  We measure radiofrequency (rf) spectra of the homogeneous unitary Fermi gas
at temperatures ranging from the Boltzmann regime through quantum degeneracy
and across the superfluid transition. For all temperatures, a single spectral
peak is observed. Its position smoothly evolves from the bare atomic resonance
in the Boltzmann regime to a frequency corresponding to nearly one Fermi energy
at the lowest temperatures. At high temperatures, the peak width reflects the
scattering rate of the atoms, while at low temperatures, the width is set by
the size of fermion pairs. Above the superfluid transition, and approaching the
quantum critical regime, the width increases linearly with temperature,
indicating non-Fermi-liquid behavior. From the wings of the rf spectra, we
obtain the contact, quantifying the strength of short-range pair correlations.
We find that the contact rapidly increases as the gas is cooled below the
superfluid transition.
