After World War II it appears at first glance as if the interest in cosmic-ray studies had declined in favor of particle physics. This may hold true for HEP, though even then objections arose saying that man-made accelerators could never reach the enormous energies that particles from cosmic sources have . Moreover, many other phenomena of cosmic radiation were studied in the meantime, though they seem to belong rather to astrophysics. Yet, today they also contribute to the list of urgent matters of astroparticle physics cited above and they are supposed to form the field of “cosmic-ray physics” [110, 205]. As one possible reason for a certain phase of stagnation in cosmic-ray physics, the aspect of funding has been brought into play. Due to the (assumed) utility of particle physics for the civil, i.e. industrial, and military use of nuclear power, there were large investments made, as well as in fundamental research [165, 126]. On the other hand, some physicists indicate that the reason for the major interest in particle physics was due to internal scientific difficulties that had to be overcome before cosmic-ray physics could gain ground again. Regrettably, this alternative view of the internal development of cosmic-ray studies has not been spelled out so far, so that this would be another point that needs deeper scrutiny.
Of course, cosmic-ray studies had always been related to x-rays, as the very early beginnings were fostered by an interest in the various aspects of radioactivity at the turn of the century. In those early days of cosmic-ray studies, cosmic rays themselves had been thought to be gamma-rays. In the 1940s and 1950s the possibility of gamma-ray production by interaction of electrons or protons with star light and interstellar matter  was being analyzed. But the detection of immense x-ray and gamma-ray sources, due to the context of their detection being connected to the field of radio astronomy, occurred in the 1960s. First, the x-ray emission of the sun was proven by photographs taken of the sun by a rocket-borne pinhole camera . Then, in 1962, not only was a major x-ray source detected in the constellation Scorpio, an already well known radio source, but researchers investigating x-ray fluorescence signals from the moon realized that the overall diffuse x-ray background was far more intense than expected. The conclusion was that there must be x-ray sources outside the solar system . It was assumed that synchrotron radiation produced by cosmic electrons accounted for the emission of these x-rays . Those findings paved the way for a new generation of rocket-borne experiments mapping x-ray sources in the universe.
The discovery of gamma-ray bursts is renowned among physicists as it is linked to one anecdote of the Cold War. In the 1960s, military satellites of the Vela type were looking for nuclear tests by the Soviets that would have broken the Test Ban Treaty. They did find strong gamma-ray emissions, but they did not come from Soviet military grounds. They came from space. So in the 1970s and 1980s satellite-borne operations were launched to learn more about the different sources of gamma-rays, one of the most astonishing results being that these sources seem to be distributed all over the sky .
The study of gamma-rays from galactic and extra-galactic sources seems to be able to provide modern cosmic-ray physicists with answers to numerous interesting questions concerning problems of the production and the possible sources of such rays, as they are supposed to be exceeding the energy of 100 billion eV .
As mentioned in the introduction, dark matter is high on the agenda of modern astroparticle physicists. Still, the idea of matter that we cannot “see” with our particular means of detection is somewhat older. Working on the velocity of rotating stars in the 1920s, Oort found that our galaxy should be much more massive than its visible matter would lead us to believe . Zwicky came to a similar result in 1933, when he analyzed the mass-to-luminosity ratio in single galaxies and in clusters. To make sure that his first estimates were not due to the measuring error of his method, he tried new ways of calculating the mass of extra-galactic nebulae . For a long time the question of the missing mass remained unsolved .
More recent approaches have tried to make use of different methods of astroparticle physics, like neutrino detection, as a possible means of learning more about dark matter. The idea is that high energy neutrinos might be produced during the decay of super-symmetric dark matter particles . Sometime before, neutrinos had even been supposed to be identical with dark matter. Today, physicists consider it safe to say that there has to be something like dark matter in order to account for a number of the currently observable gravitational phenomena in the cosmos . There are different candidates for this matter. Some baryonic as well as many non-baryonic models have been proposed to solve the problem . As the question about the nature of the matter in our universe in general is closely linked to the issues of particle physics, astroparticle and particle physicists have become regularly involved in the search for dark matter . The current opinion of researchers is tending towards the existence of non-baryonic dark matter, as the search for baryonic dark matter, like that for massive compact halo objects (MACHOs), has been unsuccessful so far , though this is not the only reason. The most promising candidates presently on the agenda are axions, a new type of particle, and WIMPs (Weak Interacting Massive Particles) .
