Jeans Mass and the
Growth of Perturbations
•For short wavelengths, the pressure term dominates, leading to a sound wave with an amplitude that slowly decays with expansion:
•
•
•
–
•
•
In the case of wavelengths greater than the Jeans length the gravity dominates:
The perturbation increases, but not exponentially due to expansion, which acts as a frictional force.  The type of growth depends on the cosmology (Ch. 13) and how R(t) evolves.