Cosmic rays generate the most energetic particles in the universe, utterly dwarfing anything we can generate in particle accelerators. Astrophysicists thought these cosmic rays were created in powerful gamma-ray bursts. Turns out they were completely, utterly wrong. So now what?
A non-result doesn't usually mean that much. But Antarctica's IceCube Neutrino Observatory has failed to find a single neutrino despite monitoring the sites of hundreds of gamma-ray bursts. That's significant because they are one of the many types of particle thought to be found in these cosmic rays, and their almost non-existent interactions with all other matter means they should travel from their source to Earth virtually undisturbed.
As such, if you want to find the source of cosmic rays, you want to look for neutrinos, and so IceCube has been studying all recorded gamma-ray bursts for signs of those neutrinos. Now, we wouldn't have expected to detect neutrinos coming from all of these bursts, or even anything close to it. Of the 190 gamma-ray bursts between May 2010 and May 2011, we would have expected to detect about eight neutrino sources. And yet...nothing.
This is a big enough null result that IceCube's principal investigator, Francis Halzen of the University of Wisconsin, has declared that the results "have put half the theorists out of business", and we can now basically say with certainty that these gamma-ray bursts are not the source of neutrinos. That goes against what had been the prevailing theoretical consensus, and so now physicists have to go looking for a new one.
For the IceCube team, this result is disappointing - indeed, team member Peter Redl of the University of Maryland uses that exact word - but that definitely doesn't make the new data useless. Indeed, if these gamma-ray bursts really are a dead-end, then we finally have a chance to get on the right track with tracking down the origins of these particles. The best available theory now points to the supermassive black holes at the center of galaxies as the only other known cosmic phenomenon powerful enough to generate such particle blasts,
We don't know whether those active galactic nuclei are in fact the source of these cosmic rays, but that's partially because we haven't really looked yet. As Halzen puts it, we could observe these super-charged neutrinos coming from supermassive black holes any day now, and then we would know where the cosmic rays come from. That's got to be considered the odds-on favorite now, but if AGN observations don't turn up any evidence of these cosmic rays, then we might be on the verge of discovering some previously unknown or unnoticed phenomenon. That, however, is definitely the dark horse here.
It's also worth noting just how ridiculously cutting-edge this work is, and how absurdly difficult it is to even get in a position where we can detect these neutrinos. The IceCube telescope is a cubic kilometer in size, and it's located near the South Pole in order to give its detectors an unimpeded view of these elusive neutrinos. Even then, it's not the neutrinos that the IceCube's 5,160 optical modules are straining to detect - rather, it's energetic muons created as a byproduct of the passing neutrinos that causes the faint blue flashes in the massive detector array.
So far, IceCube has mostly done a good job of illustrating what we don't know about the universe. As University of Wisconsin researcher Nathan Whitehorn explains, the telescope was able to conclusively contradict 15 years worth of previous predictions while still under construction, and now it's pretty much demolished one of the leading theories of extra-galactic physics. Really, all the IceCube data serve as a good reminder of how much science relies on disappointing non-results just as much as major breakthroughs - without the former to show the limits of our current understanding, we'd risk finding ourselves awash in a sea of indistinguishable false positives.
Nature via New Scientist. Image by NSF.