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

Comments (38)

  1. Chris A.

    Don’t cosmic rays’ trajectories get bent around by the galaxy’s magnetic fields, making it hard to determine where they came from originally? Or is that only true of the old, weak ones?

  2. TheWalruss

    It’s so exciting whenever there is another clue that points to dark matter!
    Whatever could it be?

    I’m hoping for licorice!

  3. Chris A.

    Aren’t the nearest stellar black holes about that close? IIRC A0620-00 (AKA V616 Mon) is right in that range.

  4. There are a few black holes that close (including Cygnus X-1) but I don’t think they are capable of producing these CRs. I’m not sure about that, though.

  5. So, should we be concerned about this? Hey, maybe Sitchin was right… it’s Niburu!

  6. 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).

    Since they’re near C, they’d also have higher mass, no?


  7. The Grey Monk

    It’s Vogons.

    This does make me want to learn more about the physics of how subatomic particles are affected by relativistic speeds. Hrm. Your first sentence brought to mind more questions than I know how to phrase.

  8. Davidlpf

    If Sitchin is right I will say George W. was the best leader in human history.

  9. Tom M

    If high velocity particles are slowed down more quickly by magnetic fields, wouldn’t they eventually slow down to the slower particles and therefore be able to travel as far as the slower ones?

  10. Daniel

    Maybe from the sun? I hope they can find the source.

  11. Should we be concerned ?

  12. As always, COOL stuff that makes me even more glad that I read your book!

  13. Daniel Hawking

    “note that light is not a subatomic particle; this is just to give you an idea of the energy”

    Wave/particle duality, photons, particle in a box, E = h(nu), etc.

    Would be less concise, but I’m dead tired of particle physics today. Had enough of it in class.

  14. Daniel, I know about wave/particle duality. I am making a point between particle radiation and electromagnetic radiation. I should have been more clear. But note that I was relating energy with velocity, and there is no such relationship for photons, since they all travel at the same speed.

  15. Photons’ energies in eV are 1240nm/wavelength, so 1eV is the energy in an infrared photon of wavelength 1240nm. Visible light is ~400nm – ~700nm (~1.8eV – ~3.1eV), so yes, still billions of times more energetic. Scary, scary stuff.

    IIRC, the biggest danger of cosmic rays isn’t that they might directly destroy cells or DNA, but that when they collide with heavy metallic nuclei, they create a cascade of high-energy X-rays and gamma-rays that then do the damage.

  16. Daniel Hawking

    Ah. Sorry bout that, then. Too excessively tired to make any real statements at the moment, heh

  17. Anne

    This experiment is actually looking at fairly low-energy cosmic rays – Auger is designed to look at cosmic rays with energies a billion times higher still. This experiment also, importantly, is looking at the electron flux, while most cosmic rays are protons (or larger nuclei).

    As for sources of high-energy electrons within a kiloparsec, most pulsars have strong relativistic electron/positron plasma winds, and there are quite a few nearby pulsars. Their concern, though, is that there’s a fairly narrow range of energies that have an electron surplus, and (for example) we think pulsar wind nebulae should have a high enough cutoff that they’d produce an excess at other energies too. But the high-energy cutoff from PWNe is not that well-understood, and because of the beaming effect there could easily be a nearby pulsar producing this effect.

    Virtually any mysterious high-energy feature – the Galactic positron annihilation distribution, for example – gets listed as “might be dark matter”. Actually detecting dark matter annihilation is going to take an awful lot of careful work ruling out other sources for whatever feature is seen.

  18. Denzel

    Do cosmic rays have any effect on the validity of techniques used to estimate the dates of archaeologic specimens?

  19. Per Hultqvist

    How come higher energy particles is affected more? I thought that because of their high speed/energy it would be harder for any magnetic field (or any other force) to act on them?

  20. Don Snow

    Now, there’s a mystery.
    I love mysteries.

  21. Meh. And again, Dark Matter comes in to the rescue like a Deus Ex Machina. I find the “blame it on Dark Matter” explenations a bit unsatisfying. Wasn’t it why the whole idea o Dark Matter was created in the first place – because they had no other explanation?

  22. Torbjörn Larsson, OM

    there is no such relationship for photons, since they all travel at the same speed

    Looking at this through the fearsome Spectacles of Pedantry™ I note that photons may travel at any speed – as long as they interact with matter. In fact, I seem to vaguely remember that Čerenkov radiation, emitted when particles travel faster than the local speed of light, is observed when your cosmic rays penetrates the Earth atmosphere. (And there are far more interesting oddities on the list of “paths photons are observed to take”.)

    Of course, what you in a nearby post call “the gas between the stars” most people call “a damn good vacuum”, so I’m sure it is a good approximation to claim the above for most photons astronomers care about.

    [Btw, out of curiosity, if anyone knows; AFAIU it takes a very long time for photons resulting from stellar core fusion to reach the surface and be emitted into space. By all accounts it is the constant collisions with, or absorption and reemission from, plasma that accounts for most of that opacity. But what would be the percentage time lost to refraction (as a net refractive index, if that makes sense; never mind about eventual actual refraction through the star material increasing travel length)?]

  23. Torbjörn Larsson, OM

    I thought that because of their high speed/energy it would be harder for any magnetic field (or any other force) to act on them?

    As I understand it, in all cases of energy loss initial higher energy means higher energy loss rate.

    If you look at synchrotron radiation losses from relativistic particles interacting with magnetic fields, the power loss rate scales as the square of the velocity.

