A single ghost-like sub-atomic particle captured on Earth after a journey of four billion light years could help solve a cosmological conundrum that has vexed scientists for more than 100 years.
The path taken by the high energy neutrino, the first of its type ever detected, was traced to its likely source, a distant galaxy with a giant black hole at its heart.
Its appearance provides a smoking gun that may help astronomers finally unravel the century-old riddle of high energy cosmic rays.
The rays, consisting of fast moving elementary particles, rain down on Earth from space posing a threat to astronauts and even the crews and passengers of commercial jets.
But how they are created and where they come from has been an enduring mystery.
Beyond cosmic rays, the discovery opens up a whole new way of looking at the universe via a “third messenger”, the first two being light photons and newly detected gravitational waves.
The neutrino was detected on September 22 last year by the IceCube observatory, a huge facility sunk a mile beneath the South Pole.
Here, a grid of more than 5,000 super-sensitive sensors picked up the characteristic blue “Cherenkov” light emitted as the neutrino interacted with the ice.
Having almost no mass and passing right through planets, stars and anything else in its way, the particle travelled in a straight line from its point of origin to Earth.
As a result, astronomers were able to track its trajectory back across billions of light years to its probable source.
A key feature of blazars is twin jets of light and elementary particles that shoot from the poles of the swirling whirlwind of material surrounding the black hole.
The neutrino detected by IceCube is thought to have been created by high-energy cosmic rays from the jets interacting with nearby material.
Professor Paul O’Brien, a member of the international team of astronomers from the University of Leicester, said: “Neutrinos rarely interact with matter. To detect them at all from the cosmos is amazing, but to have a possible source identified is a triumph.
“This result will allow us to study the most distant, powerful energy sources in the universe in a completely new way.”
Unlike high energy neutrinos, most cosmic rays carry an electric charge that causes their trajectories to be warped by magnetic fields, making it impossible to trace their origins. In contrast neutrinos are unaffected by even the most powerful magnetic fields.
The blazar believed to have generated the neutrino, code-named TXS 0506 + 056, was located in less than a minute after the IceCube team relayed co-ordinates for follow-up observations to telescopes worldwide.
Being able to detect high energy neutrinos will provide yet another window on the universe, said the scientists.
The sensational discovery of the second “messenger”, gravitational waves, or ripples in space-time, was announced in February 2016.
France Cordova, director of the US National Science Foundation (NSF) that manages the IceCube laboratory, said: “The era of multi-messenger astrophysics is here.
“Each messenger, from electromagnetic radiation, gravitational waves and now neutrinos, gives us a more complete understanding of the universe and important new insights into the most powerful objects and events in the sky.”
Cosmic rays were discovered in 1912 by physicist Victor Hess using instruments on a balloon flight.
Later research showed them to be made up of protons, electrons or atomic nuclei accelerated to speeds approaching that of light.