Thermodynamical properties of dark energy

Abstract

We have investigated the thermodynamical properties of dark energy. Assuming that the dark energy temperature T ∼ a −n and considering that the volume of the Universe enveloped by the apparent horizon relates to the temperature, we have derived the dark energy entropy. For dark energy with constant equation of state w > −1 and the generalized Chaplygin gas, the derived entropy can be positive and satisfy the entropy bound. The total entropy, including those of dark energy, the thermal radiation and the apparent horizon, satisfies the generalized second law of thermodynamics. However, for the phantom with constant equation of state, the positivity of entropy, the entropy bound, and the generalized second law cannot be satisfied simultaneously. Results from numerous and complementary observations show an emerging a paradigm 'concordance cosmology' indicating that our universe is spatially flat and composed of about 70% dark energy (DE) and about 25% dark matter. The weird DE is a major puzzle of physics now. Its nature and origin have been the intriguing subject of discussions in the past years. The DE has been sought within a wide range of physical phenomena, including a cosmological constant, quintessence or an exotic field called phantom [1]. Except the known fact that DE has a negative pressure causing the acceleration of the universe, its nature still remains a complete mystery. In the conceptual set up of the DE, one of the important questions concerns its thermodynamical properties. It is expected that the thermodynamical consideration might shed some light on the properties of DE and help us understand its nature.The topic on the DE entropy, temperature and their evolution by using the first law of thermodynamics was widely discussed in the literature [2,3,4,5,6,7,8,9,10]. It was found that the entropy of the phantom might be negative [6,7,8]. The existence of negative entropy of the phantom could be easily seen from the relation T s = ρ+p between the temperature T , the entropy density s, the energy density ρ and the pressure p. Negative entropy is problematic if we accept that the entropy is in association with the measure of the number of microstates in statistical mechanics. The intuition of statistical mechanics requires that the entropy of all physical components to be positive. Besides if we consider the universe as a ther- * Electronic address: yungui˙gong@baylor.edu † Electronic address: wangb@fudan.edu.cn ‡ Electronic address: anzhong˙wang@baylor.edu modynamical system, the total entropy of the universe including DE and dark matter should satisfy the second law of thermodynamics. The generalized second law (GSL) for phantom and non-phantom DE has been explored in [8]. It was found that the GSL can be protected in the universe with DE. The GSL of the universe with DE has been investigated in [9,10] as well. In order to rescue the GSL of thermodynamics, Bekenstein conjectured that there exists an upper bound on the entropy for a weakly self-gravitating physical system [11]. Bekenstein's entropy bou...

Dark Energy

“…Due to the lack of a compelling fundamental theory to explain the dark energy, another route, the model-independent dark energy reconstructions, have drawn more and more attentions [566,709,710,711,712,713,714,715,716,717].…”

“…In addition to w and ρ de , some other quantities, such as the deceleration parameter q [710,726,727,728], the redshift of acceleration-deceleration transition z c [729], the statefinder diagnostic (r, s) [730], the jerk parameter j [731,732] and the diagnostic Om [733,734], can also provide very useful information for the study of dark energy. Although these quantities can accurately reconstruct some dark energy models, they have difficulty to discriminate between different models of dark energy [735].…”

Cosmology with hybrid expansion law: scalar field reconstruction of cosmic history and observational constraints

“…In particular, parametrization of EoS/Hubble parameter/pressure have been extensively used in the literature [21,22,23,24]. The dynamics of realistic Universe is described by an EoS parameter which behaves differently at different epochs.…”

Particle production and universal thermodynamics

“…This is another parametrization of q(z) common in the literature [52,53]. Here the present value of the deceleration parameter is ( 1 2 + q 1 ) and the best fit values are q 0 = 1.47 and q 1 = −1.46.…”