against'It is a space mission that is likely to revolutionize our knowledge, not only of astrophysics and cosmology, but also of fundamental physics. It's called Lisa (Laser Interferometer Space Antenna), and it has just been certified by the European Space Agency (ESA), at the same time as EnVision, the Venus exploration mission.
Its goal: to push the boundaries of the scientific revolution that began on September 14, 2015, the date of the first detection of gravitational waves – tiny ripples in space-time caused by the acceleration of the most massive objects in the universe. Like two black holes ready to merge, for example. The phenomena were predicted a century ago by Albert Einstein, who defined them in an article published in 1918 as the propagation of gravity by waves, at the speed of light.
Better perception of gravitational waves
interest? It's as if the discovery of these gravitational waves has given us a sixth sense for understanding the universe and the stars that inhabit it. Before them, we only knew them through light and other electromagnetic waves. Now we have direct access to their moving mass. “The detection of gravitational waves allows us, in a way, to hear the universe,” explains Nelson Christensen, research director of the National Center for Scientific Research and head of the Artemis Laboratory (for relativistic astrophysics, theory, experiments, metrology, instrumentation and signals) from Côte d'Ivoire. The Azure Observatory specializes in detecting these cosmic signals and is heavily involved in the LISA mission.
Today, to perceive these waves that distort the fabric of space-time, like waves, a unique scientific community, consisting of astronomers and physicists, uses ground-based detectors. Observatories consisting of two perpendicular arms, each a few kilometers long, with a mirror at the end. These arms are crossed by the same laser beam divided into two parts. Two secondary beams are then recombined to see if they have traveled the same distance, within the size of an atom. In fact, if it doesn't, it's likely due to the passage of a gravitational wave, stretching spacetime in one direction and simultaneously compressing it in the opposite direction.
Except that these clever ground-based systems, giant Michelson-Morley interferometers, only provide access to a small fraction of the gravitational waves traveling through the universe. By going to discover them in space, Lisa should make it possible to open the window that these waves now provide us on the universe more widely. “In reality, there is a lot of noise on Earth: seismic, anthropogenic, electromagnetic. Whereas LISA, which also has much longer 'arms', will be able from space to detect gravitational waves of a much lower frequency,” emphasizes Nelson Christensen.
Golden cubes floating freely in space
When the American astrophysicist mentions the “arms” of LISA, it must be emphasized that they have nothing to do with the arms of ground-based interferometers such as Ligo and Virgo. In fact, the LISA mission consists of three satellites orbiting the Sun, forming an equilateral triangle with a side length of 2.5 million kilometers. “This represents 6.5 times the distance between the Earth and the Moon,” explains Antoine Petitto, a researcher at the CEA Institute for Research on Fundamental Laws of the Universe (CEA-Irfu) and co-scientific leader of the LISA project in France.
However, the basic principle of detecting gravitational waves remains the same: using the laser light that these satellites exchange with each other to measure distances and pick up on the large differences that can affect them. “This requires significant and complex work in the field of data processing, for which France bears responsibility,” Antoine Petitto emphasizes.
However, in this system there are no mirrors, and the elements that capture the passage of waves are golden cubes floating in the void, in pairs, well protected, inside each of the three satellites. “Because if there is less noise in space, there is still a significant amount of noise,” explains the scientist, “such as disturbances caused by micrometeorites or particles, especially those coming from the Sun. Our device protects these cubes from any disturbance and allows them to float freely without touching the walls of the space.” The satellite you are located in. »
If two black holes with millions of times the mass of the Sun merged somewhere in the universe, Lisa would see them.Antoine Petitto, astrophysicist at CEA-Irfu, and co-scientific leader of the LISA project for France
But then, what will we be able to observe that is so valuable thanks to this incredibly complex task? “There are a lot of interesting things that we have not been able to realize yet,” says Antoine Petitto. The merger of two supermassive black holes of the type found at the heart of our galaxy, and many more. The stars are much larger than those that can be detected from Earth. “If two black holes with millions of times the mass of the Sun merged somewhere in the universe, Lisa would see them, even if it happened on the other side of the universe!”
As for the low-solar-mass black holes already observed by LIGO and Virgo, LISA will see them too, but very early in the convergence process, the final stage of which we can only capture from Earth. LISA will also capture waves emitted by pairs of white dwarfs or neutron stars. “This will allow us to understand the distribution of these stars in the galaxy.” Scientists also hope to discover small black holes orbiting larger ones. “This would provide us with something to map the gravity around the latter. »
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Finally, researchers also believe they can capture gravitational waves emitted immediately after the Big Bang, long before the first light that could tell us about the early childhood of the universe — called the cosmic microwave background — was emitted into the universe. It is a barrier that cannot yet be overcome by observation, and it condemns science to formulating hypotheses without being able to verify them in any way. “It's the most hypothetical. Some say they exist, others say they don't exist. Regardless, if it's being transmitted, it must be in Lisa's frequency range.”
The mystery of supermassive black holes
All these cosmic events that gravitational waves will soon be able to tell us about may seem very strange, but there is nothing anecdotal about them. On the fundamental physics front, they unveiled the nature of gravity and also the relationship between the fundamental laws of physics: general relativity, which makes it possible to describe infinitely large objects, and quantum mechanics, which governs infinitely small objects.
On the side of astrophysics, Lisa will shed light on the distribution of stars in the Milky Way, and the formation of supermassive black holes. “This is a big mystery: we cannot find a clear model to explain the existence of such supermassive black holes in the universe,” says Antoine Petitto. Moreover, the discovery by the James Webb Telescope of a much larger quantity of black holes than expected could be related to the mysterious dark matter, the composition of which we still do not know yet.” Nelson Christensen adds. Finally, a better understanding of black holes also means the opportunity to better understand the dynamics of galaxy evolution.
As for cosmology, Lisa should tell us about the geometry of the universe, make a completely independent measurement of the Hubble constant corresponding to its expansion rate, and hopefully go and observe what happened right after the big bang. Which would be like touching the Holy Grail. A program so rich that we already regret having to wait until 2035 to witness its launch!
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