  Cosmological observations allow to measure the abundance of light relics
produced in the early Universe. Most studies focus on the thermal freeze-out
scenario, yet light relics produced by freeze-in are generic for models in
which new light degrees of freedom do not couple strongly enough to the
Standard Model (SM) plasma to allow for full thermalization in the early
Universe. In ultraviolet (UV) freeze-in scenarios, rates for light relic
production associated with non-renormalizable interactions typical of beyond
the SM (BSM) models grow with temperature more quickly than the Hubble rate.
Thus, relatively small couplings to the SM can be probed by current and
next-generation cosmic microwave background (CMB) experiments. We investigate
several representative benchmark BSM models, such as axion-like particles from
Primakoff production, massless dark photons and light right-handed neutrinos.
We calculate contributions to the effective number of neutrino species, $\Delta
N_{\rm eff}$, in corners of parameter space not previously considered and
discuss the sensitivity of CMB experiments compared to other probes. In
contrast to freeze-out scenarios, $\Delta N_{\rm eff}$ from UV freeze-in is
more dependent on both the specific BSM physics model and the reheating
temperature. Depending on the details of the BSM scenario, we find that the
sensitivity of next-generation CMB experiments can complement or surpass the
current astrophysical, laboratory or collider constraints on the couplings of
the SM to the light relic.
