a motivation for investigating dark energy is that the vacuum energy density is 120 orders of magnitude smaller than expected (which btw applies to the density of everything, dark matter, photons and neutrinos alike): could one not simply say that we don't understand the value of the Hubble constant? what would be a natural value, and could you make an example of units which can be easily imagined?

the questions aims basically at the cosmic coincidence problem. in earlier CQWs we already had the definition of the Hubble-system of units made from $c$, $G$ and $\Lambda$, which describes perfectly the universe today. the perplexing thing is that the critical density $\rho_\mathrm{crit}=3H_0^2/(8\pi G)$ is so close to the Hubble-density $\rho_H = c^2/\sqrt{\Lambda}/G$, or in other words, that $\sqrt{\Lambda}$ is so close to the Hubble length $\chi_H=c/H_0$. having inflation work efficiently (small first slow-roll parameter) and long enough (small second slow-roll parameter) reaches low densities, but the coincidence remains.

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