*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.

*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, it is necessary to know both the distance to a star and the speed at which it appears to be “moving 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.

**a cepheid **It is a pulsating star due to the contraction and expansion of the gas that makes up it: so its brightness varies at a very regular rate. In the early 20th century, American astronomer Henrietta Levitt discovered that the rate of pulsation depends on the intrinsic brightness of the star. Knowing this and the apparent brightness, the exact distance to the Cepheids can be determined. A boon for astrophysicists, who use it as a landmark in space.

**What will gravitational waves bring?**

They provide a third method that can resolve this disagreement. It would be ideal to observe signals like GW170817 with a gravitational wave and its electromagnetic counterpart. The gravitational wave makes it possible to directly determine the distance at which the collision occurred, and the electromagnetic equivalent, the speed at which the newly formed star moves away due to expansion.

I wish we had seen such an incident. While waiting for other copies, we have developed two methods: using properties of black holes and exploiting catalogs of galaxies. The first is to take advantage of the fact that the process of forming a black hole itself gives it a specific mass. And this mass is affected by redshift. Also, since the gravitational wave gives us the distance directly, we have two pieces of information to extract H0.

The other possibility is to rely on galaxy catalogs, such as Glade+, which give the redshifts of millions of galaxies. We detect there two black holes that were at the origin of the gravitational wave and we base ourselves on the redshift of these galaxies. The distance, here also, is given directly by the gravitational wave.

**What value do you get this way?**

Our methods give an H0 = 68 + or – 6-8 km/s/Mpc… We are not yet able to decide between inconsistent values, but we expect that as we accumulate larger numbers, We will be able to do this. events. We have the way, we need the data.

*Interview by Azar Khalatbari*

Analyst. Amateur problem solver. Wannabe internet expert. Coffee geek. Tv guru. Award-winning communicator. Food nerd.