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Precision cosmology provides a sensitive probe of extremely weakly coupledstates due to thermal freeze-in production, with subsequent decays impactingphysics during well-tested cosmological epochs. We explore the cosmologicalimplications of the freeze-in production of a new scalar $S$ via thesuper-renormalizable Higgs portal. If the mass of $S$ is at or below theelectroweak scale, peak freeze-in production occurs during the electroweakepoch. We improve the calculation of the freeze-in abundance by including allrelevant QCD and electroweak production channels. The resulting abundance andsubsequent decay of $S$ is constrained by a combination of X-ray data, cosmicmicrowave background anisotropies and spectral distortions, $N_{\rm eff}$, andthe consistency of BBN with observations. These probes constrain technicallynatural couplings for such scalars from $m_S \sim$ keV all the way to $m_S \sim100$ GeV. The ensuing constraints are similar in spirit to typical beam bumplimits, but extend to much smaller couplings, down to mixing angles as small as$\theta_{Sh} \sim 10^{-16}$, and to masses all the way to the electroweakscale.

Cosmological beam dump: constraints on dark scalars mixed with the Higgs boson