Imagine an elementary particle carrying the energy of a thrown baseball – about 50 joules. It sounds unbelievable, but physicists detected such a particle on October 15, 1991, and named it “Oh-My-God” because of its phenomenal energy of ~3×10^20 eV. This space alien challenged the established theories about the energy limits of particles from space. Scientists were stunned: Where in the Universe could such powerful energy come from? This mystery is what fuels the curiosity of physicists and astronomers. The study of Ultra-High-Energy Cosmic Rays (UHECR) has become the key to understanding the most powerful processes in the Universe. By hunting for these cosmic giants, science hopes to find clues to new physics and gain a deeper understanding of the structure of the Universe.
What are high-energy particles?
Cosmic rays are a stream of charged subatomic particles that continuously reach the Earth from all corners of the Universe. These are mostly atomic nuclei, most often protons, but also helium, carbon, or even heavier elements like iron. It is important to understand that cosmic rays cover an extensive range of energies, from moderate to truly super-scale. Most of them carry energy in the range of about 10⁷-10¹⁰ electron volts, but there are also rare representatives – high-energy particles, or, more precisely, high-energy cosmic rays. They are accelerated to speeds approaching the speed of light and can reach energies in the range of 10¹⁸-10²⁰ electron volts and even higher. For comparison, in the largest earthly accelerator, the Large Hadron Collider, particles reach 10¹²-10¹³ eV, millions of times less. And the record cosmic particle exceeded the energy of the LHC by about 40 million times! All high-energy particles are cosmic rays, but only a small fraction of cosmic rays reach such extreme energy values, and they are the object of special attention of scientists. In the scientific literature, the term “ultra-high-energy cosmic rays” (UHECR) is also used for even higher energies, meaning particles with energies above ~10¹⁸ electron volts.
*eV (electron-volt) – the energy that one electron receives when it passes through an electric voltage of 1 volt. So 3 × 10²⁰ eV is approximately 48 joules.
How do we see these invisible particles? When a cosmic ray crashes into the Earth’s atmosphere, it creates a cascade of secondary particles – the so-called air shower. Like a cosmic billiards game, a single particle knocks out whole groups of protons, pions, muons* and other debris from the atmosphere’s atoms, which rush to the surface. This avalanche can cover an area several kilometers in diameter until its energy dissipates. The Earth serves as a giant detector: we cannot see the primary beam itself, but we can catch the “rain” of secondary particles or the faint flashes of light it causes in the air. Thus, the atmosphere acts as a kind of screen on which traces of cosmic rays appear, allowing scientists to study them.
So the cosmic ray crashes into the atmosphere. The impact creates pions. The pions quickly split into muons. The muons reach the earth and pass through us (without harm).
*A pion (π meson) is an unstable particle consisting of a quark-antiquark pair. The muon (μ-meson) is a heavy analog of the electron.
Sources of cosmic rays
So what kind of astrophysical “engines” are capable of accelerating particles to such enormous energies? There are several possible candidates, from explosive events to exotic objects:
- Supernovae and their remnants – supernovae explosions create shock waves that accelerate particles to high energies within our Galaxy. Fast-moving supernova remnants (such as the Crab Nebula) are considered natural accelerators for “ordinary” cosmic rays.
- Active galactic nuclei (AGN) – the hearts of distant galaxies with supermassive black holes. A black hole, absorbing matter, ejects two powerful relativistic jets. In these jets and giant radio galaxies, particles can be accelerated to extreme energies.
- Pulsars and magnetars – rapidly rotating neutron stars with extremely strong magnetic fields. They work like cosmic dynamos: charged particles can be accelerated to enormous speeds in their electromagnetic fields.
- Gamma-ray bursts – the most powerful explosions in the Universe since the Big Bang. During a gamma-ray burst, enormous energy is released in seconds, potentially generating cosmic rays of extremely high energy.