Still, even conventional matter and dark matter combined might be unable to explain some of the phenomena observed, like the accelerated expansion of the universe that has been witnessed for the first time in different experiments in 1998. This effect is ascribed to the influence of “dark energy”, which is assumed to make up about 70% of the universe .
At first glance the use of radio waves belongs to the core methods of astronomy and cosmology (see Figure 7). Yet the analysis of the nature of radio emissions had already in the 1950s played a decisive role in answering the question of the origin of cosmic rays . Its very early history can be traced back to the 19th century, the groundwork being the pioneering work by Maxwell, Hertz, Edison and others, when the general principle of radio transmission was studied. Unfortunately, the following experiments by Wilsing and Scheiner , as well as Nordmann  on the radio emission of the sun, were unsuccessful and Planck’s theory of thermal radiation, predicting a signal of radio waves coming from the sun far too weak to be detected, seems to have discouraged further attempts . The breakthrough for the astrophysical use of waves in the radio part of the spectrum came in 1932 with Jansky. Working for Bell Laboratories and investigating short waves for means of better transatlantic radio transmission, he found a faint distant “hiss” that could not have been caused by the usual sources of static like thunderstorms . He invested further work into the problem and it soon became obvious that the signal came from stellar objects other than the sun [4, 115, 114]. He even concluded that the source could be found outside the solar system . Radio astronomy began to flourish and already in 1950 the question of how cosmic radio emission and cosmic rays consisting of charged particles might be linked arose. It was Kiepenheuer  who first suggested that radio signals might be synchrotron radiation of electrons in interstellar magnetic fields, though the idea itself had already been introduced by Alfvén and Herlofson  somewhat earlier. Just one year later, Ginzburg  suggested that synchrotron radiation might be produced in supernovae or distant galaxies.
But it took another decade for the analysis of radio signals to become profoundly intertwined with the problems of cosmic-ray physics, when radio detection became an interesting means for investigating the air showers of cosmic rays . Just recently the LOPES (Lofar Prototype Station)  project was launched in order to learn more about how to make use of radio signals to detect air showers originating from particle showers of very high energy.
The discovery of the CMB is also due to a coincidence. Penzias and Wilson, working with an antenna for the calibration of satellites in order to guarantee low noise, found a uniform 3 K background in the universe . That was the proof for a hot and dense state of the early universe and evidence for a “big bang” postulated in the 1940s and fully conceptualized by Gamov somewhat later . By and by these findings brought to an end the long-lasting debate over whether the universe was expanding. This problem had arisen from the 1920s mathematical solutions to the equations of general relativity proposed by Friedmann and Lemaître, which predicted a universe that was not static. The idea of an expanding universe had been further pushed by the detection of the red shift of galaxies, owing to Hubble’s law [22, 138].
In the 1960s, the idea was established that interactions of CMB with ultra high-energy cosmic rays (UHECR) might be responsible for the drastic rise of the cosmic-ray spectrum by more than 1020 eV [89, 228]. Current experiments on UHECR are trying to figure out the properties and possible origins of these cosmic rays. Scenarios include extra-galactic as well as galactic sources of UHECR. The most likely of those scenarios for galactic sources assumes that these rays are being accelerated by stochastic shock acceleration in supernova remnants .
Already in the 18th century, Michell  and Laplace  had thought about the possibility of objects that would not allow light to escape their surface. But typically black holes are counted as objects of relativistic cosmology. Schwarzschild’s solution to Einstein’s field equation in his gravitational theory of general relativity, proposed very soon after Einstein’s article on the matter in 1915, gave a mathematical description of such an object .
This solution was interesting on a theoretical level, as it may account for different phenomena like gravitational red shift, lensing and others . Yet, unlike some of the few stationary solutions to Einstein’s equations proposed, black holes do not only exist as a mathematical model, but also in reality. The first to predict that there were stars so massive that they would collapse under their own gravity was Landau , though Chandrasekhar is said to have come to the same conclusion at approximately the same time. Finally, in 1972, a black hole was found in the binary system Cygnus X-1 [216, 27]. Thus, it is evident that black holes are indeed formed through the collapse of massive stars, which become so dense that light cannot escape their Schwarzschild radius .
Black holes are also thought to be connected to phenomena like quasars. It is assumed that black holes, being at the nucleus of an active galaxy (see Figures 8 and 9), may accumulate matter like stellar gases in large amounts, the rotation of the black hole circumventing it that all the material is “devoured” at once. Thus the residual matter forms a large vortex-like disc, the so called accretion disc. While orbiting ever closer towards the black hole, the in-falling material loses energy, most probably in form of relativistic jets, detectable as quasars or blazars, depending on the angle of observation .
