  Cosmological data provide a powerful tool in the search for physics beyond
the Standard Model (SM). An interesting target are light relics, new degrees of
freedom which decoupled from the SM while relativistic. Nearly massless relics
contribute to the radiation energy budget, and are commonly parametrized as
variations in the effective number $N_{\rm eff}$ of neutrino species.
Additionally, relics with masses greater than $10^{-4}$ eV become
non-relativistic before today, and thus behave as matter instead of radiation.
This leaves an imprint in the clustering of the large-scale structure of the
universe, as light relics have important streaming motions, mirroring the case
of massive neutrinos. Here we forecast how well current and upcoming
cosmological surveys can probe light massive relics (LiMRs). We consider
minimal extensions to the SM by both fermionic and bosonic relic degrees of
freedom. By combining current and upcoming cosmic-microwave-background and
large-scale-structure surveys, we forecast the significance at which each LiMR,
with different masses and temperatures, can be detected. We find that a very
large coverage of parameter space will be attainable by upcoming experiments,
opening the possibility of exploring uncharted territory for new physics beyond
the SM.
