19 research outputs found
Phase structure of charged AdS black holes surrounded by exotic fluid with modified Chaplygin equation of state
No full textBy considering the concept of the modified Chaplygin gas (MCG) as a single fluid model unifying dark energy and dark matter, we construct a static, spherically charged black hole (BH) solution in the framework of General Relativity. The P–V criticality of the charged anti-de Sitter (AdS) BH with a surrounding MCG is explored in the context of the extended phase space, where the negative cosmological constant operates as a thermodynamical pressure. This critical behavior shows that the small/large BH phase transition is analogous to the van der Waals liquid/gas phase transition. Accordingly, along the P–V phase spaces, we derive the BH equations of state and then numerically evaluate the corresponding critical quantities. Similarly, critical exponents are identified, along with outcomes demonstrating the scaling behavior of thermodynamic quantities near criticality to a universal class. The use of geometrothermodynamic (GT) tools finally offers a new perspective on the discovery of the critical phase transition point. At this stage, we apply a class of GT tools, such as Weinhold, Ruppeiner, HPEM, and Quevedo classes I and II. The findings are therefore non-trivial, as each GT class metric captures at least either the physical limitation point or the phase transition critical point. Overall, this paper provides a detailed study of the critical behavior of the charged AdS BH with surrounding MCG
Topological AdS black holes surrounded by Chaplygin dark fluid: From stability to geometrothermodynamic analysis
No full textImplementing the concept of Dark Fluid with a Chaplygin-like equation of
state within General Relativity, we construct a new higher-dimensional, static,
and spherically symmetric anti-de Sitter (AdS) black hole solution. Energy
conditions are explored alongside curvature singularity tools. The inspection
at the level of the phase structure and critical behavior is carried out
in the context of the extended phase space, where the cosmological constant
appears as pressure. Our findings disclose non-trivial similarities between the
small/large phase transition of AdS black holes surrounded by Chaplygin dark
fluid and van der Waals systems' liquid/gas phase transition. This analysis
offers insights into the physical interpretation of the diagram and
identifies critical exponents that reveal the scaling behavior of thermodynamic
quantities close to criticality in a universal manner. We finally deepen our
understanding of the thermodynamic properties and microstructure of AdS black
holes by leveraging the geometrothermodynamic formalism. Specifically, we
employ tools, including Weinhold, Ruppeiner, Hendi-Panahiyan-Eslam-Momennia
(HPEM) and Quevedo classes I and II. We show that each class of metrics
predicts either the physical limitation point and/or the phase-transition
critical points, with HPEM and Quevedo formulations providing richer
information about the phase transitions. Altogether, this study contributes to
advancing our knowledge of the role of Chaplygin gas in General Relativity and
thoroughly examining the thermodynamic phase structure of high-dimensional AdS
black holes under extreme conditions.Comment: 26 pages, 11 labeled figure, accepted for publication in Phys. Dark.
Uni
Signature flips in time-varying
No full textIn this study, we investigate signature flips within the framework of cosmological models featuring a time-varying vacuum energy term . Specifically, we consider the power-law form of , where and n are constants. To constrain the model parameters, we use the MCMC technique, allowing for effective exploration of the model’s parameters. We apply this approach to analyze 31 points of observational Hubble Data (OHD), 1048 points from the Pantheon data, and additional CMB data. We consider three scenarios: when n is a free parameter (Case I), when (Case II), and when (Case III). In our analysis across all three cases, we observe that our model portrays the universe’s evolution from a matter-dominated decelerated epoch to an accelerated epoch, as indicated by the corresponding deceleration parameter. In addition, we investigate the physical behavior of total energy density, total EoS parameter, and jerk parameter. Our findings consistently indicate that all cosmological parameters predict an accelerated expansion phase of the universe for all three cases (). Furthermore, our analysis reveals that the Om(z) diagnostics for Cases I and III align with the quintessence region, while Case II corresponds to the CDM model
Cosmological constraints on time-varying cosmological terms: A study of FLRW universe models with CDM cosmology
No full textThis paper explores models of the FLRW universe that incorporate a
time-varying cosmological term . Specifically, we assume a
power-law form for the cosmological term as a function of the scale factor:
, where represents the
present value of the cosmological term. Then, we derive an exact solution to
Einstein's field equations within the framework of CDM cosmology
and determine the best-fit values of the model parameters using the combined
+ SNe Ia dataset and MCMC analysis. Moreover, the deceleration parameter
demonstrates the accelerating behavior of the universe, highlighting the
transition redshift , at which the expansion shifts from deceleration
to acceleration, with confidence levels of and . In
addition, we analyze the behavior of the Hubble parameter, jerk parameter, and
diagnostic. Our analysis leads us to the conclusion that the
CDM model is consistent with present-day observations.Comment: Advances in Space Research accepted versio
Exploring accelerated expansion in the universe: A study of gravity with parameterized EoS and cosmological constraints
No full textThe study conducted in this research paper utilizes the gravity,
where represents non-metricity and represents the trace of the
energy-momentum tensor, to investigate the accelerated expansion of the
universe. To complete the study, an effective EoS with one parameter ,
is parameterized as . The linear
version of is also considered, where is a
constant. By constraining the model with six BAO points, 57 Hubble points, and
1048 Pantheon sample datasets, the parameters and are
determined to best match the data. The cosmological parameters and energy
conditions for the model are derived and examined. The results show that the
model is in good agreement with observations, and can serve as a valuable
starting point for analyzing FLRW models in the theory of gravity.Comment: Chinese Journal of Physics accepted versio
Exploring late-time cosmic acceleration: A study of a linear cosmological model using observational data
No full textUnderstanding the evolution of dark energy poses a significant challenge in
modern cosmology, as it is responsible for the universe's accelerated
expansion. In this study, we focus on a specific cosmological model and
analyze its behavior using observational data, including 31 data points from
the CC dataset, 1048 points from the Pantheon SNe Ia samples, and 6 points from
the BAO dataset. By considering a linear model with an additional
constant term, we derive the expression for the Hubble parameter as a function
of cosmic redshift for non-relativistic pressureless matter. We obtain the
best-fit values for the Hubble constant, , and the model parameters
and , indicating a stable model capable of explaining late-time
cosmic acceleration without invoking a dark energy component. This is achieved
through modifying field equations to account for the observed accelerated
expansion of the universe.Comment: Physics of the Dark Universe published versio
Constraining gravity with bulk viscosity
No full textWe investigate the influence of bulk viscosity on late-time cosmic
acceleration within an extended gravity framework, where the
non-metricity is non-minimally coupled with the matter Lagrangian .
