## [1004.1856] The Hubble Constant

 Authors: Wendy L. Freedman, Barry F. Madore Abstract: Considerable progress has been made in determining the Hubble constant over the past two decades. We discuss the cosmological context and importance of an accurate measurement of the Hubble constant, and focus on six high-precision distance-determination methods: Cepheids, tip of the red giant branch, maser galaxies, surface brightness fluctuations, the Tully-Fisher relation and Type Ia supernovae. We discuss in detail known systematic errors in the measurement of galaxy distances and how to minimize them. Our best current estimate of the Hubble constant is 73 +/-2 (random) +/-4 (systematic) km/s/Mpc. The importance of improved accuracy in the Hubble constant will increase over the next decade with new missions and experiments designed to increase the precision in other cosmological parameters. We outline the steps that will be required to deliver a value of the Hubble constant to 2% systematic uncertainty and discuss the constraints on other cosmological parameters that will then be possible with such accuracy. [PDF]  [PS]  [BibTex]  [Bookmark]

Discussion related to specific recent arXiv papers
Syksy Rasanen
Posts: 119
Joined: March 02 2005
Affiliation: University of Helsinki

### [1004.1856] The Hubble Constant

This is a nice review of the determination of $H_0$, with an emphasis on methods independent of the cosmological model. The updated HST value is given as $73\pm2\pm4$ km/s/Mpc, with values from other methods in the same range. (To be contrasted with 0806.3018, which gives $62\pm1\pm4$ km/s/Mpc, from SNIae, Cepheids and the tip of the red giant branch.)

Cepheids remain an important calibrator for the distance scale, and there is still controversy about metallicity corrections (partly explaining the discrepant numbers mentioned above). One of the improvements reported here is changing the calibration from Cepheids in the Large Magellanic Cloud to Cepheids in our own galaxy. Since the metallicity of these Cepheids is closer to the galaxies where the supernovae used for the determination of $H_0$ reside, this should make the corrections smaller. The authors in fact write that this changes the metallicity corrections from a systematic error to a statistical error, which will decrease as more Cepheids are observed.

There is also a short review on the age of the universe, where I found the CMB discussion to be a bit misleading, because the dependence on the cosmological model is not mentioned (apart from the assumption of spatial flatness).