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The Sloan Digital Sky Survey (SDSS) is making a multi-colour, three-dimensional map of the nearby Universe. The survey is in two parts. The first part is imaging one quarter of the sky in five colours from the near ultraviolet to the near infrared. This imaging survey is expected to detect around 50 million galaxies to a magnitude limit g~23. The second part of the survey, taking place concurrently with the imaging, is to obtain spectra for up to 1 million galaxies and 100 000 quasars. From these spectra one obtains redshifts and hence distances, in order to map out the three-dimensional distribution of galaxies and quasars in the Universe. These observations will be used to constrain models of cosmology and of galaxy formation and evolution. This article describes the goals and methods used by the SDSS, the current status of the survey, and highlights some exciting discoveries made from data obtained in the first two years of survey operations.

Observations of supernova explosions halfway back to the Big Bang give plausible evidence that the expansion of the universe has been accelerating since that epoch, approximately 8 billion years ago and suggest that energy associated with the vacuum itself may be responsible for the acceleration.

Gravitational lenses, besides being interesting in their own right, have been demonstrated to be suitable as 'gravitational standard rulers' for the measurement of the rate of expansion of the Universe ([H.sub.o]), as well as to constrain the values of the cosmological parameters such as [[Omega].sub.o] and [[Lambda].sub.o] that control the evolution of the volume of the Universe with cosmic time.

How much matter is there in the universe? Does the universe have the critical density needed to stop its expansion, or is the universe underweight and destined to expand forever? We show that several independent measures, especially those utilizing the largest bound systems known - clusters of galaxies - all indicate that the mass-density of the universe is insufficient to halt the expansion. A promising new method, the evolution of the number density of clusters with time, provides the most powerful indication so far that the universe has a subcritical density. We show that different techniques reveal a consistent picture of a lightweight universe with only [approximately]20-30% of the critical density. Thus, the universe may expand forever.

According to General Relativity, the total mass density of the Universe determines its future. With too much matter, it will collapse on itself; with too little, the present expansion will continue for eternity. The density also governs the past, by determining how galaxies and stars came to form. Over the last decade, new theoretical understanding of the very early phases of creation has led to the prediction that the density must in fact have exactly the critical value which divides collapse from eternal expansion. Most excitingly, recent observational work appears to support this expectation.

Determination of distances at which the extragalatic nebulae are reducing from our own galaxy is the key to modern cosmology and to measurement of the size of the Universe. But not only are there differences between astronomers on the figures involved but also major conceptual problems. It is necessary, for example, to accept the idea of a closed space without a boundary but of finite volume.

A recent observational advance has been the mapping of the expansion pattern of the universe in the neighbourhood of the Milky Way. These studies reveal significant departures from uniform Hubble expansion, thought to be associated with the growth of superclusters. In part 1 of this review I discuss currently popular hypotheses concerning how structure in our universe might have originated and its current evolution. In part 2 I shall describe the observations and show how these can be used to test the theoretical ideas.

NOBODY WOULD ARGUE that human beings appeared out of nothing. We are complex creatures, and could not have arisen 'just by chance' out of a brew of chemicals, even in [...]

Three new measurements peg the celebrated Hubble constant at values high enough to give theorists nightmares. Is the debate finally over? Don't bet on it In life, it has been [...]

On September 6, 1992, a violent storm raged over Poland. As its relentless rains turned roads into rivers, astronomers, physicists, mathematicians and students from 13 countries descended on the land [...]

There is some evidence which suggests that the Universe is infinite. However, researchers have realised that if the Universe's curvature is negative then it can be finite. It may be possible to measure its shape and size. Knowing whether it is finite affects theories about its origin and also determines whether it will eventually collapse. If the density of the Universe is less than Einstein's equation which defines the critical density of the universe then it is finite and will continue to expand. It will collapse if it has larger than the value of Einstein's equation.

Cosmologists have a reputation as dreamers--scientists who base their ideas as much on philosophy and aesthetics as on hard facts. Hard facts about the origins and fate of the universe [...]

Einstein was wrong once. In 1917, to keep the universe from collapsing under its own gravity, Einstein added to his equations of general relativity a constant that countered the inward [...]

The branes are planes and make the cosmos wane. So says a new theory published online by Science this week (www.sciencexpress. org). Surprisingly, the theory--an alternative to the standard, inflationary [...]