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

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 Λ-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 ρΛ = Λ/8πG right in the ballpark of the rapidly diluting matter density ρm ∼ 1/a3. It should perhaps be more plausible to conceive that the vacuum energy, ρΛ, 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 ρΛ 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 behaviour. Thanks to the versatility of such cosmic fluid we can show that a composite DE system of this sort (‘ΛXCDM’) may have a key to resolving the mysterious coincidence problem.