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S. Ghaffari

Publications and source records attributed to S. Ghaffari.

At least 19 recordsLinked to original sources

Reconstructing the Aerosol State from Partial Observations with Generative Modeling

Key aerosol properties that shape climate -- such as CCN activity, scattering and absorption, and ice nucleation efficiency -- are difficult to infer from measurements that typically capture only a part of the aerosol state. We develop a conditional generative framework that maps a label (a vector of partial observations) to an ensemble of plausible aerosol states and propagates these to diagnostics, yielding mean estimates with confidence intervals. Using synthetic data, we evaluate two label configurations: a low-dimensional setup with limited number distribution and bulk-composition information, and a high-dimensional setup including complete number and total mass distributions plus species bulk masses. Generated samples maintain strong label compliance, and higher-dimensional labels markedly reduce variability. CCN activity and volume scattering are well constrained even under the low-dimensional setup, whereas dust- and BC-sensitive diagnostics (frozen fraction, absorption) benefit substantially from the additional information in the high-dimensional case. This framework clarifies which observational inputs most effectively constrain different diagnostics and demonstrates how generative machine learning can provide uncertainty-aware estimates from incomplete aerosol information.

physics.ao-ph

Black Hole Thermodynamics in Harada's inspired theory of gravity: Stability, Phase Structure and Geometrothermodynamics

In this paper, we investigate the thermodynamic properties of spherically symmetric, static black hole solutions within the framework of Conformal Killing Gravity (CKG). This is a modified theory of gravity that retains all solutions of General Relativity, while addressing some of its theoretical shortcomings and enriching gravitational phenomenology at large distances. We derive key thermodynamic quantities, including mass, temperature, heat capacity and Gibbs free energy, to examine the stability and phase structure of extended Schwarzschild-AdS and charged AdS black holes. Furthermore, employing the formalism of geometrothermodynamics, we analyze the behavior of the thermodynamic curvature scalar to identify critical points and characterize phase transitions. Our results demonstrate that the parameter \( \lambda \), which quantifies deviations from Einstein's theory, plays a pivotal role in shaping the thermodynamic behavior, resulting in new stability conditions and distinct phase transition patterns compared to those predicted by standard General Relativity.

gr-qc

Nonextensive entropies impact onto thermodynamics and phase structure of Kerr-Newman black holes

Taking the nonextensive Tsallis and R\'enyi entropies into account, we explore thermodynamic properties and phase transitions of the Kerr-Newman black holes (KNBH) in the microcanonical and canonical ensembles. We also compare our results with those obtained by attributing the Bekenstein-Hawking entropy bound to the mentioned black holes. Our analysis indicates that, similarly to the standard Boltzmann picture, isolated KNBH in the microcanonical approach are stable against axisymmetric perturbations in both Tsallis and R\'enyi models. On the other hand, in considering the case when the black holes are enveloped by a bath of thermal radiation in the canonical treatment, the KNBH based on the Tsallis and R\'enyi entropies can be stable for some values of the entropy parameters, in contrast to the traditional Boltzmann framework. For the case of R\'enyi entropy, we find that a Hawking-Page transition and a first order small black hole/large black hole transition can occur in a similar fashion as in rotating black holes in an anti-de Sitter space. Finally, we employ the Ruppeneir geometrothermodynamic technique to provide a new perspective on studying the nature of interactions between black hole microstructures, revealing a non-trivial impact of nonextensive entropies.

hep-th

Note on agegraphic dark energy inspired by modified Barrow entropy

We revisit agegraphic dark energy (ADE) model when the entropy associated with the apparent horizon is in the form of Barrow entropy, $S\sim A^{1+δ/2}$, where $0\leqδ\leq1$ indicates the amount of the quantum-gravitational deformation effects of the horizon. The modification to the entropy expression, not only change the energy density of ADE, but also modifies the Friedmann equations due to thermodynamics-gravity conjecture. Based on this, we investigate the cosmological consequences of ADE through modified Barrow cosmology and disclose the effects of Barrow exponent $δ$ on the evolutions of the cosmological parameters. We observe that, depending on the values of $δ$, the transition from early decelerated phase to the late time accelerated phase occurs, and the equation of state (EoS) parameter $ w_{de} $ varies from quintessence $ -1<w_{de}<-1/3 $ to the phantom regime $ (w_{de}<-1)$. When $δ=0$, all results of ADE in standard cosmology are restored.

gr-qc

Barrow holographic dark energy in the Brans-Dicke cosmology

We construct a holographic model for dark energy in the Brans-Dicke cosmology by using the holographic principle considering the Barrow entropy instead of the standard Bekenstein-Hawking one. The former arises from the effort to account for quantum-gravitational effects in black-hole physics and, according to the gravity-thermodynamic conjecture, in the cosmological framework. In order to explore the cosmological consequences of our model, we consider the Hubble horizon as the IR cutoff. We investigate both the non-interacting and interacting cases with the sign-changeable and linear interactions, showing that they can explain the present accelerated phase of the Universe expansion, in contrast to the standard Holographic Dark Energy model. We then perform the classical stability analysis using the squared sound speed. We find that, whilst the non-interacting model is unstable against the small perturbations, the sign-changeable interacting one can be stable only for suitable values of the model parameters. On the other hand, the linear interacting model always predicts a stable Universe. The consistency of our model with cosmological observations is discussed.

