### Imperfect Dark Energy of Kinetic Gravity Braiding

Alex Vikman

CERN

ABSTRACT

In this talk I will discuss a new class of cosmological scalar fields.
Similarly to gravity, these theories are described by actions linearly
depending on second derivatives. The latter can not be excluded
without breaking the generally covariant formulation of the action principle.
Despite the presence of these second derivatives the equations of
motion are of the second order.
Hence there are no new pathological degrees of freedom.
Because of this structure of the theory the scalar field kinetically
mixes with the metric-
the phenomenon we have called Kinetic Gravity Braiding.
These theories have rather unusual cosmological dynamics which may be
useful to model Dark Energy and Inflation.
I will discuss an equivalent hydrodynamical formulation of these
theories, stability and causality for the fluid like
configurations and cosmological applications.