arXiv subjects

Reghukrishnan Gangadharan

Publications and source records attributed to Reghukrishnan Gangadharan.

6 recordsLinked to original sources

arXiv2026

Validity of relativistic hydrodynamics beyond local equilibrium

We examine the applicability of relativistic hydrodynamics far from equilibrium by constructing formal solutions of the Boltzmann moment equations in the relaxation time approximation. These solutions naturally decompose into a divergent gradient series and exponentially decaying non-perturbative modes that encode initial conditions. The non-perturbative contributions are essential for understanding causality, the divergence of the gradient series, and the unexpected effectiveness of relativistic hydrodynamics far from equilibrium. In the 0+1D Bjorken scenario, we demonstrate that the exact evolution of non-equilibrium terms shares the same structural form as the gradient expansion, differing only through modified transport coefficients that reflect both initial data and free-streaming dynamics. Extending to 3+1D, we find that hydrodynamics remains effective not because the system is close to equilibrium, but because it interpolates smoothly between free streaming and collective behavior. This perspective offers a natural explanation for the remarkable success of hydrodynamics in modeling quark-gluon plasma evolution.

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arXiv2026

Non-Equilibrium Trace Anomaly And Bulk Viscosity in Heavy Ion Collisions From Kinetic Theory

We investigate the far-from-equilibrium dynamics and transport properties of a relativistic massive gas obeying Maxwell-Boltzmann (MB), Bose-Einstein (BE), and Fermi-Dirac (FD) statistics undergoing a boost-invariant Bjorken expansion. We solve the relativistic Boltzmann equation in the relaxation-time approximation (RTA) using the method of moments. We focus on the time evolution of the trace of the energy-momentum tensor $Θ^μ{}_μ$ and the bulk viscous pressure $Π$, which are key diagnostics of conformal-symmetry breaking in the rapidly evolving fireball created in heavy-ion collisions. We find that the non-equilibrium quantity $Θ^μ{}_μ/T^{4}$ exhibits a non-monotonic time dependence, with a local maximum at early times and a pronounced dip around the characteristic relaxation time scale $τ_{R}$. We further show that the scaled bulk pressure $Π/P_{0}$, where $P_{0}$ denotes the isotropic equilibrium pressure, depends sensitively on the particle statistics. In addition, increasing the initial chemical potential enhances the magnitudes of both $Π$ and $Θ^μ{}_μ/T^{4}$. Finally, by initializing the system with random non-equilibrium configurations, we demonstrate that the evolution of the scaled bulk pressure and the pressure anisotropy converges to a common late-time solution.

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arXiv2025

On the Approach Towards Equilibrium Through Momentum-Dependent Relaxation:Insights from Evolution of the Moments in Kinetic Theory

We investigate the impact of momentum-dependent relaxation time approximation in the Boltzmann equation within the Bjorken flow framework by analyzing the moments of the single-particle distribution function. The moment equations, which form an infinite hierarchy, provide important insights about the system dynamics and the approach towards equilibrium for systems far from equilibrium. The momentum-dependent collision kernel couples moments through both the energy exponents and the angular dependence via various-order Legendre polynomials, resulting in an intricate system of infinitely coupled equations. A naive truncation of the coupled equations results in diverging moments at late times. We outline strategies for solving the coupled system, including a novel approach for managing the divergences and non-integer moments. We show a significant influence of momentum dependent relaxation time on the time evolution of the moments, particularly for higher-order moments and system with smaller shear viscosity over entropy density, emphasizing the importance of incorporating such dependence for a more accurate description of the system dynamics with low shear viscosity such as the quark-gluon-plasma produced in high-energy heavy-ion collisions.

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arXiv2024

The Convergence Problem Of Gradient Expansion In The Relaxation Time Approximation

We obtain a formal integral solution to the 3+1 D Boltzmann Equation in relaxation time approximation. The gradient series obtained from this integral solution contains exponentially decaying non-hydrodynamic terms. It is shown that this gradient expansion can have a finite radius of convergence under certain assumptions of analyticity. We then argue that, in the relaxation time model, proximity to local thermal equilibrium is not necessary for the system to be described by hydrodynamic equations.

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arXiv2023

Study of Early time attractor with Longitudinal Forces with Bjorken Symmetry

We study the effect of external proper time-dependent longitudinal forces on the evolution of the distribution function using the Boltzmann Equation with a relaxation time collision kernel under Bjorken flow. We derive an exact solution and study the early time attractor behaviour of different components of energy-momentum tensor under Bjorken symmetry. We show that the arbitrary initial data approaches the attractor solution but is influenced by the external force with slightly broken Bjorken symmetry.

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arXiv2023

Investigating the Role of Electric Fields on Flow Harmonics in Heavy-Ion Collisions

Using the blast-wave model, we explore the effect of electric fields on spectra and flow harmonics (especially the elliptic flow) for charged pions and protons. We incorporate the first-order correction to the single-particle distribution function due to the electric fields and the dissipative effect while calculating the invariant yields of hadron in the Cooper-Frey prescription at the freezeout hypersurface. We find a noticeable correction to the directed and elliptic flow of pions and protons for unidirectional and azimuthal asymmetric electric fields in the transverse plane of magnitude $\sim m_π^{2}$. Further, we observe mass dependency of the directed flow generated due to the electric fields. The splitting of particle and antiparticle's elliptic flow is also discussed.

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