Establish whether the additional Lyman-alpha escape rate is nonzero

Determine whether cosmological observations provide definitive evidence for or against a nonzero additional Lyman-alpha resonance-line escape rate $V_{SR}$ associated with non-Hubble redshift effects.

Background

The modified CLASS/RECFAST analysis finds a best-fit value near VSRsimeq0.8×10−14 s−1V_{SR}simeq 0.8\times10^{-14}\,\mathrm{s}^{-1}, but also allows values close to zero, for which the model becomes effectively indistinguishable from standard Λ\LambdaCDM. The CMB-only data therefore yield an upper limit rather than a decisive detection of the proposed effect.

Resolving this question requires additional or extended observational datasets capable of distinguishing a nonzero non-Hubble contribution to the Lyman-alpha escape rate from the standard recombination prediction.

References

We find that using CMB data alone we cannot definitively establish a preference for or against a non-zero $V_{SR}$, constraining it to $V_{SR} <1.54\times10{-14}s{-1}$ at the $95\%$ confidence level, while noting that the best-fit region lies at $V_{SR}\sim0.8\times10{-14}s{-1}$, an $\mathcal{O}(1\%)$ deviation from the standard $Ly_\alpha$ escape rate.

— Modified hydrogen recombination rate due to non-Hubble redshift effects  (2610.21992 - Krasnov et al., 15 Oct 2026) in Conclusions