Theoretical astrophysics and plasma physics at RPC

Abstract:

A scheme is developed which enables one to trace backwards in time the cosmic density and velocity fields, and to determine accurately the current-epoch velocity field from the current-epoch density field, or vice versa. The scheme implements the idea of Giavalisco \etal\ (1993) that the principle of least action should be used to formulate gravitational instability as a two-point boundary-value problem. We argue that the Eulerian formulation of the problem is to be preferred to the Lagrangian one, on grounds of computational simplicity, of ease of interfacing with observational data, and of internal consistency at early times. The scheme is successfully tested on an exact solution in one dimension, and on currently Gaussian fields in one and two dimensions. The application of the scheme to real observational data appears to be eminently feasible, though computationally costly.

Abstract:

A long-standing problem in the theory of astrophysical accretion disks has been to determine the nature of the stress that transports orbital angular momentum outward. The recent discovery of a local MHD instability provides strong evidence that transport is through turbulent Maxwell and Reynolds stresses. Using numerical simulations we have demonstrated that a weak seed magnetic field in an accretion disk is unstable and leads to sustained MHD turbulence at dynamically important levels. The accretion disk provides a natural laboratory for the study of MHD turbulence.