(6a) The Gravitational Instability of Cosmological Pancakes
Valinia et al. (1997) studied the gravitational instability of
cosmological pancakes. We modeled these pancakes as 1D, plane-wave density
fluctuations in a collisionless gas, which collapse into sheets along the
planes of each density maximum, and subjected the pancakes to transverse
perturbations which are either symmetric (density) or antisymmetric (bending)
modes. We discovered by high resolution 2D, gravitational N-body simulations
by the PM method that pancakes are unstable to the formation of filaments
for perturbations of any wavelength lambda < lambda_p, the pancake mode
wavelength. Gas dynamical simulations using both standard SPH and the new ASPH
method developed by us have also been performed by Valinia, Shapiro and Martel
(Valinia Ph.D. thesis), revealing similar instability when
a collisional gaseous component is included. As the instability develops, this
gaseous component develops significant vorticity as a result of the presence
of curved, oblique shocks, which may be important in explaining the origin of
galactic rotation. The proportionality between this vorticity and the
magnetic field which would result in the same gas as a result of the
Biermann battery mechanism suggest that this may also be an important source
of primordial seed field necessary to explain galactic magnetic fields.