Dark matter gravity cosmology astrophysics
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 images of distant galaxies proves the existence of enormous invisible objects. For years, astronomers have sought to detect dark matter (90% of matter in the universe). Many hypotheses have been proposed to explain the nature of this medium that eludes telescopes: massive objects (brown dwarfs) and elementary particles (neutrinos). But none of these explanations have fully accounted for the phenomenon. Thus, scientists now believe this matter may consist 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 of production are not fully understood—appear in their images (...). Thanks to technological advances, new windows have opened in the infrared, ultraviolet, X-rays, and gamma rays. More recently, astronomers have begun exploring neutrino astronomy—fleeting particles that may significantly contribute to the mass of the universe. ...But theorists know well that, despite this, the bulk of the universe remains beyond the reach of the astronomical community, which cannot be satisfied with the limited observational scope—just 10% of everything—that is available to it. This is why, for many years, they have sought to detect this famous dark matter, the major constituent 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 now opened a window onto this hidden world. Just before them, a British team led by Richard Ellis (Cambridge and Caltech) and an American team led by Tyson (Bell Labs, New Jersey) confirmed parts of these findings.
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 visible itself. "Cosmic astigmatism." "In 1991," explains Yannick Mellier from the Paris Institute of Astrophysics, "the theory predicted that distant objects like galaxies could, due to the presence of large amounts of dark matter along the path of their light, appear slightly distorted and exhibit elongated elliptical shapes. But according to calculations, this cosmic astigmatism effect was so weak that detecting it seemed 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 Ludovic van Waerbeke has created specialized data-processing tools tailored to this research. After five years spent analyzing the approximately 200,000 distant galaxies photographed by the Canada-France-Hawaii Telescope, researchers have finally succeeded. Today, after appropriate processing, hundreds of small turquoise ellipses appear in the images of the deep sky taken by the CFHT—each one representing a distant galaxy. ...Can we therefore conclude that this phenomenon is indeed the result of gravitational lensing affecting light emitted by galaxies? "Absolutely," answers 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, the gravitational effect tends 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 (unlike that of the Sun or galaxy clusters), yet whose effects are still detectable due to its immense spatial 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. On the three-dimensional model reconstructed by the French team on a computer, the effect is striking. As light travels toward us, it constantly changes direction near these filaments, which form a kind of cosmic cheese in 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 gaze, 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 yet-undiscovered particles—weakly interacting massive particles (WIMPs), axions, supersymmetric particles, etc... A new door has just opened, inviting astronomers to step through. They will soon do so with the upcoming deployment, in two years, at the CFHT, of a camera four times larger—the MegaCam—developed by Saclay's CEA. In the more distant future, plans include setting up a network of about a hundred one-meter-diameter telescopes, and launching an American satellite, Snapsat, dedicated to explosive stars (supernovae), but also capable of detecting the effects of dark matter.
Jean-François Augereau