Abstract
We review dark energy models that can present non-negligible fluctuations on scales smaller than Hubble radius. Both linear and nonlinear evolutions of dark energy fluctuations are discussed. The linear evolution has a well-established framework, based on linear perturbation theory in General Relativity, and is well studied and implemented in numerical codes. We highlight the main results from linear theory to explain how dark energy perturbations become important on the scales of interest for structure formation. Next, we review some attempts to understand the impact of clustering dark energy models in the nonlinear regime, usually based on generalizations of the Spherical Collapse Model. We critically discuss the proposed generalizations of the Spherical Collapse Model that can treat clustering dark energy models and their shortcomings. Proposed implementations of clustering dark energy models in halo mass functions are reviewed. We also discuss some recent numerical simulations capable of treating dark energy fluctuations. Finally, we summarize the observational predictions based on these models.
Highlights
In the presence of homogeneous Dark Energy (DE), the Spherical Collapse Model (SCM) has to be modified to include the effects of this new component on the dynamics of the shell radius
The impact on δc was computed in Nunes and Mota [89], where differences of a few per cent with respect to ΛCDM and redshift-dependent features associated with the evolution of w were found
Its perturbations can be more important on voids, which are much larger than halos. This model presented some of the important impacts of DE fluctuations on the nonlinear regime, which were later confirmed by other studies
Summary
One of the first and most popular alternatives to Λ is the quintessence class of models In these models, a new scalar field minimally coupled to gravity and with no direct interactions to other types of matter plays the role of DE. The various types of models that can explain the accelerated expansion can be described in the Effective Field Theory framework, Gubitosi et al [28] Many of these proposals are discussed in Amendola et al [29].
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