Most matter in the universe is dark and completely different in nature from the matter that makes up stars, planets and people. Galaxies form and grow when gas cools and condenses at the center of enormous clumps of this dark matter, forming so-called dark matter haloes.

An international research team led by Professor Wang Jie from the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC) used supercomputers in China and Europe to zoom in on a typical region of a virtual universe. It was like zooming in on an image of the Moon to see a flea on its surface.

An artist’s impression of dark matter haloes with various mass in the universe [IMAGE: YU JINGCHUAN, BEIJING PLANETARIUM]

The study was published in Nature on September 2.

The biggest dark matter haloes in today's universe contain huge galaxy clusters, collections of hundreds of bright galaxies. The properties of such clusters, which weigh over a quadrillion (a million billion) times as much as our Sun, are well studied.

On the other hand, the masses of the smallest dark matter haloes are unknown. They are hypothesized to be about the mass of the Earth, according to current theories.

Such small haloes must be extremely numerous, containing a substantial fraction of all the dark matter in the universe. However, they have remained dark throughout cosmic history because stars and galaxies grow only in haloes more than a million times as massive as the Sun.

"These small haloes can only be studied by simulating the evolution of the universe in a large supercomputer," said Professor Wang.

Simulations of formation of dark matter haloes ranging in size from Earth mass to clusters of galaxies find a universal halo density structure spanning 20 orders of magnitude in mass. [IMAGE: SOWNAK BOSE, CENTER FOR ASTROPHYSICS, HARVARD UNIVERSITY]

The research team, based at the NAOC in China, Durham University in the UK, the Max Planck Institute for Astrophysics in Germany, and the Center for Astrophysics in the USA, spent five years developing, testing and carrying out their cosmic zoom.

It enabled them to study the structure of dark matter haloes of all masses between that of the Earth and that of a big galaxy cluster. In numbers, the zoom covers a mass range of 10 to 10 to the power 30 (that is one followed by 30 zeroes), which is equivalent to the number of kilograms in the Sun.

By zooming in on the virtual universe in such microscopic detail, the researchers were able to study the structure of dark matter haloes ranging in mass from that of the Earth to a big galaxy cluster.

"Surprisingly, we find that haloes of all sizes have a very similar internal structure, i.e., they are extremely dense at the center, become increasingly spread out, and have smaller clumps orbiting in their outer regions," said Professor Wang. "Without a measure scale it was almost impossible to tell an image of a dark matter halo of a massive galaxy from one with a mass a fraction of that of the Sun."

Particles of dark matter can collide near the centers of haloes, and may, according to some theories, annihilate in a burst of energetic (gamma) radiation.

Co-author Professor Carlos Frenk from Durham University said: "By zooming in on these relatively tiny dark matter haloes we can calculate the amount of radiation expected to come from different sized haloes."

Most of this radiation would be emitted by dark matter haloes too small to contain stars, but future gamma-ray observatories might be able to detect these emissions, making these small objects individually or collectively "visible".

"This would confirm the hypothesized nature of the dark matter, which may not be entirely dark after all," said co-author Simon White from the Max Planck Institute of Astrophysics. "Our research sheds light on these small haloes as we seek to learn more about what dark matter is and the role it plays in the evolution of the universe."

The simulations were carried out in the Cosmology Machine supercomputers in Guangzhou, China, Durham, England of the UK, and Munich, Germany.

This paper can be accessed at https://www.nature.com/articles/s41586-020-2642-9

Source: Chinese Academy of Sciences