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Hubble Space Telescope
One of the key tasks of the Hubble Space Telescope was to investigate the Hubble constant. (NASA)


The controversy over Ho

Hubble’s 1927 estimate of H0 (the “Hubble constant”) was 500 km s–1 Mpc–1, where 1 Megaparsec (Mpc) = 3.26 million light years. This means that a galaxy 3.26 million light years away from us would be receding at 500 km s–1, one at 6.52 million light years at 1000 km s–1 and so on.

Now H0 has the dimensions of time–1 and so 1/H0 is the expansion age of the universe – the age the universe would have if no forces were acting and therefore the expansion took place at the same rate for the whole history of the cosmos. Hubble’s value for H0 implied an age of the universe of 2 billion years and it was soon realized this was shorter than the age of the Earth, derived from radioactive isotopes.

From 1927 to 2001 the value of the Hubble constant was a matter of fierce controversy. Walter Baade pointed out in 1952 that there were two different types of Cepheid, so Hubble’s calibration had been incorrect. This reduced H0 to 200 km s–1 Mpc–1. In 1958 Allan Sandage recognized that objects that Hubble had thought were the brightest stars in some of his galaxies were in fact clouds of hot gas; Sandage arrived at the first recognizably modern value of 75 km s–1 Mpc–1. During the 1970s there was an acute disagreement between Sandage and G Tammann, on the one hand, favouring H0 = 50 km s–1 Mpc–1, and Gérard de Vaucouleurs, on the other, favouring 100 km s–1 Mpc–1.


HST Key Program

Following the launch of the Hubble Space Telescope (HST) in 1990, and the subsequent repair mission, substantial amounts of HST time were dedicated to measuring Cepheids in galaxies out to distances of 20 Mpc, to try to measure the Hubble constant accurately and to give the different distance methods a secure and consistent calibration. In 2001 the HST Key Program team, led by Wendy Freedman, announced their final result:

H0 = 72 ± 8 km s–1 Mpc–1

This, as we shall see, agreed extremely well with the first results from the WMAP Cosmic Microwave Background mission (72 ± 5 km s–1 Mpc–1). It gave an age of the universe for an Einstein–de Sitter model of 9.1 billion years. This meant that a positive cosmological constant would be required for the age of the universe to be consistent with the age of the oldest stars.

Direct evidence for a positive cosmological constant came in 1998 from studies of Type Ia supernovae that are bright enough to be seen in distant galaxies, using both ground-based telescopes and the orbiting Hubble Space Telescope. Type Ia supernovae arise in binary star systems when the more massive star starts to dump gas on a companion white dwarf, the relic of a star like the Sun when it reaches the end of its life, which reaches a critical mass and explodes.

These supernovae explode with similar maximum luminosities, so measuring their brightness allows their distance to be calculated. The way this distance varies with redshift shows that a cosmological constant is needed to get the required flat geometry for the universe.

The modern interpretation of Einstein’s cosmological constant is that it represents the energy-density of the vacuum, loosely called “dark energy”, but a problem is that the observed value is 10120 times smaller than the value predicted by particle physics theories.


This is an X-ray image of Tycho’s supernova remnant, the remains of a Type Ia supernova observed by Danish astronomer Tycho Brahe in 1572. Such explosions are used to measure distances because of their reliable brightness. (NASA/CXC/Chinese Academy of Sciences/F Lu et al.)


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