A Newtonian Approach to the Cosmological Dark Fluids - Selected Topics of Computational and Experimental Fluid Mechanics - page 509

=

The first of the two equations above can be written in the suggestive form P 0

− ˁ c ʩ ʛ

. In Ref. Aviles and Cervantes-Cota ( 2011 ) it has been shown explicitly that
the degeneracy persist at the linear cosmological perturbed level, and heuristic argu-
ments are given that point to the degeneracy is present at all orders in perturbation
theory.
We notice an important physical difference between the CDM and dark fluid

models. To describe the observed late acceleration of the Universe, in Eq. ( 37 )itis
necessary to include the cosmological constant which is then interpreted as the vac-
uum energy. This identification gives rise to the well-known cosmological constant
problem, probably the most serious inconsistency in theoretical physics. The dark
fluid model offers a possibility to avoid this problem. In fact, in Eq. ( 36 )theterm

ʛ

K
does not contain any cosmological information which should be associated with the
vacuum energy, but of course it remains the problem to explain

. This problem is
however in the arena of the microscopical theory of the dark fluid, that is not devel-
oped yet. The dynamics of the dark fluid naturally leads to an accelerated universe,
mimicking the exact behavior of the

K

CDM model, without any cosmological con-
stant. Nevertheless, the final decision about this possibility requires a more detailed
investigation.

ʛ

5 Conclusions

In this pedagogical short review we have developed the theory of cosmological
perturbations at the background and linear levels within the framework of Newtonian
gravity for matter-energy fields in the fluid approximation. It is remarkable that some
of the important aspects of the cosmic evolution of the Universe can be understood
without the use of Einstein's theory of General Relativity. Despite this fact, once
we have derived the Newtonian evolution equations we proceeded to add relativistic
corrections to arrive to the complete set of equations, and special emphasis has
been done in finding the solutions for the evolution of dark matter and dark energy
perturbations.
Due to equivalence principle and that the dark sector components—in its more
radical definition—only interact with the “visible” forms of energy through gravity,
it is arbitrary to decompose the dark sector into dark matter and dark energy. In this
work we also reviewed a model, namely the dark fluid, that is indistinguishable from
the CDM. This description results very appealing because it is based on a very

simple assumption, that is, the speed of sound of the dark fluid vanishes identically.
We explicitly show that both models are degenerated and therefore, it does not exist
any observation based on the background and linear perturbed cosmology that can
tell the correct description.
In addition, we noticed that the dark fluid model opens the possibility of avoiding
the cosmological constant problem because it can explain the late acceleration of the
Universe, without necessarily demanding the presence of a cosmological constant.

ʛ