Why the expansion of the Universe is accelerated

Abstract

There is compelling evidence, nowadays, that the present expansion of the Universe is accelerated. In Einstein gravity this acceleration is explained by invoking some exotic energy component with a huge negative pressure. More general gravity theories simply modify the Einstein–Hilbert action in ad hoc manner. Here, we show that whatever its origin, this acceleration is consistent with the second law of thermodynamics in the sense that the entropy of the causal cosmic horizon plus that of matter and fields inside it must never decrease and it should approach a finite maximum in the long run. In this light, the acceleration could have been anticipated on thermodynamic grounds well before its actual discovery.