Expanded Simons Observatory Targets Reheating Temperature to Few-Percent Precision in Warm Inflation

The arXiv preprint models how an expanded Simons Observatory measures the post-inflationary equation-of-state parameter and inflaton decay rate through the damping tail of the tensor spectrum. In plateau inflation the scale of inflation is fixed by r, converting a B-mode detection into a direct probe of the reheating temperature via the number of e-folds between horizon exit and the end of inflation. For QCD-driven warm inflation the same data set yields a few-percent constraint on both temperature and the inflaton-gluon coupling under optimistic foreground assumptions.

This approach closes a long-standing gap between CMB observables and the initial conditions of the hot Big Bang. Earlier forecasts for CMB-S4 or LiteBIRD focused on r itself; the present study isolates the reheating epoch by exploiting the lever arm provided by multiple small-aperture telescopes that improve delensing and foreground separation at the relevant multipoles. The result supplies a concrete target for axion haloscope experiments that could then search for the same inflaton candidate.

A key limitation remains foreground modeling and the assumption that r is already measured; without an actual tensor detection the reheating constraints vanish. Full end-to-end simulations that include realistic atmospheric noise and the planned 2027 instrument configuration would strengthen the case before the first light of the expanded array.

Near-term milestones include the 2025–2027 deployment phase and the first joint analysis with existing SAT data once the additional telescopes reach nominal sensitivity.