Oh-My-God Particles

Jun 13, 2011 By Steve Nerlich
Left image: The energy spectrum of cosmic rays approaching Earth. Cosmic rays with low energies come in large numbers from solar flares (yellow range). Less common, but higher energy cosmic rays originating from elsewhere in the galaxy are in the blue range. The least common but most energetic extragalactic cosmic rays are in the purple range. Right image: The output of the active galactic nucleus of Centaurus A dominates the sky in radio light - this is its apparent size relative to the full Moon. It is likely that nearly all extragalactic cosmic rays that reach Earth originate from Centaurus A.

Cosmic rays are really sub-atomic particles, being mainly protons (hydrogen nuclei) and occasionally helium or heavier atomic nuclei and very occasionally electrons. Cosmic ray particles are very energetic as a result of them having a substantial velocity and hence a substantial momentum.

The Oh-My-God particle detected over Utah in 1991 was probably a proton traveling at 0.999 (and add another 20 x 9s after that) of the speed of light and it allegedly carried the same as a baseball traveling at 90 kilometers an hour.

Its kinetic energy was estimated at 3 x 1020 (eV) and it would have had the collision energy of 7.5 x 1014 eV when it hit an atmospheric particle – since it can’t give up all its kinetic energy in the collision. Fast moving debris carries some of it away and there’s some heat loss too. In any case, this is still about 50 times the collision energy we expect the Large Hadron Collider (LHC) will be able to generate at full power. So, this gives you a sound basis to scoff at doomsayers who are still convinced that the LHC will destroy the Earth.

Now, most are low energy, up to 1010 eV – and arise locally from solar flares. Another more energetic class, up to 1015 eV, are thought originate from elsewhere in the galaxy. It’s difficult to determine their exact source as the magnetic fields of the galaxy and the solar system alter their trajectories so that they end up having a uniform distribution in the sky – as though they come from everywhere.

But in reality, these galactic probably come from supernovae – quite possibly in a delayed release process as particles bounce back and forth in the persisting magnetic field of a supernova remnant, before being catapulted out into the wider galaxy.

And then there are extragalactic cosmic rays, which are of the Oh-My-God variety, with energy levels exceeding 1015 eV, even rarely exceeding 1020 eV – which are more formally titled ultra-high-energy cosmic rays. These particles travel very close to the and must have had a heck of kick to attain such speeds.

However, a perhaps over-exaggerated aura of mystery has traditionally surrounded the origin of extragalactic cosmic rays – as exemplified in the Oh-My-God title.

In reality, there are limits to just how far away an ultra-high-energy particle can originate from – since, if they don’t collide with anything else, they will eventually come up against the Greisen–Zatsepin–Kuzmin (GZK) limit. This represents the likelihood of a fast moving particle eventually colliding with a cosmic microwave background photon, losing momentum energy and velocity in the process. It works out that extragalactic cosmic rays retaining energies of over 1019 eV cannot have originated from a source further than 163 million light years from Earth – a distance known as the GZK horizon.

Recent observations by the Pierre Auger Observatory have found a strong correlation between extragalactic cosmic rays patterns and the distribution of nearby galaxies with active galactic nuclei. Biermann and Souza have now come up with an evidence-based model for the origin of galactic and extragalactic cosmic rays – which has a number of testable predictions.

They propose that extragalactic cosmic rays are spun up in supermassive black hole accretion disks, which are the basis of active galactic nuclei. Furthermore, they estimate that nearly all extragalactic cosmic rays that reach Earth come from Centaurus A. So, no huge mystery – indeed a rich area for further research. Particles from an active supermassive black hole accretion disk in another galaxy are being delivered to our doorstep.

Explore further: Raven soars through first light and second run

More information: On a common origin of galactic and extragalactic cosmic ray, arxiv.org/PS_cache/arxiv/pdf/1106/1106.0625v1.pdf

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jibbles
not rated yet Jun 13, 2011
i don't get it. if there is no privileged frame of reference in the universe, then why should the c.m.b. tend to slow down an o.m.g. particle? or else shouldn't we detect a blue shift in the c.m.b. in the part of the sky corresponding to our direction of travel?
Graeme
not rated yet Jun 13, 2011
The CMB sets up a reference frame. From the point of view of a VHECR the CMB is blue shifted into gamma ray energies in the forward travel direction.
ubavontuba
2.6 / 5 (5) Jun 14, 2011
So, this gives you a sound basis to scoff at doomsayers who are still convinced that the LHC will destroy the Earth.
Seriously, who writes this crap? Obviously someone who never bothered to read the 2008 CERN safety analysis wherein CERN admitted that head to head LHC collisions and cosmic ray bombardment are fundamentally different types of collisions, relative to the earth!

