arXiv subjects

Anupam Mazumdar

Publications and source records attributed to Anupam Mazumdar.

285 records · Page 16Linked to original sources

arXiv2000

Brans-Dicke brane cosmology

A five dimensional brane cosmology with non-minimally coupled scalar field to gravity has been considered in a Jordan-Brans-Dicke frame. We derive an effective four dimensional field equations on a 3+1 dimensional brane where the fifth dimension has been assumed to have an orbifold symmetry. We have noticed that the evolution equation for the matter component stuck to the brane is non-trivially coupled to the scalar field living on the brane and the bulk. Finally we discuss some cosmological consequences of this set-up.

gr-qc↗

arXiv2000

Gravitino production in hybrid inflationary models

It has been recently shown that it is possible to excite gravitinos in an expanding background due to a time varying scalar field oscillating at the bottom of the inflationary potential. The two components of the gravitino, namely helicity 1/2 and helicity 3/2, are excited differently due to the presence of different time varying mass scales in the problem. In this paper we analyse the production of both the helicities in a multi-chiral scenario, in particular focusing on a general model of hybrid inflation. Fermion production in hybrid models is very much different from that of the chaotic models discussed so far in the literature. In this paper we give a full account of gravitino production analytically and numerically. It is noticed that the creation of gravitinos does not take place in the first few oscillations of the inflaton field, rather the production is a gradual and delayed process. It takes roughly 30-40 oscillations to build up the production and for the saturation to take place it can even take longer time, depending on the model parameters. We give an estimation of the reheat temperature and a brief discussion upon back-reaction on the fermionic production, which could change the gravitino abundance.

hep-ph↗

arXiv2000

A Note on Brane Inflation

We demonstrate that there exists an inflationary solution on the positive tension brane in the Randall-Sundrum scenario. Inflation is driven by a slow-rolling scalar field on the brane and is achieved within the perturbative limit of the radion field. We find that inflation on the positive tension brane results in a slight increase in the separation between the two branes. However, we show that the slow-roll inflation is not possible on the negative tension brane.

gr-qc↗

arXiv2000

Production of spin 3/2 particles from vacuum fluctuations

We study the production of spin 3/2 particles in homogeneous scalar and gravitational backgrounds from the mode-mixing Bogolyubov method. Considering only helicity $\pm 3/2$ states we can reduce the problem to a standard Dirac fermion calculation and apply the standard techniques in a straightforward way. As an example we consider a supergravity inflationary model and calculate the spectrum of gravitinos created during preheating at the end of inflation.

hep-ph↗

arXiv1999

Perturbation amplitude in isocurvature inflation scenarios

We make a detailed calculation of the amplitude of isocurvature perturbations arising from inflationary models in which the cold dark matter is represented by a scalar field which acquires perturbations during inflation. We use this to compute the normalization to large-angle microwave background anisotropies. Unlike the case of adiabatic perturbations, the normalization to COBE fixes the spectral index of the perturbations; if adiabatic perturbations are negligible then $n_{iso} \simeq 0.4$. Such blue spectra are also favoured by other observational data. Although the pure isocurvature models are unlikely to adequately fit the entire observational data set, these results also have implications for models with mixed adiabatic and isocurvature perturbations.

astro-ph↗

arXiv1999

Extra Dimensions and Inflation

It has been proposed that, without invoking supersymmetry, it is possible to solve the hierarchy problem provided the fundamental scale in the higher dimensional theory is at a much lower scale than the Planck scale. In this paper we consider a toy model where we allow $4+d$ dimensions to evolve from a region determined by the new fundamental scale, which is in our case is electro-weak scale. We further investigate whether it is possible to inflate not only the 3 uncompactified dimensions but also the extra dimensions. We require around 70 e-foldings to stabilize the size of two extra dimensions from $M_{\rm EW}^{-1}$ to the required 1 mm size. This can be easily achieved during inflation, but once inflation ends their evolution is governed by the dynamical history of the universe in a similar way that of Brans-Dicke field in generalized Einstein theories. We also show that we achieve the right level of density contrast without invoking any specific potential. The density contrast depends on the number of extra dimensions and upon the amplitude of the potential.

hep-ph↗

arXiv1999

A quantum holographic principle from decoherence

We present a fully quantum version of the holographic principle in terms of quantum systems, subsystems, and their interactions. We use the concept of environment induced decoherence to prove this principle. We discuss the conditions under which the standard (semi-classical) holographic principle is obtained from this quantum mechanical version.

