“Have you seen your body in X-rays? It looks completely different,” says astrophysicist Rashid Sunyaev. “We will do the same with the Universe.” Sunyaev, an eminent Soviet-born cosmologist at the Max Planck Institute for Astrophysics about a joint German–Russian mission called Spectrum-Roentgen-Gamma (SRG) will launch into space to create a 3D x-ray map of the cosmos that will reveal how the Universe accelerates under the mysterious repulsive force called dark energy and detect up to three million supermassive black holes.
“Within a half year, we will cover the whole sky,” says Peter Predehl, an X-ray astronomer at the Max Planck Institute for Extraterrestrial Physics, and a principal investigator for the mission.
“Cosmologists,” reports Davide Castelvecchi for Nature, “can probe dark energy through galactic clusters, whose distribution encodes the structure and history of the Universe. SRG will map a cosmic web of about 100,000 galactic clusters by detecting the X-ray glow from their intergalactic plasma and from the plasma filaments that join them. The mission will also detect supermassive black holes — many of which will be new to science — and X-rays from as many as 700,000 stars in the Milky Way.”
“It’s going to be a great survey,” says X-ray astronomer Pepi Fabbiano of the Harvard–Smithsonian Center for Astrophysics in Cambridge, Massachusetts. Its data will have a unique role in the field for a long time, she adds.
The spacecraft will lift off on a Russian-built Proton-M rocket from the Baikonur Cosmodrome in Kazakhstan. X-ray sky surveys have been conducted by previous missions, including one from Germany in the 1990s, called ROSAT. But that mission was sensitive only to ‘soft’ X-rays, with energies of about 2 kiloelectronvolts (keV). Existing missions, such as NASA’s Chandra X-ray Observatory and NuSTAR, can see higher-energy radiation and resolve tiny details of cosmic structures, but they see only small parts of the sky.
According to Nature, SRG will map the entire sky eight times, and researchers will compare the maps and look for changes: “for instance, some of the supermassive black holes at galactic centers become extremely bright when they devour matter at a high rate, and then go back to relative quiescence. Although most soft X-rays from these black holes are likely to be absorbed by surrounding dust, harder X-rays should get through, says G.V.Pavlinsky. ART-XC might see the objects appearing and then disappearing again from one year to the next, providing information about how black holes consume matter.
“We wish to observe several thousand of these events during these four years,” Sunyaev says.
SRG will also investigate the Universe’s distribution of ordinary matter and dark matter — the main engine of galaxy formation — and look for direct hints as to the nature of dark-matter particles by trying to confirm previous signals that showed peaks in X-ray emissions from some galactic centers, which some researchers have suggested come from the decay of an unknown, heavier relative of the known subatomic particles called neutrinos that could be a major component of dark matter— although this interpretation is controversial.
“So far, the dark-matter explanation is still on the table” as a potential cause of the X-ray signal, says Esra Bulbul, an astrophysicist at the Max Planck Institute for Extraterrestrial Physics and a lead scientist on the mission.
The Daily Galaxy, Max Goldberg, via Nature