arXiv subjects

Haidar Sheikhahmadi

Publications and source records attributed to Haidar Sheikhahmadi.

27 records · Page 2Linked to original sources

arXiv2019

Non-canonical scalar field in low anisotropy universe with intermediate inflation

The behaviour of a non-canonical scalar field within an anisotropic Bianchi type I, spatially homogeneous, Universe in the framework of the intermediate inflation will be studied. It will be examined on the condition that both the anisotropy and non-canonical sources come together and is there any improvement in compatibility with the observational data originated from plank $2015$?. Based on this investigation it can be observed that automatically a steep potential which can manage inflation in a better way will be obtained. Additionally, as a common procedure for an inflationary study, we shall try to calculate the related inflationary observables such as the amplitude of the scalar perturbations, scalar and tensor spectral indices, tensor-to-scalar ratio, the running spectral index, and the number of e-folds. As an exciting part of our results, we will find that our model has a good consistency compared to data risen by CMB and different Planck results. To justify our claims, the well known canonical inflationary scenario in an anisotropic Bianchi type I Universe also will be evaluated.

physics.gen-ph↗

arXiv2019

Schwinger-Keldysh mechanism in extended quasi single field inflation

We study an extension of quasi-single field model of inflation containing multiple semi-heavy isocurvaton fields using the Schwinger-Keldysh mechanism. We calculate the amplitudes and the shapes of the bispectrum and the trispectrum. We show that the diagrammatic approach associated with the Schwinger-Keldysh mechanism simplifies the analysis considerably compared to standard in-in formalism. This method is helpful to study the spectroscopy of masses and couplings of light and semi-heavy fields during inflation.

gr-qc↗

arXiv2018

Comments on $^"$Cosmic evolution in Brans-Dicke chameleon cosmology$^"$

The authors of Ref. \cite{1-2}, investigated cosmic evolution in an external interacting model of scalar tensor gravity namely Brans Dicke chameleon scenario. The procedure of this work contains novelties but, it shall be observed from this comment, their report faces three fundamental drawbacks. One of them concerns the energy conservation equation, and the other two flaws are about mathematical mistakes. In the scalar tensor gravity models, a well-known method, in order to obtain conservation equation one must combine Friedmann equations with modified Klein-Gordon equation. But in Ref. \cite{1-2}, by virtue of the mentioned approach one would not be able to achieve a correct result for conservation equation. In addition, one can readily realize that their mathematical mistakes lead to different results compared to the present report.

gr-qc↗

arXiv2016

Hamilton-Jacobi formalism for inflation with non-minimal derivative coupling

In inflation with nonminimal derivative coupling there is not a conformal transformation to the Einstein frame where calculations are straightforward, and thus in order to extract inflationary observables one needs to perform a detailed and lengthy perturbation investigation. In this work we bypass this problem by performing a Hamilton-Jacobi analysis, namely rewriting the cosmological equations considering the scalar field to be the time variable. We apply the method to two specific models, namely the power-law and the exponential cases, and for each model we calculate various observables such as the tensor-to-scalar ratio, and the spectral index and its running. We compare them with 2013 and 2015 Planck data, and we show that they are in a very good agreement with observations.

gr-qc↗

arXiv2016

The effect of de Sitter like background on increasing the zero point budget of dark energy

During this work, using subtraction renormalization mechanism, zero point quantum fluctuations for bosonic scalar fields in a de-Sitter like background are investigated. By virtue of the observed value for spectral index, $n_s(k)$, for massive scalar field the best value for the first slow roll parameter, $ε$, is achieved. In addition the energy density of vacuum quantum fluctuations for massless scalar field is obtained. The effects of these fluctuations on other components of the Universe are studied. By solving the conservation equation, for some different examples, the energy density for different components of the Universe are obtained. In the case which, all components of the Universe are in an interaction, the different dissipation functions, $\tilde{Q}_{i}$, are considered. The time evolution of ${ρ_{DE}(z)}/{ρ_{cri}(z)}$ shows that $\tilde{Q}=3 γH(t) ρ_{m}$ has best agreement in comparison to observational data including CMB, BAO and SNeIa data set.