Quasars, standing for “quasi-stellar radio source”, had first been witnessed with the help of radio astronomy in the 1950s . As they could not be linked with an object in the visible spectrum, it took until the 1960s for it to be realized that the unexplained properties of these objects were due to enormous red shifts, meaning that they were moving at very high speed at a very great distance. In 1962, Hazard  investigated the position of radio source 3C 273, which was covered by the moon. He found that the source must be a double radio source, because of its halo-like form with a much brighter center that coincided with a stellar object of thirteenth magnitude. Others started to analyze the properties of this source, as well as the radio source 3C 48. They found an immense red shift, enormous luminosity, an emission in the form of a jet and a fluctuation in brightness over the period of 80 years [187, 88, 162].
Blazars, which are based on the same phenomenon, are witnessed from the Earth’s point of view at a different angle, which first gave the impression that they were a different phenomenon. Like quasars they are supposed to be jets that are emanated from black holes when they are “swallowing” mass.
Pulsars (see Figure 10), on the other hand, are neutron stars, which give off electromagnetic waves at certain intervals . They were first detected in 1967 by Bell and Hewish, who were working on the effect the Sun’s flares have on the emission of radio sources and who were trying to develop a new means of investigating compact radio sources. When they found a pulsating object with regular radio emission , they discussed the possibility of having found an object that could be identified with the neutron stars predicted in the 1930s .
The idea of neutrinos goes back to Pauli who postulated them in 1930, together with the neutron, which he, according to Pontecorvo , for a while mistook for being identical to the remnants of beta decay. In 1933, Fermi introduced the term “neutrino”. At that time it was assumed that beta decay was the only means of neutrino production, which is not true. Neutrinos are produced in a number of different decay and scattering processes. The only possibility of detecting them during the early stages of neutrino physics was through the laws of energy and momentum conservation, as their weak interaction made them impossible to discover by the then-common means of particle detection. Yet the neutrino hypothesis was very soon transferred to astrophysical problems by Bethe, who proposed that neutrinos were emitted from the sun and other stars in their thermonuclear reactions . Though after the 1940s accelerators and reactors were to become the most important means in neutrino physics, the astrophysical aspects have always also been of great importance. Especially in the 1960s and 1970s, large experiments were conducted underground or underseas in order to learn more about cosmic sources of neutrinos.They had to be put underground to shut out the noise from other high-energy particles. But there have been neutrino experiments conducted with man-made accelerators as well .
The importance of neutrino astronomy grew over the past 40 years, after it had been found to be a means of proving fusion processes in the sun in order to verify the theory of stellar evolution, which predicted exactly such processes. Yet the number of neutrinos detected covered just one half of the predicted neutrinos, so the idea of neutrino oscillations (i.e. the change of neutrino flavor) was introduced. Though confirmed in 2001, this idea is not in line with the Standard Model of particle physics and therefore one example of how astroparticle physics or rather neutrino astronomy may go beyond that model .
Today neutrino experiments have reached a new level. For example, the Cherenkov detector of the former AMANDA  now IceCube experiment at the geographical South Pole will reach a volume of 1 km3 .
Generally, the detection of high-energy neutrinos in water or ice with energies of more than 1 TeV provides directional information about the particles. Because neutrinos are not deflected by magnetic fields as charged particles are, they may provide information of where in the universe we may find sources of highest energy particles, exceeding 1 Mio TeV. Good candidates for such sources of cosmic radiation are GRBs (Gamma-Ray Bursts) or AGN (Active Galactic Nuclei) – so called cosmic accelerators . Using the Earth as a shield makes it possible to distinguish atmospheric from galactic neutrinos, as only the latter have been accelerated to energies high enough to penetrate the entire planet. The characteristic of extremely weak interaction of the neutrinos with mass makes it difficult for researchers to detect them. The method commonly used with modern neutrino telescopes is the measurement of Cherenkov radiation  (see Figure 11), a method also used for detecting very-high-energy gamma-rays .
When a particle moves at a greater speed through a medium than the velocity of light in this medium, as is the case in water or ice, it emits Cherenkov radiation. The radiation is emitted at a well-defined angle:n is the index of refraction. Cherenkov radiation has a continuous spectrum that intensifies with higher frequencies. Therefore, Cherenkov radiation appears bluish, although most of the intensity is emitted in the ultraviolet range. This faint blue signal, named after Pavel Cherenkov, who witnessed the effect in the 1930s, can be measured with photo-multipliers and then be analyzed [22, 205].
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