arXiv subjects

Raghavan Rangarajan

Publications and source records attributed to Raghavan Rangarajan.

At least 19 recordsLinked to original sources

arXiv2022

The paradigm of warm quintessential inflation and spontaneous baryogenesis

In this paper, we consider a scenario of spontaneous baryogenesis in a framework of warm quintessential inflation where the residual inflaton field, left out after warm inflation, plays the role of quintessence field at late times and is coupled to a non-conserved baryonic current. Assuming a four fermion $(B-L)$ violating effective interaction, we have demonstrated that the required baryon asymmetry can be produced successfully in this case. We show that the post-inflationary evolution, with the underlying scalar field potential, $V(ϕ)=V^4_0 \exp{(-αϕ^n/M_{\rm Pl}^4 ) }$, $n>1$ well suited to warm inflation, exhibits scaling behaviour soon after a brief kinetic regime. We show that the coupling of the scalar field to massive neutrino matter can give rise to exit from the scaling regime to cosmic acceleration at late times as massive neutrinos turn non-relativistic. The proposed model is shown to successfully describe the cosmic history from inflation to late-time acceleration, with the evolution independent of initial conditions, along with the generation of baryon asymmetry during the post-inflationary era. A brief analysis of relic gravity waves produced in the scenario is presented.

astro-ph.CO↗

arXiv2019

A plausible solution to the problem of initial conditions for inflation

We propose yet another solution to the initial condition problem of inflation associated with homogeneity beyond the horizon at the onset of inflation, in cases where inflation is preceded by a radiation era. One may argue that causality will allow for smoothness over the causal horizon scale $H^{-1}$, but for thermal inflationary scenarios, the background inflaton field will only be correlated over the thermal correlation length $ζ\sim T^{-1}$ which is much smaller than $H^{-1}$. We argue, with examples, that if the number of relativistic degrees of freedom in the pre-inflationary era is very large $(>10^4)$ then the thermal correlation length can be of the order of the causal horizon size alleviating the initial conditions problem of inflation.

astro-ph.CO↗

arXiv2018

Study of warm inflationary models and their parameter estimation from CMB

Observations of the temperature anisotropies in the Cosmic Microwave Background (CMB) radiation show that the models of inflation with the monomial potentials are inconsistent with the allowed $n_s-r$ bounds. However certain monomial potentials of inflation are allowed in the context of \textit{Warm Inflation}, where the inflaton's coupling with other fields are significant both \textit{during and after} the inflationary phase. In our study, we consider $λϕ^4$ and $λϕ^6$ models of warm inflation with different forms of the inflaton dissipation coefficient. We parameterize the primordial power spectrum in terms of the model parameters, namely, the inflaton self coupling, $λ$, and the dissipation parameter, $Q_P$, due to inflaton's interaction with the other fields. Then we obtain the joint and marginal distributions of these parameters by carrying out a Markov Chain Monte Carlo (MCMC) analysis using the {\tt CosmoMC} numerical code. An estimation of these physical parameters is essential for model building. We also obtain the $n_s$ and $r$ values for the mean values of the parameters and find them to be consistent with the observational bounds, confirming that these simple models are viable models for describing inflation.

astro-ph.CO↗

arXiv2018

Effective chemical potential in spontaneous baryogenesis

Models of spontaneous baryogenesis have an interaction term $\partial_μθj^μ_B$ in the Lagrangian, where $j^μ_B$ is the baryonic current and $θ$ can be a pseudo-Nambu-Goldstone boson. Since the time component of this term, $\dotθ j^0_B$, equals $\dotθ n_B$ for a spatially homogeneous current, it is usually argued that this term implies a splitting in the energy of baryons and antibaryons thereby providing an effective chemical potential for baryon number. In thermal equilibrium, one {then obtains} $n_B \sim \dotθ T^2$. We however argue that a term of this form in the Lagrangian does not contribute to the single particle energies of baryons and antibaryons. We show this for both fermionic and scalar baryons. But, similar to some recent work, we find that despite the above result the baryon number density obtained from a Boltzmann equation analysis can be proportional to $\dotθ T^2$. Our arguments are very different from that in the standard literature on spontaneous baryogenesis.