gr-qc↗

arXiv2016

Interacting scalar tensor cosmology in light of SNeIa, CMB, BAO and OHD observational data sets

During this work, an interacting chameleon like scalar field scenario, by considering SNeIa, CMB, BAO and OHD data sets is investigated. In fact, the investigation is realized by introducing an ansatz for the effective dark energy equation of state, which mimics the behaviour of chameleon like models. Based on this assumption, some cosmological parameters including Hubble, deceleration and coincidence parameters in such mechanism are analysed. It is realized that, to estimate the free parameters of a theoretical model, by regarding the systematic errors it better the whole of the above observational data sets to be considered. In fact, if one considers SNeIa, CMB and BAO but disregards OHD it maybe leads to different results. Also to get a better overlap between the counters with the constraint $χ_{\rm{m}}^2\leq 1$, the $χ_{\rm{T}}^2$ function could be re-weighted. The relative probability functions are plotted for marginalized likelihood $\mathcal{L} (Ω_{\rm{m0}} ,ω_1, β)$ according to two dimensional confidence levels $68.3\%$, $90\%$ and $95.4\%$. Meanwhile, the value of free parameters which maximize the marginalized likelihoods using above confidence levels are obtained. In addition, based on these calculations the minimum value of $χ^2$ based on free parameters of an ansatz for the effective dark energy equation of state are achieved.

astro-ph.CO↗

arXiv2015

Non-commutative and commutative vacua effects in a scalar torsion scenario

In this work, the effects of non-commutative and commutative vacua on the phase space generated by a scalar field in a scalar torsion scenario are investigated. For both classical and quantum regimes, the commutative and non-commutative cases are compared. To take account the effects of non-commutativity, two well known non-commutative parameters, $θ$ and $β,$ are introduced. It should be emphasized, the effects of $β$ which is related to momentum sector has more key role in comparison to $θ$ which is related to space sector. Also the different boundary conditions and mathematical interpretations of non-commutativity are explored.

gr-qc↗

arXiv2015

Non-local scalar fields inflationary mechanism in light of Planck $2013$

A generalization of the canonical and non-canonical theory of inflation is introduced in which the kinetic energy term in action is written as non-local term. The inflationary universe within the framework of considering this non-locality will be studied. To investigate the effects of non-locality on the inflationary parameters we consider two well known models of inflationary scenario includes of chaotic and exponential inflation proposals. For such scenarios some important parameters include slow roll parameters, scalar and tensor power spectra, spectral indices, the tensor-to-scalar ratio and so on for both mentioned models, chaotic and exponential inflationary scenarios, will be calculated. Also the Hamilton-Jacobi formalism, as an easiest way to study the effect of perturbation based on e-folding number $N$, to investigate inflationary attractors will be used. The free theoretical parameters of this model will be compared with observations by means of Planck $2013$, $WMAP9+eCMB+BAO+H_0$ data sets in addition to $BICEP2$ data surveying. It will be shown that our theoretical results are in acceptable range in comparison to observations. For instance the tensor-to-scalar ratio for exponential potential, by considering $BICEP2$ is in best agreement in comparison with chaotic inflation.

gr-qc↗

arXiv2011

$γ$ Parameter and Solar System constraint in Chameleon Brans Dick Theory

The post Newtonian parameter is considered in the Chameleon-Brans-Dicke frame. In the first step, the general form of this parameter and also effective gravitational constant is obtained. An arbitrary function for $f(Φ)$, which indicates the coupling between matter and scalar field, is introduced to investigate validity of solar system constraint. It is shown that Chameleon-Brans-Dicke model can satisfy the solar system constraint and gives us an $ω$ parameter of order $10^4$, which is in comparable to the constraint which has been indicated in {\cite{17-a}}.

gr-qc↗