Trade.Information

Topic Name: Physicists have Observed the most Energetic Particles in the Universe Rarely Reach Earth at Full Strength

Category: STAR (Space, Telecommunications & Radioscience)

Research persons: Professor Pierre Sokolsky

Location: University of Utah, United States

Details

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 Physics.

“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 During a cosmic ray conference in Merida, Mexico, last summer, Auger physicists outlined preliminary, unpublished results showing that the number of ultrahigh-energy cosmic rays reaching Earth drops sharply above the cutoff.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 billion.]

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 rays.

“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 Mexico, Albuquerque.

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 stars.

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 as HiRes.

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 cosmic rays.

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 Earth.

“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 cosmic rays.”

Note for Ultra-High-Energy Cosmic Ray
In high-energy physics, an ultra-high-energy cosmic ray (UHECR) or extreme-energy cosmic ray (EECR) is a cosmic ray (subatomic particle) which appears to have extreme kinetic energy, far beyond both its rest mass and energies typical of other cosmic rays. These particles are significant because they have energy comparable to (and sometimes exceeding) the Greisen-Zatsepin-Kuzmin limit.

The first observation of a cosmic ray with an energy exceeding 1020 electronvolts was made by John Linsley at the Volcanic Ranch experiment in New Mexico in 1962.

Cosmic rays with even higher energies have since been observed, among them the Oh-My-God particle (a play on the nickname "God particle" for the Higgs boson), the nickname given to a particle observed on the evening of October 15, 1991, over Dugway Proving Grounds, Utah, estimated to have an energy of approximately 3 × 1020 electronvolts, equivalent to about 50 joules — in other words, it was a subatomic particle with macroscopic kinetic energy equal to that of a baseball (140 g) which is moving at about 27 m/s (60 mph).

These very high energy cosmic rays are however very rare and most cosmic rays possess an energy between 107 eV and 1010 eV.

It was most likely a proton travelling with velocity almost equal to the speed of light (if it was a proton, its speed would have been approximately (1 − 5 × 10−24) c; after traveling one year the particle would be only 46 nanometres behind a photon that left at the same time) and its observation was a shock to astrophysicists.

Since the first observation, by the University of Utah's Fly's Eye Cosmic Ray Detector, at least fifteen similar events have been recorded, confirming the phenomenon.

The source of such high energy particles was a mystery for many years, but results later correlated ultra high energy cosmic ray origins with extragalactic super-massive black holes at the center of nearby galaxies called active galactic nuclei. Interactions with blue-shifted cosmic microwave background radiation limit the distance that these particles can travel before losing energy (the Greisen-Zatsepin-Kuzmin limit).

Because of its energy, the Oh-My-God particle would have experienced very little influence from cosmic electromagnetic and gravitational fields, and so its trajectory should be easily calculable. However, nothing of note was found in the estimated direction of its origin.

In November 2007, the Pierre Auger Observatory announced that they had found a correlation between the 27 highest energy events thus far detected, and nearby active galactic nuclei [AGN] and that the rapid decrease in the number of events at highest energy is consistent with the GZK process. This confirming the GZK cutoff even further.

Additional data collection is expected to obtain even stronger verification of the AGN source for these highest energy particles, which are believed to be protons accelerated to those energies by magnetic fields associated with the rapidly growing black holes at the AGN centers. According to a recent study, short-duration AGN flares resulting from the tidal disruption of a star or from a disk instability can be the main source of the observed flux of super GZK cosmic rays.

Note for Supermassive Black Hole
A supermassive black hole is a black hole with a mass of an order of magnitude between 105 and 1010 (hundreds of thousands and tens of billions) of solar masses. It is currently thought that most, if not all galaxies, including the Milky Way, contain supermassive black holes at their galactic centers. There is also evidence that two supermassive black holes can co-exist in the same galaxy for a certain amount of time.

Supermassive black holes have properties which distinguish them from their relatively low-mass cousins:

The average density of a supermassive black hole (measured as the mass of the black hole divided by its Schwarzschild volume) can be very low, and may actually be lower than the density of air. This is because the Schwarzschild radius is directly proportional to mass, while density is inversely proportional to the volume. Since the volume of a spherical object (such as the event horizon of a non-rotating black hole) is directly proportional to the cube of the radius, and mass merely increases linearly, the volume increases at a greater rate than mass. Thus, density decreases for increasingly larger radii of black holes. One should be aware however that this results from scientific definitions and does not manifest as a real physical property.
The tidal forces in the vicinity of the event horizon are significantly weaker. Since the central singularity is so far away from the horizon, a hypothetical astronaut travelling towards the black hole center would not experience significant tidal force until very deep into the black hole.

