arXiv subjects

Jitesh R. Bhatt

Publications and source records attributed to Jitesh R. Bhatt.

At least 19 recordsLinked to original sources

arXiv2025

Detection of Axion Stars in Galactic Magnetic Fields

We perform a linear mode analysis of a uniformly distributed cloud of axion-like particles (ALPs) embedded in a magnetized intergalactic medium, in order to investigate the stability of axion stars under realistic astrophysical conditions. We find that when the frequency $ω$ of transverse waves is much smaller than the collision frequency $ν_c$ of the intergalactic plasma, the conversion of ALPs into photons occurs on timescales far longer than the age of the Universe, ensuring stability of the star. In the opposite regime, $ω\gg ν_c$, significant axion-to-photon conversion may occur if the condition $\tfrac{β^2}{m_a^2-ω_p^2} < 1$ is satisfied, where $β$ depends on the ALP--photon coupling and the magnetic field, $m_a$ is the ALP mass, and $ω_p$ is the plasma frequency. We have calculated up to second order in perturbations to compute the effect of an ALP star. Since the calculated value of parameter $β^2$ is extremely small in comparison with $ω^2_p$, we argue that the direct detection of an axion star is highly unlikely in experiments like NCLE. However, since the calculated $β$ is extremely small compared to $ω_p$, this requires an unrealistically fine-tuned coincidence between $m_a$ and $ω_p$. As a consequence we argue that that detection of Our results therefore suggest that axion stars remain stable in typical intergalactic environments, though extreme magnetic fields (e.g.\ near magnetars) may lead to different outcomes.

astro-ph.CO↗

arXiv2023

Formation of a Bose Star in a Rotating Cloud

In this paper, we study the evolutions of a self-gravitating cloud of bosonic dark matter with finite angular momentum and self-interaction. This is achieved by using the sixth-order pseudospectral operator splitting method to solve the system of nonlinear Schrödinger and Poisson equations. The initial cloud is assumed to have mass density randomly distributed throughout three-dimensional space. The dark matter particles in the initial cloud are in the kinetic regime, i.e., their de Broglie wavelength is much smaller than the halo size. It is shown that Bose stars are indeed formed in the numerical simulation presented here. The presence of angular momentum and self-interaction in the initial cloud can significantly influence the star formation time in a non-trivial fashion. Furthermore, the plots of the vorticity magnitude profile after the star formation time indicate that the formed star may not have any intrinsic angular momentum for the cases when the self-interaction among the particles is either negligible or attractive. These results are in agreement with the earlier analytical studies of an isolated rotating Bose star. However, for the case of repulsive self-interaction, the vorticity magnitude analysis shows a possibility that the star formed in the numerical simulations may possess intrinsic angular momentum. It is also shown that the average mass and radius diagrams of the star are strongly influenced by the presence of angular momentum in the initial cloud.

astro-ph.CO↗

arXiv2023

The causality and stability of relativistic spin-hydrodynamics

We study the causality and stability of relativistic hydrodynamics with the inclusion of the spin degree of freedom as a hydrodynamic field. We consider two specific models of spin-hydrodynamics for this purpose. A linear mode analysis for static background shows that a first-order dissipative spin-hydrodynamics remains acausal and admits instabilities. Besides, it is found that the inclusion of the spin field in hydrodynamics leads to new kinds of linear modes in the system. These new modes also exhibit instability and acausal behavior. The second model of the spin-hydrodynamics that we have considered here is equivalent to a particular second-order conventional hydrodynamics with no dissipative effects. For a static background, it is found that the linear modes of this model support the sound waves only. However, when the background has constant vorticity, then the model admits instability and acausality in certain situations. It is found that the spin-dynamics have an effect on the hydrodynamic response of the fluid. These findings point toward the need for a causal and stable theory with spin as a hydrodynamic field to describe the spin-polarized fluid.

