Disformal couplings in a $Λ$CDM background cosmology

Abstract

While the accelerated cosmological expansion suggests the presence of a dark energy component, the possibility of its interaction with other fluid components is not entirely ruled out. We investigate the cosmological dynamics of coupled quintessence (where the quintessence φ couples and interacts with a generic fluid) in a disformal framework. The novelty of this work is: (i) extend the idea of coupled quintessence mimicking the cosmological constant Λ with an uncoupled fluid, by including a disformal coupling, and (ii) to bring these conformal and disformal couplings with a ΛCDM background, to a dynamical system analysis. Using this approach, the quintessence potential does not have to be specified. Three scenarios are studied. Scenario I consists of a pressureless fluid (e.g dark matter), which is coupled to quintessence, and both fluids are mimicking exactly a ΛCDM background. Scenario II consists of a relativistic fluid, which is coupled to quintessence and both fluids are also mimicking exactly the ΛCDM background. Scenario III consists of a relativistic fluid (e.g. neutrinos), which is coupled to quintessence and both fluids are mimicking a background with a cosmological constant Λ and uncoupled radiation. The expansion history of the coupled quintessence is investigated for scenario I, which confirms that the mass scale of the quintessence influences the disformal characteristics of the dynamical system, which is portrayed by the evolution of an effective conformal coupling.

Metastable conformal dark matter

“…The expression (7) contains a conformal and a disformal transformation between the two metrics g µν and gµν induced by C and D respectively. The disformal coupling has been studied extensively in [38,39] at the cosmological level and [40,41] for WIMP and FIMP scenarii of dark matter respectively. In both cases, a Z 2 symmetry was implicitly introduced to ensure the stability of the DM candidate.…”

Coupled dark energy model inspired from general conformal transformation

“…Possible models of coupled dark energy are inspired from the frames transformation in theories of gravity. The interaction between dark energy and CDM can be inspired from the conformal transformation [11][12][13][14][15] and disformal transformation [16][17][18][19][20][21][22]. When the transformation coefficient depends on the scalar field only, the coupling due to conformal transformation leads to a coupling between field and the energy density of CDM and consequently energy transfers between the dark components [7,23].…”