How do plasmas and galaxies relax to (near) universal equilibria?

by Dr Uddipan Banik (Institute for Advanced Study, Princeton University, USA)

Wednesday 11 Feb 2026, 11:00 → 12:00 Asia/Kolkata

AG69 (https://tifr-res-in.zoom.us/j/98723226368?pwd=11WmAxith6AkZ7opMpElfYmgPqqkSG.1 Meeting ID: 987 2322 6368 Passcode: 880586)

The collective relaxation of astrophysical systems governed by long-range interactions, such as collisionless plasmas and self-gravitating systems, remains poorly understood. In this talk, I will present recent work that sheds light on the physical mechanisms responsible for driving plasmas and galaxies towards near-universal distributions that are only weakly sensitive to initial conditions. Using a quasilinear theory based on the Boltzmann-Maxwell equations, I will show how dielectric polarization or Debye screening gives rise to specific power-law distribution functions in kinetic plasmas driven by electromagnetic turbulence at super-Debye but sub-Larmor scales. I will discuss how the presence of such power-law distributions, combined with the filtering effect of gravity (velocity filtration theory), facilitates the outward escape of suprathermal particles, which can explain the observed temperature inversion in the solar corona and the abrupt rise in temperature beyond the chromosphere. Next, I will discuss the origin of large-scale magnetorotational dynamo action in magnetized accretion disks. I will present a quasilinear theory based on the ideal MHD equations that describes the long-period oscillations of large-scale magnetic fields (the butterfly diagram) in sheared rotational flows. I will show how these cycles fundamentally arise from beats due to the interference of shear Alfven waves, in agreement with unstratified shearing box simulations of the magnetorotational instability using the AthenaK code. Finally, I will discuss the collective relaxation of self-gravitating systems. I will present a novel kinetic theory based on the Vlasov-Poisson equations that demonstrates how the universal Navarro–Frenk–White profile can arise in cold dark matter halos due to the collisionless relaxation of matter accreted into a pre-assembled halo. The theory shows that such universal halo profiles are a natural consequence of hierarchical accretion and virialization.