arXiv subjects

Mohammad Hossein Namjoo

Publications and source records attributed to Mohammad Hossein Namjoo.

44 records · Page 3Linked to original sources

arXiv2013

Cosmological Perturbations in Inflationary Models with Anisotropic Space-Time Scaling in Lifshitz Background

Models of inflation in a gravitational background with an anisotropic space-time scaling are studied. The background is a higher-dimensional Lifshitz throat with the anisotropy scaling $z\neq 1$. After the dimensional reduction, the four-dimensional general covariance is explicitly broken to a three-dimensional spatial diffeomorphism. As a result the cosmological perturbation theory in this set up with less symmetries have to be formulated. We present the consistent cosmological perturbation theory for this set up. We find that the effective four-dimensional gravitational wave perturbations propagate with a different speed than the higher dimensional gravitational excitations. Depending on the model parameters, for an observer inside the throat, the four-dimensional gravitational wave propagation can be superluminal. We also find that the Bardeen potential and the Newtonian potential are different. This can have interesting observational consequences for lensing and CMB fluctuations. Furthermore, we show that at the linearized level the inflaton field excitations vanish.

hep-th↗

arXiv2013

Hemispherical Asymmetry and Local non-Gaussianity: a Consistency Condition

In this paper we provide a consistency relation between the amplitude of the hemispherical bipolar asymmetry, $A$, and the amplitude of the primordial non-Gaussianity in the squeezed limit, $f_{NL}$, as $|A| \lesssim 10^{-1} f_{NL}$. We demonstrate that this consistency condition is at work for any model of inflation in which the curvature perturbations is sourced by a single light field with the Bunch-Davies initial condition, irrespective of the number of inflation fields which contribute to the background inflationary expansion. As a non-trivial example, we show that observable hemispherical asymmetry can be generated in single field non-attractor inflationary models. We also study hemispherical asymmetry generated in the models of multiple fields inflation. We show that $A$ is controlled by the weighted sum of non-Gaussianity contribution from each field. In particular, we show that observable hemispherical asymmetry can be generated in models where inhomogeneities are generated from a light scalar field modulating the surface of end of inflation.

astro-ph.CO↗

arXiv2013

Multiple Inflationary Stages with Varying Equation of State

We consider a model of inflation consisting a single fluid with a time-dependent equation of state. In this phenomenological picture, two periods of inflation are separated by an intermediate non-inflationary stage which can be either a radiation dominated, matter dominated or kinetic energy dominated universe, respectively, with the equation of state $w=1/3$, 0 or 1. We consider the toy model in which the change in $w$ happens instantaneously. Depending on whether the mode of interest leaves the horizon before or after or between the phase transitions, the curvature power spectrum can have non-trivial sinusoidal modulations. This can have interesting observational implications for CMB anisotropies and for primordial black-hole formation.

hep-th↗

arXiv2013

Violation of non-Gaussianity consistency relation in a single field inflationary model

In this paper we present a simple, toy model of single field inflation in which the standard non-Gaussianity consistency condition is violated. In this model the curvature perturbations on super-horizon scales are not conserved and the decaying modes of perturbations are not negligible in the non-atractor phase. As a result a large local non-Gaussianity can be obtained in the squeezed limit which violates the standard non-Gaussianity consistency condition for the single field models.

astro-ph.CO↗

arXiv2013

Modulated curvaton decay

We study primordial density perturbations generated by the late decay of a curvaton field whose decay rate may be modulated by the local value of another isocurvature field, analogous to models of modulated reheating at the end of inflation. We calculate the primordial density perturbation and its local-type non-Gaussianity using the sudden-decay approximation for the curvaton field, recovering standard curvaton and modulated reheating results as limiting cases. We verify the Suyama-Yamaguchi inequality between bispectrum and trispectrum parameters for the primordial density field generated by multiple field fluctuations, and find conditions for the bound to be saturated.

hep-th↗

arXiv2012

Curvaton and the inhomogeneous end of inflation

We study the primordial density perturbations and non-Gaussianities generated from the combined effects of an inhomogeneous end of inflation and curvaton decay in hybrid inflation. This dual role is played by a single isocurvature field which is massless during inflation but acquire a mass at the end of inflation via the waterfall phase transition. We calculate the resulting primordial non-Gaussianity characterized by the non-linearity parameter, $f_{NL}$, recovering the usual end-of-inflation result when the field decays promptly and the usual curvaton result if the field decays sufficiently late.

astro-ph.CO↗

arXiv2011

DBI Lifshitz Inflation

A new model of DBI inflation is introduced where the mobile brane, the inflaton field, is moving relativistically inside a Lifshitz throat with an arbitrary anisotropic scaling exponent $z$. After dimensional reduction to four dimension the general covariance is broken explicitly both in the matter and the gravitational sectors. The general action for the metric and matter field perturbations are obtained and it is shown to be similar to the classifications made in the effective field theory of inflation literature.

hep-th↗

arXiv2011

Curvature Perturbations and non-Gaussianities from Waterfall Phase Transition during Inflation

We consider a variant of hybrid inflation where the waterfall phase transition happens during inflation. By adjusting the parameters associated with the mass of the waterfall field we arrange that the phase transition is not sharp so inflation can proceed for about 50-60 e-folds after the waterfall phase transition. We show that one can work in the limit where the quantum back-reactions are subdominant compared to the classical back-reactions. It is shown that significant amount of large scale curvature perturbations are induced from the entropy perturbations. The curvature perturbations spectral index is either blue or red depending on whether the mode of interest leaves the horizon before the phase transition or after the phase transition. This can have interesting observational consequences on CMB. The non-Gaussianity parameter $f_{NL}$ is calculated to be $\lesssim 1$ but much bigger than the slow-roll parameters.

astro-ph.CO↗