The major reason for the development of astroparticle physics might be seen in the “conceptual links”  that were established during the 1960s and 70s between cosmology and particle physics. The first of these links was the finding that the abundance of helium in the universe might account for a limit on the number of neutrinos. Another important insight was provided by the application of CP violation to Big Bang Theory, which gave a possible explanation of the relationship between matter and antimatter in the universe. Finally, as a particle theory might offer an explanation for the nature of dark matter, “dark energy and dark matter are natural targets for particle physicists” . The most promising of these explanations so far is the theory of super-symmetry, which predicts more massive counterparts to every known elementary particle .
Another point that might help explain the growth of astroparticle physics is that it is becoming more attractive for particle physicists. This is due to better working conditions on the one hand, but also for the simple reason that, especially in the U.S., funding for accelerator-based experiments has been cut down drastically . This is dependent on the fact that cosmic-ray particles reach “energies not accessible to terrestrial detectors” , thus promoting cosmic-ray physics to a new heyday as a means of investigating the properties of elementary particles. Apart from this, some physicists state that it is more rewarding for them to pursue particle physics beyond the standard model to try to answer the many open questions that have first been put by cosmology with the help of astroparticle physics .
The “First International School on Astroparticle Physics” took place in Erice, Italy in 1987 because – as the introduction of the Proceedings states, quoting the announcement of the event:
“Recent progress in particle physics, cosmology and astrophysics has given birth to a discipline that encompasses them all. These embryonary developments are not often covered in an interdisciplinary way. The purpose of this School is to cover this gap.” (, page v)
The talks given covered a wide range of topics grouped around two major problems: the standard model of particle physics including how astroparticle physics might transcend it and the question of the history of the universe, focusing on galaxy formation.
The most striking point seems to be the fact that according to the introduction, the organizers of the School had in mind the fostering of a whole new discipline, not some minor sub-discipline. Yet they had to admit that they faced methodical and pedagogic difficulties in the course of their School .
Current experiments in astroparticle physics span almost the whole spectrum of cosmic rays, as even the most superficial look at recent articles shows [1, 97, 163, 184, 210]. They make use of the detection of all types of radiation in order to learn about the sources of cosmic radiation, the nature of the particles, as well as the properties of the interstellar medium. The knowledge of astroparticle physics is understood as a vital means to learn about the nature of the smallest and largest structures in the universe at the same time, thus maybe making it possible to find a unified theory [18, 85].
As indicated above, there is still no consensus on what the current scientific standing of astroparticle physics is. While even in the most recent publications  renowned scientists call astroparticle physics an interdisciplinary area, others claim the status of a discipline for this field. The dissent might be rooted in the discrepancy between the fact that the formal aspects that mark a discipline are more or less fulfilled by astroparticle physics and the lack of an inner-disciplinary profile that makes it possible to tell the contents of this field clearly from those of particle physics and, even more so, from those of astrophysics (see Section 6.2). These external aspects have been described by historians of science as being defined by the community and its ongoing exchange on the topic, the growing tendency to form institutions and the self-reproductive character mirrored in academic publishing and teaching. This definition  is to some extent owed to the former Marxist approach, which laid emphasis on the material conditions, i.e. the social context of scientific development. But not only do historians that foster modern interpretations of that contextual method still agree upon that definition , it is also in good compliance with the Kuhnian idea of the so-called “normal sciences” .
Now first, the aspect of a large community does surely apply to astroparticle physics. For not only do even single experiments consist of cooperating groups of up to a few hundred members [43, 212], but also different experiments work together like in the German VIHKOS  cooperation, that links about ten international projects concerning “High Energy Radiation from the Cosmos”. Communication between astroparticle physicists is also enhanced by various regularly held international conferences and workshops in the field of cosmic rays and astroparticle physics. A good example of such a conference with a long tradition is the International Cosmic Ray Conference (ICRC), a bi-annual event that took place for the 30th time in 2007 . Modern astroparticle physics has formed in the course of such international meetings. Concerning institutionalization in astroparticle physics, it would exceed the scope of this article to list all the different chairs at universities and various other institutions that are connected to experiments from the field of astroparticle physics. Yet even a brief look at the physics departments of German universities shows that more than one third of them appear to have at least one chair of astroparticle physics, which by and large are subsumed under the departments of experimental physics. In some cases, astroparticle physics is one equal part of a joint chair of particle and astroparticle physics. Besides, the simple fact that there is specific teaching in astroparticle physics at all, providing the opportunity to reach “degrees of higher status”  in this field, i.e. at least the status of chair-holder, is another indicator for the “self-reproductive” element in astroparticle physics. One might argue that this is still not sufficient to grant a field the status of a discipline. Yet, as it has been only 20 years since this new field came into being, one might leave it to future analysis to judge whether this will have been just one step towards a successful fulfillment of the process of institutionalization or whether this will mark the final status of a sub-discipline or interdisciplinary field. As different institutions are dependent on the financial support they are granted for equipment and human resources, it might also be worth taking a closer look at the status the field has with the major institutions that fund sciences, like the German Federal Ministry of Education (BMBF). There, astroparticle physics is funded as one section of basic scientific research . BMBF and other European funding institutions are also closely related via certain networks like the aforementioned ASPERA, which provides a plan through the year 2009 for building up infrastructure, ensuring communication and organizing workshops and other events that promote research in astroparticle physics (see Figure 12).
