Have ‘Dark Attractors’ Shaped the Universe?

6a00d8341bf7f753ef0120a6eb04c5970b.jpg The science community doesn't know what invisible matter, dark matter is, but they know that it (whatever "it" is) is there and that without dark matter there would be no galaxies, and hence stars, planets and life as we know it. The universe is filled with large structures that are dominated by dark matter and in these dark matter spheres or 'halos' the light emitting particles form stars and gas clumps. Recent studies show that these dark matter halos have so-called attractors, which maintain their shape.

"It is somewhat surprising that we do not understand why these dark matter halos look like they do. The dark matter particles only feel the pull of gravity so it should be simple to find out, for example, how their density changes when you move from the inner to the outer part. But no one has yet understood why the temperature and density of the dark matter has the form we observed", explains astrophysicist Steen Hansen, Dark Cosmology Centre at the Niels Bohr Institute at the University of Copenhagen.

It's not known whether there is a fundamental reason for the simple structure of the temperature and density of the halos, which are observed or whether their form is determined by the collisions or random, dramatic events in the history of the universe.

As a star evolves its surface temperature and luminosity can be measured. Nearly all stars that are in the process of burning off their hydrogen fall into a main sequence with uniform states. This uniformity is an attractor for ordinary luminous particles because there is always a unique balance between radiation pressure and gravity as the hydrogen atoms fuse together to form helium.

Dark matter particles have no radiation pressure, so it was previously entirely unknown whether they could have an attractor that would explain why all of the dark matter halos look virtually the same.

"We have for the first time, through computer simulations, shown that dark matter halos have an attractor. We have found a very special relationship between the state of the temperature and the density of the dark matter from the inner part of the halo to the outer part," explains Steen Hansen.

The researchers created computer models of a wide range of different dark matter halos, all of which were in perfect balance. Then they 'knocked on' these structures so that they changed. They did so by moving the energy in them around a little bit, which you can do to stars in a computer model. But it turned out that all of the halos changed their state towards the same uniform shape. In this way dark matter halos are completely distinct from stars.

"When a star has exhausted its hydrogen, it moves away from the main sequence – it leaves its attractor. Since we have no indications that dark matter emits radiation or collide with each other, we believe that these halos will remain on their attractor forever", concludes Steen Hansen.

Casey Kazan via University of Copenhagen

Image top of page: A very high-resolution simulation of the formation of a  dark matter halo in its cosmological context. This simulation has a spatial resolution of a few hundred light years and the mass resolution of a few thousand of solar masses (the size of a small star cluster). It took 100,000 CPU hours on the ARSC Power4 to complete and will allow us to measure the density profile of a galaxy sized dark matter halo with unprecedented detail down to its very center.

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