### LUC BLANCHET LAUREATE OF THE GRAND PRIX “CNES ASTROPHYSICS AND SPACE SCIENCES” OF THE FRENCH ACADEMY OF SCIENCES

**Luc Blanchet** © Jean Mouette IAP-CNRS-SU

Luc Blanchet is the first laureate of the Grand Prix “CNES Astrophysics and Space Sciences” of the French Academy of Sciences (newly created in 2018), for his work on gravitational waves and space experiments in fundamental physics. The award ceremony will take place under the dome of the Institut de France in November 20, 2018.

Luc Blanchet is senior researcher in theoretical physics at the CNRS (“directeur de recherche de classe exceptionnelle”), and is affiliated to the Institut d’Astrophysique de Paris (CNRS, Sorbonne Université). His research activity mainly focuses on general relativity and the theory of gravitational waves. With the first detections of gravitational waves generated by the coalescence of compact binary systems^{[1]}, the theoretical work carried out for many years by Luc Blanchet and his collaborators (notably Guillaume Faye, as well as their students) has been successful (see the previous IAP Science highlight recounting the first historical detection on September 14th, 2015).

This theoretical work is based on the post-Newtonian approximation of general relativity, which consists of adding corrections to Newton’s theory as a power series in a small parameter, which is the ratio *v/c* between the velocity of the bodies on their orbit and the speed of light. The post-Newtonian calculations made it possible to determine of the gravitational waves emitted by a compact binary system up to the order (*v/c*)^{7}, beyond the famous Einstein quadrupole formula^{[2]} - an approximation level called the *3.5PN* order. The next orders, *4PN* and *4.5PN*, are currently being analyzed.

The post-Newtonian approximation describes very accurately the gravitational waves during the inspiralling phase of the compact binary system, juste before the final merger. At the instant of merger, the approximation is no longer valid and must be replaced by a numerical integration of the Einstein field equations^{[3]}.

The first detections of black holes already yielded some experimental constraints on the post-Newtonian parameters, which are in agreement with the theoretical predictions. But because the black holes are rather massive, the number of orbital cycles before the final merger is limited (typically ten) and as a result the constraint is weak. It is with the detection of gravitational waves from the coalescence of a binary system made of neutron stars (see the IAP Science highlight about this extraordinary event of August 17th, 2017), that the post-Newtonian predictions become highly important. For such systems, several thousand orbital cycles are measured, and the algorithms for searching and analyzing the signal in the LIGO and VIRGO detectors are essentially based on the post-Newtonian formulas developed by Luc Blanchet and his team.

In addition to his work on gravitational waves, Luc Blanchet is interested in the experimental tests of general relativity, and in the theoretical aspects of space experiments such as Microscope (test of the equivalence principle of equivalence in terrestrial orbit by measuring the gravitational accelerations of two masses with different compositions) and Pharao-ACES (atomic clock placed in the international space station to measure the gravitational shift of frequencies, or Einstein effect). Luc Blanchet is currently a full member of the Bureau des Longitudes; moreover, he was president of the committee for Fundamental Physics at CNES.

The “Grand Prix CNES” was created in 2018 for Astrophysics and Space Sciences. This award recognizes a person or a team chosen by the members of the French Academy of Sciences belonging to the “Sciences de l’Univers” division, for their work in astrophysics and space sciences in general, either observational or theoretical.

#### Links

The press release announcing the creation of the “Grand Prix CNES” in 2018: http://www.academie-sciences.fr/en/Press-releases/cnes-and-french-science-academy-renew-partnership-agreement-and-create-cnes-astrophysics-space-science-award.htmlThe Grand Prize winners of the Academy of Sciences in 2018: http://www.academie-sciences.fr/en/Table/Prix-et-medailles/Laureats/

#### Footnotes

[1] In Astronomy, a binary system is composed of two stars linked by the gravitational interaction, and which therefore follow a bound orbit around their common center of mass. The system is compact if its size is of the order of its mass (multiplied by*G/c*

^{2}, where

*G*is the gravitational constant and

*c*is the speed of light). In this case, the gravitational field is strong, and the two stars are either neutron stars or black holes.

[2] The dominant term in the gravitational wave when v/c is small has a quadrupole shape (whereas the electromagnetic wave is dipolar in general), and is described by the Einstein quadrupole formula of 1918.

[3] Methods such as the “effective-one-body” approach permit an analytic interpolation between the post-Newtonian calculations and the numerical results, and are used in the data analysis of the detectors in the case of black hole binaries.

Web editing: Valérie de Lapparent

Layout: Jean Mouette

*September 2018*