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SUMMARY:The discreteness-driven relaxation of collisionless gravitating sy
 stems - Beraldo e Silva (U Central Lancashire)
DTSTART:20190222T113000Z
DTEND:20190222T123000Z
UID:TALK120166@talks.cam.ac.uk
CONTACT:Martin Haehnelt
DESCRIPTION:The violent relaxation of a perturbed or collapsing collisionl
 ess\ngravitational system\, with the fast achievement of a quasi-stationar
 y\nstate\, is traditionally assumed to be governed by the Vlasov equation\
 ,\nin which case the entropy must be conserved. In this scenario\, the\nre
 conciliation with the 2nd law of thermodynamics is made through\ncoarse-gr
 aining (a subjective effect). In this talk\, I will discuss\nrecent result
 s obtained with entropy estimates in N-body simulations\nand for orbit ens
 embles in fixed external potentials. In the N-body\nsimulations\, the (slo
 w) long-term evolution is well described as\nresulting from two-body relax
 ation\, while the early evolution\ngenerates a fast (in a few crossing tim
 es) entropy production. The\nintegration of orbit ensembles in external po
 tentials shows that this\nearly collisionless relaxation is due to the dis
 creteness (finite N)\nof gravitating systems in any potential\, being a co
 nsequence of the\nNyquist-Shannon theorem\, which precludes the developmen
 t of\nphase-space structures finer than a typical scale ~ N^-1/d (for a\ns
 ample of size N in d dimensions). As a result\, a typical relaxation\ntime
  T/tau_cr ~ 0.1 * N^1/6 emerges in integrable potentials\, with\nweaker N-
 dependencies in the presence of chaotic orbits. Furthermore\,\nthis scenar
 io avoids the need for the subjective effect of\ncoarse-graining and indic
 ates that the Vlasov equation does not\nprovide an adequate kinetic descri
 ption of this fast (violent)\ncollisionless relaxation.
LOCATION:Kavli Large Meeting Room\, Kavli Building
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