Anisotropic Fluid Cosmology: an Alternative to Dark Matter?
  We use anisotropic fluid cosmology to describe the present, dark
energy-dominated, universe. Similarly to what has been proposed for galactic
dynamics, the anisotropic fluid gives an effective description of baryonic
matter, dark energy and their possible interaction, without assuming the
presence of dark matter. The resulting anisotropic fluid spacetime naturally
generates inhomogeneities at small scales, triggered by an anisotropic stress,
and could therefore be responsible for structure formation at these scales.
Solving the cosmological equations, we show that the dynamics of the scale
factor $a$ is described by usual FLRW cosmology and decouples completely from
that describing inhomogeneities. We assume that the cosmological anisotropic
fluid inherits the equation of state from that used to describe galaxy rotation
curves. We show that, in the large scale regime, the fluid can be described as
a generalized Chaplygin gas and fits well the distance modulus experimental
data of type Ia supernovae, thus correctly modelling the observed accelerated
expansion of the universe. Conversely, in the small scale regime, we use
cosmological perturbation theory to derive the power spectrum $P(k)$ for mass
density distribution. At short wavelengths, we find a $1/k^4$ behaviour, in
good accordance with the observed correlation function for matter distribution
at small scales.