gr-qc

Kaniadakis holographic dark energy in Brans-Dicke cosmology

By using the holographic hypothesis and Kaniadakis generalized entropy, which is based on relativistic statistical theory and modified Boltzmann-Gibbs entrory, we build Kaniadakis holographic dark energy (DE) model in the Brans-Dicke framework. We drive cosmological parameters of Kaniadakis holographic DE model, with IR cutoff as the Hubble horizon, in order to investigate its cosmological consequences. Our study shows that, even in the absence of an interaction between the dark sectors of cosmos, the Kaniadakis holographic dark energy model with the Hubble radius as IR cutoff can explain the present accelerated phase of the universe expansion in the Brans-Dicke theory. The stability of the model, using the squared of sound speed, has been checked and it is found that the model is unstable in non-interacting case and can be stable for some range of model parameters within the interacting case.

hep-th

Effects of Rastall parameter on perturbation of dark sectors of the Universe

In recent years, Rastall gravity is undergoing a considerable surge in popularity. This theory purports to be a modified gravity theory with a non-conserved energy-momentum tensor ({\rm EMT}) and an unusual non-minimal coupling between matter and geometry. The present work looks for the evolution of homogeneous spherical perturbations within the Universe in the context of Rastall gravity. Using the spherical Top-Hat collapse model we seek for exact solutions in linear regime for density contrast of dark matter (\rm DM) and dark energy ({\rm DE}). We find that the Rastall parameter affects crucially the dynamics of density contrasts for {\rm DM} and {\rm DE} and the fate of spherical collapse is different in comparison to the case of general relativity ({\rm GR}). Numerical solutions for perturbation equations in non-linear regime reveal that {\rm DE} perturbations could amplify the rate of growth of {\rm DM} perturbations depending on the values of Rastall parameter.

physics.gen-ph

Holographic dark energy in Rastall theory

Bearing holographic dark energy hypothesis in mind, the ability of vacuum energy in describing the current accelerated universe is studied in the framework of Rastall theory. Here, in addition to the ordinary approach in which it is expected that this energy plays the role of dark energy, we also address a new approach where the sum of this energy and Rastall term is responsible for the current accelerated universe. We also investigate the cosmological outcomes of using Tsallis entropy in quantifying the energy of fields in vacuum for both mentioned approaches. The implications of considering an interaction between the various segments of cosmic fluid have been addressed in each studied cases. The normalized Hubble parameter for the models have also been plotted and compared that with the H(z) data consisting of 41 data points in the redshift range of 0.07< z<2.36.

gr-qc

Black hole solutions and Euler equation in Rastall and generalized Rastall theories of gravity

Focusing on the special case of generalized Rastall theory, as a subclass of the non-minimal curvature-matter coupling theories in which the field equations are mathematically similar to the Einstein field equations in the presence of cosmological constant, we find two classes of black hole (BH) solutions including $i$) conformally flat solutions and $ii$) non-singular BHs. Accepting the mass function definition and by using the entropy contents of the solutions along with thermodynamic definitions of temperature and pressure, we study the validity of Euler equation on the corresponding horizons. Our results show that the thermodynamic pressure, meeting the Euler equation, is not always equal to the pressure components appeared in the gravitational field equations and satisfies the first law of thermodynamics, a result which in fact depends on the presumed energy definition. The requirements of having solutions with equal thermodynamic and Hawking temperatures are also studied. Additionally, we study the conformally flat BHs in the Rastall framework. The consequences of employing generalized Misner-Sharp mass in studying the validity of the Euler equation are also addressed.

physics.gen-ph

Tsallis holographic dark energy in Fractal Universe

We study the cosmological consequences of interacting Tsallis holographic dark energy model in the framework of the fractal universe, in which, the Hubble radius is considered as the IR cut-off. We drive the equation of state (EoS) parameter, deceleration parameter and the evolution equation for the Tsallis holographic dark energy density parameter. Our study shows that this model can describe the current accelerating Universe in both noninteracting and interacting scenarios, and also a transition occurs from the deceleration phase to the accelerated phase, at the late time. Finally, we check the compatibility of free parameters of the model with the latest observational results by using the Pantheon supernovae data, eBOSS, 6df, BOSS DR12, CMB Planck 2015, Gamma-Ray Burst.

gr-qc

The generalized and extended uncertainty principles and their implications on the Jeans mass

