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H. Mohseni Sadjadi

Publications and source records attributed to H. Mohseni Sadjadi.

At least 37 records · Page 2Linked to original sources
arXiv2019

No scalar hair in nonminimal derivative coupling model for Neumann and Dirichlet reflecting star

We investigate the existence of scalar hair in the background of a reflecting star. The kinetic part of the scalar field is nonminimally coupled to the Einstein tensor. For both the Dirichlet and the Neumann boundary conditions, we study the no-hair theorem. For the massless case, we show that there is no scalar hair, and in the massive case conditions leading to the no-hair theorem are specified.

gr-qc↗
arXiv2019

No skyrmion hair for stationary spherically symmetric reflecting stars

We investigate the existence of the skyrmion field in the background of an asymptotically flat stationary reflecting star. For this purpose, we consider the Einstein Skyrme system for which there is a skyrmion hair in the black hole case. We discuss spherically symmetric skyrmions and employ the hedgehog ansatz for the skyrmion field. We show that, in contrast to the black hole there is no skyrmion hair for a reflecting star.

hep-th↗
arXiv2019

Reheating in a modified teleparallel model of inflation

We study the cosmological inflation and reheating in a teleparallel model of gravity. Reheating is assumed to be due to the decay of a scalar field to radiation during its rapid oscillation. By using cosmological perturbations during inflation, and subsequent evolutions of the Universe, we calculate the reheating temperature as a function of the spectral index and the power spectrum.

physics.gen-ph↗
arXiv2018

Cosmic acceleration in screening hybrid quintessence model

A formalism for the appearance of dark energy in the matter dominated era, leading to a persistent de Sitter expansion at the late time is proposed. Our framework is a hybrid quintessence model with a non-minimal coupling to the Ricci scalar. Coupling to the curvature triggers the dark energy evolution, while the coupling of scalar fields drives this evolution to a de Sitter stable fixed point. These occur via successive $Z_2$ symmetry breakings.

gr-qc↗
arXiv2018

Cosmic acceleration and de Sitter expansion in hybrid mass varying neutrino model

A nonminimally coupled hybrid dark energy model is introduced. The dark energy evolution is triggered by mass varying neutrinos. Quintessence evolution begins from an initial point, with an insignificant dark energy density, and arrives at a final de Sitter attractor. While initially, for a Higgs-like potential, the non-minimal coupling keeps the system in a state with negligible dark energy density, finally the second quintessence conducts the evolution to a stable fixed point. These evolutions occur through successive $Z_2$ symmetry breakings.

gr-qc↗
arXiv2018

Symmetry breaking, and the effect of matter density on neutrino oscillation

A proposal for the neutrino mass, based on neutrino-scalar field interaction, is introduced. The scalar field is also non-minimally coupled to the Ricci scalar and hence relates the neutrino mass to the matter density. In a dense region, the scalar field obeys the $Z_2$ symmetry, and the neutrino is massless. In a dilute region, the $Z_2$ symmetry breaks and neutrino acquires mass from the non-vanishing expectation value of the scalar field. We consider this scenario in the framework of a spherical dense object whose outside is a dilute region. In this background, we study the neutrino flavors oscillation, along with the consequences of the theory on oscillation length and MSW effect. This preliminary model may shed some lights on the existing anomalies within the neutrino data, concerning the different oscillating behavior of the neutrinos in regions with different densities.

hep-ph↗
arXiv2017

Warm inflation with an oscillatory inflaton in the non-minimal kinetic coupling model

In the cold inflation scenario, the slow roll inflation and reheating via coherent rapid oscillation, are usually considered as two distinct eras. When the slow roll ends, a rapid oscillation phase begins and the inflaton decays to relativistic particles reheating the Universe. In another model dubbed warm inflation, the rapid oscillation phase is suppressed, and we are left with only a slow roll period during which the reheating occurs. Instead, in this paper, we propose a new picture for inflation in which the slow roll era is suppressed and only the rapid oscillation phase exists. Radiation generation during this era is taken into account, so we have warm inflation with an oscillatory inflaton. To provide enough e-folds, we employ the non-minimal derivative coupling model. We study the cosmological perturbations and compute the temperature at the end of warm oscillatory inflation.

gr-qc↗
arXiv2017

Symmetron and de Sitter attractor in a teleparallel model of cosmology

In the teleparallel framework of cosmology, a quintessence with non-minimal couplings to the scalar torsion and a boundary term is considered. A conformal coupling to matter density is also taken into account. It is shown that the model can describe the onset of cosmic acceleration after an epoch of matter dominated era, where dark energy is negligible, via $Z_2$ symmetry breaking. While the conformal coupling holds the Universe in a state with zero dark energy density in the early epoch, the non-minimal couplings lead the Universe to a stable state with de Sitter expansion at the late time.

