The CMB as a Unique Probe
The physics of the epoch of reionization (EOR) is very poorly constrained at present, and the CMB is the most direct way to measure the distribution of ionized gas during reionization, because electrons Thomson scatter primary CMB photons in well-understood ways. The main effects are direct screening of the primary fluctuations, generation of polarization via anisotropies, and Doppler boosting of the scattered primary photons (kSZ).
Primary Science Requirements
The statistics of CMB secondaries from the EOR, in particular via the power spectrum and tri-spectrum, will tell us about the two major features of reionization:
- History: How long did reionization take and when did it start?
- Morphology: How 'patchy' was reionization, on what scales, and how did it correlate with cosmological density fluctuations?
This in turn will tell us about the first stars, black holes and galaxies. In particular we wish to be able to separate among the following scenarios:
- Late UV - Sourced by massive galaxies in halos with M>10^9 M_sun; reionization and happens relatively quickly
- Extended UV - Sourced early by less massive galaxies with M>10^8 M_sun, giving way to more massive galaxies at end, and is more extended
- X-ray dominated - There is a substantial component from "mini-quasars", accreting intermediate mass black holes in the early universe that would reduce the kSZ signal relative to the other scenarios at fixed optical depth
Secondary Science Requirements
While harder/more ambitious, one can learn even more by combing with external data and further theoretical exploration.
- Combine measurement with 21cm observations (for example through cross-correlation) to improve redshift resolution.
- Measure physical reionization parameters such as the mean free path of ionizing photons, efficiency, escape fraction etc.