A September 2026 measurement of primordial helium-4 has made Big Bang nucleosynthesis (BBN) a more precise probe of early-universe radiation than the cosmic microwave background (CMB), helping physicists test dark-matter and hypothetical-physics models.
As the universe cooled, protons and neutrons fused into nuclei during BBN. The expansion rate, tied to the amount of radiation, affected how many neutrons remained to form helium-4; researchers use the primordial abundance to infer radiation levels. Deuterium measurements also estimate proton-and-neutron density with precision that rivals, but does not yet exceed, the CMB’s.
The Simons Observatory has begun making high-precision CMB observations. The next-generation CMB-S4 experiment was suspended indefinitely last year, and the article says there is no clear path to improved CMB measurements after Simons finishes. BBN measurements can improve with more observing time on existing or planned telescopes, without a dedicated observatory.
