Vol. 18 (2015) > lrr-2015-2

doi: 10.1007/lrr-2015-2
Living Rev. Relativity 18 (2015), 2

The Hubble Constant

1 Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, University of Manchester, Turing Building, Manchester M13 9PL, U.K.

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Article Abstract

I review the current state of determinations of the Hubble constant, which gives the length scale of the Universe by relating the expansion velocity of objects to their distance. There are two broad categories of measurements. The first uses individual astrophysical objects which have some property that allows their intrinsic luminosity or size to be determined, or allows the determination of their distance by geometric means. The second category comprises the use of all-sky cosmic microwave background, or correlations between large samples of galaxies, to determine information about the geometry of the Universe and hence the Hubble constant, typically in a combination with other cosmological parameters. Many, but not all, object-based measurements give H_0 values of around 72–74 km s^–1 Mpc^–1, with typical errors of 2–3 km s^–1 Mpc^–1. This is in mild discrepancy with CMB-based measurements, in particular those from the Planck satellite, which give values of 67–68 km s^–1 Mpc^–1 and typical errors of 1–2 km s^–1 Mpc^–1. The size of the remaining systematics indicate that accuracy rather than precision is the remaining problem in a good determination of the Hubble constant. Whether a discrepancy exists, and whether new physics is needed to resolve it, depends on details of the systematics of the object-based methods, and also on the assumptions about other cosmological parameters and which datasets are combined in the case of the all-sky methods.

Keywords: Cosmology, Hubble constant

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Article Citation

Since a Living Reviews in Relativity article may evolve over time, please cite the access <date>, which uniquely identifies the version of the article you are referring to:

Neal Jackson,
"The Hubble Constant",
Living Rev. Relativity 18,  (2015),  2. URL (cited on <date>):

Article History

ORIGINAL http://www.livingreviews.org/lrr-2007-4
Title The Hubble Constant
Author Neal Jackson
Date accepted 11 September 2007, published 24 September 2007
UPDATE http://www.livingreviews.org/lrr-2015-2
Title The Hubble Constant
Author Neal Jackson
Date accepted 14 August 2014, published 24 September 2015
Changes Major revision, updated and expanded. The number of references has increased from 179 to 242.
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