arXiv subjects

M. Mohsenzadeh

Publications and source records attributed to M. Mohsenzadeh.

At least 19 recordsLinked to original sources

arXiv2025

A Quadratic Equation of State to Cosmic Acceleration: Entropy Evolution and Phantom Crossing

This paper investigates the thermodynamic evolution of the universe within the framework of a quadratic equation of state (EoS). Building upon the basis of the quadratic EoS model, as a phenomenological extension to dark energy models, we analyze the implications for cosmic dynamics, including energy density evolution of effective dark matter and dark energy, entropy behavior, and convexity stability conditions. Our approach emphasizes the significance of thermodynamic principles in understanding the late-time acceleration and the crossing of the phantom divide, providing a cohesive description consistent with recent observational data. Moreover, we demonstrate that the $Λ$CDM model, regardless of entropy additivity, violates the convexity condition, while the quadratic model aligns with maximum entropy and may prevent a \textit{Big Rip} scenario.

astro-ph.CO↗

arXiv2024

Accelerating Universe in Terms of Hankel Function Index

In this paper, $F(ν)$ cosmology is proposed for the accelerating universe with asymptotic de Sitter expansion in terms of Hankel function index $ν$. To some extent, both the initial expansion during early inflation and the current accelerated expansion can be studied with a vacuum cosmic fluid i.e. $Λ$ in the pure de Sitter phase. Observational data further support the notion of a quasi-vacuum fluid, rather than a pure vacuum, contributing to the quasi-de Sitter acceleration in both the early and late universe. By examining the asymptotic expansion of the Henkel function as an approximate solution of the Mukhanov-Sasaki equation, we seek a more detailed study of quasi-de Sitter solutions in cosmology containing vacuum-like fluid.

gr-qc↗

arXiv2023

A possible role for the merger of clusters/voids in the cosmological expansion

In this study, we use the merger process of clusters/voids in the role of variable dark energy fluid to alleviate the Hubble tension, which can lead to a balance in the cosmological expansion rate. To reach this target, we will introduce a modified form of energy density for cosmic fluid with the quadratic equation of state, and then we obtain Hubble, deceleration parameters, and luminosity distance for this fluid. To obtain the merger factor and other parameters of our model, we utilize the NONLINEARMODELFIT function within MATHEMATICA. By consideration of the local and global measurements of $\rm H_0$, and the equation of state parameter $w$ as the priory values and fitting our model with Observational Hubble Data (OHD) measurements, we will show that the merger of clusters/voids plays the role of balancing the cosmic expansion rate. Also, it will be shown that the model is more compatible than $w$CDM with the standard model to describe the accelerating universe.

astro-ph.CO↗

arXiv2022

Surface Tension of Cosmic Voids as a Possible Source for Dark Energy

The cosmological constant is estimated by considering the surface tension of supervoids in a void-dominated cosmic fluid by which we can get a possible source of dark energy. Looking at voids as bubbles, we define the concept of surface tension which is shown to have an almost constant value for supervoids that are enclosed by superclusters. The surface tensions of voids are computed by dimensional method for galaxies and superclusters with different values for each group. At large scale which vast voids are dominant the positive cosmological constants obtained of order $ (\simeq+10^{-52} {\rm m^{-2}} )$, which are very close to those given by $Planck$.

astro-ph.CO↗

arXiv2020

A Covariant Approach for Particle Creation in Non-flat Background

Krein approach is used to study the particle creation during quasi-de Sitter inflation in different background space-times. In the conventional method for calculating the created particles spectrum, the background space-time is automatically considered flat. Selecting a flat background poses two fundamental problems: First, the method of calculating is not covariant relative to the curved space-time. Second, the number of created particles becomes negative. Krein approach can be considered as a covariant method to calculate two-point functions in curved space-time. So we extend this method for particle creation during early cosmic inflation. As a new proposal, we choose the background vacuum based on the smallest number of created particles in that space-time. The calculations will show that in order to solve the above mentioned problems, the background space-time must be non-flat and the number of particles in the background must be minimum.

gr-qc↗

arXiv2017

Higher Order Corrections to Asymptotic-de Sitter Inflation

Since trans-Planckian considerations can be associated with the re-definition of the initial vacuum, we investigate further the influence of trans-Planckian physics on the spectra produced by the initial quasi-de Sitter (dS) state during inflation. We use the asymptotic-dS mode to study the trans-Planckian correction of the power spectrum to the quasi-dS inflation. The obtained spectra consist of higher order corrections associated with the type of geometry and harmonic terms sensitive to the fluctuations of space-time (or gravitational waves) during inflation. As an important result, the amplitude of the power spectrum is dependent on the choice of $c$, i.e. the type of space-time in the period of inflation. Also, the results are always valid for any asymptotic dS space-time and particularly coincide with the conventional results for dS and flat space-time.

