February 27, 2024

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Green light for LISA, the first gravitational-wave space telescope

Green light for LISA, the first gravitational-wave space telescope

KU Leuven manages the Belgian activities

The European Space Agency (ESA) today gave the green light to the first space mission dedicated to detecting gravitational waves. With the official approval of the LISA mission by the European Space Agency, preparations for this space telescope are accelerating. After its launch in 2035, LISA is supposed to allow the first observation of gravitational waves from space. Belgian activities surrounding LISA have been managed by KU Leuven since 2016, under the leadership of cosmologist Thomas Hertog.

the Space antenna laser interferometer, or LISA for short, will consist of three separate spacecraft. Together, these spacecraft form a giant triangle, which appears to follow Earth as it orbits the sun. The sides of this triangle are 2.5 million kilometers long, which is 6 times longer than the distance between the Earth and the Moon. This makes the LISA experiment the largest ever. Using a laser, the side lengths of the triangle are closely monitored. The interferometer principle is used here: by allowing the laser beams from these spacecraft to “interference” – that is, interact with each other – the distances between the spacecraft can be precisely measured and monitored.

After all, Einstein's theory of relativity predicts that the lengths of the sides vary slightly as gravitational waves pass through the triangle. “Gravity waves are, to some extent, pure geometry,” says cosmologist Professor Hertog. “There are no particles involved.” They are tiny ripples in the fabric of space, constantly generated by many high-energy phenomena in the universe, from colliding black holes to the supernova explosions of dying stars. Then it spread through the universe at the speed of light. Gravitational waves were first observed directly in 2015 using the LIGO detector in the United States. It turns out that these waves were the result of the collision of two black holes 1.3 billion years ago

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Holy Grail

The idea of ​​detecting gravitational waves from space has been around since the 1970s, and the first drawings of the triangle shape that would become the LISA mission date back to that period. But gravitational waves are very small vibrations, and space tissue is very tough. Only now, 50 years later, do the major technological challenges facing these types of detectors appear to be under control. An important role in this was played by the LISA Pathfinder, a spacecraft that launched in 2015 and served as a test platform for LISA. Due in part to the success of the LISA Pathfinder and the confirmation of the existence of gravitational waves in 2015, the LISA idea was finally launched. The transfer to ESA on January 25 is the final step toward building LISA, which will take the next 10 years.

The very large triangle allows LISA to observe gravitational waves at wavelengths much longer than the waves captured by the LIGO detector or the future Einstein telescope on Earth. This highlights a whole host of new sources, from the merger of supermassive black holes as galaxies collide to the soft noise of gravitational waves that may have arisen shortly after the Big Bang.

This new achievement is eagerly awaited in Belgium. What is the holy grail of this new form of astronomy?

“I hope I will be surprised,” says Professor Hertog. “Eventually, I hope to use LISA to test the limits of Einstein's theory of relativity.”

“Einstein predicted the existence of gravitational waves as early as 1916,” Hertog continues. But we also know that his theory does not apply to black holes and the big bang. We are convinced that gravitational waves coming from those extreme corners of our universe can tell us something about where and how things go wrong in Einstein's theory.

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The Belgian activities surrounding LISA have been managed by KU Leuven since 2016, under the leadership of Hertog. In addition to modeling gravitational wave sources, an activity developed in collaboration with ULB, KU Leuven coordinates the various instrument activities of LISA with Belgian industry. These Belgian LISA activities are funded by the Science Policy PPS (BELSPO) through the ESA Science Program and the ESA PRODEX programme,” says Professor Hertog. Later in 2024, Professor Hertog’s team will begin developing the simulation software that will allow simulation LISA Future Notes.

“This digital twin will provide the more than 1,500 scientists participating in LISA with an ideal tool to prepare their scientific research using LISA data.” This would not be an unnecessary luxury, because the LISA triangle will permanently resonate with a symphony of gravitational waves generated by thousands of sources in the universe. To “read” this result and understand the physics of those sources, we will have to know LISA inside and out. The digital twin we are developing will play a crucial role in this. But the real answers to the big cosmological questions that scientists want to answer with LISA will require some patience. – Professor Thomas Hertog