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Title: Why is the vacuum energy density so small yet nonzero?
Plain: Quantum theory says empty space should be full of energy from fluctuating fields, which would curl up or blow apart the universe. The observed value is smaller than naive estimates by 44 to about 120 powers of ten, yet not zero.
Precise: The observed dark energy density is $\rho_{\Lambda} \sim 6e-30\ \mathrm{g}/\mathrm{cm}^{3}$, i.e. $\rho_{\Lambda}^{1/4} \sim 2.3e-3\ \mathrm{eV}$ in natural units, while Standard Model contributions (electroweak symmetry breaking ~ $(100\ \mathrm{GeV})^{4}$, QCD ~ $(0.2\ \mathrm{GeV})^{4}$) exceed it by $\sim 1e55$ and $\sim 1e44$, and a Planck-scale cutoff by $\sim 1e123$ (often quoted as 1e120, 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.
Settled by: A dynamical or symmetry mechanism that predicts $\rho_{\Lambda}$ to within its observed order of magnitude, with independently testable consequences.
Kind: phenomenon
Answer type: mechanism
References: (empty)
Literature status: open
Status note: (empty)
Posed since: (empty)