Thermodynamical properties of the Universe with dark energy

TL;DR

This paper explores the thermodynamical behavior of the universe with dark energy, emphasizing the importance of considering the relationship between volume and temperature, and confirms the validity of thermodynamic laws within the apparent horizon.

Contribution

It demonstrates that realistic volume-temperature relations resolve strange thermodynamical behaviors and affirms the apparent horizon as the physical boundary for thermodynamics in dark energy-dominated universe.

Findings

Thermodynamical behaviors become consistent when volume and temperature are related.

The apparent horizon is confirmed as the physical horizon for thermodynamics.

The holographic principle and second law hold for dark energy with w ≥ -1.

Abstract

We have investigated the thermodynamical properties of the Universe with dark energy. Adopting the usual assumption in deriving the constant co-moving entropy density that the physical volume and the temperature are independent, we observed some strange thermodynamical behaviors. However, these strange behaviors disappeared if we consider the realistic situation that the physical volume and the temperature of the Universe are related. Based on the well known correspondence between the Friedmann equation and the first law of thermodynamics of the apparent horizon, we argued that the apparent horizon is the physical horizon in dealing with thermodynamics problems. We have concentrated on the volume of the Universe within the apparent horizon and considered that the Universe is in thermal equilibrium with the Hawking temperature on the apparent horizon. For dark energy with $w\ge -1$, the holographic principle and the generalized second law are always respected.