Planck Cosmology 2015The 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 CMBdata alone

It also means that an accurateindependent measure of H0 providesa key means of constraining othercosmological parameters incombination with CMB anisotropies

Ωbh2, Ωch2 well measured

CMB@50Princeton

Wendy L.FreedmanUniversityof Chicago

June 11,2015

H0

Neff

wa

w0

σ8

H0

Planck Cosmology 2015

Weinberg et al. 2012

Improvement in H0

Fiducial Stage IV prior

H0

Neff

wa

w0

H0

σ8

H0

M. Tegmark

History of the Hubble Constant

Direct Measurements of H0: Decreasing Systematics

LMCMilky Way

Δ[3.6]

ΔW

WLF et al. 2012; Scowcroft et al. 2012; Monson et al 2015WLF et al. (2001)

RIVB

M33

WLF et al. (1991

Other Future H0 Measurements:Target H0 to 1%: (sys + stat) Overcoming Systematics

LIGO Gravitational Waves

BAO

Gaia Calibration of Cepheid/RRL/SNIae

Gravitational Lens Time Delays

For robustMeasuremen,tat least 3 independentmethods withuncertaintiesat ~1% level

Landau – “Cosmologists are often in error but never in doubt”