The First Stars and Beyond: Constraints and Predictions Across the Spectrum

Recent advances in observations of the Cosmic Dawn and the Epoch of Reionization are opening new opportunities to study the earliest stars and galaxies. In this talk, I will combine constraints from high-redshift UV luminosity functions, the CMB optical depth to reionization, hydrogen absorption in quasar spectra, and the soft cosmic X-ray background to infer the formation history and radiative properties of the earliest stellar populations. I will then show how the resulting constraints on early-Universe astrophysics sharpen predictions for the 21-cm signal and reveal the conditions under which Pop III stars may produce a distinctive signature. By reducing astrophysical uncertainties, these constraints also strengthen the role of the 21-cm signal as a probe of physics beyond the Standard Model, enabling potential new-physics signatures to be distinguished from otherwise degenerate astrophysical processes.