The generalized and extended uncertainty principles affect the Newtonian gravity and also the geometry of the thermodynamic phase space. Under the influence of the latter, the energy-temperature relation of ideal gas may change. Moreover, it seems that the Newtonian gravity is modified in the framework of the Rényi entropy formalism motivated by both the long-range nature of gravity, and the extended uncertainty principle. Here, the consequences of employing the generalized and extended uncertainty principles, instead of the Heisenberg uncertainty principle, on the Jeans mass are studied. The results of working in the Rényi entropy formalism are also addressed. It is shown that unlike the extended uncertainty principle and the Rényi entropy formalism which lead to the same increase in the Jeans mass, the generalized uncertainty principle can decrease it. The latter means that a cloud with mass smaller than the standard Jeans mass, obtained in the framework of the Newtonian gravity, may also undergo the gravitational collapse process.

gr-qc

Black hole thermodynamics in the Sharma-Mittal generalized entropy formalism

Using the Sharma-Mittal entropy, we study some properties of the Schwarzschild and Schwarzschild-de Sitter black holes. The results are compared with those obtained by attributing the Bekenstein entropy bound to the mentioned black holes. Our main results show that while the Schwarzschild black hole is always stable in the micro-canonical ensemble, it can be stable in the canonical ensemble if its mass is bigger than the mass of the coldest Schwarzschild black hole. A semi-classical analysis has also been used to find an approximate relation between the entropy free parameters. Throughout the paper, we use units $c=G=\hbar=k_B=1$, where $k_B$ denotes the Boltzmann constant.

physics.gen-ph

The extended uncertainty principle inspires the Rényi entropy

We use the extended uncertainty principle (EUP) in order to obtain the Rényi entropy for a black hole (BH). The result implies that the non-extensivity parameter, appeared in the Rényi entropy formalism, may be evaluated from the considerations which lead to EUP. It is also shown that, for excited BHs, the Rényi entropy is a function of the BH principal quantum number, i.e. the BH quantum excited state. Temperature and heat capacity of the excited BHs are also investigated addressing two phases while only one of them can be stable. At this situation, whereas entropy is vanished, temperature may take a non-zero positive minimum value, depending on the value of the non-extensivity parameter. The evaporation time of excited BH has also been studied.

gr-qc

Gravitational Collapse in Rastall Gravity

We study spherically symmetric gravitational collapse of a homogeneous perfect fluid in Rastall gravity. Considering a linear equation of state (EoS) for the fluid profiles, we examine the conditions under which the collapse scenario could end in a spacetime singularity. Depending on the model parameters, the singularity could be either naked or covered by a horizon. We find that a non-vanishing Rastall parameter could affect the formation of apparent horizon so that, naked singularities may form for those values of EoS parameter for which a homogeneous perfect fluid collapse in general relativity (GR) terminates at the black hole formation. Hence the visibility of the resulting singularity depends on the Rastall parameter. The solutions we obtain respect the weak energy condition (WEC) which is crucial for physical validity of the model.

gr-qc

Inflation in the Rényi cosmology

The description of the accelerated phases for the current and primordial cosmos has been obtained in the framework of Rényi cosmology. The results show that corrections to the Friedmann equations, originated from differences between Rényi and Bekenstein entropies, can describe both the current accelerated and inflationary eras. The effects of these corrections can be appointed to a hypothetical fluid which is classically unstable. Although inflation can be obtained in Rényi cosmology as the vacuum solution, the values found for the parameters of the hypothetical fluid which supports the current accelerated and primary inflationary cosmos differ from each other.

gr-qc

Tsallis holographic dark energy in the brane cosmology

We study some cosmological features of Tsallis holographic dark energy (THDE) in Cyclic, DGP and RS II braneworlds. In our setup, a flat FRW universe is considered filled by a pressureless source and THDE with the Hubble radius as the IR cutoff, while there is no interaction between them. Our result shows that although suitable behavior can be obtained for the system parameters such as the deceleration parameter, the models are not always stable during the cosmic evolution at the classical level.

physics.gen-ph

Tsallis holographic dark energy in the Brans-Dicke cosmology

Using the Tsallis generalized entropy, holographic hypothesis and also considering the Hubble horizon as the IR cutoff, we build a holographic model for dark energy and study its cosmological consequences in the Brans-Dicke framework. At first, we focus on a non-interacting universe, and thereinafter, we study the results of considering a sign-changeable interaction between the dark sectors of the cosmos. Our investigations show that, compared with the flat case, the power and freedom of the model in describing the cosmic evolution is significantly increased in the presence of the curvature. The stability analysis also indicates that, independent of the universe curvature, both the interacting and non-interacting cases are classically unstable. In fact, both the classical stability criterion and an acceptable behavior for the cosmos quantities, including the deceleration and density parameters as well as the equation of state, are not simultaneously obtainable.

gr-qc

Thermodynamic analysis of gravitational field equations in Lyra manifold

Considering the Einstein field equations in Lyra manifold, and applying the unified first law of thermodynamics as well as the Clausius relation to the apparent horizon of FRW universe, we find the entropy of apparent horizon in Lyra manifold. In addition, the validity of second law of thermodynamics and its generalized form are also studied. Finally, we use the first law of thermodynamics in order to find the horizon entropy of static spherically symmetric spacetimes. Some results of considering (anti)de-Sitter and Schwarzschild metrics have also been addressed.

gr-qc