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Title: Why are the dark energy and matter densities comparable today?
Plain: Matter thins out as the universe expands while vacuum energy does not, so their ratio changes enormously over cosmic history. It happens to be close to one now, when observers exist.
Precise: With $\rho_m \sim a^{-3}$ and a constant $\rho_\Lambda$, the ratio $r = \rho_\Lambda / \rho_m = (\Omega_\Lambda / \Omega_m) a^3$ is about 2.2 today and was about $2e-3$ at $a = 0.1$. Find a mechanism (tracker or coupled dark energy, or anthropic selection with an explicitly defined measure) that predicts $r$ of order one at the epoch when observers form without tuning its parameters, while staying consistent with the measured $w(z)$.
Settled by: A model that predicts r of order one at the observed epoch and makes a further prediction for w(z) or for dark-sector couplings that is later confirmed.
Kind: phenomenon
Answer type: mechanism
References: (empty)
Literature status: open
Status note: (empty)
Posed since: (empty)