Research Article
Energy Definition and Dark Energy: A Thermodynamic Analysis
H. Moradpour ,
1
J. P. Morais Graça,
2
I. P. Lobo ,
2
and I. G. Salako
3
1
Research Institute for Astronomy and Astrophysics of Maragha (RIAAM), P.O. Box 55134-441, Maragha, Iran
2
Departamento de F
´
ısica, Universidade Federal da Para
´
ıba, Caixa Postal 5008, 58051-970 Jo
˜
ao Pessoa, PB, Brazil
3
Institut de Math
´
ematiques et de Sciences Physiques (IMSP), 01 BP 613 Porto-Novo, Benin
Correspondence should be addressed to H. Moradpour; hn.moradpour@gmail.com
Received 19 May 2018; Revised 16 July 2018; Accepted 30 July 2018; Published 9 August 2018
A
c
ademic Editor: Elias C. Vagenas
Copyright © 2018 H. Moradpour et al. is is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. e
publication of this article was funded by SCOAP
3
.
Accepting the Komar mass denition of a source with energy-momentum tensor
𝜇]
and using the thermodynamic pressure
denition, we nd a relaxed energy-momentum conservation law. ereinaer, we study some cosmological consequences of the
obtained energy-momentum conservation law. It has been found out that the dark sectors of cosmos are uniable into one cosmic
uid in our setup. While this cosmic uid impels the universe to enter an accelerated expansion phase, it may even show a baryonic
behavior by itself during the cosmos evolution. Indeed, in this manner, while
𝜇]
behaves baryonically, a part of it, namely,
𝜇]
()
which is satisfying the ordinary energy-momentum conservation law, is responsible for the current accelerated expansion.
1. Introduction
Friedmann equations, the ordinary energy-momentum con-
servation law (OCL) (or the continuity equation), and its
compatibility with the Bianchi identity (BI) are the backbone
of the standard cosmology which forms the foundation of
our understanding of cosmos [1]. Since on scales larger
than about 100 Megaparsecs, cosmos is homogeneous and
isotropic [1], the FRW metric is a suitable metric to study
the cosmic evolution [1]. Relations between thermodynamics
and Friedmann equations have been studied in various setups
which help us in getting more close to the thermodynamic
origin of space-time, gravity, and related topics [2–20].
A thermodynamic analysis can also lead to a better
understanding of the origin of dark energy, responsible for
the current accelerated universe [15–33]. In fact, there are
thermodynamic and holographic approaches claiming that
the cosmos expansion can be explained as an emergent
phenomenon [18, 34–46]. Two key points in these approaches
are the denition of energy and the form of the energy-
momentum conservation law [14, 19, 34–38], and indeed their
results are so sensitive to the energy denitions that have been
employed [19].
In order to make the discussion clearer, consider a at
FRW universe with scale factor ()[1]
2
=−
2
+
2
(
)
2
+
2
2
, (1)
lled by a prefect uid source with energy density and
pressure . Einstein eld equations and energy-momentum
conservation law (or equally the Bianchi identity) lead to
2
=
8
3
,
3
2
+2
=−8,
+3+=0,
(2)
forming the cornerstone of standard cosmology. Although,
the third equation (OCL) is in full agreement with our
observations on the cosmic uid in matter and radiation
dominated eras [1, 47], its validity for the current accelerated
cosmos is questionable [48], which may encourage us to
consider the relaxed types of OCL to describe the current
universe [18, 49]. Combining the Friedmann equations with
each other, we get
=−4+,
(3)
Hindawi
Advances in High Energy Physics
Volume 2018, Article ID 7124730, 8 pages
https://doi.org/10.1155/2018/7124730