Imagine an elementary particle that carries the energy of a thrown baseball — about 50 joules. It sounds incredible, but this is exactly the particle that physicists registered on October 15, 1991 and called it “Oh-My-God” — because of its phenomenal energy of ~3×10²⁰ eV. This space alien challenged established theories about the energy limits of space particles. Scientists were stunned: where in the Universe could such powerful energy come from? It is this mystery that sparks 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 space giants, science hopes to find clues to new physics and a deeper understanding of the structure of the Universe.
What are high energy particles?
Cosmic rays are a stream of charged subatomic particles that continually reach Earth from all corners of the Universe. They are mostly nuclei of atoms, most often protons, but also nuclei of helium, carbon, or even heavier elements like iron. It is important to understand that cosmic rays cover a very wide range of energies, from moderate to truly mega-scale. Most of them carry energies 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 most powerful accelerator on Earth, the Large Hadron Collider, particles gain up to 10¹²–10¹³ eV, which is millions of times less. And the record-breaking cosmic particle exceeded the LHC energy by about 40 million times! All high-energy particles are cosmic rays, but only a small number of cosmic ray particles reach such extreme energy values - they are the object of special attention of scientists. In scientific literature, for even higher energies, there is a term “ultra-high-energy cosmic rays” (UHECR), which means particles with energies greater than ~10¹⁸ electronvolts.
*eV (electron volt) is the energy that one electron gains when passing through an electrical voltage of 1 volt. So, 3 × 10²⁰ eV is about 48 joules.
How do we notice these invisible particles? When a cosmic ray hits the Earth’s atmosphere, it creates a cascade of secondary particles called an air shower . Like a game of cosmic billiards, a single particle knocks protons, pions, muons*, and other debris out of the atoms of the atmosphere and sends them hurtling toward the surface. This avalanche can cover an area several kilometers in diameter before its energy dissipates. The Earth serves as a giant detector: we cannot see the primary beam itself, but we can detect the “rain” of secondary particles or the faint flashes of light it causes in the air. The atmosphere thus acts as a screen on which the traces of cosmic rays appear, allowing scientists to study them.
So a cosmic ray hits the atmosphere. The impact creates pions.
The pions quickly decay into muons. The muons reach Earth and pass through us (harmlessly).
*A pion (π-meson) is an unstable particle consisting of a quark-antiquark pair. A muon (μ-meson) is a heavy analogue of an electron .
Sources of cosmic rays
So what astrophysical “engines” are capable of accelerating particles to such wild energies? There are several likely candidates, ranging from explosive events to exotic objects
Supernovae and their remnants – Supernova explosions create shock waves that accelerate particles to high energies within our Galaxy. Fast-moving supernova remnants (such as the Crab Nebula) are thought to act as natural accelerators for “regular” cosmic rays.
Active galactic nuclei (AGN) are the hearts of distant galaxies with supermassive black holes. The black hole, swallowing matter, ejects two powerful relativistic jets. In these jets and giant radio galaxies, particles can be accelerated to extreme energies.
Pulsars and magnetars are madly spinning neutron stars with super-strong magnetic fields. They act like cosmic dynamos: charged particles can accelerate to enormous speeds in their electromagnetic fields.
Gamma-ray bursts are the most powerful explosions in the Universe since the Big Bang. During a gamma-ray burst, colossal energy is released in seconds; it has the potential to produce ultra-high-energy cosmic rays.
Hypothetical sources – if conventional astrophysics fails to explain all cases, exotic theories enter the arena. The decay of massive dark matter particles, the collision of primordial black holes, or cosmic strings (topological defects in space-time) have all been 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 cosmic “accelerators” are needed. The main process is considered to be the Fermi mechanism – a kind of cosmic pinball. Let’s imagine a charged particle chaotically wandering between moving magnetic clouds of gas or shock waves. Each time, reflecting from such magnetic “mirrors”, the particle gains a little energy. Repeating this process many times – and the particle accelerates to relativistic speeds. Shock waves, for example, from supernovae, provide the first Fermi acceleration : the particle crosses the front of the shock wave many times and each time receives an additional “push” forward. This is how the famous power-low spectrum of cosmic rays arises.
* Relativistic speed is very close to the speed of light. Usually, relativistic speed is considered to be more than ~10% of the speed of light (i.e. more than 30,000 km/s). At such speeds, Einstein’s laws of relativity theory come into effect, and classical (ordinary) physics no longer works.
But to reach energies like 10²⁰ eV, exceptional conditions are needed. The higher the energy of the particle, the more difficult it is to contain 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 the capabilities of such an “engine” – the particle will simply run away. To give it three additional orders of magnitude of energy, either a significantly larger accelerator or a much stronger magnetic field is needed. This criterion is known as the Hillas condition : the object must be large enough and magnetic to contain an ultrarelativistic particle. It turns out that only a very few places in the Universe meet this bar: either gigantic in size (for example, galaxy clusters, radio galaxies) or extremely “charged” magnetically (neutron stars, black holes). Accelerating a particle to 10²⁰ eV is a colossal 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 them is the Pierre Auger Observatory in Argentina. An array of 1,600 water detectors is located on an area larger than Luxembourg.
