Hot Paper #2, from the international team headed by Saul Perlmutter of the University of California, Berkeley, has cosmologists fretting that we may be living in a low-density university that will expand forever. The current concept of the inflationary universe is severely challenged by observations of the kind described in #2. For the last 20 years the standard assumption has been that the Big Bang was followed by a period of inflation, lasting 10^-35 seconds. In this fleeting moment, the nascent universe ballooned exponentially from something like 10^-26 m across to the size of a basketball. This horrendous stretching, at much faster than the velocity of light, kick-started the huge flat universe we inhabit. What inflation offers cosmologists is an explanation of how distant regions, which today are so far apart that they cannot have been in contact since the Big Bang, can nevertheless look so similar.

   To read the history, the present state, and the future of the universe, the history of expansion has to be deduced. This is read by calibrating the rate at which the universe was expanding at different times in the past, by producing the Hubble diagram which plots distance against velocity. The huge challenge for observational cosmology has been that the universe is so flat and vast that any deviations from a uniform expansion have been lost in the noise and errors of the data. Until now.

   New technologies are enabling us to see far beyond the local neighborhood. In #2 we are taken halfway to the Big Bang, while Hot Paper #10, from Matias Zaldarriaga of MIT and colleagues, brings us to the era when the expanding universe became transparent. In #2 the novel element comes from an early warning system to detect supernovas, while #10 looks forward to the launches of the Microwave Anisotropy Probe (MAP), slated for Fall 2000, and the Planck background probe in 2007.

   Perlmutter’s collaboration looks at Type Ia supernovas, which are "standard candles." Type Ia’s start as white dwarf stars accreting matter. Eventually enough new mass piles up to ignite nuclear carbon-burning, which detonates the entire star. The physics of these exploding stars is relatively simple. They have a characteristic rise and fall in the light curve from the expanding debris which enables the observer to infer the absolute magnitude (a measure of the rate of light emission) at peak brightness. A comparison of absolute magnitude with relative magnitude gives the distance, and a redshift yields the velocity. Then the Type Ia becomes another point on the Hubble diagram.

   Perlmutter’s Supernova Cosmology Project has made a major contribution to the 50 or so high-redshift Type Ia’s now studied. Paper #2 presents a landmark discovery made on March 5, 1997, with the capture of a supernova at z=0.83, or "half the age of the universe." The brightness measurements came from the ground and the Hubble Space Telescope, while the Keck II telescope did the spectroscopy. The new point on the Hubble diagram is located where the differences in cosmological models are accessible to confrontation with reality. And the reality check of the high-redshift supernovas is astonishing: the expansion of the universe is accelerating! This conclusion is a major reason for the high citation rate of #2.

   Putting it another way, these distant supernovas are seen to be dimmer than we would expect in a decelerating universe. What is flinging them away at ever-higher velocities? An acceleration in expansion indicates that the universe has dark energy with negative pressure. This harks back to Einstein’s cosmological constant, a term he arbitrarily introduced into the gravitational field equations to provide repulsion in a static universe.

   The supernova data measure the acceleration. To find the curvature of the universe, we have to use the Cosmic Microwave Background (CMB), which is where #10 enters the plot. Anisotropy in the CMB was detected by the COBE satellite in 1992. The MAP and Planck missions will produce all-sky temperature maps with a resolution of a few minutes of arc. Paper #10 shows how much more information than temperature can be extracted from the data. By using the Fisher information approach, the data can be used to constrain several cosmological parameters such as the Hubble constant, and key ratios including the matter-to-photon ratio.