Speaker
Description
Gravity theories in which the metric tensor is complemented by a minimally coupled kinetic gravity braiding type scalar field obey all present observational constraints. The k-essence part of the scalar field is either equivalent to a perfect fluid (when the scalar gradient is timelike), an inhomogeneous type I fluid with tangential pressures equal to minus its energy density (for spacelike gradient) or a type II fluid with radial heat flow (for null gradient). We show that the energy-momentum tensors of generic k-essence scalars with temporal and spatial gradients differ by a crossflow of radiation streams. The energy conditions require that a k-essence with null gradient should degenerate into null dust.
Then we carry out a similar analysis for the much more complicated proper kinetic gravity braiding scalar field. By casting its energy-momentum tensor into a 2+1+1 form, we recover many of the features identified for k-essence. In particular, the proper kinetic gravity braiding scalar with null gradient appears as null dust. In the case of temporal and spatial gradients, beyond the type of terms already present for k-essence, the fluid decomposition of the energy-momentum tensor of proper kinetic gravity braiding scalars acquire additional heat transport and pressure anisotropy contributions, scaling with the two normal fundamental scalars and the 2-accelerations of the normals entering the decomposition.