# UMICH-2015: CMB-Lensing break-out session 1

From CMB-S4 wiki

## Lensing autospectrum

Alex V. leading discussion, ~15 min

Alex's preliminary slides for discussion

- CMB lensing intro
- Sensitivity to neutrino mass and other parameters (w, $\Omega_k$)
- Foreground challenges
- Extragalactic polarized sources
- How big of an issue can they be? The CMBPol study 0811.3916 shows they give small biases under conservative assumptions, particularly if the resolution is high enough to find sources
- They can also be projected out of the estimator (bias-hardening)

- Galactic dust
- Biases from nonzero connected four-point function at high-ell? e.g. 1207.0508

- Instrumental systematics
- gain variations, monopole leakagge, quadrupole leakage, pointing/beam offsets

- Extragalactic polarized sources
- Impact of nonlinear growth of structure?
- How important are better optical depth measurements for m_\nu constraints from CMB lensing?
- Can we reconstruct large-scale lensing modes from a patchwork of small-area observations? Related issue: requirements on relative calibration across patches?

(Lloyd writing) The following figures are to explain:

- The sensitivity of the lensing potential to varying neutrino mass
- The complementary orientation of degeneracy structure in the omegam-Mnu plane from DESI BAO

The primary impact of neutrino mass on low-redshift probes of cosmology (including lensing) is the increase in expansion rate at z > 1 and its decrease at z < 1 (due to need to decrease the cosmological constant value to keep distance to last scattering unchanged.

The quantity "R sub ell" is the fractional change in the lensing power spectrum per meV change in neutrino mass.

- Starting with the green curve, the increased expansion rate suppresses structure growth on length scales below the neutrino free streaming length. It also decrease the neutrino free-streaming length, allowing for neutrinos to cluster on sufficiently large scales. On large scales these two effects cancel.
- As other parameters vary (to keep CMB temperature and polarization anisotropy likelihood near peak value), there are additional effects. The main one is due to correlation between mnu and omegam (arising indirectly via influence of lensing on temperature and polarization power spectra).
- Almost all the impact is due to growth. All geometry effect is encoded in the distance to recombination. Letting it vary or keeping it fixed makes little difference.

- omegam increases lensing power while Mnu decreases it hence they are positively correlated for CMB-S4.
- increasing omegam increases the expansion rate at z > 1, and (because the cosmological constant must decrease to preserve angular size of sound horizon) decreases it at z < 1. This is similar to the effect of Mnu increasing and so these are negatively correlated for BAO.

## Lensing by individual DM halos

Neelima leading discussion - ~10 min

- mass estimate forecasts
- instrument requirements
- systematic/foreground challenges