In 1929 Edwin Hubble confirmed that galaxies are receding from us with a speed proportional to their distance: v = H0d. As late as the mid 1990s, the value of the proportionality constant H0, the Hubble constant, was known only to be somewhere between 50 and 90 km/s per megaparsec (see the article by Mario Livio and Adam Riess, Physics Today, October 2013, page 41). With the help of space-based observatories, H0 can now be determined with a precision of about 1%. The value obtained from a detailed map of the cosmic microwave background (CMB) is 66.93 ± 0.62 km/s/Mpc. But that determination is in tension with the value of 73.24 ± 1.74 km/s/Mpc derived from standard candles (Cepheid variables and type Ia supernovae, whose luminosities are known).

Now the H0LiCOW (H0 Lenses in COSMOGRAIL’s Wellspring) collaboration has presented a...

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