jibbles
not rated yet Jun 14, 2011
The CMB sets up a reference frame. From the point of view of a VHECR the CMB is blue shifted into gamma ray energies in the forward travel direction.


ok so what direction (constellation, say) is our local group moving w.r.t. to the c.m.b.?
DavidMcC
not rated yet Jun 14, 2011
The CMB sets up a reference frame. From the point of view of a VHECR the CMB is blue shifted into gamma ray energies in the forward travel direction.


ok so what direction (constellation, say) is our local group moving w.r.t. to the c.m.b.?

It isn't, at least not at relativistic speed. If it was, c.m.b. wouldn't have such a uniform temperature over the whole sky.
jibbles
not rated yet Jun 14, 2011
The CMB sets up a reference frame. From the point of view of a VHECR the CMB is blue shifted into gamma ray energies in the forward travel direction.


ok so what direction (constellation, say) is our local group moving w.r.t. to the c.m.b.?

It isn't, at least not at relativistic speed. If it was, c.m.b. wouldn't have such a uniform temperature over the whole sky.


so then you're saying all those other galaxy clusters having significant red-shifts (i.e. most of them) should perceive a non-uniform c.m.b? sounds a bit pre-copernican to me. am i missing something?
yyz
5 / 5 (5) Jun 14, 2011
"ok so what direction (constellation, say) is our local group moving w.r.t. to the c.m.b.?"

Measurement of the CMB dipole anisotropy by COBE indicates the Local Group is moving ~600 km/s wrt the CMB rest frame, in the general direction RA=11hr 06m Dec= -27deg 20' (in central Hydra):

http://arxiv.org/...312056v1

http://en.wikiped...isotropy
vidar_lund
5 / 5 (4) Jun 15, 2011
i don't get it. if there is no privileged frame of reference in the universe, then why should the c.m.b. tend to slow down an o.m.g. particle? or else shouldn't we detect a blue shift in the c.m.b. in the part of the sky corresponding to our direction of travel?

Jibbles, the special theory of relativity does not operate with a privileged frame of reference and this causes the twin paradox. In general relativity Einstein solved this problem by stating that there is a privileged rest frame which is space itself and the matter and energy contained within this frame. Think of it as an enormous web of energy and particles. The OMG particles are moving through this web at tremendous speeds and colliding with CMB photons in violent collisions. If you are moving at a leisurely pace through your local part of space any collisions with CMB photons are very modest events. And yes, objects moving fast through their local region of space will see a blue/red shifted CMB.
Pyle
4 / 5 (4) Jun 15, 2011
Seriously, who writes this crap?
Nerlich is awesome. Shame on you Uba. I do a hunt for his articles at least weekly to make sure I don't miss any.
Obviously someone who never bothered to read the 2008 CERN safety analysis wherein CERN admitted that head to head LHC collisions and cosmic ray bombardment are fundamentally different types of collisions, relative to the earth!
No denying. I think his point was simply that these OMG particles have a much higher energy and the events happen frequently on a cosmological time scale and we're here and we aren't surrounded by BHs as far as we know. (I read into that a bit, but I think that was close to his point.)
jibbles
not rated yet Jun 16, 2011
"ok so what direction (constellation, say) is our local group moving w.r.t. to the c.m.b.?"

Measurement of the CMB dipole anisotropy by COBE indicates the Local Group is moving ~600 km/s wrt the CMB rest frame, in the general direction RA=11hr 06m Dec= -27deg 20' (in central Hydra):

http://arxiv.org/...312056v1


cool. this helped.
ubavontuba
1 / 5 (1) Jun 17, 2011
Nerlich is awesome. Shame on you Uba. I do a hunt for his articles at least weekly to make sure I don't miss any.
Sure, he's usually respectable enough. But bad editorializing is bad science. In this case, the science is readily available. He's better than this.

No denying. I think his point was simply that these OMG particles have a much higher energy and the events happen frequently on a cosmological time scale and we're here and we aren't surrounded by BHs as far as we know. (I read into that a bit, but I think that was close to his point.)
We don't know that. Micro black holes are still a viable candidate for dark matter WIMPs.