There is strong evidence that the universe was once hot and dense, some ~ 10 billion years ago. Such a beginning is often called the big bang. The proportions of light elements (Li, Be, He) in the solar system and Galaxy relative to hydrogen are nicely consistent with those expected from nuclear interactions in the hot early universe. Also, a diffuse radiation at microwave frequencies is seen to arrive from all directions of the sky. It is known as the cosmic microwave background radiation, CMB, and has a blackbody spectrum with temperature 2.7 K. This radiation is expected theoretically to be the remnant of the early hot phases, and its characteristics match the theory extraordinarily well. Also geological information yields ages of rocks and meteorites that are comparable to 1010 years, an age inferred independently from the expansion of the galaxies.

The final fate of the universe is not known. According to viable theories, it could expand forever, it could slow to a stop and start contracting, or it could be in between, slowing "critically" forever. Continuing interplay of observation and theory are providing further progress on this issue; see below.

New horizons

Another consequence of the general theory of relativity is that oscillating masses should radiate gravitational waves. (Recall that oscillating electric charges radiate electromagnetic waves.) A binary radio pulsar can consist of two neutron stars in orbit about their common center of mass. If they radiate away enough energy through gravitational radiation, they will move closer together as they orbit around their common center of mass. In so doing they lose potential energy and gain a lesser amount of kinetic energy. At this writing, the rates of decay of several such systems, including PSR 1913 + 16 (the Hulse-Taylorpulsar) and PSR 1534 + 12, confirm to very high precision the predictions of Einstein's general relativity. There are efforts underway to detect gravitational waves directly as mentioned above. Very sensitive and large detection systems are required due to the small amplitude of the expected signals.

The physics of the interior of the sun may not be completely resolved. The neutrino experiments mentioned above do not show the expected numbers of neutrinos; the observed rate is about half that expected. The neutrinos are a measure of the nuclear reactions taking place within the sun. Are the conditions of temperature, density, and composition in the nuclear-burning regions of the sun not well understood? Are the neutrinos changing form (neutrino oscillations) as they pass through the solar material so some of them become undetectable in existing instruments? Neutrino oscillations of a different type of neutrino (the muon neutrino) have been detected in the flux of neutrinos created by cosmic ray interactions in the earth's atmosphere. This is a major advance in neutrino physics. The resolution of the solar neutrino puzzle has important ramifications in astrophysics and particle physics, and ongoing experiments are addressing it.

There is a well-received theory of the early universe (the inflationary universe) wherein it expanded by many orders of magnitude very early, 10-33 s, after the "big bang". This suggests the universe is now expanding at just the critical rate where it asymptotically approaches zero expansion speed; it neither completely "escapes", nor does it start falling inward. A major difficulty with this view is that the visible matter in the universe falls far short of the mass required to yield the critical condition.

If the theory is correct, there must be some new kind of dark matter of an unknown type. Such speculation gains credence from the motions of stars in galaxies and of galaxies in clusters of galaxies. In both cases, the objects move so rapidly in a confined volume that unseen matter would seem to be holding them in their orbits. The nature of this dark matter is one of the great questions now facing astrophysicists. The picture is further complicated by indications from observations of supernovae that the expansion of the universe is increasing, due to some (unknown) type of dark energy.

The distribution of galaxies in space is found to be very clumpy with huge voids and "walls" of galaxies. This can be compared to the very smooth distribution of the CMB. It is generally believed that the galaxy clustering arises from small density fluctuations in the early universe, and these should be visible as tiny fluctuations in the brightness (temperature) of the CMB as a function of angle on the sky. In fact, such fluctuations have been detected by the Cosmic Background Explorer (COBE) and WMAP satellites, and with experiments carried out in Antarctica (high-altitude balloon and ground based). These warm and cool spots and the postulated existence of large amounts of cold dark matter could well lead to the formation of galaxies with the observed clustering. The dominant angular scale of the CMB fluctuations, ~1°, indicates that the universe is expanding at the critical rate, or equivalently that it has a "flat" geometry. This gives support to the existence of an episode of inflation in the early universe. Confidence in these ideas is growing at this writing.

These are only some of the challenges facing astronomers today. The nature of gamma-ray bursts, the most energetic explosions known to man, and the mechanisms that give rise to jets of material from many different types of celestial objects are among them. If the past has been any indication, some of the answers will prove to be quite surprising. Also serendipitous discoveries will surely provide new surprises and new questions. It is likely that many of the questions asked today will be shown to be off the mark. The truth will lie in other unexpected directions.

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