Cuscuton and low energy limit of Horava-Lifshitz gravity
  A proposal for a power-counting renormalizable theory of quantum gravity at a
Lifshitz point was recently put forth by Horava (arXiv:0901.3775), and has been
since dubbed as Horava-Lifshitz gravity. The theory explicitly breaks Lorentz
invariance, which introduces an apparent extra scalar degree of freedom. In
this note, we show that the low energy limit of (non-projectible)
Horava-Lifshitz gravity is uniquely given by the quadratic cuscuton model: a
covariant scalar field theory with an infinite speed of sound and a quadratic
potential, which is minimally coupled to Einstein gravity. This implies that
the extra scalar is non-dynamical to all orders in perturbation theory. Using
current cosmological constraints on the quadratic cuscuton model, we can
constrain the low energy Lorentz breaking parameter of Horava-Lifshitz theory
(which leads to a running of Planck mass on the Hubble scale) to |lambda-1| <
0.014 (at 95% confidence level). We also point out that, with reasonable
boundary conditions, the spatial hypersurfaces in this theory are constant mean
curvature (CMC) or uniform expansion surfaces at low energies, and introduce
geometrical symmetries that can protect the non-dynamical nature of these
theories from quantum corrections. We also notice that the theory with lambda <
1 might suffer from a non-perturbative UV instability.
