arXiv subjects

Khaled Saaidi

Publications and source records attributed to Khaled Saaidi.

At least 19 recordsLinked to original sources

arXiv2022

Brane inflation: swampland criteria, TCC, and reheating predictions

We consider inflation in a five -dimensional space time with the inflaton field confined to live on a brane world. In this scenario, we study different types of potentials for the inflaton, discuss their observational consequences, and compare with data. We find that some class of potentials are in good agreement with observation and that the value of the inflaton field can be sub-Planckian. Moreover, we investigate the swampland criteria in this scenario and determine the consistency of the model with the conjectures. Doing so, we could determine models that simultaneously satisfy both observational data and swampland criteria. More constraints are applied by studying the reheating phase where the acceptable range for the reheating temperature imposes some bounds on the models. As the last step, the result of trans-Planckian censorship conjecture for the model is considered where it is shown the constraint of TCC will be very strong and it could be used to applied limit on the brane tension.

gr-qc↗

arXiv2020

Revisiting Scalar Tensor inflation by swampland criteria and Reheating

The scenario of the slow-roll inflation is studied in the frame of the scalar-tensor theory of gravity where the scalar field has a non-minimal coupling to the geometric part. After deriving the main dynamical and perturbation equations, the model is investigated in detail for different functions of the potential and the coupling function. Considering the consistency of the model with observational model results into specific ranges for the free constants of the model. Then, the obtained ranges are reconsidered to realize whether they satisfy the swampland criteria. It is found that the swampland criteria impose another limitation and reduce the ranges. Using the final outcomes for the free constants of the model, we briefly consider the reheating phase to find out about the number of e-fold for the phase and the reheating temperature.

gr-qc↗

arXiv2020

Brane Constant-roll Inflation

The scenario of constant-roll inflation in the frame of the RSII brane gravity model is considered. Based on the scenario, the smallness of the second slow-roll parameter is released and it is assumed as a constant which could be of the order of unity. Applying the Hamilton-Jacobi formalism, the constancy of the parameter gives a differential equation for the Hubble parameter which leads to an exact solution for the model. Reconsidering the perturbation equations we show there are some modified terms appearing in the amplitude of the scalar perturbations and in turn in the scalar spectral index and tensor-to-scalar ratio. Comparing the theoretical results of the model with observational data, the free parameters of the model are determined. Then, the consistency of the model with the swampland criteria is investigated for the obtained values of the free parameters. As the final step, the attractor behavior of the model is considered.

gr-qc↗

arXiv2020

Warm Tachyon Inflation and Swampland Criteria

The scenario of two components warm tachyon inflation is considered where the tachyon field plays the role of inflaton and drives inflation. During inflation, the tachyon scalar field interacts with the other component of the Universe which is considered as photon gas, i.e. radiation. The interacting term contains a dissipation coefficient, and the study is modeled based on two different and familiar choices of the coefficient that have been {studied} in the literature. By applying the latest observational data, the acceptable ranges for the free parameters of the model are obtained. For any choice inside the {estimated} ranges, there is an acceptable concordance between the theoretical predictions and observations. Whereas the model is established based on some assumptions, it is vital to check their validity for the obtained values of the free parameters of the model. It is realized that the model is not self-consistent for all values of the ranges and sometimes the assumptions are violated. Therefore, {to have both self-consistency and agreement with data} the parameters of the model need to be {constrained} again. After that, we are going to consider the recently proposed swampland conjecture, which imposes two conditions on the inflationary models. These criteria could rule out some of the inflationary models, however, warm inflation is known as one of those models that could successfully satisfy the swampland criteria. A precise investigation determines that the proposed warm tachyon inflation could not satisfy the swampland criteria for some cases. In fact, for the first case of the dissipation coefficient, where there is dependency only on the scalar field, the model could agree with observational data ...

gr-qc↗

arXiv2020

Renyi entropy and the holographic dark energy in flat space time

Based on Renyi entropy, we study the entropy corrected version of the holographic dark energy (HDE) model in apparent horizon of spatially flat FLRW universe. Applying the generalized entropy leads to the modified version of the Friedmann evolution equations which besides pressure-less matter and HDE, there is an extra term that is purely geometric. This extra term are assumed as another part of dark energy. We assume the universe is filled by non-interacting components of ideal fluids such as dark matter and holographic dark energy. The total dark energy, which is a combination of generalized HDE and geometric part, has a density parameter that approaches one by decreasing the redshift. Considering the total equation of state parameter and deceleration parameter of the universe indicates that the universe could stays in positive accelerated expansion phase that shows an agreement with observational data, only for the specific values of the constant $\zeta$.

astro-ph.CO↗

arXiv2019

Beta-function formalism for k-essence constant-roll inflation

The beta function formalism is being used to study the constant-roll inflation in the k-essence model. Assuming the second slow-roll parameter as a constant leads to a first-order differential equation for $β$-function which is much easier to solve and find a solution than the second (non-linear) order equation that we have in corresponding standard constant-roll inflation. Many cosmological models are known as a subclass of k-essence, so we will try to consider the model as general as possible. It is determined that the second slow-roll parameter should be positive to produce a small value for the first slow-roll parameter. The scenario is considered for three well-known cosmological models, and it is clarified that for the non-canonical and tachyon model, the scalar spectral index never reaches the observational range, however for the DBI model, we could arrive at a result consistent with observational data.

