Final results from the University
of Utah's High-Resolution Fly's Eye cosmic ray observatory show that the
most energetic particles in the universe rarely reach Earth at full strength
because they come from great distances, so most of them collide with radiation
left over from the birth of the universe.
The findings are based on nine years of observations at the now-shuttered observatory
on the U.S. Army's Dugway Proving Ground. They confirm a 42-year-old prediction
– known as the Greisen-Zatsepin-Kuzmin (GZK) “cutoff,” “limit”
or “suppression” – about the behavior of ultrahigh-energy
cosmic rays, which carry more energy than any other known particle.
The idea is that most – but not all – cosmic ray particles with
energies above the GZK cutoff cannot reach Earth because they lose energy when
they collide with “cosmic microwave background radiation,” which
was discovered in 1965 and is the “afterglow” of the “big
bang” physicists believe formed the universe 13 billion years ago.
The journal Physical Review Letters published the results Friday, March 21.
The GZK limit's existence was first predicted by Kenneth Greisen of Cornell
University while visiting the University of Utah in 1966, and independently
by Georgiy Zatsepin and Vadim Kuzmin of Moscow's Lebedev Institute of
“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,” says physics
Professor Pierre Sokolsky, dean of the University of Utah College of Science
and leader of the study by a collaboration of 60 scientists from seven research
institutions. “For the first time in 40 years, that question is answered:
there is a cutoff.”
That conclusion, based on 1997-early 2006 observations at the High Resolution
Fly's Eye cosmic ray observatory (nicknamed HiRes) in Utah's western
desert, has been bolstered by the new Auger cosmic ray observatory in Argentina.
During a cosmic ray conference in Merida, Mexico, last summer, Auger physicists
outlined preliminary, unpublished results showing that the number of ultrahigh-energy
cosmic rays reaching Earth drops sharply above the cutoff.
So both the HiRes and Auger findings contradict Japan's now-defunct Akeno
Giant Air Shower Array (AGASA), which observed roughly 10 times more of the
highest-energy cosmic rays – and thus suggested there was no GZK cutoff.
Cosmic Rays: Far Out
Last November, the Auger observatory collaboration – to which Sokolsky
also belongs – published a study suggesting that the highest-energy cosmic
rays come from active galactic nuclei or AGNs, or the hearts of extremely active
galaxies believed to harbor supermassive black holes.
AGNs are distributed throughout the universe, so confirmation that the GZK
cutoff is real suggests that if ultrahigh-energy cosmic rays are spewed out
by AGNs, they primarily are very distant from the Earth – at least in
Northern Hemisphere skies viewed by the HiRes observatory. University of Utah
physics Professor Charlie Jui, a co-author of the new study, says that means
galaxies beyond our “local” supercluster of galaxies at distances
of at least 150 million light years from Earth, or roughly 870 billion billion
miles. [In U.S. usage, billion billion is correct here and in subsequent references
for 10 to the 18th power. In British usage, 10 to the 18th power should be million
However, unpublished results from HiRes do not find the same correlation that
Auger did between ultrahigh-energy cosmic rays and active galactic nuclei. So
there still is uncertainty about the true source of extremely energetic cosmic
“We still don't know where they're coming from, but they're
coming from far away,” Sokolsky says. “Now that we know the GZK
cutoff is there, we have to look at sources much farther out.”
In addition to the University of Utah, High Resolution Fly's Eye scientists
are from Los Alamos National Laboratory in New Mexico, Columbia University in
New York, Rutgers University – the State University of New Jersey, Montana
State University in Bozeman, the University of Tokyo and the University of New
Messengers from the Great Beyond
Cosmic rays, discovered in 1912, are subatomic particles: the nuclei of mostly
hydrogen (bare protons) and helium, but also of some heavier elements such as
oxygen, carbon, nitrogen or even iron. The sun and other stars emit relatively
low-energy cosmic rays, while medium-energy cosmic rays come from exploding
The source of ultrahigh-energy cosmic rays has been a mystery for almost a
century. The recent Auger observatory results have given the edge to the popular
theory they originate from active galactic nuclei. They are 100 million times
more energetic than anything produced by particle smashers on Earth. The energy
of one such subatomic particle has been compared with that of a lead brick dropped
on a foot or a fast-pitched baseball hitting the head.
“Quite apart from arcane physics, we are talking about understanding
the origin of the most energetic particles produced by the most energetic acceleration
process in the universe,” Sokolsky says. “It's a question
of how much energy the universe can pack into these extraordinarily tiny particles
known as cosmic rays. … How high the energy can be in principle is unknown.
