Measuring the expansion rate of the universe thanks to … black holes

The Hubble constant and the expansion of the universe

The expansion of the universe began 13.8 billion years ago, just after the Big Bang, when it was smaller than an atom. At that time it was extremely hot and equally dense. Partially confirmed by astronomical observations and accepted by most astronomers, the Big Bang marks the beginning of the expansion of the universe.

To fully understand the concept of the expansion of the universe, we must realize that space-time is dynamic. In reality, it is not the objects that make up the universe such as planets, stars, galaxies or nebulae that are moving, but the space between these objects that is getting bigger and bigger.

The expansion of the universe was demonstrated by Edwin Hubble (1889-1953) in 1929 using a method based on the study of the light spectrum, spectroscopy. He demonstrated that the farther a galaxy is from the observer, the higher its speed.

It is this relationship that was used to establish the Hubble constant, denoted H0. It is a constant of proportionality which relates the rate of removal of the celestial objects in the observable universe at the moment T and the distance. This constant, which today is approximately 74.3 km/s/Mpc (kilometers per second and per megaparsec), means that as a galaxy moves away from one megaparsec, its speed increases by 74 km/s.

Unfortunately, the exact rate of expansion of the universe, in other words the Hubble constant, is not determined with precision. The different calculation methods used today give slightly different answers from each other. But knowing this value precisely could improve our understanding of the universe in terms of its age, history, and even composition.

>> Read also: What happened right after the big bang?

Black holes collide in the universe

Part of the LIGO facilities in Louisiana, USA

The study by researchers at the University of Chicago offers another way to determine the Hubble constant. A black hole is a cosmic object that is invisible to the naked eye. It is a region of space where the gravitational field is so intense that it prevents all matter and radiation from escaping. Even the light attracted by the force of the black hole cannot get out. There are different types such as supermassive black holes whose mass can vary from one million solar masses to several billion such as the black hole at the center of the Holmberg 15A galaxy weighing 15 billion solar masses.

In the vast cosmos, two black holes can collide. This event is so powerful that it generates a space-time ripple that propagates throughout the universe. On Earth, these waves can be picked up by two observatories equipped to detect gravitational waves. These are the LIGO observatory (Laser Interferometer Gravitational wave Observatory) in the USA and the Virgo observatory in Italy.

The space-time ripples captured by these observatories provide valuable information about the mass of black holes. However, this ripple has traveled through space such a great distance before reaching Earth that the universe has had time to expand, changing the characteristics of the signal. By managing to measure the evolution of this signal, astronomers will be able to calculate the Hubble constant more precisely. However, in order not to introduce any risk of error, the calibration of the system is important.

>> Also read: Two supermassive black holes engaged in a cosmic waltz 9 billion light years away

Thousands of signals for precise calibration

Current knowledge of black holes allows astronomers to say that most of those discovered have a mass between five and forty solar masses. By studying the properties and measuring the masses of the nearest black holes and then looking at them much further away, they can determine how far away they are.

In fact, the relationship between the amplitude of the gravitational waves emitted during the collision of two black holes and the observed amplitude makes it possible to derive a distance. This is the so-called “spectral siren” method, by analogy with the sound of a siren, which becomes less and less powerful as it moves away.

This way of proceeding makes it possible to achieve a goal for the expansion of the universe. But for the calibration to be as accurate as possible, the astronomers need as much data as possible from colliding black holes. To date, the two detectors LIGO and Jomfruen have already collected data for a hundred pairs of black holes.

In the future, this method may make it easier to study the beginning of the universe 10 billion years ago. Until now, these young years in the cosmos have been difficult to study.

>> Read also: Detection of primordial black holes thanks to gravitational wave microlensing

Source :

Jose María Ezquiaga and Daniel E. Holz, “Spectral Sirens: Cosmology from the Full Mass Distribution of Compact Binaries”, Physical review letters129, 061102, 3 Aug. 2022,

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