gr-qc↗

arXiv2019

Observational constraints on DBI constant-roll inflation

Using the constant-roll approach, DBI inflationary scenario will be studied and it is sought to compare the result with observational data. By considering the cosmological perturbations of the model, it is realized that some extra terms appear in the amplitude of scalar perturbations which indicate that there shuold be a modified version of the scalar spectral index and tensor-to-scalar ratio. In order to compare the model with observational data, some specific functions of scalar field are assumed for the function $f(ϕ)$ function. For the power-law and exponential functions, a constant slow-roll parameter $ε$ is obtained which produced difficulties for the graceful exit from inflation. Then, a product of linear and exponential function, and also a hyperbolic function of scalar field are selected for $f(ϕ)$ that result in a $ε$ with an end for accelerated expansion phase. Considering the scalar spectral index, the amplitude of scalar perturbations and tensor-to-scalar ratio shows that for some values of the constant $η= β$ there could be a good consistency between the model predictions and observational data. In addition, based on the form of the equation of motion of the scalar field, a new interesting definition for the second slow-roll parameters is presented and the behavior of the perturbations is reconsidered. The results come to good consistency with data. Finally, the attractor behavior of these two cases is investigated and it is determined that this feature could be satisfied.

gr-qc↗

arXiv2019

Constant-roll approach to non-canonical inflation

The scenario of constant-roll inflation is studied where the inflaton is a scalar field with modified kinetic term, known as non-canonical scalar field. This modification leads to some changes in the slow-roll parameters, and also by taking the second slow-roll parameter as a constant, the differential equation for the Hubble parameter is altered as well. Assuming $η=β$ and reconsidering the perturbation equations makes it clear that there should be some modified terms in the scalar spectral index and amplitude of scalar perturbations. After finding the exact solution, the main perturbation parameters are obtained at the horizon crossing time. Then by plotting the $r-n_s$ diagram it is shown that for some specific values of $β$ one could find a good agreement with observational data. In addition, considering the attractor behavior of the model leads to this result that this feature could be appropriately satisfied.

astro-ph.CO↗

arXiv2019

Constraining chameleon field driven warm inflation with Planck 2018 data

We investigate warm inflationary scenario in which the accelerated expansion of the early Universe is driven by chameleon-like scalar fields. Due to the non-minimal coupling between the scalar field and the matter sector, the energy-momentum tensor of each fluid component is not conserved anymore, and the generalized balance equation is obtained. The new source term in the energy equation can be used to model warm inflation. On the other hand, if the coupling function varies slowly, the model reduces to the standard model used for the description of cold inflation. To test the validity of the warm chameleon inflation model, the results for warm inflationary scenarios are compared with the observational Planck2018 Cosmic Microwave Background data. In this regard, the perturbation parameters such as the amplitude of scalar perturbations, the scalar spectral index and the tensor-to-scalar ratio are derived at the horizon crossing in two approximations, corresponding to the weak and strong dissipative regimes. As a general result it turns out that the theoretical predictions of the chameleon warm inflationary scenario are consistent with the Planck 2018 observations.

gr-qc↗

arXiv2018

Anisotropy effects on Baryogenesis in $f(R)$-Theories of Gravity

We study the $f(R)$ theory of gravity in an anisotropic metric and its effect on the baryon number to entropy ratio. The mechanism of gravitational baryogenesis based on the CPT-violating gravitational interaction between derivative of the Ricci scalar curvature and the baryon-number current is investigated in the context of the $f(R)$ gravity. The gravitational baryogenesis in the Bianchi type I universe is examined. We survey the effect of anisotropy of the universe on the baryon asymmetry from point of view the $f(R)$-theories of gravity and its effect on $n_{b}/s$ for radiation dominant regime.

physics.gen-ph↗

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↗

arXiv2014

(Non-) geodesic motion in chameleon Brans Dicke model

Based on \cite{16}, we assume there is a non-minimal coupling between the scalar field and matter in Brans-Dicke model. We analyzes the motion of different matter such as, massless scalar field, photon, massless perfect fluid (dust), massive perfect fluid and point particle matter in this theory. We show that the motion of massless scalar field and photon can satisfy null geodesic motion only in high frequency limit. Also we find that the motion of the dust and massive perfect fluid is geodesic for $L_m=-P$ and is non-geodesic for $L_m=\R$. Finally, we study the motion of point particle and show that the motion of this kind of matter is non-geodesic.

gr-qc↗

arXiv2014

Brane Cosmology with the Chameleon Scalar Field in Bulk

In this work we investigate the effect of a kind of scalar field, called chameleon, on the evolution of universe. We put this scalar field in the bulk. It is displayed that this scalar field gives us an exponential expansion in early time which may concern inflation. Interaction between scalar field and matter bring some complications in our analysis, however it is shown that by defining an effective potential, we could recover conventional equation at inflation era. After inflation, and entering the universe in radiation era, the exponential expansion is omitted. Also, in late time the universe possess an accelerated expansion. In the last section, the validity of generalized second law of thermodynamic, with assumption the validity of first law is considered.

physics.gen-ph↗

arXiv2013

Nonflat time-variable dark energy cosmology

We generalize the time-variable dark energy scalar field $Φ$ model ($Φ$CDM) to nonflat space. We show that even in the space-curvature-dominated epoch the scalar field solution is a time-dependent fixed point or attractor, with scalar field energy density that grows relative to the energy density in spatial curvature. This is the first example of a physically consistent and complete model of dynamical dark energy in a nonflat geometry.

astro-ph.CO↗