- Hypothetical sources – If conventional astrophysics fails to explain all cases, exotic theories enter the arena. The decay of massive dark matter particles, collisions of primary black holes, or cosmic strings (topological defects in space-time) are all proposed as possible, albeit speculative, sources of the most energetic cosmic rays.
Acceleration mechanisms
Having a powerful source is only half the battle. Nature still needs to accelerate the particle to fantastic speeds. An ordinary star or planet cannot do this – extreme space “accelerators” are needed. The main process is the Fermi acceleration, a kind of cosmic pinball. Imagine a charged particle wandering chaotically between moving magnetic clouds of gas or shock waves. Each time it reflects off such magnetic “mirrors”, the particle gains a little energy. Repeatedly repeating this process, the particle accelerates to relativistic speeds. Shock waves, for example, from supernovae, provide the first Fermi acceleration: the particle crosses the shock wave front many times and each time receives an additional “push” forward. This is how the well-known power-law spectrum of cosmic rays appears.
*Relativistic speed is a speed that is very close to the speed of light. Usually, a velocity is considered relativistic when it is more than ~10% of the speed of light (i.e., more than 30,000 km/s). At such speeds, the laws of Einstein’s theory of relativity begin to apply, and classical (ordinary) physics no longer works.
But to reach energies like 10^20 eV, exceptional conditions are required. The higher the energy of a particle, the more difficult it is to keep it in the accelerating region – it tends to escape. Calculations show that the shock wave of an ordinary supernova can accelerate a proton to a maximum of ~10^17 eV. This is already the limit of such an “engine”, and then the particle will simply escape. To give it three additional orders of magnitude of energy, either a much larger accelerator or a much stronger magnetic field is needed. This criterion is known as the Hillas’ plot: an object must be large enough and magnetic enough to hold an ultra-relativistic particle. It turns out that only a very few places in the Universe meet this criterion: either gigantic (e.g., galaxy clusters, radio galaxies) or extremely “charged” magnetically (neutron stars, black holes). Accelerating a particle to 10^20 eV is a tremendous challenge even for nature, a kind of cosmic equivalent of building a hadron collider the size of a galaxy. That is why each such particle that reaches us is, without exaggeration, unique.
How we catch them
Special cosmic ray observatories help scientists hunt for elusive space guests. The largest of these is the Pierre Auger Observatory in Argentina. An array of one thousand six hundred water detectors is placed on an area larger than Luxembourg.
Image: ESA
When a cosmic ray passes through the atmosphere, it generates an avalanche of particles that reach the ground: Auger detectors record flashes of Cherenkov radiation in water tanks as the particles hit them. At the same time, dozens of telescopes around the perimeter monitor the night sky, picking up the ultraviolet glow of nitrogen excited by a passing rainstorm. By combining this data, scientists can recover the direction, energy, and some properties of the primary particle. In the Northern Hemisphere, the Telescope Array Observatory in Utah (USA) is performing a similar mission, although it is three times smaller than Auger. To increase the chances of catching extremely rare events, even larger installations are being designed. New methods are also in play: for example, observing radio pulses from atmospheric showers or even spacecraft that will detect cosmic rays from orbit, covering the entire Earth. Technology is constantly improving, giving “hunters” more and more sensitive tools.
What does this give to science?
Why spend so much effort on single particles? The fact is that high-energy rays open a unique window into both astrophysics and fundamental physics. First, they carry information about the most powerful cataclysms in the Universe. Each such particle is a messenger from the vicinity of a black hole, supernova explosion, or other extreme event. By capturing enough of these messengers, we will be able to map space accelerators and understand what is happening in distant galaxies inaccessible to conventional telescopes. Secondly, the beams allow us to test the laws of physics at energies unattainable on Earth. The Earth’s atmosphere turns into a natural collider: the collision of a cosmic ray with air nuclei is an experiment with energy hundreds of times higher than in the Large Hadron Collider. By studying the products of these collisions (atmospheric shower particles), physicists can test existing theories and look for signs of new physics, such as unexpected interactions, the appearance of unknown particles, or subtle violations of fundamental symmetries. Some theories suggest that such beams may arise from the decay of hypothetical superheavy particles (so-called top-centered or relic particles) left over from the early Universe. If traces of such a process could be detected, it would revolutionize our understanding of dark matter and the evolution of the cosmos.
