Unified Dark Matter models with fast transition

Abstract

We investigate the general properties of Unified Dark Matter (UDM) fluid models where the pressure and the energy density are linked by a barotropic equation of state (EoS) p = p(ρ) and the perturbations are adiabatic. The EoS is assumed to admit a future attractor that acts as an effective cosmological constant, while asymptotically in the past the pressure is negligible. UDM models of the dark sector are appealing because they evade the so-called "coincidence problem" and "predict" what can be interpreted as w DE ≈ −1, but in general suffer the effects of a non-negligible Jeans scale that wreak havoc in the evolution of perturbations, causing a large Integrated Sachs-Wolfe effect and/or changing structure formation at small scales. Typically, observational constraints are violated, unless the parameters of the UDM model are tuned to make it indistinguishable from ΛCDM. Here we show how this problem can be avoided, studying in detail the functional form of the Jeans scale in adiabatic UDM perturbations and introducing a class of models with a fast transition between an early Einstein-de Sitter CDM-like era and a later ΛCDM-like phase. If the transition is fast enough, these models may exhibit satisfactory structure formation and CMB fluctuations. To consider a concrete case, we introduce a toy UDM model and show that it can predict CMB and matter power spectra that are in agreement with observations for a wide range of parameter values.

Cosmological constraints on fast transition unified dark energy and dark matter models

“…But among the different UDE/M models, a new class has emerged that offers a way out of that difficulty: models with a fast transition between a matterdriven-like era and a dark-energy-driven-like era. They are believed to provide an alternative and defendable explanation of the accelerated expansion of the universe [25], as they can fit the observational data quite well while they display interesting and different new features. Besides, fast transitions UDE/M models with scalar fields are also compatible with observational data [26].…”

“…The theoretical reasoning on which fast transition UDE/M models rely provides hints towards well mathematically/analytically-defined expressions for their equation of state parameter w UDE/M , as discussed in [25], but this parametrization scheme turns out to be computationally expensive when tested with likelihood techniques. Taking into account this, and without a fundamental model, it is worth to consider simple phenomenological models for the fast transition UDE/M in order to achieve as much theoretical progress as possible from analytical calculations.…”

“…A simple model, gentle on computations, must be one whose most important variables required for the numerical calculations can be expressed analytically. Thus, instead of implementing the UDE/M model in the equation of state w UDE/M , as in [25], we believe a more convenient way to proceed analytically is to prescribe the evolution of the UDE/M energy density at the level of the Hubble factor itself [27]. Here we present (in section II) two parametrizations for these UDE/M set-ups with fast transition.…”

Phenomenology of Unified Dark Matter Models with Fast Transition

“…The squared Jeans wave number plays a crucial role in determining the viability of a UDM model, because of its effect on perturbations, which is then revealed in observables such as the CMB and matter power spectrum [4,2]. The explicit form of the Jeans wave number is [2] …”

“…UDM models with fast transition were introduced in [2] and show interesting features [2,3]. The single UDM component must accelerate the Universe and provide acceptable perturbations which evolve in a scale-dependent fashion.…”