14 DAY TRIAL // More than 42 years ago, Kenneth Greisen from Cornell University, Grorgiy Zatsepin and Vadim Kuzmin from Mascow Lebedev Institute of Physics independently predicted that cosmic rays emitted throughout the universe would never hit the Earth at their full strength due to collision with the Cosmic Microwave Background, remnant of the Big Bang event that led to the birth of the universe. This is called Greisen-Zatsepin-Kuzmin (GZK) suppression of ultrahigh-energy cosmic rays.
Now, University of Utah's High-Resolution Fly's Eye cosmic ray observatory studies the GZK limit for energetic particles originating in remote areas of the universe. The basic idea is that cosmic rays pack energy much higher than the GZK limit during emission, however, while traveling through space, they lose a great deal of energy through CMB collision.
University of Utah College of Science professor Pierre Sokolsky, and leader of the study, reveals: "It has been the goal of much of ultrahigh-energy cosmic ray physics for the past 40 years to find this cutoff or disprove it. For the first time in 40 years, that question is answered: there is a cutoff."
The High Resolution Fly's Eye cosmic ray observatory located in Utah's western desert conducted continuous observations on cosmic rays between 1997 and 2006, but the research was boosted by the Auger cosmic ray observatory which showed last year that the number of ultrahigh-energy cosmic rays that rain down on Earth and exceed the energies predicted by the GZK limit is rather small in comparison to high-energy cosmic rays.
The discoveries made by the Auger observatory and HiRes observatory clearly showed that the Akeno Giant Air Shower Array measurements were flawed since no GZK limit was detected. Data provided by the AGSA observatory revealed that cosmic rays ten times higher than the GZK limit are able to reach the Earth, thus contradicting the measurements made with Auger and HiRes.
Black holes, source of ultrahigh-energy cosmic rays?
In a study presented last year, professor Sokolsky suggested that the highest energy cosmic rays may come from active galactic nuclei that are believed to house supermassive black holes. The distribution of active galactic nuclei throughout the universe confirmed that the GZK limit prediction is correct and most ultrahigh-energy cosmic rays come from distant active galactic nuclei (AGN).
According to co-author of the study, professor Charlie Jui, such AGNs are consistent with galaxies located more than 150 million light years away from Earth - outside the Local Group. This doesn't mean, however, that the presumption of professor Sokolsky is necessarily true. Data collected by HiRes and that of Auger is not closely correlated, meaning that the true source of extremely energetic cosmic rays is at this time uncertain.
"We still don't know where they're coming from, but they're coming from far away. Now that we know that GZK cutoff is there, we have to look at sources much farther out," said professor Sokolsky.
The cosmic rays have been discovered nearly a century ago in the form of hydrogen nuclei and other heavy elements that are created during the life of the stars. Stars surrounding us, including the Sun, emit low energy cosmic rays, while supernova explosions determine medium-energy cosmic rays. But for more than a century, we haven't been quite able to reveal the source of ultrahigh-energy cosmic rays.
Some of the particles in these cosmic rays can reach energies up to 100 million times higher than that of the energy emitted inside particles accelerators back here on Earth. To make a slight comparison, the energy contained in one of these energetic particles could actually match that of a baseball flying through the air.
And it is not only about discovering their origins. The true question is, in fact, how much energy could be packed into a single elementary particle? Ultrahigh-energy particles are believed to have energies exceeding one billion billion electron volts, but the most powerful cosmic ray emission ever was found with the HiRes in 1991 and carried 300 times that energy.
High Resolution Fly's Eye
Fly's Eye was opened in 1981 and received upgrades in 1986. From 1994 to 1999, a second observatory was built and named High Resolution Fly's Eye. The curious thing, though, is the fact that, while AGASA is four times less sensitive than HiRes, it was able to detect 11 ultrahigh-energy cosmic rays exceeding 100 billion billion electron volts, nearly three times higher than those revealed by the HiRes. Further still, during operation HiRes detected 13 ultrahigh-energy cosmic rays exceeding 60 billion billion electron volts, which is in concordance with the GZK cutoff theory.
Without the GZK limit, the number of cosmic rays with an energy higher than the cutoff should have reached at least 43.
HiRes consists of a set of multifaceted mirrors that detects ultraviolet fluorescent flashes of light emitted when high energy cosmic rays collide with Earth's atmosphere. Doing so, researchers are able to establish the energy and direction of the cosmic radiation, while the Japanese observatory AGASA makes use of ground scintillator counters that are less reliable.
Whether ground scintillator counters are less sensitive than the optical method used by HiRes remains to be cleared out in the future. Until then, Auger can still continue the search for the source of the ultrahigh-energy cosmic rays alongside with HiRes. "The most reasonable assumption is they are coming from a class of active galactic nuclei called blazars," says Sokolsky.
Blazars may contain black holes with masses billions of time higher than that of the Sun, which emit beams of cosmic rays, as matter from the accretion disk spinning around them falls beyond the black hole's event horizon.