Helium-4 Plateau Shift

The helium-4 abundance measured in metal-poor HII regions traces the primordial helium produced by Big Bang nucleosynthesis with a small contamination from stellar processing. When the observed Y_p values are plotted against the oxygen abundance O/H of each HII region, they show a roughly linear trend that extrapolates to the primordial value at zero metallicity. Different analyses of this regression have found primordial Y_p values ranging from 0.2449 to 0.2551, with the systematic spread driven by choices in emission line modeling, electron temperature determination methods, and sample selection. The tension between the high-end values (Y_p ~ 0.255) and the standard BBN prediction (Y_p ~ 0.247) reaches approximately two sigma and has been interpreted as evidence for extra relativistic species during BBN, but the systematic uncertainties in the measurement prevent a definitive conclusion.

Successive Collision Theory explains the apparent helium-4 plateau shift through the pre-existing stellar helium contribution mixed into the primordial debris field, the same mechanism that addresses the broader helium-4 abundance tension. The collision debris contained pre-existing stellar populations that had already converted hydrogen to helium through nuclear burning across prior cosmic epochs. When this pre-processed gas was thermalized into the post-collision plasma, the thermalized baryons included helium that was not produced by the post-collision BBN epoch but was inherited from prior stellar evolution. This pre-existing helium mixed with the BBN-produced helium to set an effective initial helium fraction in the gas from which the observed metal-poor HII regions formed. The HII regions that appear to have anomalously high helium abundances given their low metallicity are those that sampled regions of the debris field with higher pre-existing stellar helium contamination — regions where the collision debris mixing ratio between pre-processed and freshly synthesized material was higher.

The linear trend of Y_p with metallicity used to extrapolate to the primordial value is itself modified in SCT because the pre-existing stellar helium and the pre-existing metals were not produced with the same yield ratio as post-collision stellar populations. Pre-existing stars that had processed gas through multiple stellar generations produced helium and metals with a helium-to-metal ratio that differs from the standard Galactic yield, introducing a non-standard slope in the Y_p–O/H relation. Regression analyses that assume the standard yield slope will therefore extrapolate to an incorrect zero-metallicity intercept, biasing the inferred primordial Y_p. SCT predicts that more sophisticated analyses that account for the two-component (pre-existing plus post-collision) stellar population will recover a primordial Y_p consistent with the standard BBN prediction from Planck baryon density, with the plateau shift arising from the pre-existing stellar contribution rather than from non-standard BBN physics.