Update available: http://www.livingreviews.org/lrr-2015-2

Jodrell Bank Centre for Astrophysics

University of Manchester

Turing Building

Manchester M13 9PL, U.K.

http://www.jodrellbank.manchester.ac.uk/~njj/

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.
In the last 20 years, much progress has been made and estimates now range between 60 and
75 km s^{–1} Mpc^{–1}, with most now between 70 and 75 km s^{–1} Mpc^{–1}, a huge improvement
over the factor-of-2 uncertainty which used to prevail. Further improvements which gave a
generally agreed margin of error of a few percent rather than the current 10% would be vital
input to much other interesting cosmology. There are several programmes which are likely to
lead us to this point in the next 10 years.

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