Nonconformal kinetic theory and hydrodynamics for Bjorken flow

Far-from-equilibrium kinetic dynamics of $λϕ^4$ theory in an expanding universe

“…One may also check if the generating function method introduced here is useful when investigating the dynamics of systems of more relevance to heavy-ion collisions, such as QCD effective kinetic theory [30][31][32][33][34][35] and models where the particles in the gas have a temperature-dependent mass [36][37][38][39][40][41][42]. Furthermore, a better understanding of how the vector and tensor parts of the spectrum behave in our system would be relevant when studying the emergence of hydrodynamic attractors [43][44][45] and the characterization of the far-fromequilibrium properties of kinetic theory systems [21,22,25,[46][47][48][49][50][51][52][53][54][55][56][57][58][59][60][61][62][63]. We hope to report on our progress in some of the topics mentioned above in the near future.…”

Hydrodynamic attractors for the speed of sound in holographic Bjorken flow

“…Attractors were found within QCD and kinetic theory [62][63][64][65]. Anisotropic attractors were considered [66], and attractors (including early-time attractors at weak coupling) have been further studied in the context of Bjorken flow with higher-order viscous fluid dynamics [67] (also for Gubser flow [68]), non-conformal systems [69], and in non-conformal kinetic theory [70], see also [71]. 7 In this work, we thus utilize the confirmed hydrodynamic attractors of N = 4 SYM theory to provide a leading order resummation of the hydrodynamic expectation of the speed of sound, and find excellent agreement between the exact numerical evolution and the hydrodynamic attractor expectation.…”

New developments in relativistic hydrodynamics

“…In particular, this means that the system is assumed to be near local equilibrium, implying that the so-called Knudsen number Kn≡ l mfp /l hydro , where l mfp is the mean free path and l hydro is a scale characterizing inverse gradients of hydrodynamic fields, is sufficiently small. However, recent developments suggest that hydrodynamics may give a useful description of a system even beyond the mentioned limits, extending hydrodynamic frameworks in order to include far-from-equilibrium dynamics [3,4,5,6,7,8,9,10,11,12], short-wavelength phenomena like chiral symmetry breaking [13,14,15,16,17] or quantum phenomena like spin [18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35]. Furthermore, there has been important progress in the past years in formulating causal and stable first-order hydrodynamics, known as Bemfica-Disconzi-Noronha-Kovtun (BDNK) theories [36,37,38,39,40,41].…”

“…In this context, the existence of so-called attractor solutions [3], which determine the early-as well as late-time behavior of these systems, turned out to be of significant importance. In the following, we will review as an example studies on attractors and fixed points in kinetic theory and hydrodynamics for boost invariant systems both in the conformal limit [4,8,10] and in the massive case [11,12].…”

“…We now turn to the case of a nonconformal, boost-invariant expanding system considered in Refs. [11,12]. When studying the transition from the free-streaming to the hydrodynamic regime, it is found that, in contrast to the massless case, no early-time attractor for the pressure Π nor for the shear stress π (which is in Bjorken symmetry the only nonzero component of the shear-stress tensor π µν ) exists.…”