arXiv subjects

Sampurn Anand

Publications and source records attributed to Sampurn Anand.

10 recordsLinked to original sources

arXiv2026

Resonant Dynamics of Gravitational Wave and Chiral Alfv\'en wave in the Early Universe

In this work, we investigate the resonant interaction between stochastic gravitational waves (GWs) and chiral Alfv\'en waves in a magnetized chiral plasma in the early Universe. Starting from covariant chiral magnetohydrodynamics coupled to linearized gravity, we derive a closed system of equations for the coupled chiral-Alfv\'en velocity and magnetic-field perturbations. Both analytics and numerics show a parametric resonance at the sum frequency of the two CVE-split branches, with an instability band that widens as the GW strain grows. This CVE-to-Alfv\'en ratio is not a free parameter. In the early Universe, it is fixed by the Standard Model's relativistic degrees of freedom. Including the plasma's backreaction on the GW makes the energy exchange nonlinear, breaking the usual single-frequency Manley-Rowe picture. For a self-consistent choice of parameters, where backreaction is only a small correction, the unsuppressed resonance still drives the system to a finite-time blow-up within a few Hubble times. We then work out the resonant frequencies and growth rates involved, mapping out what current and future gravitational-wave detectors could observe.

astro-ph.CO↗

arXiv2021

Generating Seed magnetic field à la Chiral Biermann battery

Cosmological and astrophysical observations indicate the presence of magnetic field over all scales. In order to explain these magnetic fields, it is assumed that there exists a seed magnetic field that gets amplified by dynamos. These seed fields may have been produced during inflation, at phase transitions, or some turbulent phase of the early universe. One well-known mechanism to get the seed field is the Biermann battery, which was originally discussed in the context of generation in an astrophysical object. Requirements for this mechanism to work are (i) non-zero gradient of the electron number density and pressure, (ii) they are non-parallel to each other. In the present article, we propose a similar mechanism to generate the seed field but in inhomogeneous chiral plasma. Our mechanism works, in presence of chiral anomaly, by the virtue of inhomogeneity in the chiral chemical potential and temperature. We will discuss various scenarios where inhomogeneities in the chemical potential and temperature can arise. We found that, depending on the epoch of generation, the strength of the seed magnetic fields varies from a few nano-Gauss (nG) to a few hundred nG.

astro-ph.CO↗

arXiv2020

Accounting for the time evolution of the equation of state parameter during reheating

One of the important parameters in cosmology is the parameter characterizing the equation of state (EoS) of the sources driving the cosmic expansion. Epochs that are dominated by radiation, matter, or scalar fields, whether they are probed either directly or indirectly, can be characterised by a unique value of this parameter. However, the EoS parameter during reheating---a phase succeeding inflation which is supposed to rapidly defrost our universe---remains to be understood satisfactorily. In order to circumvent the complexity of defining an instantaneous EoS parameter during reheating, an effective parameter $w_\mathrm{eff}$, which is an average of the EoS parameter over the duration of reheating, is usually considered. The value of $w_\mathrm{eff}$ is often chosen arbitrarily to lie in the range $-1/3 \leq w_\mathrm{eff} \leq 1$. In this work, we consider the time evolution of the EoS parameter during reheating and relate it to inflationary potentials $V(ϕ)$ that behave as $ϕ^p$ around the minimum, a proposal which can be applied to a wide class of inflationary models. We find that, given the index $p$, the effective EoS parameter $w_{\rm eff}$ is determined uniquely. We discuss the corresponding effects on the reheating temperature and its implications.

astro-ph.CO↗

arXiv2019

Chiral plasma instability and primordial Gravitational wave

It is known that cosmic magnetic field, if present, can generate anisotropic stress in the plasma and hence, can act as a source of gravitational waves. These cosmic magnetic fields can be generated at very high temperature, much above electroweak scale, due to the gravitational anomaly in presence of the chiral asymmetry. The chiral asymmetry leads to instability in the plasma which ultimately leads to the generation of magnetic fields. In this article, we discuss the generation of gravitational waves, during the period of instability, in the chiral plasma sourced by the magnetic field created due to the gravitational anomaly. We have shown that such gravitational wave will have a unique spectrum. Moreover, depending on the temperature of the universe at the time of its generation, such gravitational waves can have a wide range of frequencies. We also estimate the amplitude and frequency of the gravitational waves and delineate the possibility of its detection by future experiments like eLISA.

