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

Abstract

With the advent of space experiments itwas demonstrated that cosmic sources emit energy practicallyacross all the electromagnetic spectrum via different physicalprocesses. Several physical quantities give witness to theseprocesses which usually are not stationary; those physicalobservable quantities are then generally variable. Therefore simultaneous multifrequency observations are strictly necessaryin order to understand the actual behaviour of cosmic sources.

Space experiments have opened practically all the electromagneticwindows on the Universe. A discussion of the most importantresults coming from multifrequency photonic astrophysicsexperiments will provide new inputs for the advance of theknowledge of the physics, very often in its more extremeconditions.

A multitude of high quality data across practically the wholeelectromagnetic spectrum came at the scientific community'sdisposal a few years after the beginning of the Space Era. Withthese data we are attempting to explain the physics governing theUniverse and, moreover, its origin, which has been and still is amatter of the greatest curiosity for humanity.

In this paper we will try to describe the last steps of theinvestigation born with the advent of space experiments, to noteupon 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 hadthe problem of how to plan the exposition. Indeed, the expositionof the results can be made in different ways, following severalpoints of view, according to:

- a division in diffuse and discrete sources;

- different classes of cosmic sources;

- different spectral ranges, which implies inturn a sub-classification in accordance with different techniquesof observations;

- different physical emission mechanisms ofelectromagnetic radiation;

- different vehicles used for launching the experiments(aircraft, balloons, rockets, satellites, observatories).

In order to exhaustively present The Impact of SpaceExperiments on our Knowledge of the Physics of the Universe itwould then have been necessary to write a kind of Encyclopaedia ofthe Astronomical Space Research, which is not our desire. On thecontrary, since our goal is to provide an useful tool for thereader who has not specialized in space astrophysics and for thestudents, we decided to write this paper in the form of a review,the length of which can be still considered reasonable, takinginto account the complexity of the arguments discussed.

Because of the impossibility of realizing a complete picture ofthe physics governing the Universe, we were obliged to select howto proceed, the subjects to be discussed the more or the less, orthose 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 `astronomicaltradition' 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 sourcesas: radio, infrared, optical, ultraviolet, X-ray, γ-rayemitters), they do determine the methods of study, and from thetechnical point of view they define the technology employed in therelative experiments. However, since then we have decided to avoida deep description of the experiments, satellites, andobservatories, simply to grant a preference to the physicalresults, rather than to technologies, however fundamental forobtaining those results.

The exposition, after an introduction (Section 1) and some crucialresults from space astronomy (Section 2), has been focussed intothree parts: the physics of the diffuse cosmic sources deducedfrom space experiments (Section 3), the physics of cosmic raysfrom ground- and space-based experiments (Section 4), and thephysics of discrete cosmic sources deduced from space experiments(Section 5). In this first part of the paper we have used thelogic of describing the main results obtained in different energyranges, which in turn characterize the experiments on board spacevehicles. Within each energy range we have discussed thecontributions to the knowledge of various kind of cosmic sourcescoming from different experiments. And this part is mainly derivedby the bulk of the introductory part of LSG's Ph.D. thesis.

In the second part of the paper, starting from Section 6, we havepreferred to discuss several classes of cosmic sourcesindependently of the energy ranges, mainly focussing the resultsfrom a multifrequency point of view, making a preference for theknowledge of the physics governing the whole class. This wasdecided also because of the multitude of new space experimentslaunched in the last fifteen years, which would have renderedalmost impossible a discussion of the results divided into energyranges without weakening the construction of the entire puzzle.

We do not pretend to cover every aspect of every subjectconsidered under the heading of the physics of the universe.Instead a cross section of essays on historical, modern, andphilosophical topics are offered and combined with personal viewsinto tricks of the space astrophysics trade.

The reader is, then, invited to accept this paper even though itobviously lacks completeness and the arguments discussed arecertainly biased by a selection effect owed essentially to ourknowledge, and to it being of a reasonable length.

Some parts of it could seem, in certain sense, to belong to anolder paper, in which the `news' is not reported. But this is owedto our own choice, just in full accord with the goals of the text:we want to present those results which have, in our opinion, beenreally important, in the development of the science. Theseimpacting results do not necessarily constitute the last news.

This text was formally closed just on the day of the launch of theINTEGRAL satellite: October 17, 2002. After that date onlyfinishing touches have been added.