There are several models for the formation of black holes of this size. The most obvious is by slow accretion of matter starting from a black hole of stellar size. Another model of supermassive black hole formation involves a large gas cloud collapsing into a relativistic star of perhaps a hundred thousand solar masses or larger. The star would then become unstable to radial perturbations due to electron-positron pair production in its core, and may collapse directly into a black hole without a supernova explosion, which would eject most of its mass preventing it from leaving a supermassive black hole as a remnant. Yet another model involves a dense stellar cluster undergoing core-collapse as the negative heat capacity of the system drives the velocity dispersion in the core to relativistic speeds. Finally, primordial black holes may have been produced directly from external pressure in the first instants after the Big Bang.

The difficulty in forming a supermassive black hole resides in the need for enough matter to be in a small enough volume. This matter needs to have very little angular momentum in order for this to happen. Normally the process of accretion involves transporting a large initial endowment of angular momentum outwards, and this appears to be the limiting factor in black hole growth, and explains the formation of accretion disks.

Currently, there appears to be a gap in the observed mass distribution of black holes. There are stellar-mass black holes, generated from collapsing stars, which range up to perhaps 33 solar masses. The minimal supermassive black hole is in the range of a hundred thousand solar masses. Between these regimes there appears to be a dearth of objects. Such a gap would suggest qualitatively different formation processes. However, some models suggest that ultraluminous X-ray sources (ULXs) may be black holes from this missing group.

About High Resolution Fly's Eye
The High Resolution Fly's Eye or HiRes detector was an ultra-high-energy cosmic ray observatory that operated in the western Utah desert from May 1997 until April 2006. HiRes utilized the atmospheric fluorescence technique that was pioneered by the Utah group first in tests at the Volcano Ranch experiment and then with the original Fly's Eye experiment. Dr. Pierre Sokolsky and Dr. George Cassidy, both of the University of Utah, received the 2007 Panofsky Prize for their work on this.

The High Resolution Fly's Eye used larger mirrors and smaller pixels as compared with the original Fly's Eye, hence the name. A prototype of the HiRes experiment operated between 1993–1996 at the original Fly's Eye-I site (Five Mile Hill). It was configured in a tower viewing a narrow wedge of sky from 3–73 degrees in elevation. First the Utah ground array and later the CASA and MIA (ground array and muon array) experiments were placed on the surface in the view of the HiRes prototype. This then became the first "hybrid experiment" collecting information both on the development of the air shower induced by the incident cosmic ray, but also measuring the shower's footprint at the Earth's surface and 3 m below surface (with the buried muon array). The HiRes prototype was disassembled early in 1997 to become part of the final HiRes configuration.

In its final configuration, HiRes was composed of two sites separated by 12.6 km. The sites were located on hilltops in Dugway Proving Grounds, a U.S. Army test facility in the west Utah desert. HiRes-I (located on Five Mile Hill or Little Granite Mountain) had one ring of 22 telescopes viewing from 3–17 degrees in elevation. HiRes-I was instrumented with sample and hold electronics which took a "snapshot" of the extensive air shower generated when the incident cosmic ray interacted with the atmosphere. Meanwhile, HiRes-II (located on Camel's Back Ridge) had two rings of telescopes to provide viewing higher into the atmosphere. It observed from 3 to 31 degrees in elevation. HiRes-II was instrumented with an FADC (Flash Analog to Digital Converter) so that it essentially made movies of the cosmic ray events. Both observatory sites provided full azimuthal coverage (360 degrees in azimuth). They were operated independently on moonless clear nights. The duty cycle of HiRes was close to 10%.

The HiRes experiment made the first observation of the GZK cut-off which is an indication of the highest energy cosmic rays interacting with the Cosmic Microwave Background and the universe becoming opaque to their propagation.

About Akeno Giant Air Shower Array
The Akeno Giant Air Shower Array (AGASA) is a very large surface array designed to study the origin of ultra-high energy cosmic rays. It covers an area of 100km² and consists of 111 surface detectors and 27 muon detectors. The array is operated by the Institute for Cosmic Ray Research, University of Tokyo at the Akeno Observatory. Array experiments such as this one are used to detect air shower particles. The results from the AGASA were used to calculate the energy spectrum and anisotropy of cosmic rays. The results helped to confirm the existence of ultra-high energy cosmic rays (>5 x 1019eV), such as so-called Oh-My-God particle that was observed by the Fly's Eye atmospheric fluorescence detector experiment run by the University of Utah.

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 Lawrence Wiencke.