Unified Dark Matter in Scalar Field Cosmologies

Modern Physics Letters A (Impact Factor: 1.2). 04/2007; 22(38). DOI: 10.1142/S0217732307025893
Source: arXiv


Considering the general Lagrangian of k-essence models, we study and classify them through variables connected to the fluid equation of state parameter w_\kappa. This allows to find solutions around which the scalar field describes a mixture of dark matter and cosmological constant-like dark energy, an example being the purely kinetic model proposed by Scherrer. Making the stronger assumption that the scalar field Lagrangian is exactly constant along solutions of the equation of motion, we find a general class of k-essence models whose classical trajectories directly describe a unified dark matter/dark energy (cosmological constant) fluid. While the simplest case of a scalar field with canonical kinetic term unavoidably leads to an effective sound speed c_s=1, thereby inhibiting the growth of matter inhomogeneities, more general non-canonical k-essence models allow for the possibility that c_s << 1 whenever matter dominates.

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Available from: Daniele Bertacca, Oct 17, 2012
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    • "As we have seen in section 4.1 the UDM model [65] [66] at the level of Noether symmetries (but also as a dynamical system) is equivalent to the unharmonic oscillator [53]. In the case of two scalar fields this happens if the fields interact in their kinematic part so that 2d the interaction metric H AB admits so(3) as the Killing algebra. "
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    • "Scalar fields are also important in cosmology, since they are at the basis of most inflationary theories [2] [3] [4] [5] and are candidates for dark energy [6] [7]. Moreover, non-canonical scalar fields can have in some instances vanishing effective speed of sound, thus being able to model cold dark matter [8] [9] or even the entire dark sector of the universe [10] [11] [12] [13] [14] [15] [16] [17]. "
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    • "However, if perturbations in the ghost condensate model [38] [39] ever become non-linear, they will behave in a way that is very similar to the sort of phenomenology we describe in this paper. Generalised Chaplygin gas [40] [41] and unified dark-matter models [42] [43] [44] have an equation of state that is very close to dust until late times and therefore yet again the physics described in this paper would be realized in these models at least until the onset of acceleration. "
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