What is cosmic energy
Cosmic rays and the most energetic celestial bodies
The earth is constantly bombarded by high-energy particles. Even if this cosmic radiation is not directly visible, it affects us in many ways. With new instruments, physicists are looking for the origin of the high-energy particles and are thus opening up fascinating insights into the most energetic processes in the universe.
Victor Hess before one of his research flights
The Austrian physicist Victor Hess discovered cosmic rays in 1912 in a very adventurous way. Hess was concerned with the question of what the electrical conductivity of the air is based on. There was already the assumption that high-energy particles from the cosmos cause this. Then, Hess concluded, the electrical conductivity would have to increase at great heights. Between 1911 and 1913 the daring researcher ascended with free balloons to heights of over 5000 meters and was actually able to prove the predicted effect. He thus provided evidence that cosmic rays penetrate the atmosphere and ionize atoms in it. The American physicist Robert Millikan coined the term “cosmic radiation” in 1924.
Today it is known that cosmic rays essentially consist of atomic nuclei with very different energies. These are sometimes many powers of ten higher than in the largest particle accelerators on earth. The most energetic atomic nuclei that have been registered so far had almost as much kinetic energy as a hard hit tennis ball.
Energy spectrum of cosmic rays
Cosmic radiation is certainly important for the development of the cosmos. This is supported by the fact that there is just as much energy in it as in the entire starlight or in all cosmic magnetic fields of the universe combined.
In search of the cosmic accelerators
With all the advances made over the past few decades, it is still largely unknown which celestial bodies accelerate the particles and how this happens. One thing is clear, however: the physical processes must differ fundamentally from those that are responsible for the visible light from stars, for example. The latter is called "thermal radiation". Their energy grows with the temperature of the luminous celestial body. Apart from the Big Bang, there is no known object in space that has temperatures even remotely high enough to be considered a source of particles of cosmic rays. Other mechanisms that are called non-thermal must be responsible for this.
The most energetic event in the universe, the Big Bang, is ruled out as a source of origin. From the proportion of radioactive nuclei with “short” decay times in cosmic rays, one can conclude that a maximum of a few million years can have elapsed between the creation of the particles and their arrival in the earth's atmosphere. It is therefore also clear that a large part of the cosmic radiation has to be generated in our own galaxy. Only with the very highest energies one speculates on an origin in distant galaxies.
Particle shower caused by cosmic rays
Studying cosmic rays is still a difficult endeavor almost a hundred years after Hess' balloon flights. When a high-energy particle penetrates the atmosphere, it collides with an atomic nucleus at a height of twenty to thirty kilometers. The two nuclei burst, and new particles are released, which continue to race towards the ground. These hit atomic nuclei again and trigger further particles. Only when the energy of the primary particle in this cascade has been used up does the process come to a standstill. A so-called air shower arrives on earth. But this also means that the primary particles themselves do not even arrive on earth. How do they go to study then?
In principle, this is possible with space experiments. But it is precisely the most interesting particles with the very high energies that are extremely rare. At relatively “modest” energies of a few billion electron volts, a satellite will still be hit by many particles per second. For the cosmic accelerators in our galaxy, however, energies are up to 1015 Electron volts (eV) characteristic. Of them, just one particle per year and square meter arrives at the upper edge of the atmosphere. At the very highest energies above 1020 eV reaches us one particle per square kilometer every hundred years or so. A satellite instrument with a measuring area of around one square meter would have to wait a hundred million years for the first "hit".
Trajectories of cosmic particles
The only viable alternative to space experiments today is to set up stations on the ground that can be used to detect the air shower particles. From their properties, conclusions can be drawn about the type and energy of the primary particle. But this type of observation does not make it possible to determine the source of a primary particle or even to produce images of celestial bodies. Magnetic fields in interstellar space deflect the electrically charged particles from their orbits and thus obscure any directional information. At best, with the most energetic particles, there is hope of being able to localize individual sources in the sky.
Riddle of the highest energies
The origin of the most energetic particles in the universe is one of the key questions in modern astrophysics. The hottest candidates are supernovae. In such an event, an explosion or shock wave moves away from the precursor star at a few tens of thousands of kilometers per second. Atomic nuclei are trapped in the magnetic fields that such waves carry and gain enormous energies over time, much like a surfer on a high wave. The final stages of the exploded stars, so-called pulsars, can also accelerate particles in their extremely strong magnetic fields.
According to current knowledge, the acceleration in supernova shock waves is not sufficient to explain the very highest energies measured in cosmic rays. Larger and stronger shock waves are likely to occur in particle beams from active galaxies. These so-called jets can reach hundreds of thousands of light years into intergalactic space. They are currently candidates for the cosmic “super particle accelerator”.
But this hypothesis poses a problem: the particles collide with the microwave radiation on their way through intergalactic space. As a result, they are constantly losing energy. It is assumed that the particles can therefore travel a few hundred million light years at most. However, within this distance radius there are very few galaxies around our Milky Way system that could even be considered as cosmic accelerators. A recently published hypothesis suggests a completely different origin for some of the cosmic rays. It could perhaps arise from the decay of very heavy and previously unknown elementary particles or other relics from the Big Bang.
The new instruments
Probably the most powerful particle accelerator
With a new generation of measuring instruments, astroparticle physicists are studying these mysterious particles today and in the near future. The Pierre Auger Observatory is currently being built in Argentina, a huge facility with numerous detector stations being distributed over an area of over 3,000 square kilometers. With it, the air showers are registered in order to precisely measure the energies, directions of incidence and other properties of the cosmic radiation particles at the highest energies. Here one can hope to find some springs in the sky.
At lower energies this is not possible because of the deflection in the interstellar magnetic fields. This is where new gamma and neutrino telescopes come into play. The trick is that you don't observe the particles themselves, but their by-products, neutrinos and gamma quanta. They arise when the particles of cosmic radiation interact with interstellar gas or radiation in the vicinity of their place of origin. Unlike the particles themselves, the gamma quanta and neutrinos spread in a straight line. So they allow the sources to be identified and even sky photographs to be taken.
High-energy gamma radiation is observed with modern systems. They use the atmosphere as a kind of luminous screen. If a gamma quantum penetrates the atmosphere, collisions with atomic nuclei produce electrically charged secondary particles. These emit a bluish light for a fraction of a second. It is as if a fluorescent tube were briefly lit up in the high atmosphere. This so-called Cherenkov radiation can be observed with large reflector telescopes on the ground. In this way it is possible to create images of sources of high-energy gamma radiation.
This generation of new instruments will solve many cosmic ray mysteries in the years to come - and it is very likely that they will raise new questions. The many synergies with other areas of astroparticle physics, particle physics and astrophysics are also interesting in this research area. This type of research reveals a previously largely unknown facet of the universe.
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