gr-qc↗
arXiv2017

Mass varying neutrinos, symmetry breaking, and cosmic acceleration

We introduce a new proposal for the onset of cosmic acceleration based on mass-varying neutrinos. When massive neutrinos become nonrelativistic, the $Z_2$ symmetry breaks, and the quintessence potential becomes positive from its initially zero value. This positive potential behaves like a cosmological constant at the present era and drives the Universe's acceleration during the slow roll evolution of the quintessence. In contrast to $Λ$CDM model, the dark energy in our model is dynamical, and the acceleration is not persistent. Contrary to some of the previous models of dark energy with mass-varying neutrinos, we do not use the adiabaticity condition which leads to instability.

gr-qc↗
arXiv2017

Parameterized post Newtonian approximation in a teleparallel model of dark energy with a boundary term

We study the parameterized post-Newtonian approximation in teleparallel model of gravity with a scalar field. The scalar field is non-minimally coupled to the scalar torsion as well as to the boundary term introduced in [1]. We show that, in contrast to the case where the scalar field is only coupled to the scalar torsion, the presence of the new coupling affects the parameterized post-Newtonian parameters. These parameters for different situations are obtained and discussed.

gr-qc↗
arXiv2016

Symmetry breaking and the onset of cosmic acceleration in scalar field models

We propose a new scenario for the onset of positive acceleration of our Universe based on symmetry breaking in coupled dark energy scalar field model. In a symmetry breaking process where the scalar field rolls down its own potential, the potential reduction is not in favor of acceleration. In our model, when dark matter density becomes less than a critical value, the shape of the effective potential is changed and, the quintessence field climbs up along {\it{its own potential}} while rolls down the effective potential. We show that this procedure may establish the positivity of the potential required for the Universe to accelerate. In addition, we show that by choosing an appropriate interaction between dark sectors there is the possibility that the scalar field resides in a new vacuum giving rise to a positive cosmological constant which is responsible for a permanent late time acceleration.

gr-qc↗
arXiv2016

Thawing in a coupled quintessence model

We consider the thawing model in the framework of coupled quintessence model. The effective potential has $Z_2$ symmetry which is broken spontaneously when the dark matter density becomes less than a critical value leading the quintessence equation of state parameter to deviate from -1. Conditions required for this procedure are obtained and analytical solution for the equation of state parameter is derived.

gr-qc↗
arXiv2015

Temperature in warm inflation in non minimal kinetic coupling model

Warm inflation in the non minimal derivative coupling model with a general dissipation coefficient is considered. We investigate conditions for the existence of the slow roll approximation and study cosmological perturbations. The spectral index, and the power spectrum are calculated and the temperature of the universe at the end of the slow roll warm inflation is obtained.

gr-qc↗
arXiv2014

Rapid Oscillatory quintessence with variable equation of state parameter in non minimal derivative coupling model

The rapid oscillating scalar field is considered as the quintessence in the framework of nonminimal kinetic coupling model. The scalar field behaves like a perfect fluid with a variable equation of state parameter which can be expressed as a function of the Hubble parameter. This feature enables us to study its evolution via continuity equation. The scalar field may behave as dark energy in the present epoch, while it behaves like dark matter in the future. This characteristic allows the occurrence of a decelerated expansion after the present acceleration, without invoking any interaction. The model is also studied in the presence of an interaction between dark sectors alleviating the coincidence problem. The stability of the model is investigated and the stable attractor solutions are studied.

gr-qc↗
arXiv2014

Oscillatory inflation in non-minimal derivative coupling model

Inflation during rapid oscillation of a scalar field in non-minimal derivative coupling model is discussed. Cosmological perturbations originated in this stage are studied and consistency of the results with observational constraints coming from Planck 2013 data are investigated.

astro-ph.CO↗
arXiv2013

On solutions of loop quantum cosmology

Loop quantum cosmology is considered in inflation era. A slow roll scalar field solution with power law potential is presented in the neighborhood of transition time, i.e. when the universe enters inflation phase from super-inflation. The compatibility of the model with Planck 2013 data is discussed. The domain of validity of second and the generalized second laws of thermodynamics for this solution and some other examples is studied.

gr-qc↗
arXiv2013

Reheating temperature in non-minimal derivative coupling model

We consider the inflaton as a scalar field described by a non-minimal derivative coupling model with a power law potential. We study the slow roll inflation, the rapid oscillation phase, the radiation dominated and the recombination eras respectively, and estimate e-folds numbers during these epochs. Using these results and recent astrophysical data we determine the reheating temperature in terms of the spectral index and the amplitude of the power spectrum of scalar perturbations.

gr-qc↗