nucl-th↗

arXiv2022

Rotating Scalar Field and Formation of Bose Stars

We study numerical evolutions of an initial cloud of self-gravitating bosonic dark matter with finite angular momentum and self-interaction in kinetic regime. It is demonstrated that such a system can undergo gravitational condensation and form a Bose star. The results show that the gravitational condensation time is strongly influenced by the presence of finite angular momentum or the strength of self-interaction. We find that in the cases related with attractive or no self-interaction, there is no significant transfer of angular momentum from the initial cloud to the formed star. However, for the case repulsive interaction our results indicate that such a angular-momentum transfer is possible. These results are consistent with the earlier analytical work where the stability of the rotating boson star was considered [Dmitriev et al. 2021].

astro-ph.CO↗

arXiv2020

EDGES signal in presence of magnetic fields

We study the 21-cm differential brightness temperature in the presence of primordial helical magnetic fields for redshift $z=10-30$. We argue that the $α$-effect that sets in at earlier time can be helpful in lowering the gas temperature to 3.2 degrees Kelvin at $z=17$. This effect can arise in the early Universe due to some parity violating high energy processes. Using the EDGES (Experiment to Detect the Global Epoch of Reionization Signature) results, we find the upper and lower limits on the primordial magnetic field to be $6\times 10^{-3}~{\rm nG} $ & $5\times 10^{-4}~{\rm nG}$ respectively. We also discuss the effect of Ly$α$ background on the bounds. Our results do not require any new physics in terms of dark matter.

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

Viscous dark matter and 21 cm cosmology

The EDGES experiment has detected the global absorption signal of 21 cm line at $z\sim17$ in cosmic dawn era and reported its amplitude larger than the standard cosmological prediction. One of the possible explanation requires that the baryons were much cooler than the standard scenario. This requires an interaction between the dark and baryonic sectors with some appropriate cross-section, $ \hatσ $. In this work, we examine the role that dissipative effects of cosmic fluid might play in influencing the 21 cm signal. We show that the presence of viscous dissipation of dark matter can significantly affect the energy transfer between the baryonic and dark matter fluids. It is demonstrated that the inclusion of the dissipative mechanism in the dark sector, strongly modify the earlier constraints on dark matter mass and $ \hatσ $ obtain from EDGES observation. Further, we argue that EDGES absorption signal can put an independent bound on dark matter viscosity which is many order of magnitude larger than the maximum viscosity allowed by the structure formation.

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↗

arXiv2019

Thermoelectric effect and Seebeck coefficient for hot and dense hadronic matter

We investigate the thermoelectric effect for baryon rich plasma produced in heavy ion collision experiments. We estimate the associated Seebeck coefficient for the hadronic matter. Using kinetic theory within relaxation time approximation we calculate the Seebeck coefficient of a hadronic medium with a temperature gradient. The calculation is performed for hadronic matter modeled by hadron resonance gas model with hadrons and resonance states up to a cutoff in the mass as 2.25 GeV. We argue that the thermoelectric current produced by such effect can produce magnetic field in heavy ion collision experiments.

hep-ph↗

arXiv2018

Viscous Self Interacting Dark Matter Cosmology For Small Redshift

The viscosity of dark matter in cosmological models may cause an accelerated expansion and when this effect is sufficiently large, it can explain the dark energy. In this work, attributing the origin of viscosity to self-interaction of dark matter, we study the viscous cosmology at small redshift $(0\leq z\leq2.5)$. Assuming the cluster scale to be virialized and by modeling a power law behavior of velocity gradients, we calculate the Hubble expansion rate, $H(z)$ and the deceleration parameter, $q(z)$. We then perform a $χ^{2}$ analysis to estimate the best fit model parameters. By using the best fit values, we explain the cosmic chronometer and type Ia supernova data. We conclude that if the dissipative effects become prominent only at the late time of cosmic evolution and are smaller at higher redshift, we can explain the observational data without requiring any dark energy component. Our analysis is independent of any specific model of self interacting dark matter.