Analyzing the function , we derive exact
solutions under non-relativistic matter domination. Using observational
datasets (, Pantheon supernovae, and their combination), we constrain the
model parameters , , , and . The deceleration
parameter transitions from positive to negative values around redshifts to , indicating current accelerated expansion. Moreover,
the effective equation of state parameter, , resembles
quintessence dark energy (), with
corresponding values from respective datasets. Finally, we use the
diagnostic, which confirms that our model demonstrates quintessence-like
behavior. Our findings underscore the significant role of bulk viscosity in
understanding accelerated expansion in the universe within alternative gravity
theories.Comment: 12 pages, 8 figure
Observational constraints on cosmic expansion in f(R,Lm,T) gravity
No full textThe late-time acceleration of the universe remains one of the central challenges in modern cosmology, motivating both dynamical dark energy models and extensions of general relativity. In this work, we investigate the cosmology of the recently proposed f(R,Lm,T) gravity framework, which generalizes the f(R,T) and f(R,Lm) models by incorporating the Ricci scalar R, the matter Lagrangian Lm, and the trace of the energy-momentum tensor T within a unified gravitational action. By adopting the simplest functional form f(R,Lm,T)=R+αLm+βT+γ, we derive modified Friedmann equations in a dust-dominated FLRW background and constrain the parameters (H0,α,β,γ) using cosmic chronometers (CC), Type Ia supernovae from the PantheonPlus (PP)+SH0ES compilation, and their joint dataset. The inferred Hubble constant values are H0=68.69±0.52 km/s/Mpc (CC), 72.75±0.15 km/s/Mpc (PP+SH0ES), and 72.68−0.12+0.15 km/s/Mpc (Joint), while the corresponding deceleration parameters are q0∼−0.561, q0∼−0.451, and q0∼−0.458, respectively. Notably, the obtained H0 values lie between early-universe and local measurements, indicating that the model can accommodate both low- and high-redshift datasets, partially easing the Hubble tension. The Om(z) diagnostic analyses reveal a quintessence-like evolution of the cosmic energy density, distinguishing this framework from the standard ΛCDM model while preserving late-time acceleration
Quasiperiodic oscillation around charged black holes in Einstein–Maxwell-scalar theory
No full textIn the present paper, first, we study the event horizon properties of charged black holes (BHs) in Einstein Maxwell-scalar (EMS) gravity. Then, we investigate the circular motion of test particles’ around the BH in the EMS gravity. We also analyze the effects of the EMS parameters on the position of innermost circular orbits (ISCOs), energy, and angular momentum of the test particles corresponding to circular orbits. We provide detailed studies of the efficiency of energy release from EMS BHs based on the Hartle–Thorne model and fundamental frequencies of oscillations of particles along their circular stable orbits. Moreover, we have explored possible values of upper and lower frequencies of twin-peak quasiperiodic oscillations (QPOs) around the BHs. Finally, we obtain relationships between the BH charge and the EMS parameters using observational data from the QPOs detected in the microquasars: GRS 1905+105, GRO J 1655-40, H 1745+322, and XTE 1550-564
Constraints on metric-Palatini gravity from QPO data
No full textIn this work, we study metric-Palatini gravity extended by the antisymmetric part of the affine curvature. This gravity theory leads to general relativity plus a geometric Proca field. Using our previous construction of its static spherically-symmetric AdS solution (Eur Phys J. C 83(4):318, 2023), we perform a detailed analysis in this work using the observational quasiperiodic oscillations (QPOs) data. To this end, we use the latest data from stellar-mass black hole GRO J1655-40, intermediate-mass black hole in M82-X1, and the super-massive black hole in SgA* (our Milky Way) and perform a Monte-Carlo-Markov-Chain (MCMC) analysis to determine or bound the model parameters. Our results shed light on the allowed ranges of the Proca mass and other parameters. The results imply that our solutions can cover all three astrophysical black holes. Our analysis can also be extended to more general metric-affine gravity theories
