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Arun Kumar Pandey

Publications and source records attributed to Arun Kumar Pandey.

14 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↗
arXiv2022

Thermal SZ effect in a magnetized IGM dominated by interacting DM decay/annihilation during dark ages

During cosmic dawn, the thermal history of the universe is well studied, and a study of this era can give us some of the most useful insight into the universe before the recombination epoch. Its precise modeling and future high-precision measurements will be a valuable tool for determining the thermal history of the universe. In the present work, we study the thermal and ionization history of IGM in the presence of decaying magnetic fields via ambipolar and turbulent decay, Baryon-Dark matter (BDM) interaction, including the DM decay/annihilation. The BDM interaction cross-sections considered are of the form $σ=σ_0 v^{n}$, where $n=-2$ and $n=-4$. In this work, we show that in the current scenario, the decay/annihilation of the DM particles have a considerable impact on the temperature and ionization histories at low redshift. With the addition of the concept of fractional interaction, which states that if a fraction of the DM particles interacts with the baryons, the temperature and ionization fraction of the baryons show a strong dependence on the percentage of DM particles interacting with the baryons. We have also studied the interesting consequences of the present scenario on the thermal Sunyaev-Zeldovich (tSZ) effect. We show that the highest value of the absolute value of the mean $y-$parameter in the current DM decay/annihilation scenario is well within the values derived from experimental data such as PLANCK, FIRAS, and PIXIE. Later we calculate the bound on the ordinary magnetic fields originating from the Dark photons.

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↗
arXiv2021

Gravitational waves in neutrino plasma and NANOGrav signal

The recent finding of the gravitational wave (GW) signal by the NANOGrav collaboration in the nHZ frequency range has opened up the door for the existence of stochastic GWs. In the present work, we have argued that in a hot dense neutrino asymmetric plasma, GWs could be generated due to the instability caused by the finite difference in the number densities of the different species of the neutrinos. The generated GWs have amplitude and frequency in the sensitivity range of the NANOGrav observation. We have shown that the GWs generated by this mechanism could be one of the possible explanations for the observed NANOGrav signal. We have also discussed generation of GWs in an inhomogeneous cosmological neutrino plasma, where GWs are generated when neutrinos enter a free streaming regime. We show that the generated GWs in an inhomogeneous neutrino plasma cannot explain the observed NANOGrav signal. We have also calculated the lower bound on magnetic fields' strength using the NANOGrav signal and found that to explain the signal, the magnetic fields' strength should have atleast value $\sim 10^{-12}$ G at an Mpc length scale.

astro-ph.CO↗
arXiv2020

Viscosity in cosmic fluids

The effective theory of large-scale structure formation based on $Λ$CDM paradigm predicts finite dissipative effects in the resulting fluid equations. In this work, we study how viscous effect that could arise if one includes self-interaction among the dark-matter particles combines with the effective theory. It is shown that these two possible sources of dissipation can operate together in a cosmic fluid and the interplay between them can play an important role in determining dynamics of the cosmic fluid. In particular, we demonstrate that the viscosity coefficient due to self-interaction is added inversely with the viscosity calculated using effective theory of $Λ$CDM model. Thus the larger viscosity has less significant contribution in the effective viscosity. Using the known bounds on $\,σ/m$ for self-interacting dark-matter, where $\,σ\,$ and $m$ are the cross-section and mass of the dark-matter particles respectively, we discuss role of the effective viscosity in various cosmological scenarios.

astro-ph.CO↗
arXiv2020

Implications of baryon-dark matter interaction on IGM temperature and tSZ effect with magnetic field

We show that the combined effect of cosmic magnetic field and a possible non-standard interaction between baryons and dark matter has interesting consequences on the thermal Sunyaev$-$Zel${'}$dovich (tSZ) effect depending on the temperature and the ionization state of the intergalactic medium. The drag force between the baryons and dark matter due to the relative velocity between them, and their temperature difference results in heat transfer between these two species. At the same time the ambipolar diffusion and the decaying magnetic turbulence tends to heat up the baryons. This interplay of these two processes give rise to different evolution histories of the thermal and ionization state of the universe and hence influences the CMB spectrum at small scales through the tSZ effect. In this work, we have computed the evolution of the temperature, ionization fraction and the y-parameter of the CMB for different strengths of the magnetic field and the interaction cross-section. We note that an interaction cross-section of $σ_0=10^{-42}$ m$^{2}$ (with a magnetic field strength of $3.0$ nG) reduces the y-parameter by an order of magnitude as compared to the case with similar strength of magnetic field but where there is no such interaction between the baryons and dark matter.

