  $\beta$-decay, a process that changes a neutron into a proton (and vice
versa), is the dominant decay mode of atomic nuclei. This decay offers a unique
window to physics beyond the standard model, and is at the heart of
microphysical processes in stellar explosions and the synthesis of the elements
in the Universe. For 50 years, a central puzzle has been that observed
$\beta$-decay rates are systematically smaller than theoretical predictions.
This was attributed to an apparent quenching of the fundamental coupling
constant $g_A \simeq $ 1.27 in the nucleus by a factor of about 0.75 compared
to the $\beta$-decay of a free neutron. The origin of this quenching is
controversial and has so far eluded a first-principles theoretical
understanding. Here we address this puzzle and show that this quenching arises
to a large extent from the coupling of the weak force to two nucleons as well
as from strong correlations in the nucleus. We present state-of-the-art
computations of $\beta$-decays from light to heavy nuclei. Our results are
consistent with experimental data, including the pioneering measurement for
$^{100}$Sn. These theoretical advances are enabled by systematic effective
field theories of the strong and weak interactions combined with powerful
quantum many-body techniques. This work paves the way for systematic
theoretical predictions for fundamental physics problems. These include the
synthesis of heavy elements in neutron star mergers and the search for
neutrino-less double-$\beta$-decay, where an analogous quenching puzzle is a
major source of uncertainty in extracting the neutrino mass scale.
