  For several decades, astronomers have measured the electromagnetic emission
in the universe from the lowest to the highest energies with incredible
precision. The lowest end of the spectrum, corresponding to radio waves, is
fairly well studied and understood. Yet there is a long standing discrepancy
between measurements and predictions, which has prompted the construction of
many new models of radio emitters. Here we show that remnant electron-proton
interactions, leading to photon production in the early universe, also referred
to as cosmic free-free emission, solves the discrepancy between theory and
observations. While the possibility of cosmic free-free emission has been
postulated for several decades, this is the first time that the amplitude and
shape of the signal has been computed and its existence demonstrated. Using
current measurements we estimate this emission to become important from around
a redshift of $z \simeq 2150$. This contribution from fundamental particles and
interactions represents the lowest energy test from the early universe of one
of the pillars of modern physics, Quantum Electrodynamics. The next generation
of deep radio surveys will be able to measure primordial signals from this
cosmic era with greater precision and further solidify our understanding of the
radio sky.