hep-ph↗

arXiv2018

Revisiting CMB constraints on Warm Inflation

We revisit the constraints that Planck 2015 temperature, polarization and lensing data impose on the parameters of warm inflation. To this end, we study warm inflation driven by a single scalar field with a quartic self interaction potential in the weak dissipative regime. We analyse the effect of the parameters of warm inflation, namely, the inflaton self coupling $λ$ and the inflaton dissipation parameter $Q_P$ on the CMB angular power spectrum. We constrain $λ$ and $Q_P$ for 50 and 60 number of e-foldings with the full Planck 2015 data (TT, TE, EE + lowP and lensing) by performing a Markov-Chain Monte Carlo analysis using the publicly available code {\tt CosmoMC} and obtain the joint as well as marginalized distributions of those parameters. We present our results in the form of mean and 68 \% confidence limits on the parameters and also highlight the degeneracy between $λ$ and $Q_P$ in our analysis. From this analysis we show how warm inflation parameters can be well constrained using the Planck 2015 data.

astro-ph.CO↗

arXiv2018

Current Status of Warm Inflation

Warm inflation is an inflationary scenario in which a thermal bath coexists with the inflaton during inflation. This is unlike standard cold inflation in which the Universe is effectively devoid of particles during inflation. The thermal bath in warm inflation is maintained by the dissipation of the inflaton's energy through its couplings to other fields. Many models of warm inflation have been proposed and their predictions have been compared with cosmological data. Certain models of inflation that are disallowed in the context of cold inflation by the data are allowed in the warm inflationary scenario, and vice versa.

astro-ph.CO↗

arXiv2017

Gravitino production in a thermal Universe revisited

We study the production of spin 1/2 gravitinos in a thermal Universe. Taking into account supersymmetry breaking due to the finite thermal energy density of the Universe, there is a large enhancement in the cross section of production of these gravitino states. We consider gravitinos with zero temperature masses of 0.1 eV, 1 keV, 100 GeV and 30 TeV as representative of gauge mediated, gravity mediated and anomaly mediated supersymmetry breaking scenarios. We find that the abundance of gravitinos produced in the early Universe is very high for gravitinos of mass 1 keV and 100 GeV. The gravitino abundances can be sufficiently suppressed if the reheat temperature is less than 100 GeV and $4\cdot 10^4$ gev respectively. However such low reheat temperatures will rule out many models of baryogenesis including those via leptogenesis.

astro-ph.CO↗

arXiv2016

Constraints on just enough inflation preceded by a thermal era

If the inflationary era is preceded by a radiation dominated era in which the inflaton too was in thermal equilibrium at some very early time then the CMB data places an upper bound on the comoving temperature of the (decoupled) inflaton quanta. In addition, if one considers models of "just enough" inflation, where the number of e-foldings of inflation is just enough to solve the horizon and flatness problems, then we get a lower bound on the Hubble parameter during inflation, $H_{\rm inf}$, which is in severe conflict with the upper bound from tensor perturbations. Alternatively, imposing the upper bound on $H_{\rm inf}$ implies that such scenarios are compatible with the data only if the number of relativistic degrees of freedom in the thermal bath in the pre-inflationary Universe is extremely large (greater than $10^9$ or $10^{11})$. We are not aware of scenarios in which this can be satisfied.

