arXiv subjects

Raghavan Rangarajan

Publications and source records attributed to Raghavan Rangarajan.

At least 37 records · Page 2Linked to original sources

arXiv2013

On the power spectrum generated during inflation

Recently there have been differing viewpoints on how to evaluate the curvature power spectrum generated during inflation. Since the primordial curvature power spectrum is the seed for structure formation and provides a link between observations and inflationary parameters, it is important to resolve any disagreements over the expression for the power spectrum. In this article we discuss these differing viewpoints and indicate issues that are relevant to both approaches. We then argue why the standard expression is valid.

astro-ph.CO↗

arXiv2013

The Quantum Phase of Inflation

Inflation models can have an early phase of inflation where the evolution of the inflaton is driven by quantum fluctuations before entering the phase driven by the slope of the scalar field potential. For a Coleman-Weinberg potential this quantum phase lasts 10^{7-8} e-foldings. A long period of fluctuation driven growth of the inflation field can possibly take the inflaton past phi_*, the value of the field where our current horizon scale crosses the horizon; alternatively, even if the field does not cross phi_*, the inflaton could have high kinetic energy at the end of this phase. Therefore we study these issues in the context of different models of inflation. In scenarios where cosmological relevant scales leave during the quantum phase we obtain large curvature perturbations of O(10). We also apply our results to quadratic curvaton models and to quintessence models. In curvaton models we find that inflation must last longer than required to solve the horizon problem, that the curvaton models are incompatible with small field inflation models and that there may be too large non-gaussianity. A new phase of thermal fluctuation driven inflation is proposed, in which during inflation the inflaton evolution is governed by fluctuations from a sustained thermal radiation bath rather than by a scalar field potential.

astro-ph.CO↗

arXiv2012

Flat directions and gravitino production in SUSY models

Flat directions in supersymmetric models can get large vacuum expectation values in the early Universe which leads to a large mass for gauge bosons and gauginos. We point out that this can then result in enhanced gravitino production because the cross-section for the production of the +/- 1/2 helicity states of the gravitino is proportional to the square of the gaugino masses. We consider gravitino production after inflation in such a scenario and find that the abundance in some cases can be much larger than the upper bound on the gravtino abundance from cosmological constraints unless the flat direction field has a very small vacuum expectation value when it commences oscillating.

astro-ph.CO↗

arXiv2012

Remarks on non-gaussian fluctuations of the inflaton and constancy of ζoutside the horizon

We point out that the non-gaussianity arising from cubic self interactions of the inflaton field is proportional to ξN_e where ξ~ V"' and N_e is the number of e-foldings from horizon exit till the end of inflation. For scales of interest N_e = 60, and for models of inflation such as new inflation, natural inflation and running mass inflation ξis large compared to the slow roll parameter ε~ V'^{2}. Therefore the contribution from self interactions should not be outrightly ignored while retaining other terms in the non-gaussianity parameter f_{NL}. But the N_e dependent term seems to imply the growth of non-gaussianities outside the horizon. Therefore we briefly discuss the issue of the constancy of correlations of the curvature perturbation ζoutside the horizon. We then calculate the 3-point function of the inflaton fluctuations using the canonical formalism and further obtain the 3-point function of ζ_k. We find that the N_e dependent contribution to f_{NL} from self interactions of the inflaton field is cancelled by contributions from other terms associated with non-linearities in cosmological perturbation theory.

astro-ph.CO↗

arXiv2009

Perturbative Reheating and Gravitino Production in Inflationary Models

The low reheat temperature at the end of inflation from the gravitino bound constrains the creation of heavy Majorana neutrinos associated with models of leptogenesis. However, a detailed view of the reheating of the Universe at the end of inflation implies that the maximum temperature during reheating, $\Tmax$, can be orders of magnitude higher than the final reheat temperature. This then allows for the production of the heavy Majorana neutrinos needed for leptogenesis. We carry out the complementary calculation of the gravitino production during reheating and its dependence on $\Tmax$. We find that the gravitino abundance generated during reheating for a quartic potential is comparable to the standard estimate of the abundance generated after reheating and study its consequences for leptogenesis.

hep-ph↗

arXiv2008

Gravitino production in an inflationary Universe and implications for leptogenesis

