Tag Archives: dark matter

Positrons from normal, not dark, matter

Gamma-rays from electron-positron annihilations in center of Milky Way
Gamma-rays from electron-positron annihilations in center of Milky Way

The ESA space observatory Integral had observed gamma-rays from the center of the galaxy, which indicated the presence of positrons distributed in a way that couldn't quite be explained with known phenomenon. Hence some physicists speculated that the positrons may have been the result of dark matter annihilation. Not only did the distribution of positrons within our galaxy turn out to be lopsided, arguing against dark matter annihilation as the source, but it has now been explained how supernovae could be responsible for the distribution of positrons.

Some had thought that supernovae could not be the source of most of the positrons because it was assumed that they would all annihilate very close to their origin, which would not match the observed distribution of positrons. But it turns out that the positrons from supernovae, which are the result of the decay of heavy elements from the stellar explosion, travel nearly at the speed of light and can travel for thousands of light-years before slowing down and annihilating with an electron. By considering how electrons move in galactic magnetic fields, they were able to model how positrons would travel before being annihilated, and the results seem to be consistent with the Integral observations.

This deals a blow to the hypothesis that dark matter annihilation may be responsible for the positron distribution. I wonder if the same implication can be inferred for the PAMELA observations?

Supermassive black holes from dark gulping

I had thought that dark matter should be responsible for only a very small portion of the mass of supermassive black holes in the centers of galaxies, for the same reason that dark matter is distributed much more evenly than ordinary matter. But it turns out I could be very wrong about this.

A young but heavy galaxy
A young but heavy galaxy in the background

Researchers from the University College London modeled the gravitational interactions between dark matter halos and gas embedded in the dark matter. They found out that depending on the thermal properties of dark matter, a small disturbance could cause a rapid gravitational collapse of dark matter into a black hole. Appropriately enough, this rapid collapse of dark matter is called "dark gulping".

Dark matter being what it is, there would hardly be any electromagnetic radiation from the collapse. This would avoid the blasting away of matter by a normal accretion disk which slows down the growth of a black hole, not that dark matter would be affected by radiation in the first place. It would also explain how supermassive black holes could have existed when the universe was less than a billion years old.

If it turns out that dark gulping is indeed responsible for the formation of supermassive black holes, it could provide an interesting look into the properties of dark matter. Because dark gulping is determined by the thermal properties of dark matter, which depends on the degrees of freedom of each dark matter particle, i.e. the number of ways that a dark matter particle could move, rotate, etc., this could give hints to the microscopic interactions of dark matter or even the number of extra dimensions our universe could have.

Why dark matter is more diffuse than ordinary matter

It appears that galaxies form around clumps of dark matter, so the large scale structure of the universe is determined by the distribution of dark matter, and ordinary matter is attracted to these clumps of dark matter to form the galaxies we see today. Given that it is dark matter that clumps together first, and only then is ordinary matter attracted to these clumps to form galaxies, why is it ordinary matter that ended up being clumped in much more dense objects such as stars or galaxies? It's been a question that has been plaguing me for a while, so I asked about it on Astronomy Cast.

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