Spectral Dimension of the Universe in Quantum Gravity at a Lifshitz Point

We extend the definition of ``spectral dimension'' dₒ (usually defined for fractal and lattice geometries) to theories in spacetimes with anisotropic scaling. We show that in gravity with dynamical critical exponent z in D+1 dimensions, the spectral dimension of spacetime is dₒ=1+Dz. In the case of gravity in 3+1 dimensions with z=3 in the UV which flows to z=1 in the IR, the spectral dimension changes from dₒ=4 at large scales to dₒ=2 at short distances. Remarkably, this is the behavior found numerically by Ambjrn et al. in their causal dynamical triangulations approach to quantum gravity.