Viability of post-inflationary freeze-in with precision cosmology

Prediction of inflationary observables from the temperature fluctuation of Cosmic Microwave Background (CMB) can play a pivotal role in predicting the reheating dynamics in the early universe. In this work, we highlight how the inflationary observables, in particular the spectral index $n_s$, can play a potential role in constraining the post-inflationary dark matter (DM) production. We demonstrate a novel way of constraining the non-thermal production of DM via UV freeze-in which is otherwise elusive in terrestrial experiments. We consider a scenario in which DM is produced from this thermal plasma via a dimension-five operator. The mutual connection between $n_s$ and relic density of DM via the reheating temperature, $T_{\rm RH}$, enables us to put constraints on the DM parameter space. For the minimal choice of the inflationary model parameters and DM mass between $1\,\rm MeV$ to $1\,\rm TeV$, we found that Planck alone can exclude the cut-off scale of the dimension-five operator $\Lambda \lesssim 10^{12}\,\rm GeV$ which is significantly stronger than any other existing constraints on such minimal scenario. If we impose the combined prediction form Planck and recently released data by ACT, the exclusion limit can reach up to the Planck scale for TeV-scale dark matter.