Interacting Dark Matter and Dark Energy

TL;DR

The paper investigates cosmological models in which a scalar field driving dark energy also sets dark matter masses through a Yukawa coupling, introducing a dark-sector fifth force. It develops a framework for one- and two-family dark matter scenarios, analyzes background evolution and linear perturbations, and identifies parameter regions where departures from ΛCDM are possible yet observationally viable. A key result is the potential suppression of the fifth force via field locking in a two-family setup, yielding near-Lambda behavior at early times with richer late-time dynamics. The work highlights how such interacting-dark-sector models can be constrained or distinguished by structure formation and CMB observations, offering a cautionary perspective on interpreting cosmological parameters under beyond-ΛCDM physics.

Abstract

We discuss models for the cosmological dark sector in which the energy density of a scalar field approximates Einstein's cosmological constant and the scalar field value determines the dark matter particle mass by a Yukawa coupling. A model with one dark matter family can be adjusted so the observational constraints on the cosmological parameters are close to but different from what is predicted by the Lambda CDM model. This may be a useful aid to judging how tightly the cosmological parameters are constrained by the new generation of cosmological tests that depend on the theory of structure formation. In a model with two families of dark matter particles the scalar field may be locked to near zero mass for one family. This can suppress the long-range scalar force in the dark sector and eliminate evolution of the effective cosmological constant and the mass of the nonrelativistic dark matter particles, making the model close to Lambda CDM, until the particle number density becomes low enough to allow the scalar field to evolve. This is a useful example of the possibility for complexity in the dark sector.

Figures (6)

• Figure 1: Evolution of the DE field in models with the power law exponent $\alpha = -2$ in the potential (eq. [\ref{['eq:V']}]). The solutions are fixed by the initial value of $f=G^{1/2}\phi$ listed in the first four rows of Table 1. The initial values are close to the field values at the left-hand edge of the plot.
• Figure 2: The same as Fig. 1 for solutions with $\alpha =6$. The initial values of $\phi$ are listed in the last four entries in Table 1.
• Figure 3: Evolution of the mass density contrast in linear perturbation theory in models with $\alpha = -2$. The density contrast has been multiplied by the redshift factor $1+z$ to scale out the evolution when the expansion is matter-dominated. The short dashed curve is the solution for the $\Lambda$CDM model. The line types of the other curves match Fig. 1, where the initial field values are close to what is plotted at the left side of the figure.
• Figure 4: The same as Fig. 3 for solutions with $\alpha = 6$
• Figure 5: The evolution of the equation of state parameter (Fig. [\ref{['eq:w']}]) in solutions with $\alpha = -2$. The line types match Fig. 1.