(ORDO NEWS) — A team of astronomers studied two nearby globular clusters, 47 Tucanae and Omega Centauri, looking for signals produced by annihilating dark matter.
Although the search was empty, it was not unsuccessful. The lack of detection put strict upper limits on the mass of the hypothetical dark matter particle.
Dark matter makes up about 80% of all mass in the universe, although it is completely invisible. It simply does not interact with electromagnetic forces, so it does not glow, does not reflect, does not absorb, and so on.
So far, the only evidence of its existence is the gravitational effect on the rest of the universe. Because of this, astronomers are not entirely sure what dark matter is, although many physicists believe it is some new kind of particle previously unknown to the standard model of particle physics.
One suggestion includes the possibility that dark matter is composed of some kind of ultralight particle, such as the axion.
And although these particles do not interact with ordinary matter, they can very rarely interact with themselves, colliding with each other and annihilating. If the collision energy is high enough, then the result can be gamma radiation, which then splits into an electron and a positron.
These electrons and positrons can stick together to form bound states called positronium. However, positronium atoms are not stable, and eventually they decay, leaving behind a burst of radio emission.
Thus, even if dark matter does not directly interact with electromagnetism, there is still the possibility that we will see radio emission from the collision and decay of dark matter particles.
For this to work, a large amount of dark matter is needed. If dark matter particles collided easily enough, we would have seen it by now.
So collisions should be rare. The density of dark matter in our galactic terrain is too low for the emission to be detected, but the dense cores of galaxies may offer better access.
A natural place to look is our galactic core, but it’s littered with all sorts of radio emissions, so it’s hard to tell if a particular signal is coming from annihilating dark matter or something more mundane. That is why a group of astronomers turned their attention to two nearby globular clusters.
These two clusters, 47 Tucanae and Omega Centauri, are only a few thousand light-years away, making them relatively easy to observe.
Astronomers believe that they are the remnants of dwarf galaxies, the main part of the stars of which were removed from them as a result of interaction with the Milky Way.
This makes clusters ideal laboratories as they are balls of dense dark matter with very little contamination. A team of astronomers set out to look for a unique radio signal from decaying positronium using the Parkes Observatory in Australia.
Based on their observations, they were able to establish better upper limits on the mass and cross section (a measure of how often particles interact) of these models of light dark matter.
Of course, it would be great to see a confirmed signal and finally do away with this mystery of dark matter, but new knowledge in any direction is always welcome and always useful.
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