Models of leptogenesis are constrained by the low reheat temperature at the end of reheating associated with the gravitino bound. However a detailed view of reheating, in which the maximum temperature during reheating, $\Tmax$, can be orders of magnitude higher than the reheat temperature, allows for the production of heavy Majorana neutrinos needed for leptogenesis. But then one must also consider the possibility of enhanced gravitino production in such scenarios. In this article we consider gravitino production during reheating, its dependence on $\Tmax$, and its relevance for leptogenesis. Earlier analytical studies of the gravitino abundance have only considered gravitino production in the post-reheating radiation dominated era. We find that the gravitino abundance generated during reheating is comparable to that generated after reheating. This lowers the upper bound on the reheat temperature by a factor of 4/3.

hep-ph↗

arXiv2007

WMAPping the Inflationary Universe

An epoch of accelerated expansion, or inflation, in the early universe solves several cosmological problems. While there are many models of inflation only recently has it become possible to discriminate between some of the models using observations of the cosmic microwave background radiation and large-scale structure. In this talk, we discuss inflation and its observational consequences, and then the status of current cosmological observations and their implications for different models of inflation.

hep-ph↗

arXiv2007

Features in the Primordial Spectrum from WMAP: A Wavelet Analysis

Precise measurements of the anisotropies in the cosmic microwave background enable us to do an accurate study on the form of the primordial power spectrum for a given set of cosmological parameters. In a previous paper (Shafieloo and Souradeep 2004), we implemented an improved (error sensitive) Richardson-Lucy deconvolution algorithm on the measured angular power spectrum from the first year of WMAP data to determine the primordial power spectrum assuming a concordance cosmological model. This recovered spectrum has a likelihood far better than a scale invariant, or, `best fit' scale free spectra (Δln L = 25 w.r.t. Harrison Zeldovich, and, Δln L = 11 w.r.t. power law with n_s=0.95). In this paper we use Discrete Wavelet Transform (DWT) to decompose the local features of the recovered spectrum individually to study their effect and significance on the recovered angular power spectrum and hence the likelihood. We show that besides the infra-red cut off at the horizon scale, the associated features of the primordial power spectrum around the horizon have a significant effect on improving the likelihood. The strong features are localised at the horizon scale.

astro-ph↗

arXiv2007

Thermalisation in Thick Wall Electroweak Baryogenesis

In models of thick wall electroweak baryogenesis a common assumption is that the plasma interacting with the expanding Higgs bubble wall during the electroweak phase transition is in kinetic equilibrium (or close to it). We point out that, in addition to the requirement of low wall velocity, kinetic equilibrium requires that the change in the momentum of the particles due to the force exerted by the wall should be much less than that due to scattering as the plasma passes through the wall. We investigate whether this condition is satisfied for charginos and neutralinos participating in thick wall supersymmetric electroweak baryogenesis

hep-ph↗

arXiv2006

Temperature of the inflaton and duration of inflation from WMAP data

If the initial state of the inflaton field is taken to have a thermal distribution instead of the conventional zero particle vacuum state then the curvature power spectrum gets modified by a temperature dependent factor such that the fluctuation spectrum of the microwave background radiation is enhanced at larger angles. We compare this modified cosmic microwave background spectrum with Wilkinson microwave anisotropy probe data to obtain an upper bound on the temperature of the inflaton at the time our current horizon crossed the horizon during inflation. We further conclude that there must be additional e-foldings of inflation beyond what is needed to solve the horizon problem.

hep-ph↗

arXiv2005

Studying Electroweak Baryogenesis using Evenisation and the Wigner Formalism

We derive the kinetic equation for fermions and antifermions interacting with a planar Higgs bubble wall during the electroweak phase transition using the `evenisation' procedure and the Wigner formalism for a Lagrangian with the phase of the complex fermion mass rotated away. We obtain the energy, velocity and force for the particles in the presence of the Higgs bubble wall. Our results using both methods are in agreement. This indicates the robustness of evenisation as a method to study quantum corrections to the velocity and force for particles in the Higgs wall during the electroweak phase transition. We also derive the transport equations from the zeroth and first moment of the kinetic equation.

hep-ph↗

arXiv2005

The Kinetic Equation for Electroweak Baryogenesis

We derive the kinetic equation for fermions and antifermions interacting with a planar Higgs bubble wall during the electroweak phase transition using the `evenisation' procedure. Equations of motion in a relativistic quantum theory do not mirror classical relations unless one uses evenised operators. We give a brief introduction to evenisation and then use the evenised Heisenberg equations of motion to obtain the velocity and force for the particles in the presence of the Higgs bubble wall. Keeping quantum contributions to $O(\hbar)$ in the equations of motion we obtain the semi-classical force obtained earlier by other techniques.