gr-qc↗

arXiv2016

Large Angular Scale CMB Anisotropy from an Excited Initial Mode

According to inflationary cosmology, the CMB anisotropy gives an opportunity to test predictions of new physics hypotheses. The initial state of quantum fluctuations is one of the important options at high energy scale, as it can affect observables such as the CMB power spectrum. In this study a quasi-de Sitter inflationary background with approximate de Sitter mode function built over the Bunch-Davies mode is applied to investigate the scale-dependency of the CMB anisotropy. The recent Planck constraint on spectral index motivated us to examine the effect of a new excited mode function (instead of pure de Sitter mode) on the CMB anisotropy at large angular scales. In so doing, it is found that the angular scale-invariance in the CMB temperature fluctuations is broken and in the limit $ \ell<200 $ a tiny deviation appears. Also, it is shown that the power spectrum of CMB anisotropy is dependent on a free parameter with mass dimension $H<<M_{*}<M_{p}$ and on the slow-roll parameter $ε$.

hep-th↗

arXiv2016

Inflation in Non-de Sitter Background with Coherent States

We use the excited coherent states built over the initial non-de Sitter modes, to study the modification of spectra of primordial scalar fluctuation. Non-de Sitter modes are actually the asymptotic solution of the inflaton field equation[JHEP 09(2014) 020]. We build excited coherent states over the non-de Sitter modes and despite the lack of interactions in the Lagrangian, we find a non-zero one-point function. It is shown that the primordial non-Gaussianity resulting from excited-de Sitter modes depend both of time and background space-time. It is very tiny of order $(\leq{10}^{-24})$, at the Planck initial fixed time that confirmed by resent observations for single field inflation but it grows in the present epoch. Moreover, our results at the leading order are similar to what obtained with general initial states and in the dS limit leads to standard results [JCAP 1202(2012) 005]. We will show that the non-dS modes and its resulting spectrum are more usable for far past time limit.

gr-qc↗

arXiv2015

Creation of Non-minimal Coupled Particle in Asymptotic de-Sitter Background

A general form of quasi-de Sitter(dS) modes in a dynamical background is used to study the creation of particle during the inflation. Actually, by considering the general form of inflaton field equation as a function of the Hankel function index and by using the Planck 2015 constraint on spectral index, we obtain the possible new constraints for the values of coupling constant in the era with quasi-dS space-time. Then, we explicitly calculate the general form of expectation value of the created particles in terms of the Hankel function index and the conformal time. As an important result, we see that the number of created particles and the value of coupling constant can be dependent to the selection of the background space-time and it's dynamics. Our result is general and confirm the conventional special results for the Minkowski and dS background.

hep-th↗

arXiv2014

An Asymptotic Method for Selection of Inflationary Modes

We present some features of early universe cosmology in terms of Hankel functions index ($ν$). Actually, the recent data from observational cosmology indicate that our universe was nearly de Sitter space-time in the early times which results in an approximate scale-invariant spectrum. This imposes some constrains on index $ν$ [1]. These constrains stimulate us to use general solution of inflaton field equation for $ν\neq{\frac{3}{2}}$. To obtain the general solution for the inflationary background, we use asymptotic expansion of Hankel functions up to non-linear order of $\frac{1}{kη}$. We consider the non-linear modes as the fundamental modes for early universe during the inflation. In this paper, we obtain the general form of the inflationary modes, scale factor expansion, equation of state and some non-linear corrections of power spectrum in terms of index ν. These results are general and in the quasi-de Sitter and de Sitter limit confirm the conventional results.

hep-th↗

arXiv2014

Particle Creation and Excited-de Sitter Modes

Recently, in Ref.\cite{moh1}, we introduced exited-de Sitter modes to study the power spectrum which was finite in Krein space quantization and the trans-Plankian corrections due to the exited modes were non-linear. It was shown that the de Sitter limit of corrections reduces to what obtained via the several previous conventional methods, moreover, with such modes the space-time symmetry becomes manifest. In this paper, inspired by Krein method and using exited-de Sitter modes as the fundamental initial states during the inflation, we calculate particle creation in the spatially flat Robertson-Walker space-time. It is shown that in de Sitter and Minkowski space-time in the far past time limit, our results coincides to the standard results.

