Anisotropick-essence cosmologies

Abstract

We investigate a Bianchi type-I (BTI) cosmology with k-essence and find the set of models which dissipate the initial anisotropy. There are cosmological models with extended tachyon fields and k-essence having constant barotropic index. We obtain the conditions leading to a regular bounce of the average geometry and the residual anisotropy on the bounce. For constant potential, we develop purely kinetic k-essence models which are dust dominated in their early stages, dissipate the initial anisotropy and end in a stable de Sitter accelerated expansion scenario. We show that linear k field and polynomial kinetic function models evolve asymptotically to FriedmannRobertson-Walker (FRW) cosmologies. The linear case is compatible with an asymptotic potential interpolating between V l ∝ φ −γ l , in the shear dominated regime, and V l ∝ φ −2 at late time. In the polynomial case, the general solution contains cosmological models with an oscillatory average geometry. For linear k-essence, we find the general solution in the BTI cosmology when the k field is driven by an inverse square potential. This model shares the same geometry as a quintessence field driven by an exponential potential.

A critical review of classical bouncing cosmologies

“…This model, however intrinsically problematic because relying on an unstable negative energy field, may be seen as a precursor of a ghost condensate: the temporary effective violation of the NEC merely permits the bounce to occur, but the negative energy component is otherwise subdominant for most of the universe history. No implementation was suggested in practice (see also [366] for a bounce introduced by K-essence, where the conditions for a regular bounce, as well as anisotropies, were discussed): A more general phase space analysis of models with generalized kinetic terms can be found in [367], where conditions under which a bounce is possible in the NEC violating regime are derived.…”

“…Models whereby the bounce is realized via galileons, in which the Lagrangian contains higher order derivatives, avoid the appearance of ghosts while the extra additional degrees of freedom violate the NEC. Promising models which address the anisotropy problem in Galileon cosmology and might generate a scale-invariant spectrum include [71,225,226,366], see Sec. II F.…”

Tachyonization of the ΛCDM cosmological model

“…(16). In those cases, the tachyon energy density has a more complicated dependence with the cosmic scale factor than the perfect fluid one ρ ϕ = C/a 3γ0 .…”

“…It means that the particular set of solution, corresponding the linear tachyon field solution along with the limit γ 0 → 1, is the general exact cosmic scale factor solution of the ΛCDM model. In addition, for the case where the tachyon energy density has the perfect fluid form ρ ϕ = C/a 3γ0 , with γ 0 = 1, the time evolution of the cosmic scale factor is given by the solution (21) of the Friedmann equation (20) and the tachyon field solution has the linear form (16) with b = γ 0 . This represent the tachyonization of the flat FRW universe with cosmological constant filled with a perfect fluid.…”

“…[14] it was shown that a positive sound speed is not a sufficient condition for the theory to be stable. Hence, non standard tachyon fields have been generated assuming a sound speed proportional to that of the standard tachyon field in [15] for FRW and in [16] for Bianchi type I cosmologies. In this work it is analyzed the evolution of a Universe whose gravitational sources are a cosmological constant and a tachyon field, and its aim is to determine a large set of cosmological models which contain the ΛCDM model as a special case.…”