gr-qc↗

arXiv1999

Preheating in Generalized Einstein Theories

We study the preheating scenario in Generalized Einstein Theories, considering a class of such theories which are conformally equivalent to those of an extra field with a modified potential in the Einstein frame. Resonant creation of bosons from oscillating inflaton has been studied before in the context of general relativity taking also into account the effect of metric perturbations in linearized gravity. As a natural generalization we include the dilatonic/Brans-Dicke field without any potential of its own and in particular we study the linear theory of perturbations including the metric perturbations in the longitudinal gauge. We show that there is an amplification of the perturbations in the dilaton/Brans-Dicke field on super-horizon scales (k -> 0) due to the fluctuations in metric, thus leading to an oscillating Newton's constant with very high frequency within the horizon and with growing amplitude outside the horizon. We briefly mention the entropy perturbations generated by such fluctuations and also mention that there is a possibility to excite the Kaluza-Klein modes in the theories where the dilatonic/Brans-Dicke field is interpreted as a homogeneous field appearing due to the dimensional reduction from the higher dimensional theories.

hep-ph↗

arXiv1999

Generalized Assisted Inflation

We obtain a new class of exact cosmological solutions for multi-scalar fields with exponential potentials. We generalize the assisted inflation solutions previously obtained, and demonstrate how they are modified when there exist cross-couplings between the fields, such as occur in supergravity inspired cosmological models.

astro-ph↗

arXiv1999

Resonant production of fermions in an axial background

We consider the resonant production of fermions from an oscillating axial background. The classical evolution of the axial field is given by that of a massive pseudovector field, as suggested by the renormalizability of the theory. We look upon both the massive and the massless fermion production from a perturbative point of view. We obtain the corresponding spectrum and angular distributions for the different spins or helicities in the particular case of a spatial-like axial field. We also extend our study to the non-perturbative regime in the massless case and compare the results with the perturbative ones.

hep-ph↗

arXiv1998

Instantons from Low Energy String Actions

We look for instanton solutions in a class of two scalar field gravity models, which includes the low energy string action in four dimensions. In models where the matter field has a potential with a false vacuum, we find that non-singular instantons exist as long as the Dilaton field found in string theory has a potential with a minimum, and provide an example of such an instanton. The class of singular instanton solutions are also examined, and we find that depending on the parameter values, the volume factor of the Euclidean region does not always vanish fast enough at the singularity to make the action finite.

hep-th↗

arXiv1998

Assisted inflation

In inflationary scenarios with more than one scalar field, inflation may proceed even if each of the individual fields has a potential too steep for that field to sustain inflation on its own. We show that scalar fields with exponential potentials evolve so as to act cooperatively to assist inflation, by finding solutions in which the energy densities of the different scalar fields evolve in fixed proportion. Such scaling solutions exist for an arbitrary number of scalar fields, with different slopes for the exponential potentials, and we show that these solutions are the unique late-time attractors for the evolution. We determine the density perturbation spectrum produced by such a period of inflation, and show that with multiple scalar fields the spectrum is closer to the scale-invariant than the spectrum that any of the fields would generate individually.

astro-ph↗

arXiv1998

Inflation during oscillations of the inflaton

Damour and Mukhanov have recently devised circumstances in which inflation may continue during the oscillatory phase which ensues once the inflaton field reaches the minimum of its potential. We confirm the existence of this phenomenon by numerical integration. In such circumstances the quantification of the amount of inflation requires particular care. We use a definition based on the decrease of the comoving Hubble length, and show that Damour and Mukhanov overestimated the amount of inflation occurring. We use the numerical calculations to check the validity of analytic approximations.

astro-ph↗

arXiv1998

Extended Inflation with an Exponential Potential

In this paper we investigate extended inflation with an exponential potential $V(σ)= V_0 e^{-κσ}$, which provides a simple cosmological scenario where the distribution of the constants of Nature is mostly determined by $κ$. In particular, we show that this theory predicts a uniform distribution for the Planck mass at the end of inflation, for the entire ensemble of universes that undergo stochastic inflation. Eternal inflation takes place in this scenario for a broad family of initial conditions, all of which lead up to the same value of the Planck mass at the end of inflation. The predicted value of the Planck mass is consistent with the observed value within a comfortable range of values of the parameters involved.

gr-qc↗

arXiv1998

The radiation-matter transition in Jordan-Brans-Dicke theory

We study the transition from radiation domination to matter domination in Jordan-Brans-Dicke theory, in particular examining how the Hubble length at equality depends on the coupling parameter $ω$. We consider the prospects for using high-accuracy microwave anisotropy and large-scale structure data to constrain $ω$ more strongly than by conventional solar system gravity experiments.

astro-ph↗