Reproduction of the article from Le Monde, Friday, March 17, 2000
If dark matter bends the path of light, then it must exist
The distortion of distant galaxy images proves the existence of vast invisible objects. For years, astronomers have been trying to detect dark matter (90% of the matter in the universe). Many hypotheses have been proposed to explain the nature of this elusive substance that evades telescopes: massive objects (brown dwarfs) and elementary particles (neutrinos). But none of these explanations have been fully convincing. Therefore, scientists now believe this matter might be composed of theoretical particles yet to be discovered. Astronomers are certain: 90% of the universe's matter remains invisible to their telescopes. Only galaxies and the billions of stars that compose them, dark or bright nebulae scattered across the sky, and gigantic bursts of energy whose mechanisms are not fully understood appear in the images (...). Thanks to technological advances, new windows have opened in the infrared, ultraviolet, X-ray, and gamma-ray ranges. More recently, astronomers have begun exploring neutrino astronomy—fleeting particles that could significantly contribute to the mass of the universe. ...But theorists know well that, despite these advances, the bulk of the universe still eludes the astronomical community, which cannot be satisfied with the limited 10% of the total universe accessible to observation. This is why they have been searching for years to detect this famous dark matter, a major component of our universe. A team from the Paris Institute of Astrophysics, joined by French astronomers (CEA Saclay, Canada-France-Hawaii Telescope (CFHT), and Marseille Space Astronomy Laboratory) and international colleagues (Canada, Germany, United States), has just opened a new window onto this world. Shortly before, a British team led by Richard Ellis (Cambridge and Caltech) and an American team led by Tyson (Bell Labs, New Jersey) confirmed these results, at least in part.
How did researchers overcome the invisible and confirm the existence of dark matter? By relying on a principle stating that light bends near enormous masses of matter (the Sun, galaxy clusters) due to gravity. This hypothesis has been repeatedly verified. But astronomers wondered whether the same effect could be observed with dark matter, which is thought to be very diffuse yet present in vast quantities. If so, this dark matter would betray its presence without being directly visible. "Cosmic astigmatism." "In 1991," explains Yannick Mellier of the Paris Institute of Astrophysics, "theory predicted that distant objects like galaxies could appear slightly distorted and exhibit elongated elliptical shapes due to the presence of large amounts of dark matter along the path of their light. However, according to calculations, this cosmic astigmatism effect was so weak that detecting it seemed nearly impossible." Moreover, researchers at the time lacked a theoretical model to validate potential measurements, as well as cameras powerful enough to carry out such observations. Since then, the CFH12K camera has been developed, and Canadian researcher Ludovic van Waerbeke has created specialized data-processing tools for this research. After five years of analyzing the approximately 200,000 distant galaxies photographed by the Canada-France-Hawaii Telescope, researchers have finally succeeded. Today, after applying appropriate processing to deep-sky images taken by the CFHT, hundreds of small turquoise ellipses appear—each representing a galaxy. ...Can we therefore conclude that this phenomenon is indeed due to gravitational lensing of light emitted by galaxies? "Absolutely," responds Yannick Mellier. "In the absence of matter along the path of light—thus without gravitational influence—elliptical galaxies still appear as small, round dots. In contrast, when gravitational effects are present, the image becomes covered with tiny ellipses. Furthermore, gravitational effects tend to organize these galaxies, much like a magnet aligns iron filings along magnetic field lines." These imperceptible distortions and the reorganization of galaxies allow scientists to conclude that light has been deflected from its original path by diffuse, invisible filaments of matter. A matter whose density is low (in contrast to that of the Sun or galaxy clusters), yet whose effects are perceptible due to its immense extent: between 100 million and 1 billion parsecs (1 parsec equals 3.36 light-years). For comparison, our galaxy measures only 34,000 parsecs in its greatest diameter. ...The three-dimensional model reconstructed by the French team on a computer reveals a striking effect. As light travels toward us, it constantly changes direction near these filaments, which form a kind of cosmic "Swiss cheese" structure within the space they occupy. This structure tells the story of the universe and reveals the initial conditions of its formation. For dark matter, which escapes our sight, is not of the same nature (baryonic) as the matter from which stars and we ourselves are made. According to theorists, it is composed of unknown particles—weakly interacting massive particles (WIMPs), axions, supersymmetric particles, etc.—still awaiting discovery. ...A new door has just opened, inviting astronomers to step through. They will soon do so with the upcoming deployment, in two years, of a camera four times larger than the current one at the CFHT—the MegaCam, developed by Saclay's CEA. In the more distant future, a network of about a hundred one-meter-diameter telescopes is envisioned, along with the launch of an American satellite, Snapsat, dedicated to explosive stars (supernovae), but also capable of tracking the effects of dark matter.
Jean-François Augereau
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