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arXiv2015

Thawing quintessence from the inflationary epoch to today

Using the latest observational data we obtain a lower bound on the initial value of the quintessence field in thawing quintessence models of dark energy. For potentials of the form V(ϕ) ϕ^{\pm2} we find that the initial value |ϕ_i|>7x10^{18}\gev. We then relate ϕ_i to the duration of inflation by assuming that the initial value of the quintessence field is determined by quantum fluctuations of the quintessence field during inflation. From the lower bound on $ϕ_i$ we obtain a lower bound on the number of e-foldings of inflation, namely, N>2x10^{11}. We obtain similar bounds for other power law potentials for which too we obtain |ϕ_{i}|>O(M_{P}.

astro-ph.CO↗

arXiv2015

Falsifying leptogenesis for a TeV scale $W^{\pm}_{R}$ at the LHC

We point out that the discovery of a right-handed charged gauge boson $W_R^\pm$ with mass of around a few TeV, for example through a signal of two leptons and two jets that has been reported by CMS to have a 2.8$σ$ local excess or through a signal of a resonance decaying into a pair of standard model (SM) gauge bosons showing a local excess of 3.4$σ$ (2.5$σ$ global) reported by ATLAS search, will rule out all possibilities of leptogenesis in all classes of the left-right symmetric extensions of the Standard Model (LRSM) with both triplet and doublet Higgs scalars due to the unavoidable fast gauge mediated $B-L$ violating interactions $e_{R}^{\pm} W_{R}^{\mp} \rightarrow e_{R}^{\mp} W_{R}^{\pm}$. Our conclusions are very general in the sense that they do not necessarily demand for a lepton number violating detection signal of $W_R^\pm$.

hep-ph↗

arXiv2015

Inflation after Planck and BICEP2

We discuss the inflationary paradigm, how it can be tested, and how various models of inflation fare in the light of data from Planck and BICEP2. We introduce inflation and reheating, and discuss temperature and polarisation anisotropies in the cosmic microwave background radiation due to quantum fluctuations during inflation. Fitting observations of the anisotropies with theoretical realisations obtained by varying various parameters of the curvature power spectrum and cosmological parameters enables one to obtain the allowed ranges of these parameters. We discuss how to relate these parameters to inflation models which allows one to rule in or out specific models of inflation.

astro-ph.CO↗

arXiv2015

Revisiting a pre-inflationary radiation era and its effect on the CMB power spectrum

We revisit the scenario where inflation is preceded by a radiation era by considering that the inflaton too could have been in thermal equilibrium early in the radiation era. Hence we take into account not only the effect of a pre-inflationary era on the inflaton mode functions but also that of a frozen thermal distribution of inflaton quanta. We initially discuss in detail the issues relevant to our scenario of a pre-inflationary radiation dominated era and then obtain the scalar power spectrum for this scenario. We find that the power spectrum is free from infrared divergences. We then use the WMAP and Planck data to determine the constraints on the inflaton comoving `temperature' and on the duration of inflation. We find that the best fit value of the duration of inflation is less than 1 e-folding more than what is required to solve cosmological problems, while only an upper bound on the inflaton temperature can be obtained.

astro-ph.CO↗

arXiv2015

Nonthermal $CP$ violation in soft leptogenesis

Soft leptogenesis is a mechanism which generates the matter-antimatter asymmetry of the Universe via the out-of-equilibrium decays of heavy sneutrinos in which soft supersymmetry breaking terms play two important roles: they provide the required $CP$ violation and give rise to the mass splitting between otherwise degenerate sneutrino mass eigenstates within a single generation. This mechanism is interesting because it can be successful at the lower temperature regime $T \lesssim 10^9$ GeV in which the conflict with the overproduction of gravitinos can possibly be avoided. In earlier works, the leading $CP$ violation is found to be nonzero only if finite temperature effects are included. By considering generic soft trilinear couplings, we find two interesting consequences: (1) the leading $CP$ violation can be nonzero even at zero temperature realizing nonthermal $CP$ violation, and (2) the $CP$ violation is sufficient even far away from the resonant regime allowing soft supersymmetry breaking parameters to assume natural values at around the TeV scale. We discuss phenomenological constraints on such scenarios and conclude that the contributions to charged lepton flavor violating processes are close to the sensitivities of present and future experiments.

