This article is taken from the journal Les Indispensables de Sciences et Avenir n°209 dated April/June 2022.
Science et Avenir: How is the rate of expansion of the universe determined?
Simone Mastrogiovanni: This rate, also called Hubble’s constant – denoted H0 – determines the speed at which each megaparsec (Mpc) of the universe swells: one megaparsec corresponds to 3.26 million light-years. This constant is expressed in kilometers per second and per MPC. To calculate this, you need to know both the distance to a star and the speed at which it seems to “go away” due to the expansion of space. The distance is very hard to determine, because no one knows the star’s intrinsic brightness: it’s not because it seems so faint to us that it’s far away. It is necessary to use indirect methods. As for the removal speed, it is given by the redshift: when a light source moves away from the observer, its light is shifted towards a longer wavelength (reddish in the visible spectrum).
What is its value today?
Cosmologists obtain different values based on observations. The study of the cosmic microwave background – this first light of the universe, specifically probed by the European Planck satellite – gives a value of 67 km/s per Mpc, with a measurement uncertainty of 0.5 km/s per Mpc. Whereas observations of stars such as Cepheids (see box below), whose distances we know, result in 72! These two values seem to be irreplaceable. This could mean that the universe is not expanding as expected, and we may need new physics! This would disturb current knowledge. But before getting there, we need to be sure of the measurement of H0.
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