The applied aspect is no less important. Cosmic rays of all energies are part of the space environment in which the Earth is located. They affect atmospheric chemistry, can disable satellite electronics, and even cause computer malfunctions (some memory failures are associated with the ingress of cosmic particles). Understanding these phenomena is an important component of space security. If humanity aspires to long-distance space travel, we need to know what kind of particle showers we may encounter in outer space and how to protect ourselves from them. Data from observatories help to refine models of space radiation, which is important for the safety of astronauts and satellites. Finally, the technology of detecting weak signals from space itself stimulates the development of new tools, from ultrafast photodetectors to distributed computing networks, which can be used in other industries.
Problems and puzzles
Ultra-high energy cosmic rays leave more questions than answers. One of the main mysteries is the so-called Greisen–Zatsepin–Kuzmin limit (GZK limit). The theory predicts that protons with energies above ~5×10^19 eV inevitably lose energy as they pass through the all-encompassing “fog” of relic radiation photons (the microwave background of the Universe). Interacting with these photons, an ultra-energetic proton produces pions and gradually slows down like a bullet flying through water. This means that cosmic rays of extreme energies should not come to us from distances greater than ~100-200 million light-years – they would “melt” on the way. And yet, we do detect particles that exceed the GR limit. The “Oh-My-God” particle is a vivid example of such energy. How is this possible? There are assumptions that the sources of these rays are located relatively close to us, within the local supercluster of galaxies, so the protons do not have time to lose energy. Another bold idea is that perhaps at extreme energies, the special theory of relativity (Lorentz invariance) is somewhat violated, and particles travel through space without loss. So far, there is no direct evidence for this, but the very emergence of such assumptions shows how mysterious cosmic rays of ultrahigh energies are.
Another problem is trajectory distortion. Cosmic rays are charged particles, and intergalactic space is permeated by magnetic fields. Just like a compass pointer that loses its orientation during a storm, a ray on its way to Earth deviates from the straight line many times. As a result, the direction from which it came says almost nothing about its place of origin. The detectors have shown that the most energetic particles come from almost everywhere – there are no clear “rays” or clusters that can be used to identify a particular star or galaxy. This seriously complicates the hunt for sources: imagine looking at a spot of light on the wall and trying to guess which side the beam is coming from.
Finally, the rarity of such events makes us patient. The flux of ultra-high-energy rays is extremely small: it is estimated that a particle with an energy of more than 10^19 eV arrives, on average, once a year on an area of one square kilometer. And at energies of ~10^20 eV, we are talking about decades or even centuries in the same area. So to catch at least a few of these “space projectiles”, scientists have to build giant detectors and collect data for years. Despite all these difficulties, rather, because ultra-high-energy cosmic rays remain one of the hottest topics in astrophysics, each new sample may be a clue.
We are just beginning to unravel the mysteries of the cosmic giants of energy. Each captured super-energetic particle is like a thread that leads scientists through a maze of questions to understanding the fundamental laws of nature. So far, there are few of these threads, but every year, there are more of them: observatories are being modernized, new facilities are being built, and international collaborations are joining forces for a common goal. We may have some high-profile discoveries ahead of us – from identifying specific sources of the rays to, quite possibly, discovering phenomena beyond the realm of known physics.
The optimism of scientists is supported by the very fact of the existence of such particles: The universe has already demonstrated that it is capable of surpassing our wildest imaginations. So we can hope that the solution is not far off. The hunt is on, and every reader can become an observer. One has only to look up into the sky – perhaps right now, another cosmic arrow of record energy is flying through the atmosphere somewhere, bringing us new knowledge.
