Thermodynamics, spectral distribution and the nature of dark energy

Recent astronomical observations suggest that the bulk of energy in the universe is repulsive and appears like a dark component with negative pressure (omega equivalent to p(x) / rho(x) < 0).In this Letter we investigate thermodynamic and statistical properties of such a component. It is found that its energy and temperature grow during the evolution of the universe since work is done on the system. Under the hypothesis of a null chemical potential, the case of phantom energy (omega < -1) seems to be physically meaningless because its entropy is negative. It is also proved that the wavelengths of the omega-quanta decrease in an expanding universe. This unexpected behavior explains how their energy may be continuously stored in the course of expansion. The spectrum and the associated Wien-type law favors a fermionic nature with omega naturally restricted to the interval -1 less than or equal to omega < -1/2. Our analysis also implies that the ultimate fate of the universe may be considerably modified. If a dark energy dominated universe expands forever. it will become increasingly hot. (C) Elsevier B.V. All rights reserved.