Unified Dark Matter in Scalar Field Cosmologies

Abstract

Considering the general Lagrangian of k-essence models, we study and classify them through variables connected to the fluid equation of state parameter wκ. 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 cs = 1, thereby inhibiting the growth of matter inhomogeneities, more general non-canonical k-essence models allow for the possibility that cs ≪ 1 whenever matter dominates.

Halos of unified dark matter scalar field

“…Moreover, the resulting UDM scalar field Lagrangian must be able to provide cosmological solutions that yield an acceptable description of the cosmological background (see, e.g., Ref. [7]) and low effective speed of sound (see for example Refs. [2,3,6]) so that cosmic structure formation successfully takes place and CMB anisotropies fit the observed pattern [4,5,8].…”

“…For example, the generalized Chaplygin gas ( [9,10,11] (see also [12]), the Scherrer [13] and generalized Scherrer [7] solutions, the single dark perfect fluid with a simple 2-parameter barotropic equation of state [14], or the homogeneous scalar field deduced from the galactic halo space-times [15].…”

“…Moreover, one can build up scalar field models for which the constraint that the Lagrangian is constant along the classical trajectories allows to describe a UDM fluid [7] (see also Ref. [16], for a different approach).…”

Observational constraints to a unified cosmological model

“…II A. There is a plethora of cosmological models based on the same idea [18]; they are build either based on theoretical motivations [19][20][21][22][23][24][25][26][27][28][29] or on phenomenological ones [30][31][32][33][34][35][36][37][38]. Our model is built on phenomenological grounds.…”

Metric-Independent Spacetime Volume-Forms and Dark Energy/Dark Matter Unification

“…Since more than a decade a principal challenge in modern cosmology is to understand theoretically from first principles the nature of both "dark" species of the universe's substance as a manifestation of the dynamics of a single entity of matter. Among the multitude of approaches to this seminal problem proposed so far are the (generalized) "Chaplygin gas" models [10]- [13], the "purely kinetic k-essence" models [18]- [21] based on the class of kinetic "quintessence" models [14]- [17], and more recently -the so called "mimetic" dark matter model [22,23] and its extensions [24,25], as well as constant-pressure-ansatz models [26].…”