Teaching – or more generally speaking institutionalized academic reproduction – as an aspect of discipline formation, has not so far been investigated in the case of astroparticle physics. Being a potential topic for a sociological study of its own, it is beyond the scope of this article to look at the contents of the curricula of academic teaching in astroparticle physics. But, next to teaching, publishing is another indicator of the reproductive character of scientific disciplines . So at this point a representative selection of the available publications in the field will provide a first, though by no means sufficient, clue as to whether one might speak here of “self-reproduction” in any sense, not taking into account the numerous articles on the various topics related to astroparticle physics. A spontaneous query for articles on databases like the Smithsonian and NASA Astrophysics Data System (ADS)  or the SPIRES Database  will give you a few hundred hits for keywords such as ‘astroparticle physics’ and even a few thousands for keywords like ‘neutrino’ or ‘cosmic rays’. Books on astroparticle physics do not come in such huge numbers, but they do span the past decades, treating various topics and intellectual levels, from textbooks for beginners to those for advanced students [22, 29, 56, 57, 92, 93, 121, 149, 186, 202, 205], to journals (Astropart. Phys., JCAP), to conference proceedings [12, 23, 21, 68, 28, 31, 35, 39, 40, 45, 46, 50, 51, 52, 157, 91, 152, 150, 151, 153, 154, 166, 167, 171, 176, 180, 198, 203, 118, 218], to specialists’ accounts of different problems [8, 24, 25, 42, 54, 55, 81, 78, 86, 90, 99, 111, 145, 132, 141, 142, 143, 144, 148, 156, 182, 195, 196, 204, 209, 226]. So, the reproductive character seems to be quite evident, though this aspect still requires investigation. As historians of science have pointed out, it is exactly this self-reproductive element that makes scientific disciplines self-evolutionary systems and thus makes it difficult to distinguish the indicators of disciplinary “self-reproduction” from those aspects that mark the foundation of a totally new discipline:
“The internal mechanism of reproduction that maintains the unity of a discipline through time thus becomes at the same time the elementary mechanism of its development, providing stability and a direction.” 1 (, p. 41)
As a consequence of this interpretation, it would be rather difficult, if not impossible, to tell a certain stage of development within one established discipline from the first steps of an evolving new discipline. This problem, among others, might account for the problem pointed out in Section 5.3.
Though the external factors for reaching the status of a scientific discipline are all more or less fulfilled by astroparticle physics, so that one might speak of a discipline in the making, or at least of something more than just an interdisciplinary field of study, this is exactly the term used in the latest publications . So what might be the reason for this discrepancy? One of the major problems with defining astroparticle physics as either a field or a discipline lies in the lack of a definition of astroparticle physics. As indicated above, terms like ‘high-energy astrophysics’, ‘cosmic-ray physics’, ‘extraterrestrial physics’ and other variations are used synonymously, giving no clue as to whether they are intended to have identical or slightly differing meanings. This seems to be closely connected to the conceptual links that linked cosmology and particle physics in the last decades and became the solid ground that modern astroparticle physics is based on. For even those neighboring disciplines have become “inextricably intertwined” . So it may become ever more difficult to differentiate astroparticle physics from astrophysics or particle physics, not to mention the problem of telling astroparticle physics from related fields such as cosmic-ray astrophysics and others. Thus, it becomes either nearly impossible to grant astroparticle physics the status of a discipline of its own or it requires deeper scrutiny as to whether the current means of dividing different disciplines from each other are profound enough to do so in a satisfying way. On the other hand, this blurring of disciplines and fields might be just a natural development owing to successful interdisciplinary work that has helped to attenuate the “segmenting force”  of the formation of disciplines. For this is exactly what some see as the major task of interdisciplinary projects: to exceed the limitations of knowledge caused by the fragmentation of the sciences into disciplines and to compensate for the loss of the unity of science .
A possible solution to this problem might be the analysis of the internal factors of theory formation in astroparticle physics, though this approach has its own pitfalls, because one of the decisive shortcomings of analyzing the different factors of theory building in general is the difficulty of telling the internal from the external ones. First of all, without the internal factors building the core of the theory, one cannot speak about a theory and its external conditionality at all. But also the interrelations between, say biographical, institutional and technical conditions on the one hand and the progress of the internal aspects of a theory on the other hand are inextricably intertwined. As shown above (2.4), the success of Hess’ experiments that paved the way for the acceptance of the hypothesis of the cosmic origin of penetrating rays, was to a great part owed to the good technical condition of the balloons he used, whereas others seem to have been less lucky . From an historical perspective the transition between internal and external factors is even more blurred, depending especially on the sources one consults. In any scientific publication the reasons given for or against pursuing a certain train of thought will be mainly logical, inherent ones, whereas private communications, laboratory journals and so forth might disclose a multitude of different reasons that need not necessarily be linked with internal structures of the theory at all. For historians it is sometimes even more complex to distinguish external from internal factors, for, knowing the result of the whole process already, one might evaluate an external fact as an absolute condition, that otherwise would have not been taken into consideration at all. Taking again early cosmic-ray studies as an example, one might ask the question, how we would judge the work of Wilson, Elster and Geitel concerning the residual effect caused by cosmic rays today, had Hess stumbled upon this radiation accidentally, not being acquainted with their findings.
Defining the current scientific standing of astroparticle physics will therefore only be possible after a deeper scrutiny of its internal history. What theoretical basis, what experimental results, what mathematical methods were needed before a sufficient theory of cosmic radiation could be developed at all? What is the core theory or the common concept that is shared by all those working in the field of modern astroparticle physics, independent from the field of physics they used to work in? These questions will provide interesting work for historians, philosophers of science, physicists and sociologists alike.
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