Using a phenomenological model to test the coincidence problem of dark energy

Understand

By assuming a phenomenological form for the ratio of the DE and DM densities $\rho_X\propto \rho_{m} a^{\xi}$ (Dalal et al.2001), we discuss the cosmic coincidence problem in light of current observational data.

• $\xi$ is a key parameter to denote the severity of the coincidence problem.
• In this scenario, $\xi=3$ and $\xi=0$ correspond to $\Lambda$CDM and the self-similar solution without coincidence problem, respectively.
• Hence, any solution with a scaling parameter $0<\xi<3$ makes the coincidence problem less severe.