hep-ph↗

arXiv2015

The $eejj$ Excess Signal at the LHC and Constraints on Leptogenesis

We review the non-supersymmetric (Extended) Left-Right Symmetric Models (LRSM) and low energy $E_6$-based models to investigate if they can explain both the recently detected excess $eejj$ signal at CMS and leptogenesis. The $eejj$ excess can be explained from the decay of the right-handed gauge bosons ($W_R$) with mass $\sim \rm{TeV}$ in certain variants of the LRSM (with $g_{L}\neq g_{R}$). However such scenarios can not accommodate high-scale leptogenesis. Other attempts have been made to explain leptogenesis while keeping the $W_{R}$ mass almost within the reach of the LHC by considering the resonant leptogenesis scenario in the context of the LRSM for relatively large Yukawa couplings. However this may not be feasible due to washout of the lepton asymmetry by certain processes. Therefore we consider three effective low energy subgroups of the superstring inspired $E_{6}$ model having a number of additional exotic fermions which provides a rich phenomenology to be explored. We however find that these three effective low energy subgroups of $E_6$ too cannot explain both the $eejj$ excess signal and leptogenesis simultaneously.

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arXiv2015

Explaining the CMS $eejj$ and $e\ \rm{missing}\ p_T \ jj$ excess and leptogenesis in superstring inspired $E_6$ models

We show that superstring inspired $E_6$ models can explain both the recently detected excess $eejj$ and $e\ \rm{missing}\ p_T \ jj$ signals at CMS, and also allow for leptogenesis. Working in a R-parity conserving low energy supersymmetric effective model, we show that the excess CMS events can be produced via the decay of exotic sleptons in Alternative Left-Right Symmetric Model of $E_6$, which can also accommodate leptogenesis at a high scale. On the other hand, either the $eejj$ excess or the $e\ \rm{missing}\ p_T \ jj$ excess can be produced via the decays of right handed gauge bosons, but some of these scenarios may not accommodate letptogenesis as there will be strong $B-L$ violation at low energy, which, along with the anomalous fast electroweak $B+L$ violation, will wash out all baryon asymmetry. Baryogenesis below the electroweak scale may then need to be implemented in these models.

hep-ph↗

arXiv2014

Supersymmetric flat directions and resonant gravitino production

We study resonant gravitino production in the early Universe in the presence of SUSY flat directions whose large VEVs break some but not all gauge symmetries. We find that for a large region of parameter space the gravitino abundance is several orders of magnitude larger than the cosmological upper bound. Since flat directions with large VEVs are generically expected in supersymmetric theories this result further exacerbates the gravitino problem.

astro-ph.CO↗

arXiv2014

Resolving the Inflationary Power Spectrum

Recently there have been differing viewpoints on how to evaluate the curvature power spectrum generated during inflation. In a series of papers by some authors it has been argued that the renormalization scheme adopted for the inflaton field phi(x) to make finite should also be applied to |phi_k|^2. But this then modifies the curvature power spectrum in a non-trivial way. On the other hand, others have criticized this approach and suggested alternatives, which have been further countered by the original authors. We discuss these differing viewpoints and indicate inconsistencies in both approaches. We then resolve the issue by showing why the standard expression, without any non-trivial regularization, is still valid.

astro-ph.CO↗

arXiv2013

Kinetic and chemical equilibrium of the Universe and gravitino production

Flat directions in generic supersymmetric theories can change the thermal history of the Universe. A novel scenario was proposed earlier where the vacuum expectation value of the flat directions induces large masses for all the gauge bosons and gauginos. This delays the thermalization of the Universe after inflation and solves the gravitino problem. In this article we perform a detailed calculation of the above scenario. We include the appropriate initial state particle distribution functions, consider the conditions for the feasibility of the non-thermal scenario, and investigate phase space suppression of gravitino production in the context of heavy gauge bosons and gauginos in the final state. We find that the total gravitino abundance generated is consistent with cosmological constraints.

astro-ph.CO↗