arXiv subjects

Y. Bisabr

Publications and source records attributed to Y. Bisabr.

8 recordsLinked to original sources

arXiv2026

Scalar Photon Coupling and Modified Photon Evolution: CMB Signatures and Cosmological Constraints

We investigate the constant-$\epsilon$ limit of a photon-sector modification derived from an early-time interaction between a scalar field and radiation. Starting from the underlying scalar--photon coupling, the parameter $\epsilon$ quantifies the departure of the photon energy density from the standard adiabatic scaling, leading in the constant-$\epsilon$ limit to $\rho_{\gamma}\propto a^{-4+\epsilon}$ and to the modified CMB temperature--redshift relation $T(z)=T_{0}(1+z)^{1-\epsilon/4}$. The standard photon sector is recovered for $\epsilon=0$. We implement the corresponding constant-$\epsilon$ background evolution in CLASS and verify that the numerical photon-density evolution reproduces the analytic scaling. We then examine the resulting changes in the recombination history, visibility function and CMB temperature-anisotropy spectrum. These CMB spectra are used only as diagnostic outputs since the coupled scalar--photon perturbation equations are not implemented self-consistently in the numerical calculation and a full CMB likelihood analysis is beyond the scope of the present work. As a preliminary statistical application, we interface the modified CLASS implementation with MontePython and analyze the Pantheon+SH0ES supernova likelihood together with BAO measurements. Within the adopted constant-$\epsilon$ framework, prior choices, data combination and fixed early Universe parameter setup, we obtain $\epsilon=0.0230\pm0.0065$. The posterior is therefore centered on a positive value within this restricted analysis. Because the BAO predictions depend on the model-dependent drag scale and several early-Universe quantities are held fixed, this result should be interpreted as a conditional and preliminary constraint rather than as a full CMB-level determination or evidence for a resolution of the Hubble tension.

physics.gen-ph↗

arXiv2024

Hubble Tension in Power-Law f(R) Gravity and Generalized Brans-Dicke Theory

We introduce a theoretical framework to alleviate the Hubble tension. This framework is based on dynamics of a minimally coupled scalar field which either belongs to the Brans-Dicke theory with a self-interacting potential or is the scalar partner of f(R) gravity. These two theories are dynamically equivalent when the Brans-Dicke parameter is zero. We will use this dynamical equivalence to interpret the Hubble tension in the same theoretical framework. For both theories, we write one set of field equations in which the value of a parameter distinguishes between the two theories. We will show that H0 actually evolves with redshift so that its value is consistent with that measured from the local distance ladder and it drops to the value measured from CMB at high redshift. We argue that even though both theories exhibit this behaviour, Brans-Dicke theory with an exponential potential is more successful than power-law f(R) gravity to relieve the Hubble tension.

gr-qc↗

arXiv2020

Deflation of Vacuum Energy During Inflation Due to Bulk-Brane Interaction

We consider a brane world inflationary model in which inflation is driven by dynamics of a self-interacting scalar field living in the five-dimensional bulk. The scalar field is non-minimally coupled to matter fields on the brane and acts as an inflaton which induces a slow-roll inflation. We show that although the Friedmann equation is modified at early times due to effects of the extra dimension, the slow-roll condition is the same as that of the four-dimensional case. Due to the non-minimal coupling of matter with the bulk scalar, there is an energy transfer between the two components. We investigate the conditions under which the direction of this energy transfer can be from matter onto the bulk scalar. There are at least two advantages in this case: 1) It establishes a mechanism by which a large effective cosmological term on the brane is deflated during the inflation period. 2) The energy flow onto the bulk inflaton gives a more strongly damped evolution of the scalar field in the slow-roll region for a given potential. We then show that our results are supported by numerical estimations with quadratic and exponential potentials.

gr-qc↗

arXiv2017

Effect of the Chameleon Scalar Field on Brane Cosmological Evolution

We have investigated a brane world model in which the gravitational field in the bulk is described both by a metric tensor and a minimally coupled scalar field. This scalar field is taken to be a chameleon with an appropriate potential function. The scalar field interacts with matter and there is an energy transfer between the two components. We find a late-time asymptotic solution which exhibits late-time accelerating expansion. We also show that the Universe recently crosses the phantom barrier without recourse to any exotic matter. We provide some thermodynamic arguments which constrain both the direction of energy transfer and dynamics of the extra dimension.

gr-qc↗

arXiv2001

Mechanism for a Decaying Cosmological Constant

A mechanism is introduced to reduce a large cosmological constant to a sufficiently small value consistent with observational upper limit. The basic ingradient in this mechanism is a distinction which has been made between the two unit systems used on cosmology and particle physics. We have used a conformal invariant gravitational model to define a particular conformal frame in terms of the large scale properties of the universe. It is then argued that the contributions of mass scales in particle physics to the vacuum energy density should be considered in a different conformal frame. In this manner a cancellation mechanism is presented in which the conformal factor plays a key role to relax the large effective cosmological constant.

gr-qc↗

arXiv2001

Hadamard States and Two-dimensional Gravity

We have used a two-dimensional analog of the Hadamard state-condition to study the local constraints on the two-point function of a linear quantum field conformally coupled to a two-dimensional gravitational background. We develop a dynamical model in which the determination of the state of the quantum field is essentially related to the determination of a conformal frame. A particular conformal frame is then introduced in which a two-dimensional gravitational equation is established.

hep-th↗

arXiv2000

Conformal Anomaly and Large Scale Gravitational Coupling

We present a model in which the breackdown of conformal symmetry of a quantum stress-tensor due to the trace anomaly is related to a cosmological effect in a gravitational model. This is done by characterizing the traceless part of the quantum stress-tensor in terms of the stress-tensor of a conformal invariant classical scalar field. We introduce a conformal frame in which the anomalous trace is identified with a cosmological constant. In this conformal frame we establish the Einstein field equations by connecting the quantum stress-tensor with the large scale distribution of matter in the universe.

hep-th↗

arXiv2000

Scalar Tensor Theories and Hadamard State Condition

The Hadamard state condition is used to analyze the local constraints on the two-point function of a quantum field conformally coupled to a background geometry. Using these constraints we develop a scalar tensor theory which controls the coupling of the stress-tensor induced by the two-point function of the quantum field to the conformal class of the background metric. It is then argued that the determination of the state-dependent part of the two-point function is connected with the determination of a conformal frame. We comment on a particular way to relate the theory to a specific conformal frame (different from the background frame) in which the large scale properties are brought into focus.

hep-th↗