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**Session 86 - Cosmology: Theory.**

*Display session, Friday, January 09*

*Exhibit Hall, *

## [86.01] Gravitational Instability of Interpenetrating Fluids

*A. Casti (Columbia University), P. Morrison (Institute for Fusion Studies / U.Texas at Austin), E. Spiegel (Columbia University)*

We examine the gravitational instability of two interpenetrating,
barotropic fluids interacting only through gravity. Previous
work on the problem (Fridman and Polyachenko, 1984) indicates
that two fluids with initial relative motion are unstable
to small perturbations at all wavenumbers in some range of Mach
numbers. We expand upon these results by exploiting
the analogy between this problem and the two-stream instability
of plasma physics, and demonstrate the existence of negative energy
modes from the indefiniteness of the energy functional. This allows
for explosive nonlinear growth even in situations for which the linear
theory predicts absolute stability. We further investigate the
effects of negative energy modes in a non-conservative situation
where one of the fluids is radiatively damped.

reference: "Physics of Gravitating Systems I and II," by A.M. Fridman
amp; V.L. Polyachenko, Springer-Verlag, 1984.

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