The Kelvin-Helmholtz instability in an expanding universe and its effect on dark matter

¹ GRECO, Institut d'Astrophysique de Paris, CNRS, 98bis Boulevard Arago, 75014 Paris, France
² Institute of Cosmology and Gravitation, University of Portsmouth, Portsmouth PO1 2EG, UK

Corresponding author: K. A. Malik, malik@iap.fr

Received: 16 January 2003
Accepted: 12 May 2003

Abstract

We extend the Kelvin-Helmholtz instability to an expanding background. We study the evolution of a non-viscous irrotational fluid and find that for wavelengths much smaller than the Hubble scale small perturbations of the fluid are unstable for wavenumbers larger than a critical value. We then apply this result in the early universe, treating cold dark matter as a classical fluid with vanishing background pressure.