Decaying dark matter mimicking time-varying dark energy

TL;DR

Decaying dark matter can mimic time-varying dark energy in cosmological models, leading to potential misinterpretations of observational data, but differences in structure growth can distinguish these scenarios.

Contribution

This paper demonstrates how decaying dark matter models can produce apparent dark energy behavior, highlighting the importance of growth of structure in breaking degeneracies.

Findings

Decaying dark matter models mimic various dark energy equations of state.

The evolution of the effective equation of state depends on dark matter density assumptions.

Growth of density perturbations can differentiate true dark energy from decaying dark matter effects.

Abstract

A $\Lambda$CDM model with dark matter that decays into inert relativistic energy on a timescale longer than the Hubble time will produce an expansion history that can be misinterpreted as stable dark matter with time-varying dark energy. We calculate the corresponding spurious equation of state parameter, $\widetilde w_\phi$, as a function of redshift, and show that the evolution of $\widetilde w_\phi$ depends strongly on the assumed value of the dark matter density, erroneously taken to scale as $a^{-3}$. Depending on the latter, one can obtain models that mimic quintessence ($\widetilde w_\phi > -1$), phantom models ($\widetilde w_\phi < -1$) or models in which the equation of state parameter crosses the phantom divide, evolving from $\widetilde w_\phi > -1$ at high redshift to $\widetilde w_\phi < -1$ at low redshift. All of these models generically converge toward $\widetilde w_\phi \approx -1$ at the present. The degeneracy between the $\Lambda$CDM model with decaying dark matter and the corresponding spurious quintessence model is broken by the growth of density perturbations.