Cosmological Friedmann equation in infrared modified Hořava–Lifshitz gravity via generalized Misner–Sharp mass

Abstract

In various gravity theories, Friedmann equations can be cast to a form of the first law of thermodynamics in a Friedmann–Robertson–Walker (FRW) cosmological setup. However, this result failed in recent infrared (IR) modified Hořava–Lifshitz (HL) gravity. The difficulty stems from the fact that HL gravity is Lorentz-violating. Motivated by this problem, we use the Misner–Sharp mass to investigate the thermodynamics near the apparent horizon in HL cosmology. We find that the Friedmann equations can be derived from the first law of thermodynamics. The Misner–Sharp mass used here inherits the specific properties of HL gravity since it is directly from the gravitational action of HL theory. We also prove that the first law of thermodynamics with logarithmic entropy still holds at the apparent horizon in FRW. The results suggest that the general prescription of deriving the field equation from thermodynamics still works in the HL cosmology.