astro-ph.CO↗

arXiv2017

Viscous Self Interacting Dark Matter and Cosmic Acceleration

Self interacting dark matter (SIDM) provides us with a consistent solution to certain astrophysical observations in conflict with collision-less cold DM paradigm. In this work we estimate the shear viscosity $(η)$ and bulk viscosity $(ζ)$ of SIDM, within kinetic theory formalism, for galactic and cluster size SIDM halos. To that extent we make use of the recent constraints on SIDM crossections for the dwarf galaxies, LSB galaxies and clusters. We also estimate the change in solution of Einstein's equation due to these viscous effects and find that $σ/m$ constraints on SIDM from astrophysical data provide us with sufficient viscosity to account for the observed cosmic acceleration at present epoch, without the need of any additional dark energy component. Using the estimates of dark matter density for galactic and cluster size halo we find that the mean free path of dark matter $\sim$ few Mpc. Thus the smallest scale at which the viscous effect start playing the role is cluster scale. Astrophysical data for dwarf, LSB galaxies and clusters also seems to suggest the same. The entire analysis is independent of any specific particle physics motivated model for SIDM.

astro-ph.CO↗

arXiv2017

Collective excitations of a hot anisotropic QCD medium with Bhatnagar-Gross-Krook collisional kernel within an effective description

Collective modes of an anisotropic hot QCD medium have been studied within the semi-classical transport theory employing Bhatnagar-Gross-Krook (BGK) collisional kernel. The modeling of the isotropic medium is primarily based on a recent quasi-particle description of hot QCD equation of state where the medium effects have been encoded in effective gluon and quark/anti-quark momentum distributions that posses non-trivial energy dispersions. The anisotropic distribution functions are obtained in a straightforward the way by stretching or squeezing the isotropic ones along one of the directions. The gluon self-energy is computed using these distribution functions in a linearized transport equation with Bhatnagar-Gross-Krook (BGK) collisional kernel. Further, the tensor decomposition of gluon self-energy leads to the structure functions which eventually controls the dispersion relations and the collective mode structure of the medium. It has been seen that both the medium effects and collisions induce appreciable modifications to the collective modes and plasma excitations in the hot QCD medium.

nucl-th↗

arXiv2017

Neutrino induced vorticity, Alfven waves and the normal modes

We consider plasma consisting of electrons and ions in presence of a background neutrino gas and develop the magneto hydrodynamic equations for the system. We show that electron neutrino interaction can induce vorticity in the plasma even in the absence of any electromagnetic perturbations if the background neutrino density is left-right asymmetric. This induced vorticity support a new kind of Alfvén wave whose velocity depends on both the external magnetic field and on the neutrino asymmetry. The normal mode analysis show that in the presence of neutrino background the Alfvén waves can have different velocities. We also discuss our results in the context of dense astrophysical plasma such as magnetars and show that the difference in the Alfvén velocities can be used to explain the observed pulsar kick. We discuss also the relativistic generalization of electron fluid in presence of asymmetric neutrino background.

hep-ph↗

arXiv2017

Manybody aspects of gravity in compact stars

Compact stars such as neutron stars and black holes are gravitationally bound many body systems. We investigate the importance of short and long range part of gravity for such systems. From our analysis, we conclude that the true essence of gravity lies with the long range nature of the interaction. At the end we show how these arguments in the manybody theory consistently leads to Dvali-Gomez picture of a black holes as a collective bound state of long wavelength gravitons.

gr-qc↗

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

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↗

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↗

arXiv2016

Electromagnetic instability induced by neutrino interaction

We consider the generation and evolution of magnetic field in a primordial plasma at temperature T < 1 MeV in presence of asymmetric neutrino background i.e. the number densities of right- handed and left-handed neutrinos are not same. Semi-classical equations of motion of a charged fermion are derived using the effective low-energy Lagrangian. It is shown that the spin degree of freedom of the charged fermion couples with the neutrino background. Using this kinetic equation we study the collective modes of the plasma. We find that there exist an unstable mode. This instability is closely related with the instability induced by chiral-anomaly in high temperature T > 80 TeV plasma where right and left-handed electrons are out of equilibrium. We find that at the temperatures below the neutrino decoupling the instability can produce magnetic field of 10 Gauss in the Universe. We discuss cosmological implications of the results.

hep-ph↗