Something powerful lurks nearby

By Phil Plait | November 19, 2008 4:21 pm

Cosmic rays are subatomic particles streaming through space at almost the speed of light. They are actually different kinds of particles, including protons and helium nuclei (two protons and two neutrons bound together). Their exact speed determines how much energy they have; a faster particle is said to have higher energy (or conversely a higher energy particle is moving more quickly).

Many cosmic ray sources have been identified. Most appear to get their start in the expanding debris of a violent supernova explosion. Shock waves rip to and fro in the material, and particles trapped in the gas can be accelerated to phenomenal velocities.

But there’s a problem: the higher the energy of the cosmic ray, the more its travel through the galaxy wears it down. A relatively slow-moving cosmic ray has no difficulty traveling millions of light years (coming, for example, from supermassive black holes in the centers of other galaxies), but the faster they move, the more they are at the mercy of forces like the intergalactic magnetic field. Extremely high-energy cosmic rays can’t travel very far before having their energy sapped away.

However, a new study using the balloon-borne instrument called the Advanced Thin Ionization Calorimeter (ATIC) shows that there is an excess of particles coming in with energies of 300-800 billion electron Volts. To give you an idea of the energy involved, a photon of visible light has an energy of 1 eV. So these puppies are screaming in with billions of times the energy of light we can see (note that light is not a subatomic particle; this is just to give you an idea of the energy). In fact this is thousands of times the energy of even X-rays.

Cosmic rays at this energy should slow down so much that the source of these particles can’t be more than 3000 or so light years away. That’s pretty close, on a galactic scale (the Milky Way is 100,000 light years across). Whatever the power source for these particles is — a pulsar, a black hole, or something more exotic — it’s practically in our back yard.

Anything that close capable of producing such prodigiously propelled particles should, I would think, be relatively easy to find. I have not heard of anything that close, however. The scientists who conducted the study therefore have an alternative idea: dark matter. One possible candidate for this mysterious matter that fills the Universe is a type of particle that, if it collides with another dark matter particle, can produce cosmic rays in this energy range. That’s still speculative, but it’s awfully interesting. Since dark matter permeates space, the cosmic rays could be coming from pretty close by; even inside the solar system!

That’s pretty weird to think about.

It’s too early to speculate much about them. ATIC only detected the particles, but is not sensitive to direction. If a detector were used that could see where these cosmic rays were coming from, that would give a big clue to their origin. If they all come from one spot in space, for example, then we know it’s probably a black hole or pulsar. But if they come from everywhere, well, wouldn’t that be interesting?

CATEGORIZED UNDER: Astronomy, Cool stuff, Science

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