  In this work we explore the intriguing connections between searches for
long-lived particles (LLPs) at the LHC and early universe cosmology. We study
the non-thermal production of ultra-relativistic particles (i.e. dark
radiation) in the early universe via the decay of weak-scale LLPs and show that
the cosmologically interesting range $\Delta N_\text{eff} \sim 0.01-0.1$
corresponds to LLP decay lengths in the mm to cm range. These decay lengths lie
at the boundary between prompt and displaced signatures at the LHC and can be
comprehensively explored by combining searches for both. To illustrate this
point, we consider a scenario where the LLP decays into a charged lepton and a
(nearly) massless invisible particle. By reinterpreting searches for promptly
decaying sleptons and for displaced leptons at both ATLAS and CMS we can then
directly compare LHC exclusions with cosmological observables. We find that the
CMB-S4 target value of $\Delta N_\text{eff}=0.06$ is already excluded by
current LHC searches and even smaller values can be probed for LLP masses at
the electroweak scale.