By the time they get to us, they have lost that energy.”
He adds: “Looking at energy processes at the very edge of what's
possible in the universe is going to tell us how well we understand nature.”
Ultrahigh-energy cosmic rays are considered to be those above about 1 billion
billion electron volts (1 times 10 to the 18th power).
The most energetic cosmic ray ever found was detected over Utah in 1991 and
carried an energy of 300 billion billion electron volts (3 times 10 to the 20th
power). It was detected by the University of Utah's original Fly's
Eye observatory, which was built at Dugway during 1980-1981 and improved in
1986. A better observatory was constructed during 1994-1999 and named the High
Resolution Fly's Eye.
Jui says that during its years of operation, HiRes detected only four of the
highest-energy cosmic rays – those with energies above 100 billion billion
electron volts. AGASA detected 11, even though it was only one-fourth as sensitive
The new study covers HiRes operations during 1997 through 2006, and cosmic
rays above the GZK cutoff of 60 billion billion electron volts (6 times 10 to
the 19th power). During that period, the observatory detected 13 such cosmic
rays, compared with 43 that would be expected without the cutoff. So the detection
of only 13 indicates the GZK limit is real, and that most ultrahigh-energy cosmic
rays are blocked by cosmic microwave background radiation so that few reach
Earth without losing energy.
The discrepancy between HiRes Fly's Eye and AGASA is thought to stem
from their different methods for measuring cosmic rays.
HiRes used multifaceted (like a fly's eye) sets of mirrors and photomultiplier
tubes to detect faint ultraviolet fluorescent flashes in the sky generated when
incoming cosmic ray particles hit Earth's atmosphere. Sokolsky and University
of Utah physicist George Cassiday won the prestigious 2008 Panofsky Prize for
developing the method.
HiRes measured a cosmic ray's energy and direction more directly and
reliably than AGASA, which used a grid-like array of “scintillation counters”
on the ground.
The Search Goes On
University of Tokyo, University of Utah and other scientists now are using
the new $17 million Telescope Array cosmic ray observatory west of Delta, Utah,
which includes three sets of fluorescence detectors and 512 table-like scintillation
detectors spread over 400 square miles – in other words, the two methods
that produced conflicting results at HiRes and AGASA. One goal is to figure
out why ground detectors gave an inflated count of the number of ultrahigh-energy
The Telescope Array also will try to explain an apparent shortage in the number
of cosmic rays at energies about 10 times lower than the GZK cutoff. This ankle-shaped
dip in the cosmic ray spectrum is a deficit of cosmic rays at energies of about
5 billion billion electron volts.
Sokolsky says there is debate over whether the “ankle” represents
cosmic rays that run out of “oomph” after being spewed by exploding
stars in our galaxy, or the loss of energy predicted to occur when ultrahigh-energy
cosmic rays from outside our galaxy collide with the big bang's afterglow,
generating electrons and antimatter positrons.
The Telescope Array and Auger observatories will keep looking for the source
of rare ultrahigh-energy cosmic rays that evade the big bang afterglow and reach
“The most reasonable assumption is they are coming from a class of active
galactic nuclei called blazars,” Sokolsky says.
Such a galaxy center is suspected to harbor a supermassive black hole with
the mass of a billion or so suns. As matter is sucked into the black hole, nearby
matter is spewed outward in the form of a beam-like jet. When such a jet is
pointed at Earth, the galaxy is known as a blazar.
“It's like looking down the barrel of a gun,” Sokolsky says.
“Those guys are the most likely candidates for the source of ultrahigh-energy
The new study's 60 co-authors include Sokolsky, Jui and 31 other University
of Utah faculty members, postdoctoral fellows and students: Rasha Abbasi, Tareq
Abu-Zayyad, Monica Allen, Greg Archbold, Konstantin Belov, John Belz, S. Adam
Blake, Olga Brusova, Gary W. Burt, Chris Cannon, Zhen Cao, Weiran Deng, Yulia
Fedorova, Richard C. Gray, William Hanlon, Petra Huntemeyer, Benjamin Jones,
Kiyoung Kim, the late Eugene Loh, Melissa Maestas, Kai Martens, John N. Matthews,
Steffanie Moore, Kevin Reil, Robertson Riehle, Douglas Rodriguez, Jeremy D.
Smith, R. Wayne Springer, Benjamin Stokes, Stanton Thomas, Jason Thomas and