RSAA Colloquia / Seminars / Feast-of-Facts: Friday, 18 November 2016, 14:30-14:50; Duffield Lecture Theatre

Bonnie Zhang

"A blinded determination of the Hubble constant from supernovae'"

Presently an over 3sigma tension exists between values of the Hubble constant H0 derived from analysis of fluctuations in the Cosmic Microwave Background by Planck and local measures of the expansion using calibrators of type Ia supernovae (SNe Ia). Calibration of SNe Ia requires a series of often subtle choices, and the need to preclude human bias is more compelling that ever before. We perform a blind reanalysis of Riess et al. (2011) to measure the H0 using SNe Ia from CfA3 and LOSS, calibrated by Cepheid variables and the geometric distance to NGC 4258. We obscure the value of H0 throughout our analysis and the referee process. Our analysis departs from that of Riess et al. (2011) by incorporating the covariance matrix method adopted in SNLS and JLA to quantify SN Ia systematics, and includes a simultaneous fit of all SN Ia and Cepheid data using MultiNest, thus capturing the full dependences and interactions of all parameters. The measured value of H0 is yet to be revealed; however, we find a relative uncertainty in H0 marginally larger than the errors in Riess et al. (2011) and in the Efstathiou (2014) reanalysis. Our error budget is dominated by statistical noise in the supernova data, whilst the systematic contribution to the error in H0 is dominated by uncertainties in the host galaxy mass dependence and Malmquist bias.