Day 864

Monday, Tuesday.


1504.00718
Local gravitational redshifts can bias cosmological measurements
Wojtak, Davis, Wiis

Measurements of cosmo params via the distance-redshift relation usually rely on models that assume a homogeneous universe.  It is commonly presumed that the LSS evident in our Universe has a negligible impact on the measurement if distances probed in observations are sufficiently large (compared to the scale of inhomogeneities) and are averaged over different directions on the sky.  This presumption does not hold when considering the effect of the gravitational redshift caused by the local gravitational potential, which alters light coming from all distances and directions in the same way.  Despite its small magnitude, this local gravitational redshift gives rise to noticeable effects in cosmo inference using SN Ia data.  Assuming conservative prior knowledge of the local potential given by sampling a range of gravitational potentials at locations of MW-like galaxies identified in cosmo sims, show that ignoring the gravitational z effect in a standard data analysis leads to an additional systematic error of ~1% in the determination of density parameters and the DE EoS.  Conclude that the local gravitational field affects the cosmo inference at a level that is important in future observations aiming to achieve %-level accuracy.

1504.00733
Diagnosing multiplicative error by lensing magnification of type Ia supernovae
Zhang

WL causes spatially coherent fluctuations in flux of SNe Ia.  This lensing magnification allows for WL measurement independent of cosmic shear.  It is free of shape measurement errors associated with cosmic shear and can therefore be used to diagnose and calibrate multiplicative error.  Although this lensing magnification is difficult to measure accurately in auto correlation, its cross correlation with cosmic shear and galaxy distribution in overlapping area can be measured to significantly higher accuracy.  Therefore these cross correlations can put useful constrain on multiplicative error, and the obtained constrain is free of cosmic variance in WL field.  Present two methods implementing this idea and estimate their performances.  Find that, with ~1M SNe Ia that can be achieved by the proposed D2k survey with the LSST telescope, multiplicative error of ~0.5% for source galaxies at z_s~1 can be detected and larger multiplicative error can be corrected to the level of 0.5%.  It is therefore a promising approach to control the multiplicative to the sub-percent level required for stage IV projects.  The combination of the two methods even has the potential to diagnose and calibrate galaxy intrinsic alignment, which is another major systematic error in cosmic shear cosmology.