CMB large angular scale, full sky polarization anisotropy has been measured moderately well, but not well enough – The value of reionization fraction derived from CMB polarization measurements has decreased steadily since WMAP2003 (WMAP2005, WMAP2007, WMAP2011, and Planck 2015) – The final word from Planck is not out yet, and whenever it is out, it will surely not be “a 100-sigma measurement” of τ – We don’t want to leave the field with this hole – Planck, in polarization, has now reached the noise floor at large angular scales at L>4; this was a remarkably hard job – CMB large angular scale, full sky polarization anisotropy SHOULD be measured well in order to – Minimize sample variance – Improve foreground knowledge (from the present perspective, especially on the low-ν side) – Necessary for quality measurements of reionization bump CMB large angular scale, full sky polarization anisotropy can only be measured well from space platform – Access to all frequencies, all sky, freedom from ground and atmospheric emission, lunar, solar, etc. emissions

Noise performance, i.e. overall sensitivity of the experiment – Noise has to be low, but this is not the hardest part – systematics and foreground residuals have to be lower at least in proportion Systematics hit harder with S/N growing; lessons learned from Planck – We found early on that the low-l polarization was indeed the hardest part of the job; we’re still working on it Foregrounds – Legacy of WMAP and Planck Unprecedented multi-frequency dust emission mapping Relatively under-measured low-ν side of the foreground emission Still, this is the best we have in hand – and it is a lot Does low-l effort require a dedicated experiment design? Can such measurement be made at relatively modest, as space missions go, cost? – E.g. is the MIDEX platform sufficient? BTW – what about data analysis?

353 P 217 P 143 P 100 P 70 P 44 P 30 P CMB P CMB - a realization from the τ =0.06 and r=0.10 model All maps at FWHM = 3deg Commander polarized foreground maps: frequency scaled and added together - WMAP-K synchrotron map plus - Planck 353 dust map (only foregrounds shown) ALL PLOTS: Polarization Amplitude

CMB U CMB P 100 Q 100 U CMB Q 100 P τ =0.06 and r=0.10 model Difference between r=0.10 and r=0 models (same phases) Difference between r=0.05 and r=0 models (same phases) All maps FWHM=3deg

Planck foregrounds in the «Southern hole» at 90 GHz TotalSynchrotronThermal dust Synchrotron is comparable to thermal dust at 90 GHz, even in the cleanest regions of the sky SPIDER BICEP2 SPIDER BICEP2 SPIDER BICEP2

Foreground-to-CMB B-mode ratio