Things looking down for cutting-edge cosmic observatories
Opinion High energy neutrinos are the coolest particles in astrophysics. Born in distant cosmic cataclysms, they speed through the universe almost as if it wasn’t there. With no charge and a truly tiny rest mass – perhaps a million times lighter than an electron, but who knows – they interact with virtually nothing.
Which makes them virtually impossible to detect, and simultaneously highly desirable to questing humans. In the words of John Lennon, they’ve got to be good looking because they’re so hard to see.
What makes physicists, technologists, and cosmologists come together to seek the elusive neutrino is that it fares much better across space and time than photons and cosmic rays, especially at the ultra-high energies of the most enigmatic intergalactic events. The gamma rays and cosmic rays produced by extremely energetic events lose energy or are scattered by the cosmic medium and magnetic fields. Neutrinos fly straight and true through galaxies and across billions of light years. See them and you have an incomparably clear window on things.
Making them visible has been the focus of one of the newest fields in astronomy, neutrino observation. It has to think big. If the chances of a particle interacting with your detector are very, very small, you build a very, very big detector. This principle has led to some gargantuan machinery with surreal attributes. Not many astronomical observatories need to be buried miles below ground to operate in total darkness, but that’s exactly how it’s done. Even weirder things are on the way.
The highest profile neutrino observatory is IceCube, which sits close to the South Pole. It’s basically 5,000 light detectors hung in long strings down holes in the ice. The holes are between a mile and a mile and a half deep, forming a huge cube. They detect one neutrino every five minutes or so in that cubic kilometer of ice. This happens when one hits another particle to generate additional highly energetic charged particles. These in turn give off visible light photons, Cherenkov radiation, as they fly through the ice.
One event every five minutes doesn’t sound much. It isn’t, especially when IceCube also detects a million times more cosmic ray events in the same time. The giant gadget generates a terabyte of data per day in the process, which is cut down in a local server cluster to a mere 100 GB before being shipped out by satellite. The light burst from a neutrino reveals how energetic it was and where it came from.
It’s phenomenal, but there’s better to come. IceCube has spotted events and steady neutrino sources across the universe, but the really interesting neutrinos are the ultra-high energy ones, because they carry evidence about events at the edge of physics that make scientists swoon.
But ultra-high energy neutrinos are ultra rare, making their detection rate pitifully small even by the standards of the tribe. In fact, the most prized neutrinos have yet to be seen. Unsurprising, as the stats suggest one detectable event every 70 years per cubic kilometer of ice.
Enter the balloon. Modeling ultra-high energy neutrino collisions showed that while there was indeed visible light, there was a lot more radio. Moreover, it was coherent, making it easier to recognize and use. Finally, ice is transparent at radio frequencies. So, if you hang an extraordinary radio receiver off a balloon 40 km above the Antarctic ice, it can see a horizon of some 700 km. With the ice sheet several kilometers thick, that puts the odds back in humanity’s favor.
The experiment, called Pueo [PDF], or Payload for Ultra-high Energy Observations, has 96 antennas covering about a gigahertz of UHF spectrum, plus an additional 16 looking at lower frequencies. These detect both the characteristics and the direction of the radio pulse from a collision, which is only about a nanosecond wide. The radio receivers are exquisitely sensitive and very low noise, with the data being analyzed to some extent onboard before being squirreled away in 128 TB of local storage. Like IceCube, Pueo will also detect cosmic ray atmospheric collisions, mostly reflecting off the ice sheet below. It will also pick up on background radio noise and interference, even in Antarctica.
There’s another downside to Pueo. Flights only last about 30 days, which is limiting, and they can only happen every three years or so, making it even more challenging. So, the same techniques are being deployed at ground level, this time in the center of Greenland with an observatory called RNO-G. This is a multi-site operation, each site having 16 antennas, some near the surface and some down 100 meter holes. There are eight sites in operation with a total of 35 planned spaced around 1.25 km apart. The Greenland ice is a bit deeper and a little less radio transparent than that in Antarctica, with the system not being as sensitive to the highest energy neutrinos as Pueo but more controllable and available.
Another great advantage is that RNO-G is inherently scalable. The next-generation IceCube 2 plans to have a bigger light collecting area, but also to deploy radio detectors straight out of the RNO-G playbook.
Perhaps the coolest thing about neutrino observation is that it remains utterly aloof from going into space. Everyone else wants to get above the atmosphere to see better, with even gravitational wave mavens planning laser-linked deep space constellations of craft. Neutrino peeking needs millions if not billions of tons of dense, transparent matter, of which Earth has, for now, a monopoly supply. Air, ice, and oceans are the key to seeing the universe’s most extraordinary events in a completely new light. No spacesuit required, but do wrap up warm.
You can see a fine talk on Pueo and RNO-G, including why you’d want to iron ice with a clothes iron at 3am, here. ®