hep-th↗

arXiv2014

Non-Linear Trans-Planckian Corrections of Spectra due to the Non-trivial Initial States

Recent Planck results motivated us to use non-Bunch-Davies vacuum. In this paper, we use the excited-de Sitter mode as non-linear initial states during inflation to calculate the corrected spectra of the initial fluctuations of the scalar field. First, we consider the field in de Sitter space-time as background field and for the non-Bunch-Davies mode, we use the perturbation theory to the second order approximation. Also, unlike conventional renormalization method, we offer de Sitter space-time as the background instead Minkowski space-time. This approach preserve the symmetry of curved space-time and stimulate us to use excited mode. By taking into account this alternative mode and the effects of trans-Planckian physics, we calculate the power spectrum in standard approach and Danielsson argument. The calculated power spectrum with this method is finite, corrections of it is non-linear, and in de Sitter limit corrections reduce to linear form that obtained from several previous conventional methods.

hep-th↗

arXiv2014

Scale-dependent power spectrum from initial excited-de Sitter modes

In this paper, we calculate corrections of scalar perturbations spectra resulting from excited-de Sitter modes as the nontrivial initial states. To obtain these modes, we consider the asymptotic expansion of the Hankel functions up to the higher order of 1/kτ . Actually the Planck and WMAP data impose some constrains on the Hankel function index. These observational constraints and back-reaction effects stimulate us to use excited-de Sitter modes. Finally, we nominate these nontrivial general solutions as the fundamental mode functions during inflation and we calculate the corrected form of scale-dependent power spectrum with trans-Planckian corrections, and in de Sitter space-time limit the results reduce to the scale-invariant power spectrum.

astro-ph.CO↗

arXiv2014

Power Spectrum with Auxiliary Fields in de Sitter Space

We use the auxiliary fields and (excited-) de Sitter solutions to study the standard power spectrum of primordial fluctuations of a scalar field in the early universe. The auxiliary fields are the negative norm solutions of the field equation and as it is shown, with a fixed boundary condition, utilizing these states results in a finite power spectrum without any infinity. The power spectrum is determined by the de Sitter solutions up to some corrections and the space-time symmetry is not broken in this point of view. The modulation to the power spectrum is of order $ (\frac{H}Λ)^{2} $, where $ H $ is the Hubble parameter and $ Λ$ is the energy scale, e.g., Planck scale.

hep-th↗

arXiv2013

Evolution of Bulk Scale Factor in Warped Space-time

In this work the role of extra dimensions in the accelerated universe through the scenario of higher-dimensional Friedmann-Robertson-Walker (FRW) cosmology has been studied. For this purpose, we first consider warped space-time in the standard flat brane scenario as the modified form of Robertson-Walker (RW) metric in five-dimension (5D) space-time and then the variation of the bulk scale factor (warp factor), with respect to both space-like and time-like extra dimensions is obtained. Finally, it is shown that both of two types of extra dimensions are important in this scenario and also the bulk scale factor plays two different roles.

astro-ph.CO↗

arXiv2012

Friedmann-like equations for High Energy Area of Universe

In this paper, evolution of the high energy area of universe, through the scenario of 5 dimensional (5D) universe, has been studied. For this purpose, we solve Einstein equations for 5D metric and 5D perfect fuid to derive Friedmann-like equations. Then we obtain the evolution of scale factor and energy density with respect to both space-like and time-like extra dimensions. We obtain the novel equations for the space-like extra dimension and show that the matter with zero pressure cannot exist in the bulk. Also, for dark energy fuid and vacuum fluid, we have both accelerated expansion and contraction in the bulk.

gr-qc↗

arXiv2012

Trans-Planckian Scale and Krein Space Quantization

In this work, Krein space quantization method is applied to eliminate the Ultraviolet divergence of Green functions. This paper shows that the power spectrum of scalar field fluctuations can be calculated in the limit of short distance physics, trans-Planckian physics, without using the usual re-normalization process.

gr-qc↗

arXiv2012

Spectrum of Gravitational Waves in Krein Space Quantization

The main goal of this paper is to derive the primordial power spectrum for the scalar perturbations generated as a result of quantum fluctuations during an inflationary pe- riod by an alternative approach of field quantization[1-3]. Formulas are derived for the gravitational waves, special cases of which include power law inflation and inflation in the slow roll approximation, in Krein space quantization.

gr-qc↗