A long-standing challenge in physics has been finding evidence of dark matter, the material that supposedly constitutes a substantial part of the mbad of the universe. Its existence seems to be responsible for the structure of the universe and the formation and evolution of galaxies. But physicists have yet to observe this mysterious material.
The results reported today by a space science mission led by China provide a tantalizing clue, but not firm evidence, for dark matter. Perhaps most significantly, the first observational data produced by China's first mission dedicated to astrophysics show that the country will become a force in space science, says David Spergel, an astrophysicist at Princeton University. China is now "making significant contributions to astrophysics and space science," he says.
Physicists have inferred the existence of dark matter from its gravitational effect on visible matter. But it has never been observed.
China's dark matter particle explorer (DAMPE) was designed to try to fill that gap, looking for a signal of indirect diminution of a hypothetical dark matter candidate called mbadive weak-interacting particles (WIMP). The researchers launched the spacecraft from the Jiuquan Satellite Launch Center in the Gobi desert, about 1600 kilometers west of Beijing, in December 2015. Its primary instrument – a stack of thin, criss-crossed strips – is set to observe the direction, energy and electrical charge of the particles that make up the cosmic rays, particularly the electrons and positrons, the antimatter counterparts of the electrons. Cosmic rays emanate from conventional astrophysical objects, such as supernova explosions in the galaxy. But if the dark matter consists of WIMP, they would occasionally annihilate each other and create electron-positron pairs, which could be detected as an excess over the expected abundance of particles of conventional objects.
In its first 530 days of scientific observations, DAMPE detected 1.5 million electrons of cosmic rays and positrons above a certain threshold of energy. When researchers plan the number of particles against their energy, they would expect to see a smooth curve. But previous experiments have hinted at an anomalous break in the curve. Now, DAMPE has confirmed that deviation. "It may be evidence of dark matter," but the break in the curve "may be from another source of cosmic rays," says astrophysicist Chang Jin, who leads the collaboration in Purple Mountain Observatory (PMO) of the Chinese Academy of Science ( CAS)) in Nanjing. The results of DAMPE appear online today in Nature .
More data will be needed to confirm what you may see DAMPE. But there is good news on that front. "We expected a 3-year life for the satellite," says Chang. But given the proper functioning of the spacecraft and its instruments, "now we expect it to last 5 years," he says. That will allow the satellite to record more than 10 billion cosmic ray events. Fan Yizhong, a mission astrophysicist also at PMO, adds that the DAMPE observations will complement those of other space-based and ground-based instruments to finally clarify whether there is a connection between the anomalous signals and the annihilation of dark matter.
The DAMPE collaboration comprises four institutes under CAS, including the National Space Science Center in Beijing; also participate the University of Science and Technology of China in Hefei, the University of Geneva and the Italian universities in Bari, Lecce and Perugia. The satellite has been called Wukong, after the character of the Monkey King in the 16th century Chinese novel Journey to the West . DAMPE was also China's first mission dedicated to astronomy and astrophysics, although it was joined in space in June by the Hard-Lightning Modulation Telescope, aimed at observing X-ray and gamma-ray emissions from black holes, neutron stars, active galactic nuclei, and other phenomena.
Even if the DAMPE data does not solve the enigma of dark matter, Spergel says: "These measurements will inform our understanding of the acceleration of cosmic rays [and] will inform us about the physical processes in the collisions around the supernova and the physics of pulsars. "