    If you look at bremsstrahlung losses from particles interacting with matter such as gas, the power loss rate seems to scale between the 4th and 6th power of the velocity depending on geometry. And I assume that the analogous process from interaction with the ubiquitous CMB photon ‘gas’ scales something similar.

    So the faster you go, the harder you hit. Hmm, maybe that is a modern time analogue to “the taller you stand, the harder you fall”?

  24. Nigel Depledge

    Incidentally, for anyone wondering how galactic magnetic fields can slow down highly energetic particles more than less-energetic particles…

    A charged particle moving through a magnetic field emits a type of radiation called synchrotron radiation (so called because it was first observed in synchrotrons). It is EM radiation, and the energy of the radiation depends on the energy of the charged particle. If the magnetic field is removing energy from a moving particle by this mechanism, and if it removes more energy from faster-moving particles, then it slows fast-moving particles significantly more than slow-moving particles.

    Given that (relatively) low-energy cosmic rays can travel vast distances without being slowed, I would guess that there is a threshhold of particle energy below which synchrotron radiation is not emitted at all.

  25. Per Hultqvist

    Nigel, thanks! I kind of already knew that, but I had stored it way back in my head :-) It’s been too many years since school for me…

  26. @Davidlpf:
    As Colbert often says:
    George W Bush – Great President, or greatest President ever….. ;)

  27. Astronomynut

    How can we determine the probable distance to be around 3000 light years based on the energies observed if they change based on original energy and distance traveled? How can we rule out a higher energy source at a greater distance?

  28. rob

    how does the intergalactic magnetic field sap energy from a cosmic ray? i thought magnetic fields only changed the direction, not the velocity of charged paricles. unless a relativistic change of reference frame changes the pure magnetic field to an electric field and magnetic field, leaving an electric field to do work on the particle?

  29. Gary Ansorge

    Synchrotron radiation occurs when a particle is accelerated, ie,when its vector(direction of travel) is altered. Any charged particle passing thru a magnetic field will have it’s direction of travel(vector) altered, which then causes it to emit energy, resulting in a loss of energy(speed).

    Gary 7

  30. Gary Ansorge

    PS: To be even more precise, a vector is actually composed of two quantities, speed and direction of travel,ie, m/sec and direction of travel. Acceleration can be positive(increase in speed) or negative(decrease in speed)or change of direction(left/right, up/down, etc). X-ray machines use electrons that experience a rapid (negative acceleration) change of speed, as they slam into a tungstun target they radiate x-rays at a right angle to their direction of travel.

    Hey, I’m still working on my first three cups of coffee,,,

    Gary 7

  31. Gary Ansorge

    Krystian Majewski:

    Dark matter was proposed, not as deus ex machina, but as something we can’t detect thru normal electromagnetic radiation interactions,ie, it doesn’t radiate in the em spectrum. It’s obviously there, which we CAN detect thru its gravitational interaction, we just don’t know (yet) what could do that w/o leaving an E.M. trace.

    GAry 7

  32. rob


    aaah, yes. i forgot about synchotron radiation and how it goes as v^4. relativistic charges will radiate much better than lower speed charges, which Phil mentioned above.

  33. Gary Ansorge:

    What I meant with Deus Ex Machina that it seem like an easy thing to do to “invent” some invisible stuff which explains all the troublesome observations we come across: rotation speed of galaxies, gravitational lensing and NOW even high energy cosmic rays? And although that invisible stuff makes up most of the universe, it just happens not to be where we live? Meh. Remember Luminiferous Aether back when they were trying to construct a model of light? Well, it didn’t turn out so great – the modern explanation is QUITE the can of worms.

    Just wanted to vent some skepticism. I’ll shut my mouth as soon as I can buy a plank of dark matter at the DIY superstore. ;-)

  34. Tim

    Mysterious particles coming from everywhere reminds me of Philip Pullman’s Dust.

  35. Brian

    Graphs of the excess electrons vs. energy very closely approximate the pattern predicted for a 620 GEV Kaluza-Klein paerticle. Kaluza-Klein theory was developed in an early attempt to unify gravity and electromagnetism. These attempts preceded the advent of the concept of dark matter, so the postulation of the existence of such particles was in no way related to atempts to support dark-matter theories. Although the discovery of a stable Kaluza-Klein particle would provide a dark matter candidate, the significance of the discovery to the field of particle physics would in many or most ways dwarf the particle’s status as a dark matter candidate.

  36. Brian

    You said, “What I meant with Deus Ex Machina that it seem like an easy thing to do to “invent” some invisible stuff which explains all the troublesome observations we come across: rotation speed of galaxies, gravitational lensing and NOW even high energy cosmic rays?”

    But this is exactly what theorists are supposed to do: conjecture explanations for anomalous data. Other theories (MOG, MOND, TeVeS) have also been proposed. The various candidate explanations are constantly being examined and reexamined as more and better data becomes available. Maybe a descendent of TeVeS or some new theory will eventually prevail. But, since no explanation is ever considered incontrovertible, all conclusions are really just tentative, vulnerable to modification or rejection. Interested people are forever trying to think of other theories that seem to match at least some of the data and to look for flaws in the existing theories. I very much enjoy this approach.

  37. Torbjörn Larsson, OM

    What I meant with Deus Ex Machina that it seem like an easy thing to do to “invent” some invisible stuff which explains all the troublesome observations we come across: rotation speed of galaxies, gravitational lensing and NOW even high energy cosmic rays?

    But more importantly it has IIRC been tested, to many sigmas (wasn’t it like 13 or so?); it’s the only mechanism that explain the Bullet Cluster observations and some others recently. A nice property of mechanisms (and theories) is that you don’t need direct observations of their constituents to test them.


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