Gravitational Waves: New Universe Insights – Exclusive Interview

Ten years ago, humanity took a major step forward in understanding the Universe with the detection of the first gravitational waves. This phenomenon was predicted by Albert’s theory of general relativity. Einsteinformulated in 1915.

This theory envisaged disturbances in the fabric of space-time, caused by astrophysical events generated by compact stars, such as the collision of black holes orneutron stars. Einstein theorized that these events must generate waves that propagate to the speed of lightcreating minute variations in the gravitational field.

A new window on the Universe

The first direct detection of gravitational waves took place in 2015 thanks to the observatory Ligo. This event not only confirmed one of the great predictions of the general relativitybut also opened a new era in the field of astronomy.

On September 14, 2015, researchers from the Ligo-Virgo collaboration recorded a signal from the coalescence of two black holes, each having approximately thirty times the masse of soleil. Ce signal represented not only the first direct detection of gravitational waves, but also the first confirmation of the existence of black holes by “direct” detection. On the other hand, this is undoubtedly the first detection of a system of two black holes. The observed wave shape matched exactly what general relativity predicts.

In addition to electromagnetic waves – for example, visible light, radio waves and x-rays – which make it possible to observe ever more distant cosmic objects, gravitational waves open a new window on the Universe. They offer the possibility of exploring extreme events and diving deeper into the history of the Universe, by probing objects located in very remote places in the Universe at the edge of black holes.

The era of gravitational astronomy

However, detecting these waves has been a real challenge. During the fusions black holes, variations in distance between two mirrors can reach the order of a billionth of a billionth of a meter (10⁻¹⁸ m), a measurement a thousand times smaller than the size of an atomic nucleus.

To achieve such detection, it is essential to overcome many noise sources. From the first gravitational wave detectors to today, many innovations have been necessary to reduce this ambient noise and identify gravitational wave signals. The technological advances necessary to achieve these measures took several decades of development.

Until 1950, the astronomers even doubted the existence physique gravitational waves. In 1960, their detection was considered, but the instruments available at the time did not achieve the required precision. Gravitational waves act by stretching space-time, similar toelasticity of an elastic band.

To measure the passage of a gravitational wave, it is necessary to measure the variation in distance between masses separated by several kilometers, while reducing surrounding disturbances, such as seismic noise or acoustic noise. This required significant technological advances, making it possible to detect variations with incredible precision.

What is a gravitational wave? These new messengers from the Universe. © Dunod

The word to Matteo Barsuglia, physicistresearch director at CNRS and expert in gravitational waves. He has contributed for more than 25 years to the detection of gravitational waves and was scientific manager for France of the project Virgo. He is also author of the book “ THE Waves of space time », recently released in pocket format by Dunod.

Futura: What were the main reasons, both technological and scientific, which prevented the detection of gravitational waves before September 2015?

Matteo Barsuglia: Gravitational waves, although originating from extremely violent astrophysical phenomena such as merging black holes or exploding stars, manifest as weak disturbances in the geometry of space-time. Indeed, although space-time has elastic properties, it is extremely rigid, which means that the gravitational waves generated by these events are particularly weak. When black holes merge, for example, they cause a change in distance between two mirrors three kilometers apart of the order of a billionth of a billionth of a meter (10⁻¹⁸ m), or a thousand times smaller than the size of an atomic nucleus. This infinitesimal variation is masked by a multitude of sources of ambient noise.

Thus, from the design of the first gravitational wave detectors to their successful detection, it took half a century of technological improvements to minimize this environmental noise and make the capture of gravitational wave signals possible.

Futura: What questions have been answered by the confirmation of the existence of gravitational waves?

Matteo Barsuglia : The detection of gravitational waves not only confirmed general relativity in conditions where it had never been tested, notably in extreme gravitational fields, but it also proved the existence of black holes in binary systems. The nearly 200 detections, carried out so far, have measured various properties of black holes, such as their masses, their amount of rotation and their merger rate, revealing very massive black holes that have never been observed before.

In addition, this detection has opened new perspectives on the expansion of the Universe, by providing measurements distinct from those obtained by methods based on electromagnetic waves.

Finally, the simultaneous observation of gravitational waves and light on August 17, 2017 established a link between startles short gamma rays and neutron star mergers, as well as between phenomena called “kilonovae” and these same mergers. This helped to clarify the origin of certain chemical elements heavy, like gold.

Futura: What new questions arise following the discovery of gravitational waves?

Matteo Barsuglia: The detection of gravitational waves has opened a true “new window on the Universe”. This allows us to explore a wide range of questions in astrophysics and fundamental physics. Unlike electromagnetic waves, gravitational waves transmit different but complementary information to us, which enriches our understanding of the Universe.

Futura: Technically, what type of instruments would be needed to improve the detection of gravitational waves and broaden their frequency?

Matteo Barsuglia: It will be essential to refine existing detectors, such as Virgo and Ligo, by continuing to reduce the noise that limits their sensitivity. At the same time, projects for new terrestrial detectors are underway, which would be longer and therefore more sensitive, such as the European project Einstein Telescope and the American project Cosmic Explorer.

Furthermore, theESA is developing a project to detect gravitational waves in space, called Lisawhich consists of a trio of satellites intended to detect gravitational waves in a frequency band low, inaccessible to terrestrial observatories.

All these projects are based on a similar technique: measuring the variation in distance between “test” or free masses, caused by the passage of a gravitational wave, using a light beam. Finally, there are also initiatives, such as “timekeeping of pulsars ”, which use a different approach to detect these waves.

Futura: What do you think could be the next “discovery of the century”?

Matteo Barsuglia: This is a difficult question, but I think understanding the nature ofdark energy or the dark matter could truly constitute such a discovery. And it could shed light on many fundamental questions about the evolution of the Universe.

Progress in this area would revolutionize our understanding of cosmology and physics in general. And, as already mentioned, gravitational waves could contribute to this discovery of the century.

Today in pocket format, the book “ Waves of space-time », written by Matteo Barsuglia explains the epic story of the detection of the first gravitational wave, which will allow us to unravel certain mysteries of the Universe, and perhaps even to understand the nature of dark matter and dark energy. This book received the Ciel&Espace astronomy book prize 2020. © Dunod publisher