arXiv subjects

H. R. Fazlollahi

Publications and source records attributed to H. R. Fazlollahi.

9 recordsLinked to original sources

arXiv2026

Van der Waals Gravity Theory

In this study, we propose an extension of general relativity inspired by the van der Waals equation of state, incorporating non-ideal thermodynamic effects into the gravitational sector. Our approach is based on the thermodynamic interpretation of gravity introduced by Jacobson, in which the field equations arise from the Clausius relation. Within this framework, we obtain modified gravitational field equations in which the effective gravitational coupling is no longer constant, but instead evolves with the properties of the underlying spacetime system. This dynamical behavior leads to significant consequences in high-energy regimes. In particular, it provides a natural mechanism for avoiding the initial singularity of standard Big Bang cosmology and gives rise to non-singular black hole solutions. These findings indicate that incorporating non-ideal thermodynamic features into the description of spacetime may offer a consistent route toward resolving fundamental singularities in classical gravitational theory.

gr-qc↗

arXiv2026

Extended Gravity Theories from a Thermodynamic Perspective

We extend the thermodynamic derivation of gravity in the Jacobson framework by generalizing the Clausius relation through a nontrivial entropy functional. We show that entropy deformations appear as modifications of the effective gravitational coupling, defining a broad class of modified gravity theories. However, conventional entropy corrections are insufficient to resolve spacetime singularities within this approach. We then propose a new entropy form by incorporating quantum properties at the level of horizon degrees of freedom. Implementing this entropy in the modified gravitational framework, we study its cosmological implications at both early and late times. In the early Universe, the model predicts a nonsingular phase with a finite Hubble parameter, leading to a de Sitter-like inflationary expansion with finite entropy and temperature. At late times, the theory reproduces, at leading order, the effective dynamics of loop quantum cosmology.

gr-qc↗

arXiv2026

Natural Emergence of LCDM Cosmology within General Relativity from Two Alternative Frameworks Without Fine-Tuning and Coincidence

In this study, by revisiting the quantum interpretation of the cosmological constant, we introduce its formal representation within standard General Relativity. Examining its behavior in a Friedmann-Robertson-Walker spacetime reveals a mechanism in which the symmetry between energy and momentum is dynamically broken. Applying this concept naturally leads to the derivation of the familiar LCDM model, while simultaneously alleviating both the fine-tuning and coincidence problems. Comparison of the ground-state energy behavior in the Friedmann equations with a dust matter field further indicates that large-scale matter exhibits the same symmetry-breaking behavior. Remarkably, due to this broken symmetry, the interactions between local regions of matter in the large-scale structure generate effective pressure, driving late-time acceleration and reproducing the LCDM expansion history without invoking exotic fields or negative-pressure components. This framework provides a self-consistent realization of LCDM within General Relativity, emerging entirely from the intrinsic dynamics of standard matter without fine-tuning and coincidence problems.

gr-qc↗

arXiv2026

A New Perspective on the Cosmological Constant and Its Core Problems

One of the most enduring and unresolved challenges in modern theoretical and observational cosmology is the fine-tuning and coincidence problems associated with the cosmological constant. Rather than attempting to reconcile these issues within the standard LCDM framework where they remain effectively frozen, we adopt a fundamentally different viewpoint based on alternative theories of gravity. We argue that the root of these problems lies in a deep misinterpretation of the cosmological constant, particularly its identification with the quantum ground-state energy of spacetime. In this work, we propose a novel physical interpretation of the cosmological constant and introduce a new mechanism, termed Breaking Energy-Momentum Symmetry. This framework provides a natural and unified route toward alleviating, and potentially resolving, both the fine-tuning and coincidence problems, offering a compelling alternative to the conventional cosmological paradigm.

gr-qc↗

arXiv2025

Dark Matter and Energy-Momentum Squared Gravity

In this study, we examine the energy-momentum squared modified theory of gravity, where the squared term $T_{μν}T^{μν}$ is incorporated into the conventional gravitational Lagrangian. This modification aims to account for dark matter effects on galactic scales. Specifically, we analyze the model near general relativity solutions for spherically symmetric and static metrics. By fixing the components using the rotational velocities of galaxies, the model demonstrates the emergence of a flat rotation curve in the galactic halo. Additionally, we investigate the proposed model's predictions for the light deflection angle and radar echo delay.

gr-qc↗

arXiv2022

Renyi Entropy Correction to Expanding Universe

The Renyi entropy coprises a group of data estimates that sums up the well-known Shannon entropy, acquiring a considerable lot of its properties. It appears as unqualified and restrictive entropy, relative entropy, or common data, and has found numerous applications in information theory. in the Renyi's argument, the area law of the black hole entropy plays a significant role. However, the total entropy can be modified by some quantum effects, motivated by the randomness of a system. In this note, by employing this modified entropy relation, we have derived corrections to Newton's law of gravitation. Taking this entropy associated with the apparent horizon of the Friedmann-Robertson-Walker Universe and assuming the first law of thermodynamics, dE=TdS+WdV, satisfies the apparent horizon, we have reconsidered expanding Universe and find modified Friedmann equations. Also, the second thermodynamics law has been examined.

gr-qc↗

arXiv2022

Holographic Dark Energy from Acceleration of Particle Horizon

Following the holographic principle, which suggests that the energy density of dark energy may be proportional inversely to the area of the event horizon of the Universe, we have proposed a new energy density of dark energy through the acceleration of particle horizon scaled by the length of this parameter. This model depends only on one free parameter beta=0-1.99. For beta's near zero, the deviation of the model compared with the Lambda CDM model is tangible while for values beta=1.99, the suggested model has no conflict with Lambda CDM theory. Regardless of the value of beta, the model presents dark energy that behaves such as matter with positive pressure in high redshifts =0.33, while for present and near-future Universe treats like the cosmological constant model and phantom field. Comparing the model with Ricci dark energy illustrates our model alleviates Ricci dark energy errors in calculating the age of old supernovae and the evolution of different cosmic components in high redshifts. Moreover, we have calculated matter structure formation parameters such as CMB temperature and matter power spectrum of the model to consider the effects of matter-like dark energy during the matter-dominated era.

gr-qc↗

arXiv2018

Chaplygin Gas models without Chaplygin Gas EoS

The equation of state of Chaplygin Gas is one of the simplest ways to illustrate dark energy effects during the late time. Indeed, while one uses the equation of state of Chaplygin Gas, the continuity equation gives specific energy density which satisfies both deceleration (matter dominated) and acceleration (dark energy epoch). In other words, for the earlier universe, the energy density of whole energy-matter component behaves as matter era (baryonic and dark matter) and treats such as dark energy in the standard model of cosmology Lambda-CDM when a goes to infinity. Here, by using the general form of F(R) gravity while the pressure of matter is non-zero, we show that even for Einstein gravity (GR), one obtains a viable cosmological model with the same energy density of Chaplygin Gas while Chaplygin Gas EoS is not used. Further, we investigate cosmic evolution through two other different forms of F(R) gravity and show general behavior of Hubble parameter, cosmological scale factor and equation of state throughout cosmic time.

physics.gen-ph↗

arXiv2018

F(R) cosmology via Noether symmetry and lambda-Chaplygin Gas like model

In this work, we consider F(R) alternative theories of gravity with an eye to Noether symmetry through the gauge theorem. For non-vacuum models, one finds Λ like gravity with energy density of Chaplygin Gas. We also obtain the effective equation of state parameter for corresponding cosmology and scale factor behavior with respect to cosmic time which show that the model provides viable EoS and scale factor with respect to observational data.

gr-qc↗