Polarization Bump

The CMB EE polarization spectrum contains a feature at multipoles ℓ ~ 3–7 known as the reionization bump, produced by rescattering of CMB photons during the epoch of reionization. The bump amplitude constrains the reionization optical depth τ, and the precise shape of the bump at the lowest multipoles encodes the history of hydrogen reionization. Tensions exist between the τ value inferred from the polarization bump and that inferred from Lyman-alpha forest observations and from the kinematic Sunyaev-Zel'dovich effect. In SCT, the pre-existing matter within the two colliding pockets — which included prior generations of massive stars and ionizing radiation — means that reionization was not a single clean epoch driven solely by Population II and III stars forming after the Big Bang. Instead, reionization began earlier, with a non-standard optical depth contribution from pre-existing ionized gas that was thermalized in the collision.

This extended and multi-component reionization history modifies the shape of the EE polarization bump. The bump becomes broader and its peak shifts slightly, because the effective reionization redshift is a weighted average over a more complex ionization history than ΛCDM assumes. The SCT prediction is that a detailed fit to the polarization bump shape will require a two-component reionization model — one component from pre-existing matter thermalizing at z > 20 and one from standard post-collision star formation — and that this model will simultaneously resolve tensions with the Lyman-alpha forest and kSZ inferences of τ that a single-epoch reionization model cannot accommodate.