Coupled Dark Energy and Dark Matter for DESI: An Effective Guide to the Phantom Divide

TL;DR

This paper explores interacting dark energy models where a scalar field couples to dark matter, explaining apparent phantom behavior without actual phantom fields, guided by DESI observations.

Contribution

It identifies conditions for viable models that mimic phantom crossing through effective coupling, avoiding actual phantom scalar fields.

Findings

Effective coupling can produce apparent phantom behavior without phantom fields.

Models can evolve from $w_{eff} oughly -1.2$ at $z=1$ to $w_{eff} oughly -0.9$ at $z=0.4$.

Constraints from CMB require the scalar field to originate from a frozen phase in the radiation era.

Abstract

Motivated by the recent Dark Energy Spectroscopic Instrument (DESI) DR2 preference for dynamical dark energy, we study interacting dark energy models in which a canonical quintessence field couples to cold dark matter through a field-dependent mass $m(\phi)$. In such scenarios, the effective equation of state inferred under the assumption of non-interacting dark sectors, $w_{\rm eff}(z)$, can differ from the intrinsic scalar-field equation of state $w_\phi(z)$, making an apparent phantom crossing $w_{\rm eff}<-1$ possible without introducing a phantom scalar. We show that a viable realization of this mechanism requires the scalar field to originate from a frozen phase deep in the radiation era, in order for the effective coupling to remain sufficiently suppressed before recombination to evade cosmic microwave background constraints, and for the late-time evolution to become strong enough to reproduce the apparent behavior of $w_{\rm eff}(z)$ preferred by DESI. We identify the general conditions that allow these requirements to be satisfied simultaneously, and present an illustrative phenomenological realization in which $w_{\rm eff}(z)$ evolves from $w_{\rm eff}\approx -1.2$ at $z \approx 1.0$ to $w_{\rm eff}\approx -0.9$ at $z\approx 0.4$. These conditions and requirements serve as a guide for designing future models of this kind which can safely navigate the phantom divide at $w=-1$ in an effective way without phantom fields.