HISTORICAL PERSPECTIVE

New Windows on the Universe: Cosmic Discoveries

A Cosmic Ray Accelerator
The Remnant of Supernova 1006AD
Imaged by ROSAT

New Windows on the Structure and Evolution of the Universe

Overview

There are four known types of carrier that transport information to us from the outer reaches of space. These broad categories are electromagnetic radiation (radio, infra-red, visible, ultraviolet, x-ray, gamma-ray), solid bodies (e.g., meteors), elementary ìcosmic rayî particles (e.g., protons and heavier nuclei, electrons, neutrinos and antiparticles) and gravitational waves. Until this century all of our knowledge about the cosmos was derived from just two of these, the visible optical band of the electromagnetic spectrum and solid bodies. We refer to all the other channels for observing the universe as ìnew windowsî, vitally complementing and extending recent significant advances in optical capability.

At least a third of all known major cosmic phenomena have been discovered in the last couple of decades. In particular, about twenty can now be added to the list of forty-three compiled in 1981 by Martin Harwit for his book Cosmic Discovery (1981 Basic Books, Inc. New York). To a large extent this current richness of cosmic discoveries is due to the exploitation of new windows for observing the universe made possible by the opportunities provided by the space program for making unobscured observations from platforms above the earthís atmosphere. With the new missions planned and under study we are now well positioned to obtain answers for many of the most fundamental cosmic questions. As summarized in this pamphlet, some of the far-reaching underlying issues to be addressed include:

• Is Einsteinís general relativity theory right?
• What is the origin of the universe and its destiny?
• Why is the universe lumpy?
• How did the known structures of the universe evolve?
• How do galaxies evolve?
• How did the elemental composition of the universe evolve?
• What is the structure and behavior of matter in the extreme?

Astronomy is replete with examples in which the most significant advances or the most astounding discoveries arose with the opening of new observational windows, some by design and some by chance. Entire sub-branches of astronomy now taken for granted, such as radio astronomy and X-ray astronomy, came about accidently. Once the observational window was opened by technological advances, completely unexpected discoveries were made. For historical perspective, a representative sample of some outstanding comparisons in this regard are shown on the chart shown above, categorized here as being ìexpectedî or ìunexpectedî.

We expected an interstellar medium, but we did not expect to encounter a local hot X-radiating plasma, now known from ROSAT (Roentgen Satellite) mapping to be a lasting imprint of past explosive events in our region of the Galaxy. What are the precise constituents of this plasma? High resolution X-ray spectroscopy is on the brink of answering this question. Who would have thought that the first direct evidence for the shock acceleration of high energy cosmic rays would come via X-ray astronomy? Yet, this was clearly exhibited in a recent ASCA (Advanced Satellite for Cosmology & Astrophysics) and ROSAT X-ray observations of the supernova remnant SN1006.

A half-century ago Phil Morrison and Bruno Rossi pointed out that cosmic ray particles traversing the interstellar gas would, via high-energy interactions with this ambient matter, produce a significant diffuse flux of gamma radiation. In effect, they invented gamma-ray astronomy, but the real opening of this observational window awaited the space age.

Although diffuse galactic gamma-rays were studied with satellites before the CGRO (Compton Gamma Ray Observatory), this current mission has shown us that there are many other pronounced cosmic phenomena to be explored as well in gamma-ray astronomy. The spectacular blazars that dominate the extragalactic gamma-ray sky are indicative of cosmic particle accelerators yet to be understood. What are the many unidentified discrete gamma-ray sources discovered with CGRO in our galaxy? And the biggest surprise of all in gamma-ray astronomy is the inexplicable phenomenon of celestial gamma-ray bursts discovered several years ago via a satellite program intended to detect possible nuclear explosions; this is the most persistent unsolved mystery in modern astronomy. From extensive studies with CGRO we now know that these bursts are either cosmological or the only observable manifestation of some unseen matter in a highly extended halo of our galaxy.

From the seminal work of Einstein, Schwartzschild and others we expected the existence of stellar black holes. And quite early in the history of X-ray astronomy, strong candidates were found in binary stellar systems. We did not, however, anticipate the supermassive black holes now known to exist in many galactic nuclei. Typically, they involve the equivalent of a hundred-million solar masses in a volume less than that of our solar system. What is the origin of these amazing objects? For stellar black holes, by contrast, something can be said about their origin. In particular, we know that stellar black holes and neutron stars can be produced via supernovae. Stars of nuclear density (ìneutron starsî) were predicted by J. R. Oppenheimer and F. Zwicky well before there was any real evidence for their existence. Even so, the discovery of radio pulsars was so unanticipated that they were first interpreted as intelligent signals from outer space.

A number of promising and exciting new windows are either on the horizon or on the verge of reaching fruition, including gravitational radiation, low-frequency radio waves, and ultra high-energy cosmic rays. Frequently, opening a new observational window leads to such novel discoveries that even some very relevant questions could not have been asked beforehand. Nevertheless, key questions should be asked. An outstanding example is the well-focused COBE (Cosmic Background Explorer) determination of the precise black body spectrum of the cosmic microwave background (CMB) that is so fundamental to our understanding of the early universe. More refined future studies of the small angular variations of the CMB noted with COBE hold the promise of revealing the complete overall geometry of the universe. In addition to such basic underlying issues we also address how our new windows can be exploited to answer specific questions posed by nature, many in the guise of unexpected phenomena. These pertain to some of todayís most challenging cosmic mysteries, viz:

• What are gamma ray bursts?
• What is the origin of supermassive black holes?
• What can the (dark) matter be?
• How do the great cosmic particle accelerators work?
• Are the highest energy cosmic rays cosmological or indicative of new physics or both?
• What are the unidentified gamma ray sources in our galaxy?
• What is the origin and distribution of galactic cosmic rays?

New Windows: Future Matching Missions (planned or under study)

Other Acronyms