allowed sensitivity level of the
first shadow image of Sgr A*. Actually, using the bounds released within
uncertainty for the shadow size of Sgr A* and whose fractional
deviation from standard Schwarzschild, we set upper limits for the two
different combinations of spatial diagonal coefficients and the time-time
coefficient of the SME, as well. The best upper bound is at the
level, which should be interpreted differently from those constraints
previously extracted from well-known frameworks since unlike standard SME
studies it is not obtained from a Sun-centered celestial frame but comes from
probing the black hole horizon scale.Comment: 16 pages, 5 figures. v3:discussion improved, references added,
figures revised; matches the version accepted for publication in PR
Lifetime of scalar cloud formation around a rotating regular black hole
Does circumventing the curvature singularity of the Kerr black hole affects
the timescale of the scalar cloud formation around it? By definition, the
scalar cloud, forms a gravitational atom with hydrogen-like bound states, lying
on the threshold of a massive scalar field's superradiant instability regime
(time-growing quasi-bound states) and beyond (time-decaying quasi-bound
states). By taking a novel type of rotating hollow regular black hole proposed
by Simpson and Visser which unlike its standard rivals has an asymptotically
Minkowski core, we address this question. The metric has a minimal extension
relative to the standard Kerr, originating from a single regularization
parameter , with length dimension. We show with the inclusion of the
regularization length scale into the Kerr spacetime, without affecting
the standard superradiant instability regime, the timescale of scalar cloud
formation gets shorter. Since the scalar cloud after its formation, via energy
dissipation, can play the role of a continuum source for gravitational waves,
such a reduction in the instability growth time improves the phenomenological
detection prospects of new physics because the shorter the time, the more
astrophysically important.Comment: 13 pages (two column), 6 figures, 1 tabl
Black holes with scalar hair in light of the Event Horizon Telescope
Searching for violations of the no-hair theorem (NHT) is a powerful way to
test gravity, and more generally fundamental physics, particularly with regards
to the existence of additional scalar fields. The first observation of a black
hole (BH) shadow by the Event Horizon Telescope (EHT) has opened a new direct
window onto tests of gravity in the strong-field regime, including probes of
violations of the NHT. We consider two scenarios described by the
Einstein-Maxwell equations of General Relativity and electromagnetism, to which
we add a scalar field. In the first case we consider a minimally-coupled scalar
field with a potential, whereas in the second case the field is
conformally-coupled to curvature. In both scenarios we construct charged BH
solutions, which are found to carry primary scalar hair. We then compute the
shadows cast by these two BHs as a function of their electric charge and scalar
hair parameter. Comparing these shadows to the shadow of M87* recently imaged
by the EHT collaboration, we set constraints on the amount of scalar hair
carried by these two BHs. The conformally-coupled case admits a regime for the
hair parameter, compatible with EHT constraints, describing a so-called mutated
Reissner-Nordstr\"{o}m BH: this solution was recently found to effectively
mimic a wormhole. Our work provides novel constraints on fundamental physics,
and in particular on violations of the no-hair theorem and the existence of
additional scalar fields, from the shadow of M87*.Comment: 33 pages, 6 figures, 1 table, references added, version accepted for
publication in JCA
Constraining the Lorentz-Violating Bumblebee Vector Field with Big Bang Nucleosynthesis and Gravitational Baryogenesis
By keeping the cosmological principle i.e., an isotropic and homogeneous
universe, we consider the cosmology of a vector-tensor theory of gravitation
known as the \textit{bumblebee} model. In this model a single Lorentz-violating
timelike vector field with a nonzero vacuum expectation value (VEV) couples to
the Ricci tensor and scalar, as well. Taking the ansatz for
the time evolution of the vector field we derive the relevant dynamic equations
of the Universe, where is a free parameter. In particular, by employing
observational data coming from the Big Bang Nucleosynthesis (BBN) and the
matter-antimatter asymmetry in the Baryogenesis era, we impose some constraints
on the VEV of the bumblebee timelike vector field i.e., , and the
exponent parameter . The former and the latter limit the size of Lorentz
violation, and the rate of the time evolution of the background
Lorentz-violating bumblebee field, respectively.Comment: 18 pages (two columns), 8 figures.v2: discussion and analysis
improved, some references and figures added, version accepted in EPJ
Evolution of Spherical Overdensities in Energy-Momentum-Squared Gravity
Employing the spherical collapse (SC) formalism, we investigate the linear
evolution of the matter over-density for energy-momentum-squared gravity
(EMSG), which in practical phenomenological terms, one may imagine as an
extension of the {\Lambda}CDM model of cosmology. The underlying model, while
still having a cosmological constant, is a non-linear material extension of the
general theory of relativity (GTR) and includes correction terms that are
dominant in the high-energy regime, the early universe. Considering the
Friedman{Robertson{Walker (FRW) background in the presence of a cosmological
constant, we find the effects of the modifications arising from EMSG on the
growth of perturbations at the early stages of the universe. Considering both
possible negative and positive values of the model parameter of EMSG, we
discuss its role in the evolution of the matter density contrast and growth
function in the level of linear perturbations. While EMSG leaves imprints
distinguishable from {\Lambda}CDM, we find that the negative range of the ESMG
model parameter is not well-behaved, indicating an anomaly in the parameter
space of the model. In this regard, for the evaluation of the galaxy cluster
number count in the framework of EMSG, we equivalently provide an analysis of
the number count of the gravitationally collapsed objects (or the dark matter
halos). We show that the galaxy cluster number count decreases compared to the
{\Lambda}CDM model. In agreement with the hierarchical model of structure
formation, in EMSG cosmology the more massive structures are less abundant,
meaning that form at later times.Comment: 9 pages, 8 figures. v3: version accepted for publication in PRD: the
title change