astro-ph.CO↗
arXiv2020

Magnetic fields in a hot dense neutrino plasma and the Gravitational Waves

In the present work, we have studied the spectrum of the primordial gravitational waves due to magnetic instability in the presence of neutrino asymmetry. The magnetic instability generates a helical magnetic field on a large scale. The anisotropic stress generated by the magnetic field shown to be a source of primordial gravitational waves (GWs) at the time of matter-neutrino decoupling. We expect that the theoretically predicted GWs by this mechanism may be detected by Square Kilometer Array (SKA) or pulsar time array (PTA) observations. We also compare our findings with the results obtained by the earlier work where the effect of magnetic instability was not considered.

astro-ph.CO↗
arXiv2020

Baryon-Dark matter interaction in presence of magnetic fields in light of EDGES signal

We have shown that in presence of a cosmic magnetic field the bounds on baryon dark matter cross-section ($\hat σ$), dark-matter mass ($m_d$) and values of the magnetic field ($B_0$) can strongly influence each other. This requires to rework the bounds on $\hat σ\,$, $m_d$ and $B_0$ which can explain the observed absorption signal by EDGES collaboration. The upper limit on the magnetic field strength can modify in presence of baryon-dark matter interaction cross-section. In the presence of a strong magnetic field, a large baryon-dark matter interaction cross-section is required to balance magnetic heating of gas to explain the EDGES signal as compared to a weak magnetic field. Subsequently, the strong magnetic-fields can even erase the 21 cm signal--this gives an upper bound on the strength of magnetic-fields, dark-matter mass and baryon-dark matter cross-section. In the special case when $\hat σ$=0, one can recover the bound on magnetic field strength calculated in [1]. In this work we find that the allowed range of the primordial magnetic field can increase by three orders of magnitude in comparison with [1]. We get upper bound on the magnetic field strength: $3.48\times10^{-6}$ G for the dark matter mass $\lesssim 10^{-2}$ GeV.

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

Origin and dynamics of the Primordial Magnetic field in a parity violating plasma

In this thesis, we have studied the generation and evolution of the magnetic field in the early Universe. We investigated the generation of magnetic fields in the presence of chiral symmetry and gravitational anomaly. We have used modified kinetic theory by Berry curvature and derived expressions for magnetic and vortical conductivities. We have also shown that there are two length scales: kinetic and viscous, which is one of the important results. In the latter part of the thesis, we have shown the effect of second-order viscous effects on the modes of the magnetic fields.

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↗
arXiv2016

Primordial magnetic field and kinetic theory with Berry curvature

We study the generation of magnetic field in the primordial plasma of the standard model (SM) particles at temperature $T>80$~TeV much higher than the electroweak scale. It is assumed that there is an excess number of right-handed electrons over left-handed positrons in the plasma. Using the Berry-curvature modified kinetic theory to incorporate the effect of the Abelian anomaly, we show that this chiral-imbalance leads to generation of hyper-magnetic field in the plasma in both the collision dominated and the collisionless regimes. It is shown that in the collision dominated regime the chiral-vorticity effect can generate finite vorticity in the plasma together with the magnetic field. Typical strength of the generated magnetic field is $10^{27}$~Gauss at $T\sim 80$~TeV with the length scale $10^5/T$ whereas the Hubble length scale is $10^{13}/T$. Further the instability can also generate the magnetic field of order $10^{31}$~Gauss at typical length scale $10/T$. But there may not be any vorticity generation in this regime. We show that the estimated values of the magnetic field are consistent with the bounds obtained from present observations.

astro-ph.CO↗
arXiv2015

Primordial Generation of Magnetic Fields

We reexamine generation of the primordial magnetic fields, at temperature $T>80$TeV, by applying a consistent kinetic theory framework which is suitably modified to take the quantum anomaly into account. The modified kinetic equation can reproduce the known quantum field theoretic results upto the leading orders. We show that our results qualitatively matches with the earlier results obtained using heuristic arguments. The modified kinetic theory can give the instabilities responsible for generation of the magnetic field due to chiral imbalance in two distinct regimes: a) when the collisions play a dominant role and b) when the primordial plasma can be regarded as collisionless. We argue that the instability developing in the collisional regime can dominate over the instability in the collisionless regime.

gr-qc↗