Astronomers at the University of Tartu’s Tartu Observatory have discovered how to measure the speed of dark matter. If the method proves valid in practice, it would help get a better idea of what 85% of the matter in the universe, which so far has only signaled its existence through gravity, is like.
So how does measuring the speed of dark matter work? The premise is to find a galaxy in the Universe that moves relative to dark matter. Since everything in the Universe is in motion and there is a lot of dark matter, it is not difficult to find such galaxies.
Heavy objects, like galaxies, attract all types of matter, whether it’s dark matter or the visible matter we encounter every day. As dark matter moves past the galaxy, the galaxy also begins to attract dark matter particles towards itself. However, it takes time for the particles to change direction. Before their trajectory curves towards the galaxy, they have already passed the galaxy.
Therefore, dark matter particles do not enter the galaxy, but instead move behind the galaxy, (Watch the video). Behind the galaxy, therefore, the density of matter increases and this leads to the slowing down of the galaxy, a phenomenon called dynamical friction. The strength of the dynamic friction, in turn, depends on the speed with which the dark matter particles pass through the galaxy, that is, how long the galaxy has time to change the trajectory of the dark matter particles. If the particles pass slowly, the density of matter builds up closer to the galaxy and slows it down further.
The green dot represents the galaxy and the upper panels show the movement of dark matter particles through the galaxy (if the galaxy is present in the corresponding panel). The lower panels show the shape of all trajectories, from which it can be concluded that the gravitational field of galaxies influences the overdensity behind the galaxy by creating matter particles. The overdensity again slows down the galaxy and distorts its shape.
Suppose for a moment that the galaxy causing the dynamic friction is not small, but massive. In this case the overdensity behind it creates a friction of different intensity in different points of the galaxy, as can be seen in Figure 1. The difference in friction makes the shape of the galaxy more wavy. On Earth we experience a shape-shifting similar to that of the tides: the ebb and flow caused by the Moon’s gravity.
The panels depict sparse, light-dense areas of the Universe in dark colors. The top panels show the density around the galaxy when the galaxy’s gravity bends (left) or does not bend (right) the trajectories of dark matter particles. The bottom panel shows their difference, i.e. what the galaxy’s contribution to the distribution of dark matter is. The arrows represent the acceleration caused by the overdensity behind the galaxy, which compensates for the friction on the galactic center. Since the arrows have different directions and intensities in different regions, the northwestern forces are able to change the shape of the galaxy.
Figure 1. Author/source: Tartu Observatory
It doesn’t matter how large the dark matter particles eventually get: their orbit still bends behind the galaxy. The method may not provide accurate results if the particles were comparable in size to the galaxies themselves. However, these dark matter models have already been ruled out.
Faint galaxies are not difficult to find, as they make up about 30% of all galaxies in space. Of course, a lot depends on how far you look into the outer parts of the galaxy and what level of lopergus you consider a lopergus galaxy to be.
Furthermore, dynamical friction may not be the only reason behind the shape of the galactic halo. There are other reasons. For example, galaxies formed by the collision of multiple galaxies can be asymmetric. In this case, we might notice the core of another galaxy or a larger stellar halo somewhere within the galaxy. The asymmetry of the galaxy can also be caused by the constant influx of gas. In such situations, the shape of the galaxy recovers within a few billion years.
Therefore, to measure the speed of dark matter, we need a dwarf galaxy, as isolated as possible from other galaxies. In this case it is more certain that nothing else happened to it other than the passage of dark matter.
With current work, we have discovered how to precisely explain the forces acting on galaxies in the Northwest. The next step will be to find galaxies in the Universe that are faint enough to study the speed of dark matter relative to galaxies.
Cosmology is an important test bed for theoretical physics. Calculating the speed of dark matter may be important for testing new models of dark matter and lifting the veil of mystery about the nature of dark matter.
In the long term, measuring the speed of dark matter could also tell us something about the mass of dark matter particles. Initially, it rather contributes to the study of the possibility of different interactions of dark matter. However, with newer methods, it is never entirely certain what exactly will be done with them.
The research was published in a scientific article in the journal Astronomy & Astrophysics.
2024-01-09 10:04:00
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