Back in 1999, as the Millennium was approaching, I wrote an AV column entitled “What We Don’t Understand”, listing what I then considered to be the top seven major end-of-century unsolved problems in physics and astrophysics. Here is my 1999 list or problems: (1) Vacuum Energy and Dark Matter in Cosmology; (2) The Arbitrary Parameters of the Standard Model of Particle Physics; (3) The Origin of Gamma Ray Bursts; (4) The Origin of Ultra-High Energy Cosmic Rays; (5) The Solar Neutrino Problem; (6) The Origin of Matter/Antimatter Asymmetry in the Universe; and (7) The Origin of the Arrow of Time. That list was published about seven years ago, and in the intervening time, some of these unsolved problems have been falling like dominoes. In this column I want to describe our latest gain in understanding.
The first falling domino
was Item 5, the Solar Neutrino Problem. It involved the
discrepancy between the intensity of neutrinos from nuclear fusion in the Sun as
predicted by our best astrophysics models and as measured with large underground
neutrino detectors (which was about three times smaller).
The SNO experiment in
The next domino to fall was Item 3 on my list, the Origin of Gamma Ray Bursts. Gamma ray bursts (GRB) were accidentally discovered in 1969 by the VELA military satellites. They were built to look for clandestine nuclear explosion, but instead they detected huge bursts of gamma radiation that occurred a few times per day and came from outside the solar system. The origin of these events, which became a major scientific mystery of 20th century astrophysics, has now been identified. NASA's Swift satellite, launched in November 2004 for the explicit purpose of investing GRB, included a gamma-ray detector to identify and locate the burst and to point on-board X-ray and optical telescopes in the burst direction in under a minute. Swift was able to observe the X-ray and optical components of such events even before the gamma-ray emission has stopped. Distant galaxies were observed to “light up” in the visible and X-ray regions as they emitted these huge bursts of gamma radiation.
The conclusions are that the GRB sources are billions of light years away and that the majority of the long-duration GRB are probably the result of the core collapse of a rapidly rotating, high-mass star into a black hole. Shorter duration GRB are believed to be caused by the “merging” of two neutron stars that spin down and collide. The details of gamma ray production are still a mystery however, because the observations do not fit well with theoretical models.
Now problem 4 on my list, the origin of ultra-high energy cosmic rays, has joined the group of falling dominoes. The Pierre Auger Collaboration announced last month in a paper published in the November 8, 2007 issue of the magazine Science that the source of these highest energy particles has been identified.
Let me start by providing some background about cosmic rays. The most energetic particles observed come not from large particle accelerators but from the cosmos itself. These particles from space, usually protons, have energies up to 10 billion times higher than the most energetic protons we can produce with particle accelerators. One super-energetic particle (probably a single proton) recorded at the Fly's Eye detector in Utah was estimated to have an energy of about 3 ´ 1020 electron volts (or 300 EeV). This is a huge amount of energy, which corresponds to 50 joules or the kinetic energy of a baseball thrown at 60 mph.
In the past few years, the Pierre Auger Collaboration, an international consortium of universities and scientific institutions partly funded by the U.S. National Science Foundation and Department of Energy, has been building the Auger Observatory, an array of 1,600 large particle detectors spaced 1.5 kilometers apart and occupying 3,000 square kilometers of the Argentine pampas. The detector system is designed to record “extended air showers”, the large-area cascades of many thousands of particles created at ground level by a single ultra-energetic particle interacting with the upper atmosphere. In the past few years as it has come into operation, the Auger Observatory has recorded the arrival of 77 ultra-high-energy cosmic rays with energies in excess of 4.0 ´ 1019 electron volts (40 EeV), including 27 particles with energies in excess of 5.7 ´ 1019 electron volts (57 EeV)
The observation of such ultra-high energy (UHE) particles creates a problem. They should not be able to reach us unless they come from fairly nearby. The universe is permeated with cosmic microwave background radiation, low energy photons released about 500,000 years after the Big Bang when the protons and electrons paired off and matter and light went their separate ways. Protons with energies above about 5 EeV, on colliding with these photons, should produce pi-mesons and should rapidly lose energy. This process, called the GZK cutoff, should result in a sharp drop in cosmic rays above 5 EeV. (See my column “Ultra-Energetic Cosmic Rays and Gamma Ray Bursts” in the 01/96 Analog). This “cutoff” is not observed in the data, and more cosmic rays are observed above this energy than extrapolation from lower energies would predict. The only plausible resolution of this paradox is the assumption that these UHE particles are being produced relatively close to the Earth, within around 200 million light years.
The universe is permeated with magnetic fields, and these fields bend the paths of cosmic ray charged particles at low energies. This deflection prevents back-tracking these particles to see where they came from. However, as the cosmic ray particle energy goes up, the deflection goes down. The result is that when cosmic rays reach the 40 EeV level they can be back-tracked to an accuracy of a few degrees, providing information on possible sources.
One possible source of energetic cosmic rays is an “active galactic nucleus” (AGN), the small fraction of known galaxies with a compact region at the center that has a high luminosity over some or all of the electromagnetic spectrum (in the radio, infrared, optical, ultra-violet, X-ray, and gamma ray wavelengths). An AGN is believed to be a result of a super-massive black hole at the galactic center that is devouring large quantities of matter and converting some fraction of it to the energetic emission of particles and radiation. Our Milky Way galaxy is believed to have a black holeat its center with a mass 3 million time that of our Sun, but it is not an AGN, probably because its intake of matter is not large. There is some evidence that AGNs result from the collision of two galaxies. The massive disruption that follows, allows prodigious quantities of matter to be swallowed by the black holes at the two galactic cores, leading to vast amounts of gravitational energy release and radiation. It is plausible that such an environment could produce ultra-high-energy cosmic rays.
Therefore, with the GZK cutoff in mind, the Pierre Auger Collaboration compared the sky coordinates of their 27 most energetic cosmic ray events (all with energies greater than 57 EeV) with the positions of known AGN that are within about 240 million light years (or 75 megaparsecs) from Earth.
The correspondence between events and AGN locations is remarkable. Almost all of the cosmic ray locations have an AGN site nearby. Centaurus A, (also known as NGC 5128), a lenticular galaxy about 14 million light-years away in the constellation Centaurus, is the location of one of the closest AGNs to the Earth. The Auger study shows two ultra-high-energy cosmic ray events within a few degrees of this object.
This study provides strong evidence that the highest energy cosmic rays are coming from AGNs. But as usual, this discovery brings with it more questions. There is no known mechanism by which any known physical process, with or without a black hole involved, could accelerate particles to such high energies. So the question is, is there new physics involved in the actions of ultra-massive black holes? Or is there simply some subtle process involving the physics of very strong gravitational fields that no one has yet though of? Watch the column for further developments.
UHE Cosmic Rays
“Correlation of the highest energy cosmic rays with nearby extragalactic objects”, The Pierre Auger Collaboration, Science 318, pp. 938-943 (2007), available online at http://arxiv.org/PS_cache/arxiv/pdf/0711/0711.2256v1.pdf
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