London, March 19 : The High Resolution Fly's Eye (HiRes) cosmic-ray observatory in Utah, which had earlier captured tell-tale signs of cosmic rays, has got a negative result after some experiments, which has in turn confirmed a decades-old prediction that there is a critical threshold of energy beyond which these cosmic rays dwindle in number.
This is the reason that the finding of the absence of cosmic rays by the now defunct observatory, is far from a disappointment for the researchers.
According to a report in Nature News, this energy 'cut-off' was predicted in 1966 by Kenneth Greisen of Cornell University in Ithaca, New York, and in the same year by Soviet physicists Georgiy Zatsepin and Vadim Kuzmin of the Lebedev Institute of Physics in Moscow.
They predicted that there would be very few cosmic rays with energies greater than about 6-1019 electronvolts (eV) because of energy losses through interactions with the ubiquitous photons of the cosmic microwave background, the radiation that fills the Universe.
But scientists studying high-energy cosmic rays have failed to observe the so-called "GZK cut-off."
Such cosmic rays are mainly protons, thought to be generated by awesomely energetic astrophysical phenomena such as supernovae or supermassive black holes.
The new HiRes results, reported by Bergman and colleagues, have described a cosmic-ray spectrum that drops sharply at around the predicted GZK cut-off energy.
"They've pretty clearly seen the effect," said astrophysicist Alan Watson of the University of Leeds, UK.
HiRes's two telescopes searched the sky for the characteristic flashes of ultraviolet light produced when a cosmic ray collides with a molecule in Earth's atmosphere and creates a shower of secondary particles.
The two 'eyes' - hemispheres covered in photomultiplier tubes that look like a fly's compound eyes - capture just about all the light in the shower, giving a good measure of the original particle's energy.
"To see the GZK cut-off, it is vitally important to have good energy resolution," said Bergman.
The new Pierre Auger Observatory, operating while still under construction in the Argentinian pampas, has also seen an apparent dip in the spectrum that is consistent with the GZK cut-off.