China’s Hunt for Signals From the Dark Universe




“So far we have collected about 1.8 billion cosmic rays, among them more than 1 million particles are high energy electrons,” Professor Fan Yizhong, a member of the mission team at the Purple Mountain Observatory in Nanjing, under the Chinese Academy of Sciences (CAS), told gbtimes. China’s dark matter-hunting satellite DAMPE celebrated its one year anniversary in space over the weekend, with the team now looking for unexpected results among collected data.

Launched on December 17, 2015, the 1,900kg DArk Matter Particle Explorer (DAMPE) has spent the year measuring the spectra of extremely energetic gamma-rays and cosmic ray with the aim of identifying possible Dark Matter signatures. DAMPE, which is also known as Wukong, after the monkey king in the Chinese fairytale Journey to the West, was carried on a Long March 2D booster, and placed in a 500km-altitude orbit.

Scientists reported on Monday, Dec. 21, 2015 that China’s ground stations received its first data DAMPE. The image above is an artistic rendering imagines the filaments of dark energy that make up parts of the cosmic web. Monstrous galaxies are thought to form at the nexuses of these filaments. (ALMA/ESO/NAOJ/NRAO) A Kashgar station situated in Xinjiang tracked and obtained data from “Wukong,”taking around seven minutes to receive and record the data. It was then transmitted to the National Space Science Center, reported the Chinese Academy of Sciences (CAS) in a statement.

DAMPE boasts of a massive surface area, not only capably observing high cosmic ray volumes but also surveying the sky at high energies. It uses four instruments for capturing the high-energy particles and tracing them back to their origin: a BGO calorimeter, a plastic scintillator detector, a neutron detector and a silicon-tungsten tracker. The particle sources are believed to be dark matter collisions, possibly giving scientists new insight into the dark matter that can potentially help scientists follow a wealth of scientific pursuits, including studying oceanic depths on icy moons and mapping out layers of celestial bodies.

“[It’s] an exciting mission,” said Princeton University’s David Spergel of the DAMPE mission. A recent study in the Astrophysical Journal proposed that the solar system might be growing dark matter “hairs,” speculated to exist and sprout from Earth.

“When gravity interacts with the cold dark matter gas during galaxy formation, all particles within a stream continue traveling at the same velocity,” explained Gary Prézeau of NASA’s Jet Propulsion Laboratory, Pasadena, California, who proposes the existence of long filaments of dark matter, or “hairs.”

Based on many observations of its gravitational pull in action, scientists are certain that dark matter exists, and have measured how much of it there is in the universe to an accuracy of better than one percent. The leading theory is that dark matter is “cold,” meaning it doesn’t move around much, and it is “dark” insofar as it doesn’t produce or interact with light.

Galaxies, which contain stars made of ordinary matter, form because of fluctuations in the density of dark matter. Gravity acts as the glue that holds both the ordinary and dark matter together in galaxies.

According to calculations done in the 1990s and simulations performed in the last decade, dark matter forms “fine-grained streams” of particles that move at the same velocity and orbit galaxies such as ours. A stream can be much larger than the solar system itself, and there are many different streams crisscrossing our galactic neighborhood,” Prézeau said.

Prézeau likens the formation of fine-grained streams of dark matter to mixing chocolate and vanilla ice cream. Swirl a scoop of each together a few times and you get a mixed pattern, but you can still see the individual colors.

“When gravity interacts with the cold dark matter gas during galaxy formation, all particles within a stream continue traveling at the same velocity,” Prézeau said.

But what happens when one of these streams approaches a planet such as Earth? Prézeau used computer simulations to find out. His analysis finds that when a dark matter stream goes through a planet, the stream particles focus into an ultra-dense filament, or “hair,” of dark matter. In fact, there should be many such hairs sprouting from Earth.

A stream of ordinary matter would not go through Earth and out the other side. But from the point of view of dark matter, Earth is no obstacle. According to Prézeau’s simulations, Earth’s gravity would focus and bend the stream of dark matter particles into a narrow, dense hair.

Hairs emerging from planets have both “roots,” the densest concentration of dark matter particles in the hair, and “tips,” where the hair ends. When particles of a dark matter stream pass through Earth’s core, they focus at the “root” of a hair, where the density of the particles is about a billion times more than average. The root of such a hair should be around 600,000 miles (1 million kilometers) away from the surface, or twice as far as the moon. The stream particles that graze Earth’s surface will form the tip of the hair, about twice as far from Earth as the hair’s root.

“If we could pinpoint the location of the root of these hairs, we could potentially send a probe there and get a bonanza of data about dark matter,” Prézeau said.

A stream passing through Jupiter’s core would produce even denser roots: almost 1 trillion times denser than the original stream, according to Prézeau’s simulations.

“Dark matter has eluded all attempts at direct detection for over 30 years. The roots of dark matter hairs would be an attractive place to look, given how dense they are thought to be,” said Charles Lawrence, chief scientist for JPL’s astronomy, physics and technology directorate.

Another fascinating finding from these computer simulations is that the changes in density found inside our planet – from the inner core, to the outer core, to the mantle to the crust – would be reflected in the hairs. The hairs would have “kinks” in them that correspond to the transitions between the different layers of Earth.

Theoretically, if it were possible to obtain this information, scientists could use hairs of cold dark matter to map out the layers of any planetary body, and even infer the depths of oceans on icy moons.

DAMPE is testing the theory that dark matter particles may annihilate or decay and then produce high energy gamma-rays or cosmic rays – in particular electron/positron pairs – and DAMPE, with the widest observation spectrum and highest energy resolution of any dark matter probe in the world, will collect the evidence.

The data analysis of the DAMPE collaboration, which includes institutions from across China and international partners from Italy and Switzerland, has been concentrated on the high energy cosmic rays, in particular the electrons.

“We are looking forward to find something “unexpected” in the cosmic ray and gamma-ray spectra,” Fan says. The team is looking to publish their first results in early 2017.

DAMPE meanwhile will continue to scan in all directions for the second year of its three-year mission, before switching to focus on areas where dark matter may most likely to be observed in the third. The space craft carries four science payloads in total and has the potential to advance the understanding of the origin and propagation mechanism of high energy cosmic rays, as well as new discoveries in high energy gamma astronomy.

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