Anisotropic dark energy: dynamics of the background and perturbations

Abstract

Abstract. We investigate cosmologies where the accelerated expansion of the Universe is driven by a field with an anisotropic equation of state. We model such scenarios within the Bianchi I framework, introducing two skewness parameters to quantify the deviation of pressure from isotropy. We study the dynamics of the background expansion in these models. A special case of anisotropic cosmological constant is analyzed in detail. The anisotropic expansion is then confronted with the redshift and angular distribution of the supernovae type Ia. In addition, we investigate the effects on the cosmic microwave background (CMB) anisotropies for which the main signature appears to be a quadrupole contribution. We find that the two skewness parameters can be very well constrained. Tightest bounds are imposed by the CMB quadrupole, but there are anisotropic models which avoid this bound completely. Within these bounds, the anisotropy can be beneficial as a potential explanation of various anomalous cosmological observations, especially in the CMB at the largest angles. We also consider the dynamics of linear perturbations in these models. The covariant approach is used to derive the general evolution equations for cosmological perturbations taking into account imperfect sources in an anisotropic background. The implications for the galaxy formation are then studied. These results might help to make contact between the observed anomalies in CMB and large scale structure and fundamental theories exhibiting Lorentz violation.

Probing dark energy anisotropy

“…Other work has explored dark energy anisotropy in terms of the small scale spatial inhomogeneities in its density [1], large scale anisotropies giving an overall ellipticity to the universe [2], and within specific models such as vector dark energy [3][4][5][6][7][8][9][10], elastic dark energy [11][12][13], noncommutativity [14], etc. Our approach uses exact solutions, similar to [15], as well as phenomenological line of sight anisotropy but global isotropy, similar to [16,17], testing the difference, exploring further probes, considering sources of astrophysical systematics, and motivating the phenomenology with comparisons to exact Bianchi solutions.…”

Future CMB constraints on early, cold, or stressed dark energy

“…To parameterize viscosity in a dark component one can introduce the viscous sound speed c 2 vis , which controls the relationship between velocity/metric shear and the anisotropic stress [12,15,16]. A value of c 2 vis = 1/3, for example, is what one expects for a relativistic component, where anisotropic stress is present and approximates the radiative viscosity of a relativistic fluid.…”

Spatial and temporal variations of the fine-structure constant in the Finslerian universe

“…Then, the Killing equations of Randers space F Ra [27,28] tells us that the translational symmetry of space and the rotational symmetry of x − y plane are preserved. This means that the symmetry of Finsler spacetime (2) is the same to that of Bianchi type I spacetime [29,30]. However, we will show in the following that the gravitational field equations of Finsler spacetime (2) …”