Unified Dark Matter in Scalar Field Cosmologies

Modern Physics Letters A (Impact Factor: 1.11). 04/2007; DOI: 10.1142/S0217732307025893
Source: arXiv

ABSTRACT 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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    ABSTRACT: We present a framework for discussing the cosmology of dark energy and dark matter based on two scalar degrees of freedom. An effective field theory of cosmological perturbations is employed. A unitary gauge choice renders the dark energy field into the gravitational sector, for which we adopt a generic Lagrangian depending on three-dimensional geometrical scalar quantities arising in the ADM decomposition. We add to this dark-energy associated gravitational sector a scalar field $\phi$ and its kinetic energy $X$ as dark matter variables. Compared to the single-field case, we find that there are additional conditions to obey in order to keep the equations of motion for linear cosmological perturbations at second order. For such a second-order multi-field theory we derive conditions under which ghosts and Laplacian instabilities of the scalar and tensor perturbations are absent. We apply our general results to models with dark energy emerging in the framework of the Horndeski theory and dark matter described by a k-essence Lagrangian $P(\phi,X)$. We derive the effective coupling between such an imperfect-fluid dark matter and the gravitational sector under the quasi-static approximation on sub-horizon scales. By considering the purely kinetic Lagrangian $P(X)$ as a particular case, the formalism is verified to reproduce the gravitational coupling of a perfect-fluid dark matter.
    02/2014; 89(6).
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    ABSTRACT: We restrict purely kinetic k-essence. Assuming that the equation of state is a power law of the kinetic energy, i.e. w = w0Xα, we find that α must be positive to obtain accelerated phases, constrained from the conditions for stability and causality. In this case the k-essence behaves like a phantom. We also study the evolutions of the equation of state and the speed of sound with numerical simulations.
    Chinese Physics Letters 10/2011; 28(10):109502. · 0.92 Impact Factor
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    ABSTRACT: Models in which the Lagrangian contains only a kinetic factor and does not depend explicitly on the field itself in k-essence cosmology are considered. In the case of a constant potential, we obtain an exact analytic solution of k-essence, 𝑤𝑘=−(3𝛼−2)/3𝛼, under a simple hypothesis, 𝑎=𝑎0(𝑡/𝑡0)𝛼, but without any assumption about the form of 𝐹(𝑋). In purely kinetic k-essence model, the acceleration can only be induced after the matter-dominated epoch; the Universe is about 33.5+4.2−3.4 Gyr old now and about 3.1 Gyr old at 𝑧=3.9 which is consistent with the fact of quasar observation, while this observation contradicts with the prediction of ΛCDM model, and the year where a transition from deceleration to acceleration expansion is about from 18.43 Gyr to 25.2 Gyr after the beginning of the Universe. These results may offer clues to test observationally the k-essence scenario in the future.
    ISRN Astronomy and Astrophysics. 01/2012; 2011.

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