Post a Comment On: Backreaction

As the year 2014 is nearing its end, lists with the most read stories are making the rounds. Everything in there, from dinosaurs over miracle cures, disease scares, Schadenfreude, suicide, the relic gravitational wave signal that wasn't, space-traffic accidents, all the way to a comet landing.

For the high energy physicists, this was another year of non-news though, not counting the one or the other baryon that I have a hard time getting excited about. No susy, no dark matter detection, no quantum gravity, no beyond the standard whatsoever. My non-news of the year that probably passed you by is that the origin of highly energetic cosmic rays descended back into mystery. If you recall, in 2007, the Pierre Auger Collaboration announced that they had found a correlation between the directions from which they saw the highly energetic particles coming and the positions of galaxies with supermassive black holes, more generally referred to as active galactic nuclei. (Yes, I've been writing this blog for that long!) This correlation came with some fineprint because highly energetic particles will eventually, after sufficiently long travel, scatter at one of the very dispersed photons of the cosmic microwave background. So you would not expect a correlation with these active galactic nuclei beyond a certain distance, and that seemed to be exactly what they saw. They didn't at this point have a lot of data so that the statistical significance wasn't very high. However, many people thought this correlation would become stronger with more data, and the collaboration probably thought so too, otherwise they wouldn't have published it.

But it didn't turn out this way. The correlation didn't become stronger. Instead by now it's pretty much entirely gone. In October, Katia Moskvitch at Nature News summed it up:

"Working with three-and-a-half years of data gleaned from 27 rays, Auger researchers reported that the rays seemed to preferentially come from points in the sky occupied by supermassive black holes in nearby galaxies. The implication was that the particles were being accelerated to their ultra-high energies by some mechanism associated with the giant black holes. The announcement generated a media frenzy, with reporters claiming that the mystery of the origin of cosmic rays had been solved at last.

But it had not. As the years went on and as the data accumulated, the correlations got weaker and weaker. Eventually, the researchers had to admit that they could not unambiguously identify any sources. Maybe those random intergalactic fields were muddying the results after all. Auger “should have been more careful” before publishing the 2007 paper, says Avi Loeb, an astrophysicist at Harvard University in Cambridge, Massachusetts."
So we're back to speculation on the origin of the ultra high energetic cosmic rays. It's a puzzle that I've scratched my head over for some while - more scratching is due.