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27 April 2016

That’s odd: Weirdly energetic arrivals from outer space


South Pole
Neutrinos captured at the South Pole carry more energy than we can explain

S. Lidstrom/NSF

Bert, Ernie and Big Bird came from outer space. Not the Sesame Street characters, but three neutrinos with anomalously high energies spotted by the IceCube neutrino detector at the South Pole in 2013 and 2014. They have since been joined by dozens more, all with energies in the petaelectronvolt region – that is, a million billion electronvolts.

In the annals of mysterious cosmic apparitions, these neutrinos have joined the “fast radio bursts” that have been buzzing our telescopes over the past 15 years. Each of these bursts lasts just a few milliseconds, but contains energy equivalent to the sun’s output over the course of a month.

In neither case do we have any clue what could have made anything that energetic. In the case of the neutrinos, the existence of these ones, suggests that even more energetic ones should too – or, at least, we can’t think of a reason why they shouldn’t.

Neutrinos are created by extremely high energy processes. Many were born in the moments after the big bang. Others come from supermassive black holes at the centres of galaxies or hypernova star explosions. Suspicions that the flux of neutrinos from outer space drops off before it reaches petaelectronvolt energies only come from never having seen anything more energetic than Bert, Ernie and co. Nobel-prizewinner Sheldon Glashow has even used the standard model of particle physics to predict a flux of neutrinos with even higher energies of 6.4 PeV being produced when an antineutrino and an electron collide, creating a W boson and sloughing off a neutrino.

So why haven’t we yet seen anything like this? “Confusing?” says Thomas Weiler of Vanderbilt University in Nashville, Tennessee. “Damn right.”

The Arecibo radio astronomy dish
The Arecibo receiver has seen repeated bursts of searing radio energy

Roger Ressmeyer/Corbis

Given we have multiple detectors capable of detecting such high-energy particles, either we need to rethink the idea they can exist in the first place, or propose some mechanism that would slow them to lower energies. Together with University of Hawaii physicist John Learned, Weiler has taken the second tack. Every quantum particle has an associated wavelength. The higher the energy of the particle, the shorter that wavelength. One possibility is that the wavelengths of the high-energy neutrinos are so short that they are encountering a kind of friction from a pixelated, quantised structure of space-time.

“If we are right, the super-short wavelengths of the neutrinos are sampling the structure of space, and there will not be higher energy neutrinos seen,” says Learned. It would also be our first experimental clue about what a theory that melds quantum theory and general relativity should look like. That is a huge leap. “Probably this is too crazy, and wrong,” says Learned. “But still there is a delightful anomaly here.”

With the fast radio bursts, crazy has been the order of the day for possible explanations. Tinfoil-hatted options such as alien spacecraft or one of our own military satellites are now more or less ruled out thanks to the Arecibo observation, says Learned. “The Arecibo multiple detection over a period of many days all came from the same location in the sky, so forget a human or alien satellite,” he says.

There have been speculations that the bursts arise as quantum gravity effects cause a black hole to explode. Another possibility is that they come from an as-yet-unknown class of objects, says James Cordes of Cornell University in Ithaca, New York. His own favoured explanation is at least in the realm of the known: a neutron star, perhaps orbiting a black hole and certainly in a galaxy outside our own. If so, what exactly a neutron star is doing to emit the bursts is unknown.

That's odd: speeding galaxies

It’s another big ‘un – studies of galaxies and galaxy clusters consistently indicate that they are rotating too fast for the amount of matter that meets the eye. Try as we might, we haven’t yet been able to pin down the culprit – non-interacting “dark matter”, which seemingly outweighs normal matter by five to one.


Read more: “The 6 biggest glitches in physics

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