The Impact of Space Experiments on our Knowledge of the Physics of the Universe

Abstract

With the advent of space experiments it was demonstrated that cosmic sources emit energy practically across all the electromagnetic spectrum via different physical processes. Several physical quantities give witness to these processes which usually are not stationary; those physical observable quantities are then generally variable. Therefore simultaneous multifrequency observations are strictly necessary in order to understand the actual behaviour of cosmic sources. Space experiments have opened practically all the electromagnetic windows on the Universe. A discussion of the most important results coming from multifrequency photonic astrophysics experiments will provide new inputs for the advance of the knowledge of the physics, very often in its more extreme conditions. A multitude of high quality data across practically the whole electromagnetic spectrum came at the scientific community's disposal a few years after the beginning of the Space Era. With these data we are attempting to explain the physics governing the Universe and, moreover, its origin, which has been and still is a matter of the greatest curiosity for humanity. In this paper we will try to describe the last steps of the investigation born with the advent of space experiments, to note upon the most important results and open problems still existing, and to comment upon the perspectives we can reasonably expect. Once the idea of this paper was well accepted by ourselves, we had the problem of how to plan the exposition. Indeed, the exposition of the results can be made in different ways, following several points of view, according to: - a division in diffuse and discrete sources; - different classes of cosmic sources; - different spectral ranges, which implies in turn a sub-classification in accordance with different techniques of observations; - different physical emission mechanisms of electromagnetic radiation; - different vehicles used for launching the experiments (aircraft, balloons, rockets, satellites, observatories). In order to exhaustively present The Impact of Space Experiments on our Knowledge of the Physics of the Universe it would then have been necessary to write a kind of Encyclopaedia of the Astronomical Space Research, which is not our desire. On the contrary, since our goal is to provide an useful tool for the reader who has not specialized in space astrophysics and for the students, we decided to write this paper in the form of a review, the length of which can be still considered reasonable, taking into account the complexity of the arguments discussed. Because of the impossibility of realizing a complete picture of the physics governing the Universe, we were obliged to select how to proceed, the subjects to be discussed the more or the less, or those to be rejected. Because this work was born in the Ph.D. thesis of one of us (LSG) (Sabau-Graziati, 1990) we decided to follow the `astronomical tradition' used there, namely: the spectral energy ranges. Although such energy ranges do not determine physical objects (even if in many cases such ranges are used to define the sources as: radio, infrared, optical, ultraviolet, X-ray, γ-ray emitters), they do determine the methods of study, and from the technical point of view they define the technology employed in the relative experiments. However, since then we have decided to avoid a deep description of the experiments, satellites, and observatories, simply to grant a preference to the physical results, rather than to technologies, however fundamental for obtaining those results. The exposition, after an introduction (Section 1) and some crucial results from space astronomy (Section 2), has been focussed into three parts: the physics of the diffuse cosmic sources deduced from space experiments (Section 3), the physics of cosmic rays from ground- and space-based experiments (Section 4), and the physics of discrete cosmic sources deduced from space experiments (Section 5). In this first part of the paper we have used the logic of describing the main results obtained in different energy ranges, which in turn characterize the experiments on board space vehicles. Within each energy range we have discussed the contributions to the knowledge of various kind of cosmic sources coming from different experiments. And this part is mainly derived by the bulk of the introductory part of LSG's Ph.D. thesis. In the second part of the paper, starting from Section 6, we have preferred to discuss several classes of cosmic sources independently of the energy ranges, mainly focussing the results from a multifrequency point of view, making a preference for the knowledge of the physics governing the whole class. This was decided also because of the multitude of new space experiments launched in the last fifteen years, which would have rendered almost impossible a discussion of the results divided into energy ranges without weakening the construction of the entire puzzle. We do not pretend to cover every aspect of every subject considered under the heading of the physics of the universe. Instead a cross section of essays on historical, modern, and philosophical topics are offered and combined with personal views into tricks of the space astrophysics trade. The reader is, then, invited to accept this paper even though it obviously lacks completeness and the arguments discussed are certainly biased by a selection effect owed essentially to our knowledge, and to it being of a reasonable length. Some parts of it could seem, in certain sense, to belong to an older paper, in which the `news' is not reported. But this is owed to our own choice, just in full accord with the goals of the text: we want to present those results which have, in our opinion, been really important, in the development of the science. These impacting results do not necessarily constitute the last news. This text was formally closed just on the day of the launch of the INTEGRAL satellite: October 17, 2002. After that date only finishing touches have been added.