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Amir Ghalee

Publications and source records attributed to Amir Ghalee.

10 recordsLinked to original sources
arXiv2021

Remarks on an extended $R^2$ model

Observation of the cosmic microwave background is consistent with the $R^2$ model. In the $R^2$ model, there exists a dimensionless constant which must be assigned a very large value to reconcile the model with the data. We show that the large value for this parameter can be used to propose an extended model. The scalar spectral index and the tensor-to-scalar ratio of the extended model have been obtained. Also, we obtain the three-point correlation function of the curvature perturbation to estimate the primordial non-Gaussianities of the extended model. It has been shown that the predictions of the extended model are in agreement with the observations.

gr-qc↗
arXiv2021

Refined Ephemeris for Four Hot Jupiters using Ground-Based and TESS Observations

WASP-12 b, WASP-33 b, WASP-36 b, and WASP-46 b are four transiting planetary systems which we have studied. These systems' light curves were derived from observations made by the Transiting Light Exoplanet Survey Satellite (TESS) and some ground-based telescopes. We used Exofast-v1 to model these light curves and calculate mid-transit times. Also, we plotted TTV diagrams for them using derived mid-transit times and those available within the literature. O-C analysis of these timings enables us to refine the linear ephemeris of four systems. We measured WASP-12's tidal quality factor based on adding TESS data as Q*'=(2.13+-0.29)*10^5. According to the analysis, the orbital period of the WASP-46 b system is increasing. The WASP-36 b and WASP-33 b systems have not shown any obvious quadratic trend in their TTV diagrams. The increase in their period is most likely due to inaccurate liner ephemeris that has increased over time. So, more observations are needed to evaluate whether or not there is an orbital decay in the WASP-36 b and WASP-33 b systems.

astro-ph.EP↗
arXiv2021

Observational and Theoretical Studies of 27 $δ$ Scuti Stars with Investigation of the Period-Luminosity Relation

The multi-color CCD photometric study of 27 Delta Scuti stars is presented. By using approximately three years of photometric observations, we obtained the times of maxima and magnitude changes during the observation time interval for each star. The ephemerides of our Delta Scuti stars were calculated based on the Markov Chain Monte Carlo (MCMC) method using the observed times of maxima and the period of the stars' oscillations. We used the Gaia EDR3 parallaxes to calculate the luminosities and also the absolute magnitudes of these Delta Scuti stars. The fundamental physical parameters of all the stars in our sample such as masses and radii were estimated. We determined the pulsation modes of the stars based on the pulsation constants. Moreover, the period-luminosity (P-L) relation of Delta Scuti stars was investigated and discussed. Then, by using a machine learning classification, new P-L relations for fundamental and overtone modes are presented.

astro-ph.SR↗
arXiv2019

Condensation of a scalar field non-minimally coupled to gravity in a cosmological context

We present a new mechanism to condense a scalar field coupled to the Gauss-Bonnet term. We propose a scenario in which the condensed state will emerge from the background energy density in the late-Universe. During the radiation and dust-dominated eras, the energy density of the scalar field, $ρ_φ$, decreases at a slower rate than the background density. Eventually, $ρ_φ$ dominates over the energy density of dust and the scalar field could be condensed. In the condensed phase, we have the de Sitter phase for the universe with $ρ_φ=3H_0^2M_P^2$. Moreover, we study the cosmological perturbations of the model and explore predictions of the model.

gr-qc↗
arXiv2016

Notes on diffeomorphisms symmetry of $f(R)$ gravity in the cosmological context

We study the metric perturbations in the context of restricted $f(R)$ gravity, in which a parameter for deviation from the full diffeomorphisms of space-time is introduced. We demonstrate that one can choose the parameter to remove the induced anisotropic stress, which is present in the usual $f(R)$ gravity. Moreover, to prevent instability for the vector and tensor metric perturbations, some constraints on the model are obtained.

gr-qc↗
arXiv2014

Notes on a scalar field with a kinetic term non-minimally coupled to gravity

We explore the inflationary phase of a scalar field with a kinetic term non-minimally coupled to gravity. We find that one of the slow-roll conditions is naturally consequence of the equation of motion of the scalar field. Thus, slow-roll conditions impose fewer constraints on potentials than other inflationary models. Moreover, it is demonstrated that the inflationary phase can be described by just one slow-roll parameter. By investigating the metric perturbations, it is shown that except for one potential, almost all potentials have the same pattern in the ($n_{s}$, $r$) plane. We provide an exact solution for the exceptional case. The exact solution represents the condensed scalar field and results in an accelerated expansion.

astro-ph.CO↗
arXiv2013

Curiosity about the dust matter in the cosmological context

We propose a model for the dust matter in the cosmological context. The model contains a scalar field with a kinetic term nonminimally coupled to gravity. By investigating the background and perturbative equations, it is demonstrated that the scalar field has the same dynamics as the dust matter. We have also considered the cosmological constant in the model. It turns out that the model has not exotic behavior. Thus, a universe including the scalar field and the cosmological constant evolves just as our Universe. Moreover, we have added the quadratic term in the action. It is shown that the quadratic term can be ruled out by its consequences.

astro-ph.CO↗
arXiv2013

A new phase of scalar field with a kinetic term non-minimally coupled to gravity

We consider the dynamics of a scalar field non-minimally coupled to gravity in the context of cosmology. It is demonstrated that there exists a new phase for the scalar field, in addition to the inflationary and dust-like (reheating period) phases. Analytic expressions for the scalar field and the Hubble parameter, which describe the new phase are given. The Hubble parameter indicates an accelerating expanding Universe. We explicitly show that the scalar field oscillates with time- dependent frequency. Moreover, an interaction between the scalar field in the new phase and other fields is discussed. It turns out that the parametric resonance is absent, which is another crucial difference between the dynamics of the scalar field in the new phase and dust-like phase.

astro-ph.CO↗
arXiv2012

A new perspective on Gauge-flation

Recently Maleknejad and Sheikh-Jabbari have proposed a new model for inflation with non-Abelian gauge fields (Gauge-flation), and they have studied the model by numerical methods\cite{Sheikh}. In this model, the isotropy of space-time is recovered by suitable combination of gauge configurations, and a scalar field is constructed by gauge field and the scale factor, which produces inflation period. In this work, exact solutions for the scalar field and the Hubble parameter are presented and we provide analytic solutions for the numerical results. We explicitly present Hubble parameter and fields as functions of time and it is also demonstrated that in some conditions they are damped oscillator. Moreover, reheating period in the model is discussed.

gr-qc↗