Cosmologies with variable parameters and dynamical cosmon: implications on the cosmic coincidence problem

TL;DR

This paper explores models of dynamical dark energy involving a mixture of vacuum energy and a 'cosmon', proposing that such models can address the cosmic coincidence problem by explaining why dark energy and matter densities are comparable today.

Contribution

It introduces a composite dark energy model with a dynamical component called the 'cosmon' that can mimic a cosmological constant and potentially resolve the coincidence problem.

Findings

The LXCDM model can mimic a cosmological constant at present.

Dynamical DE models can naturally explain the coincidence of matter and dark energy densities.

The cosmon component allows for a versatile dark energy behavior.

Abstract

Dynamical dark energy (DE) has been proposed to explain various aspects of the cosmological constant (CC) problem(s). For example, it is very difficult to accept that a strictly constant Lambda-term constitutes the ultimate explanation for the DE in our Universe. It is also hard to acquiesce in the idea that we accidentally happen to live in an epoch where the CC contributes an energy density value right in the ballpark of the rapidly diluting matter density. It should perhaps be more plausible to conceive that the vacuum energy, is actually a dynamical quantity as the Universe itself. More generally, we could even entertain the possibility that the total DE is in fact a mixture of vacuum energy and other dynamical components (e.g. fields, higher order terms in the effective action etc) which can be represented collectively by an effective entity X (dubbed the ``cosmon''). The ``cosmon'', therefore, acts as a dynamical DE component different from the vacuum energy. While it can actually behave phantom-like by itself, the overall DE fluid may effectively appear as standard quintessence, or even mimic at present an almost exact CC behavior. Thanks to the versatility of such cosmic fluid we can show that a composite DE system of this sort (``LXCDM'') may have a key to resolving the mysterious coincidence problem.