Nonadiabatic perturbations

Suppose that
the matter Lagrangian depends on
two degrees of freedom,
e.g., a Born-Infeld scalar field *θ* and one additional scalar field *φ*. In this
case, instead of a simple barotropic
form *p=p*(*ρ*), the equation of state involves the entropy density (entropy per particle) *s* and may be
written in the
parametric form

• *p=p*(*θ**,**φ*) *s=s*(*θ**,**φ*)

•

•The
corresponding perturbations