Gradient instability for w < -1

Hsu, Stephen D. H.; Jenkins, Alejandro; Wise, Mark B.

Gradient instability for w < -1

Authors

Hsu, Stephen D. H.

Jenkins, Alejandro

Wise, Mark B.

Description

We show that in single scalar field models of the dark energy with equations of state satisfying $w \equiv p / ρ< -1$, the effective Lagrangian for fluctuations about the homogeneous background has a wrong sign spatial kinetic term. In most cases, spatial gradients are ruled out by microwave background observations. The sign of $w+1$ is not connected to the sign of the time derivative kinetic term in the effective Lagrangian.
revtex4, 8 pages, 1 figure. v2: corrected typo in Eq. 16, added references and a paragraph on quintessence models; v3: reordering of references. To appear in Phys. Lett. B