© 2019 The Author(s). The second law of thermodynamics is known to hold at small scales also when gravity plays a leading role, as in the case of black holes and self-gravitating radiation spheres. It has been suggested that it should as well at large scales. Here, by a purely kinematic analysis - based on the history of the Hubble factor and independent of any cosmological model - we explore if this law is fulfilled in the case of homogeneous and isotropic universes regardless of the sign of the spatial curvature.
- cosmology: Theory