When a cosmic ray passes through the atmosphere, it creates an avalanche of particles that reach the ground: Auger detectors record flashes of Cherenkov radiation in water tanks as those particles hit them. At the same time, dozens of telescopes around the perimeter monitor the night sky, capturing the ultraviolet glow of nitrogen excited by the passing shower. By combining this data, scientists can reconstruct the direction, energy, and some properties of the primary particle. In the Northern Hemisphere, a similar mission is carried out by the Telescope Array observatory in Utah (USA), although it is a third of the size of Auger. Even larger installations are being designed to increase the chances of catching extremely rare events. 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 with a glance. Technology is constantly improving, giving the “hunters” ever more sensitive instruments.
What does this give to science?
Why spend so much effort on single particles? The point is that high-energy beams open a unique window into both astrophysics and fundamental physics. First, they carry information about the most powerful cataclysms of the Universe . Each such particle is a messenger from the vicinity of a black hole, a supernova explosion, or another extreme event. By catching enough of these messengers, we will be able to map cosmic accelerators and understand what is happening in distant galaxies that are inaccessible to telescopes of the usual range. Second, beams allow us to test the laws of physics at energies unattainable on Earth. The Earth’s atmosphere is effectively turning into a natural collider : the collision of a cosmic ray with air nuclei is an experiment with an energy hundreds of times higher than in the Large Hadron Collider. By studying the products of these collisions (air shower particles), physicists can test existing theories and look for signs of new physics – for example, unexpected interactions, the appearance of unknown particles, or subtle violations of fundamental symmetries. Some theories suggest that such beams could be produced by the decay of hypothetical superheavy particles (called topocentric 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 failures (some memory failures are associated with cosmic particles). Understanding these phenomena is an important component of space safety . If humanity strives for long-distance space travel, we must know what kind of particle “rains” we may encounter in outer space and how to protect ourselves from them. Data from observatories helps 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 ultra-fast photodetectors to distributed computing networks, which may find application in other industries.
Problems and mysteries
Ultra-high-energy cosmic rays leave behind more questions than answers. One of the main mysteries is the so-called Greisen-Zatsepin-Kuzmin limit (GZK limit). The theory suggests that protons with energies above ~5×10 km² eV inevitably lose energy as they fly through the all-encompassing “fog” of relic radiation photons (the microwave background of the Universe). Interacting with these photons, the ultra-energetic proton produces pions and gradually slows down, like a ball flying through water. This means that extreme-energy cosmic rays should not fly to us from distances greater than ~100–200 million light years — they would “melt” along the way. And yet, we register particles exceeding the GZK limit. The “Oh-My-God” particle is a striking example of such energy. There are suggestions 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 special relativity (Lorentz invariance) is perhaps violated at extreme energies, and particles travel through space without losses. There is no direct evidence for this yet, but the very appearance of such suggestions shows how mysterious ultra-high-energy cosmic rays are.
Another problem is refraction . Cosmic rays are charged particles, and intergalactic space is riddled with magnetic fields. Like a compass needle that loses its bearings during a storm, the beam deviates from a straight line many times on its way to Earth. As a result, the direction from which it arrived tells us almost nothing about its birthplace. Detectors have shown that the most energetic particles come from almost everywhere – there are no clear “beams” or clusters that could be used to identify a specific star or galaxy. This makes hunting for sources very difficult: imagine looking at a spot of light on a wall and trying to guess which side the beam is coming from.
Finally, the rarity of such events forces one to be patient. The flux of ultra-high-energy rays is extremely small: according to estimates, a particle with an energy of over 10 19 eV arrives on average once a year over an area of one square kilometer. And at energies of ~10²⁰ eV, we are talking about decades or even centuries over the same area. Therefore, in order to catch at least a few of these “cosmic projectiles”, scientists have to build giant detectors and collect data for years. Despite all these difficulties – or rather, because of them – ultra-high-energy cosmic rays remain one of the hottest topics in astrophysics: each new sample can become the key to the mystery.
We are only just beginning to unravel the mysteries of the cosmic giants of energy. Each captured super-energetic particle is like a thread leading scientists through a labyrinth of questions to an understanding of the fundamental laws of nature. So far, these threads are few, but every year there are more: observatories are being modernized, new installations are being built, and international collaborations are joining forces for a common goal. Ahead of us, perhaps, there are big events – from identifying specific sources of rays to, quite possibly, discovering phenomena beyond 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 ideas. Therefore, we can hope that the solution is not far off. The hunt continues, and every reader can become its observer. You only have to raise your eyes to the sky – perhaps right now another cosmic arrow of record energy is flying through the atmosphere somewhere, bringing us new knowledge.