Damping of relativistic shocks in an expanding universe

Abstract

Recently the behaviour of strong relativistic shock waves has attracted a lot of at tent ion (1.4). One area where they play an important role is in theories of pregalactic fluctuations (5). In a series of interesting papers (1,2.6) LIANG and co-workers have tackled the problem of the propagation and damping of these shocks, one of their key points being the conjecture that the damping time for a shock propagating into a uniform medium tends to infinity as the velocity jump Iv] tends to c (2). If confirmed, this unexpected effect could have important consequences for the theory of galaxy formation (6.7). However, LIANG and BAKER obtained their result by an extremely heuristic method which is open to criticism. In particular their procedure of comparing the shocked tr iangular pulse with a simple wave pulse having Iv] as peak velocity is unsatisfactory, because for large values of Iv] the simple wave has a much larger contrast of rest mass density than is allowed by the Rankine-Hugoniot relations for the shocked pulse. Therefore, the comparison is not formally valid. Furthermore, LIA•G and BAKER give results for the damping of strong shocks only for the case of Minkowski space-time,