Mimicking Phantom Dark Energy with Evolving Dark Matter Mass

Abstract

We present a general method to reproduce a given cosmological background through energy exchange between dark energy (DE) and dark matter (DM). This can be simply realized with a standard quintessence scalar field that controls the DM mass. In particular a background with phantom crossing can be effectively realized without introducing ghosts or other pathologies. For example one can reproduce exactly the background that gives the best fit to the recent DESI+CMB+DESY5 data, within the Chevallier-Polarski-Linder (CPL) parametrization of DE. Although the background evolution is identical, the perturbations differ, leading to modified growth of structures. If the DM mass varies at late times, early-time observables are not modified and can reproduce the main predictions of the target model, but late-time observables are affected. We discuss in particular the effects on the matter power spectrum, CMB lensing and ISW effect. When reproducing the best fit CPL background model, this scenario generically predicts $\mathcal{O}(10\%)$ deviations in such observables. However, for suitable choices of parameters, effects on the matter power spectrum can be smaller, motivating a detailed study. In general, energy exchange between DE and DM generates a mismatch between the matter power spectrum and the gravitational potential amplitudes compared to the decoupled case, that can lead to deviations observable in future experiments.

Figures (8)

• Figure 1: DM mass in Linear and Smoothed-Linear models. On the left panel the time dependent mass of the two models. Also shown in gray the region that should not be crossed in order to avoid phantom behavior, for the CPL best fit considered. On the right panel we plot the mass as a function of the quintessence field obtained by solving eq. \ref{['eq:reconstruction']}; $\varphi_i$ is a reference field value at $a_i=0.01$.
• Figure 2: DE energy density in coupled scenarios. On the left we show the energy density as a function of the scale factor, normalized to the DE energy density in the CPL uncoupled case. Also shown the ratio of DE over DM density that becomes negligible at early times. On the right we report the equation of state of the coupled DE and of the uncoupled model ("CLASS"), that all give the same best-fit background .
• Figure 3: Potential of coupled DE models. On the left the potential that reproduces the CPL bestfit background, as function of the field over a reference value at $a_i = 0.01$. The evolution of the field as function of scale factor is shown on the right.
• Figure 4: DM overdensity and gravitational potential. On the left the DM overdensity relative to the same quantity in the uncoupled model as computed in CLASS. On the right the gravitational potential.
• Figure 5: Density contrast of non-relativistic matter. Left panel shows the increased clustering of matter in the linear model at $z=0$, with respect to the uncoupled model implemented in CLASS. On the right the smoothed scenario is presented where it is possible to realize both reduced and enhanced clustering.