hep-ph↗

arXiv2004

Working Group Report: Neutrino and Astroparticle Physics

This is the report of neutrino and astroparticle physics working group at WHEPP-8. We present the discussions carried out during the workshop on selected topics in the above fields and also indicate progress made subsequently. The neutrino physics subgroup studied the possibilites of constraining neutrino masses, mixing and CPT violation in lepton sector from future experiments. Neutrino mass models in the context of abelian horizontal symmetries, warped extra dimensions and in presence of triplet Higgs were studied. Effect of threshold corrections on radiative magnification of mixing angles was investigated. The astroparticle physics subgroup focused on how various particle physics inputs affect the CMBR fluctuation spectrum, and on brane cosmology. This report also contains an introduction on how to use the publicly available code CMBFAST to calculate the CMBR fluctuations.

hep-ph↗

arXiv2003

Out of Equilibrium Dynamics of the Inflaton Re-examined

In a series of papers Boyanovsky et al. have studied the evolution of an inflaton with a negative mass squared and a quartic self coupling using the Closed Time Path (CTP) formalism relevant for out-of-equilibrium dynamics. In this paper we comment on various aspects of these works. We first compare their approach to alternate approaches to study inflaton dynamics and point out that the use of the CTP formalism gives the same results as standard field theory in the Hartree and leading order large N approximations. We then rederive using the WKB approximation the large momentum mode functions of the inflaton needed for renormalisation and point out some differences with the previously obtained results. We also argue that the WKB approximation is valid only for large $k/a$ and not for large $k$ as apparently assumed in the above mentioned works. We comment on the renormalisation prescription adopted in these works and finally discuss how it differs from another more commonly used prescription.

hep-ph↗

arXiv2001

Baryon Number Violation in Particle Decays

It has been argued in the past that in baryogenesis via out-of-equilibrium decays one must consider loop diagrams that contain more than one baryon number violating coupling. In this note we argue that the requirement with regard to baryon number violating couplings in loop diagrams is that the interaction between the intermediate on-shell particles and the final particles should correspond to a net change in baryon number and that this can be satisfied even if the loop diagram contains only one baryon number violating coupling. Put simply, we show that to create a baryon asymmetry there should be net B violation to the right of the `cut' in the loop diagram. This is of relevance to some earlier works involving the out-of-equilibrium decay scenario.

hep-ph↗

arXiv2001

Inflationary Baryogenesis

In this letter we explore the possibility of creating the baryon asymmetry of the universe during inflation and reheating due to the decay of a field associated with the inflaton. CP violation is attained by assuming that this field is complex with a phase that varies as the inflaton evolves. We consider chaotic and natural inflation scenarios. In the former case, the complex decaying field is the inflaton itself and, in the latter case, the phase of the complex field is the inflaton. We calculate the asymmetry produced using the Bogolyubov formalism that relates annihilation and creation operators at late time to the annihilation and creation operators at early time.

hep-ph↗

arXiv2000

Leptogenesis with Heavy Majorana Neutrinos Reexamined

The mass term for Majorana neutrinos explicitly violates lepton number. Several authors have used this fact to create a lepton asymmetry in the universe by considering CP violating effects in the one loop self-energy correction for the decaying heavy Majorana neutrino. We compare and comment on the different approaches used to calculate the lepton asymmetry including those using an effective Hamiltonian and resummed propagators. We also recalculate the asymmetry in the small mass difference limit.

hep-ph↗

arXiv2000

Electroweak Baryogenesis in a Cold Universe

We discuss the possibility of generating the baryon asymmetry of the Universe when the temperature of the Universe is much below the electroweak scale. In our model the evaporation of primordial black holes or the decay of massive particles re-heats the surrounding plasma to temperatures above the electroweak transition temperature leading to the restoration of electroweak symmetry locally. The symmetry is broken again spontaneously as the plasma cools and a baryon asymmetry is generated during the phase transition. This mechanism generates sufficient asymmetry for a second order electroweak phase transition. For a first order phase transition, sufficient asymmetry is generated if viscous effects slow down the heated plasma as it moves away from the black hole. In our scenario there is no wash-out of the asymmetry after the phase transition as the plasma rapidly cools to lower temperatures thereby shutting off the sphaleron processes.

hep-ph↗