Planck Cosmology 2015 The nearly exact degeneracy -- i.e., nearly the same CMB anisotropies in models with different geometries but the same matter content – Is a limit to deriving parameters such as the Hubble constant from CMB data alone It also means that an accurate independent measure of H 0 provides a key means of constraining other cosmological parameters in combination with CMB anisotropies Ω b h 2, Ω c h 2 well measured Princeton Wendy L. Freedman University of Chicago June 11,2015 H0H0 N eff wawa w 0 σ8σ8 H0H0

Planck Cosmology 2015 Weinberg et al Improvement in H 0 Fiducial Stage IV prior H0H0 N eff wawa w 0 H0H0 σ8σ8 H 0   M. Tegmark

History of the Hubble Constant

Direct Measurements of H 0 : Decreasing Systematics LMC Milky Way Δ[3.6] ΔW WLF et al. 2012; Scowcroft et al. 2012; Monson et al 2015 WLF et al. (2001) R I V B M33 WLF et al. (1991

Other Future H 0 Measurements: Target H 0 to 1%: (sys + stat) Overcoming Systematics LIGO Gravitational Waves BAO Gaia Calibration of Cepheid/RRL/SNIae Gravitational Lens Time Delays For robust Measuremen,t at least 3 independent methods with uncertainties at ~1% level Landau – “Cosmologists are often in error but never in doubt”