One second after the Big Bang, an unfathomable number of neutrinos were liberated from the surrounding chaos and started traveling through the universe at nearly the speed of light. Cosmologists believe a person is bombarded with something on the order of a quadrillion of these 'relic neutrinos' every second, dwarfing, unbelievably, the number that come from other relatively nearby sources, such as nuclear fusion in the sun or radioactive decay. "You hear this, and you're driven to ask whether it's really true," says Princeton University physics professor Christopher Tully. "How can we know it's true?"
Tully is the principal investigator of the Princeton Tritium Observatory for Light, Early-Universe, Massive-Neutrino Yield (PTOLEMY) project, which aims to accomplish the first direct detection of relic neutrinos. As a byproduct, PTOLEMY may also help answer some questions about the relic neutrinos' properties, such as density and mass.
When physicists first theorized the existence of neutrinos in the 1930s, they believed the particles had no mass. However, work that won Takaaki Kajita and Arthur B. McDonald the 2015 Nobel Prize in physics revealed a surprising behavior: Neutrinos can oscillate between three different types, or 'flavors,' suggesting that they have mass after all.
PTOLEMY is based at the Princeton Plasma Physics Laboratory, a national laboratory on Princeton's Forrestal campus, but over the two-year course of the project, researchers will work with other groups, including the Savannah River National Laboratory, the Goddard Space Flight Center and Argonne National Laboratory, to install, build or improve the sensors and substrates needed to start detecting neutrinos. Unlike current neutrino experiments — which use spectrometry — PTOLEMY will use cryogenic calorimetry to detect relic neutrinos, aided by a key piece of equipment installed shortly after Simons Foundation support began in September: a cryogenic refrigerator that can reach temperatures of 7 millikelvins, just a hair above absolute zero.
Once its detector is in place, PTOLEMY will make precise measurements of the radioactive decay of tritium (hydrogen-3) to helium-3 (an isotope of helium with two protons and one neutron). Usually, tritium decay to helium-3 produces 18.6 kiloelectron volts of energy, an electron, and an antineutrino; however, if the tritium molecule interacts with a relic neutrino from the Big Bang, the amount of energy is slightly boosted. By precisely measuring this increase in energy, PTOLEMY will not only verify the presence and density of relic neutrinos but determine their mass as well.
If PTOLEMY is successful, it will also provide proof of concept for ultra-low-energy electron calorimetry as an effective way to make precise measurements, which could change the way cosmologists do experiments. PTOLEMY's findings could also have far-reaching theoretical implications. They might add to experimental evidence about neutrino masses, which could help cosmologists determine whether neutrinos are Majorana or Dirac fermions. This finding could in turn help explain why there is more matter than antimatter in the universe. The results could even have implications for the quest to understand dark energy. "It's been a huge technical challenge to build up the infrastructure," Tully says. "But once you start on a path of new technologies that can see things you've never seen before, there's no telling what you might learn from it."