Computer simulations reveal similar structures for dark matter halos large and small
Most of the matter in the universe is dark and thus cannot be directly observed. In the findings just published in Nature, an international research team harnessed supercomputers in China and Europe to magnify a model region of the hypothetical universe with a completely unprecedented factor equivalent to that required to identify a flea on the surface of a full moon. This allowed the team to create detailed images of hundreds of hypothetical dark matter halos from largest to very small expected in our world.
This image shows a slide through the main simulation over two billion light years on a side. Both of these interior shapes are successive enlargements of areas of 700,000 and then 600 light-years on one side. The largest individual masses in the main image correspond to galaxy clusters, while the smaller masses on the second magnification are similar to the mass of the Earth.
© MPA
Dark matter plays an important role in cosmic evolution. Galaxies grew as the gas cooled and condensed into the center of massive masses of dark matter, called dark matter halos. The halos themselves separated from the entire expansion of the universe as a result of the gravitational pull of their dark matter. Astronomers can deduce the structure of large dark matter halos from the properties of galaxies and the gases within them, but they have no information about halos that may be too small to contain a galaxy.
The largest dark matter halos in today’s world contain massive galaxy clusters and clusters of hundreds of bright galaxies. Their properties are well studied, and they weigh more than one quadrillion (1015thTimes as much as our sun. On the other hand, the masses of the smallest dark matter halos are unknown. The dark matter theory underlying the new supercomputers’ approximation suggests that it may be comparable in mass to Earth. Such small halos would be very numerous, containing a large portion of all dark matter in the universe, but they would remain dark throughout cosmic history because stars and galaxies only grow in halos at least a million times the mass of the sun.
The research team, based in China, Germany, the United Kingdom and the United States of America, took five years to develop, test and implement cosmic magnification. It enabled them to study the structure of dark matter halos for all masses between those on Earth and those of the LNG. In numbers: the zoom covers a collective range from 10 to the 30th power (i.e. 1 followed by 30 zeros), which is equivalent to the number of kilograms in the sun.
The importance of radiation detection of small auras
Surprisingly, astrophysicists have found that all halos have very similar internal structures: they are very dense at the center, and they become increasingly diffuse outward, with smaller masses spinning in their outer regions. Without a scale, it is nearly impossible to discern an image of the dark matter halo of a massive galaxy from a corona less than one solar mass. “We were really surprised by our results,” says Simon White of the Max Planck Institute for Astrophysics. “Everyone thought that the smallest lumps of dark matter would look very different from the large ones we know most. But when we were finally able to calculate their properties, they looked exactly the same.”
The result has a potential practical application. Dark matter particles can collide near the centers of auras and, according to some theories, perish in a burst of energetic gamma rays. The new zoom simulation allows scientists to calculate the expected amount of radiation for halos of different masses. Much of this radiation can come from dark matter halos that are too small to contain stars. Future gamma-ray observatories may be able to detect this emission, making small objects “visible” individually or collectively. This would confirm the supposed nature of dark matter, which may not be completely dark in the end!
