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The observed value is smaller than naive estimates by 44 to about 120 powers of ten, yet not zero.","posed_since":"","precise":"The observed dark energy density is $\\rho_{\\Lambda} \\sim 6 \\times 10^{-30}\\ \\mathrm{g}/\\mathrm{cm}^{3}$, i.e. $\\rho_{\\Lambda}^{1/4} \\sim 2.3 \\times 10^{-3}\\ \\mathrm{eV}$ in natural units, while Standard Model contributions (electroweak symmetry breaking ~ $(100\\ \\mathrm{GeV})^{4}$, QCD ~ $(0.2\\ \\mathrm{GeV})^{4}$) exceed it by $\\sim 10^{55}$ and $\\sim 10^{44}$, and a Planck-scale cutoff by $\\sim 10^{123}$ (often quoted as $10^{120}$, using the reduced Planck mass). Find a mechanism that yields the observed value without tuning to that precision, or show that environmental selection in a multiverse is the only consistent explanation.","problem_ref":"f6f1c7f06f70ce689102eeb83f79e5dc64af84ac1df00f4318f27cc5708dfba1","references":"","settled_by":"A dynamical or symmetry mechanism that predicts $\\rho_{\\Lambda}$ to within its observed order of magnitude, with independently testable consequences.","status_note":"","title":"Why is the vacuum energy density so small yet 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