Abstract
When black holes merge - a very frequent phenomenon - gravitational waves are emitted, and their detection has been in the pipeline for years. Gravitational waves are ripples in space-time, fluctuations in curvature caused by the rapid motion of masses. They propagate at the speed of light, and have two possible polarizations, at 45 degrees. If electromagnetic waves are represented by an oscillating dipole, this is a quadrupole. We know that these waves exist, having been detected indirectly by R. A. Hulse and J. H. Taylor in 1975 (Nobel Prize 1993), thanks to pulsar timing. The most useful pulsars for these measurements are millisecond pulsars, generally evolved pulsars where the magnetic field is weakened, but rejuvenated and accelerated by gas accretion from a binary. In this case, their lifetime is much longer. Pulsar timing is one of the most precise measurements in astrophysics. A network of pulsars is currently being built to map the space around us, and detect curvature fluctuations due to the merging of supermassive black holes. It will then be possible to quantify the number of black hole mergers, even if these are not active cores, and therefore without electromagnetic radiation. In February 2016, the detection of gravitational waves due to the merger of two black holes of stellar mass (30Mb) was announced for the first time. This detection was made possible by LIGO, in collaboration with the analogous European instrument Virgo. The measurement is established by the interference of laser waves, travelling on interferometer arms 4 km long. Even multiplied by several reflections (in a cavity), this length corresponds to the wavelength emitted by stellar black holes. To start detecting more massive black holes, with much longer wavelengths, we'll need to go into space (LISA interferometer, Laser Interferometer Space Antenna).