astro-ph.CO↗

arXiv2017

Bounds on Neutrino Mass in Viscous Cosmology

Effective field theory of dark matter fluid on large scales predicts the presence of viscosity of the order of $10^{-6} H_0 M_P^2$. It has been shown that this magnitude of viscosities can resolve the discordance between large scale structure observations and Planck CMB data in the $σ_8$-$Ω_m^0$ and $H_0$-$Ω_m^0$ parameters space. Massive neutrinos suppresses the matter power spectrum on the small length scales similar to the viscosities. We show that by including the effective viscosity, which arises from summing over non linear perturbations at small length scales, severely constrains the cosmological bound on neutrino masses. Under a joint analysis of Planck CMB and different large scale observation data, we find that upper bound on the sum of the neutrino masses at 2-$σ$ level, decreases from $\sum m_ν\le 0.396\,$eV (normal hierarchy) and $\sum m_ν\le 0.378 \,$eV (inverted hierarchy) to $\sum m_ν\le 0.267\,$eV (normal hierarchy) and $\sum m_ν\le 0.146\,$eV (inverted hierarchy) when the effective viscosities are included.

astro-ph.CO↗

arXiv2017

Cosmic viscosity as a remedy for tension between PLANCK and LSS data

Measurements of $σ_8$ from large scale structure observations show a discordance with the extrapolated $σ_8$ from Planck CMB parameters using $Λ$CDM cosmology. Similar discordance is found in the value of $H_0$ and $Ω_m$. In this paper, we show that the presence of viscosity in cold dark matter, shear or bulk or combination of both, can remove the above mentioned conflicts simultaneously. This indicates that the data from Planck CMB observation and different LSS observations prefer small but non-zero amount of viscosity in cold dark matter fluid.

astro-ph.CO↗

arXiv2017

Chiral Battery, scaling laws and magnetic fields

We study the generation and evolution of magnetic field in the presence of chiral imbalance and gravitational anomaly which gives an additional contribution to the vortical current. The contribution due to gravitational anomaly is proportional to $T^2$ which can generate a seed magnetic field irrespective of plasma being hirally charged or neutral. We estimate the order of magnitude of the magnetic field to be $10^{30}$~G at $T\sim 10^9$ GeV, with a typical length scale of the order of $10^{-18}$ cm, which is much smaller than the Hubble radius at that temperature ($10^{-8}$ cm). Moreover, such a system possesses scaling symmetry. We show that the $T^2$ term in the vorticity current along with scaling symmetry leads to more power transfer from lower to higher length scale as compared to only chiral anomaly without scaling symmetry.

astro-ph.CO↗

arXiv2017

Effects of QCD Equation of State on the Stochastic Gravitational Wave Background

Cosmological phase transitions can be a source of Stochastic Gravitational Wave (SGW) background. Apart from the dynamics of the phase transition, the characteristic frequency and the fractional energy density $Ω_{\rm gw}$ of the SGW depends upon the temperature of the transition. In this article, we compute the SGW spectrum in the light of QCD equation of state provided by the lattice results. We find that the inclusion of trace anomaly from lattice QCD, enhances the SGW signal generated during QCD phase transition by $\sim 50\%$ and the peak frequency of the QCD era SGW are shifted higher by $\sim 25\%$ as compared to the earlier estimates without trace anomaly. This result is extremely significant for testing the phase transition dynamics near QCD epoch.

hep-ph↗

arXiv2016

Cosmological Implications of QGP Bulk Viscosity

Recent studies of the hot QCD matter indicate that the bulk viscosity ($ζ$) of quark-gluon plasma (QGP) rises sharply near the critical point of the QCD phase transition. In this work, we show that such a sharp rise of the bulk viscosity will lead to an effective negative pressure near the critical temperature, $T_{c}$ which in turn drives the Universe to inflate. This inflation has a natural graceful exist when the viscous effect evanesce. We estimate that, depending upon the peak value of $ζ$, universe expands by a factor of $10$ to $80$ times in a very short span ($Δt\sim 10^{-8}$ seconds). Another important outcome of the bulk viscosity dominated dynamics is the cavitation of QGP around $T \sim 1.5T_{c}$. This would lead to the phenomenon of formation of cavitation bubbles within the QGP phase. The above scenario is independent of the order of QCD phase transition. We delineate some of the important cosmological consequences of the inflation and the cavitation.

hep-ph↗

arXiv2014

A Geometric Approach to Modulus Stabilization

Modulus stabilization, a must for explaining the hierarchy problem in the context of the Randall-Sundrum (RS) scenario, is traditionally achieved through the introduction of an extra field with {\em ad hoc} couplings. We point out that the stabilization can, instead, be achieved in a purely geometrodynamical way, with plausible quantum corrections in the gravity sector playing the key role. The size of the corrections that lead to acceptable phenomenology is also delineated.

hep-th↗