Tuesday, July 29, 2008

More evidence for the GZK cosmic ray cut-off

Last December we had a rather detailed discussion of ultrahigh-energy cosmic rays (UHECRs). The occasion for this was an important announcement of cosmic ray observations from the Pierre Auger Observatory. Science magazine ranked this result as the third most important "breakthrough" of 2007. (See here.)

The results reported then actually included a variety of important tentative conclusions from the data. Two in particular stood out. One was a statistical analysis that indicated some likelihood that the UHECRs had originated in the nuclei of active galaxies. This conclusion is still controversial, as the statistics involved have been disputed.

A second conclusion seems to be more secure, and has since received additional confirming evidence. This is the conclusion that something known as the GZK cutoff has been verified.

This predicted phenomenon is rather easy to understand at a general level, because it rests on well-known assumptions of special relatively. We know that the universe is suffused with a cosmic microwave background of photons that have an equivalent "temperature" of about 2.725 K. These photons are in the microwave part of the spectrum, which means they have fairly low energy. The energy of these photons is as low as it is because their wavelength has been stretched by a factor of about 1000 since they were last scattered, about 380,000 years after the big bang. This stretching is a result of the expansion of the universe itself.

Now consider a particle moving through this background at a very high velocity – such as a UHECR. According to special relativity, a photon observed from the reference frame of the fast-moving particle will have the same velocity (299,792,458 m/s) regardless of the particle's velocity. However, the wavelength of the photon will appear to be shortened by a very large factor, depending on the particle velocity. This is equivalent to a "blue shift", as if the source of the photon were moving towards the particle at the same velocity.

The net result is that the energy carried by the photon – as perceived by a UHECR – will be extremely high. High enough to destroy the particle (or at least consume a substantial portion of its energy). Hence UHECRs with energies above a certain limit should be observed very infrequently. This limit is called the GZK cutoff. It is about 6×1019 eV.

(In fact, there is some low probability of UHECRs with higher energy being observed, if the UHECR happened to come from a source very close to us, so that it was unlikely to interact with a CMB photon. Credible events attributable to UHECRs having energies as high as 3×1020 eV have been reported.)

It is rather important that the GZK cutoff be verified, since it rests on the assumption that special relativity is valid. If the GZK cutoff were not observed, either our understanding of cosmic rays would be very flawed, or else special relativity itself would be threatened. The latter would require a massive rethinking of contemporary physics – something that wouldn't be attempted without extremely good reason.

Fortunately, evidence for the GZK cutoff continues to grow:

Do cosmic rays get bogged down in the cosmos? (7/8/08)
Physicists are closer to understanding how ultrahigh-energy cosmic rays make their way to Earth thanks to new measurements made at the Pierre Auger Observatory in Argentina. The study shows that the number of such cosmic rays reaching Earth drops off rapidly for rays with energies of more than about 4×1019 eV.

The observations are consistent with a 40-year-old theory that ultrahigh-energy cosmic rays cannot travel very far through the universe without losing energy as they scatter off the cosmic microwave background.

This is not the first confirmation of the GZK cutoff since last November. In March, a similar conclusion was reached based on observations from a completely different cosmic ray detection facility – the University of Utah’s High-Resolution Fly’s Eye cosmic ray observatory. See here, here.

Further reading:

Observation of the suppression of the flux of cosmic rays above 4x10^19eV – technical paper at the arXiv reporting the result discussed above

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