A Newtonian Approach to the Cosmological Dark Fluids - Selected Topics of Computational and Experimental Fluid Mechanics - page 507

as shown in Sect. 2 , implies that this fluid permeates all the space homogeneously,
giving it the alias of non-evolving dark energy. Any departure of w

=−

1 would
imply that their perturbations evolve and hence possibly give rise to the formation
of dark energy structure.

4 The Dark Fluid

Is it possible that the properties of dark energy and dark matter to be different mani-
festations of the same dark fluid? Several unified dark models of the dark sector have
appeared in the literature, the prototype of these is the generalized Chaplygin gas
(Kamenshchik et al. 2001 ; Bento et al. 2002 ), which is defined as a barotropic fluid
with EoS P Chap =−

/ˁ Chap with 0 A <ʱ ≤ 1. Integrating the continuity equation

( 3 ) we obtain

A

1
1 + ʱ

B
a 3 ( 1 + ʱ)

ˁ Chap ( a ) = + (31)

where B is an integration constant. This model describes a smooth interpolation
between an early phase dominated by dust , with a − 3 and an asymptotical future ˁ ∝

with ˁ = constant. The intermediate phase is well described by an EoS P = ʱˁ .The

tightest constraints on the parameter come from comparisons to the observed large

scale matter power spectrum obtaining

ʱ

10 − 5 (Gorini et al. 2008 ), and therefore
making the model effectively indistinguishable from

ʱ<

CDM model.
Other unified models that have recently attracted the attention of the cosmolog-
ical community includes scalar fields, modifications to Einstein's theory of gravity,
among others; see, for example Aviles and Cervantes-Cota ( 2011 ), De-Santiago and
Cervantes-Cota ( 2011 ) and Khoury ( 2014 ).
We now specialize to a specific model that is totally degenerated with

ʛ

CDM at
least at zero and first order in cosmological perturbation theory, the dark fluid which
was introduced in Ref. Hu and Eisenstein ( 1999 ) and further studied in Aviles and
Cervantes-Cota ( 2011 ), Luongo and Quevedo ( 2014 ) and Aviles et al. ( 2012 ).
We define the dark fluid as in Aviles and Cervantes-Cota ( 2011 ), that is, a
barotropic perfect fluid with adiabatic speed of sound equal to zero. 3 Gravitational
instability is driven by the competition between gravitational attraction and pressure
support. From Eq. ( 26 ) it follows that the condition for vanishing sound speed allows
perturbations of the fluid to grow at all scales, as cold dark matter does. For the dark
fluid we can write the equation of state without loss of generality as

ʛ

P d = w d (ˁ)ˁ d , (32)

3 Other definitions are possible. In Hu and Eisenstein ( 1999 ) the barotropic condition is not con-
sidered but additional conditions on its EoS are imposed. In Luongo and Quevedo ( 2012 , 2014 )it
is defined as an ideal gas with vanishing speed of sound.