One of the science drivers behind the Hubble Space Telescope (HST) was the need, in the mid-1980s, to measure H₀ to an accuracy of 10%, and this goal was chosen as a Key Project of HST. The group responsible for Hot Paper #7 was awarded the project in 1986, with most of the observations being carried out from 1993. The methodology was first to define the distance scale accurately out to about 25 Mpc, then use that knowledge to calibrate secondary distance indicators further out. Extragalactic distances are mostly determined by observing "standard candles," that is to say objects for which the luminosity can be directly inferred. For example, all Type Ia supernovae have the same value for the luminosity at the peak of the outburst. This peak luminosity is calibrated from previous observations of seven supernova explosions in local galaxies. The calibration is then used for distance assessment of any galaxies further out that happen to have a Type Ia outburst. This method has already suggested that the expansion rate of the universe is accelerating.

Wendy Freedman (Observatories of the Carnegie Institution of Washington, Pasadena, California) and her numerous collaborators used observations of Cepheid variable stars in 17 nearby galaxies to provide a calibration system for the secondary methods of distance determination. Cepheids are young, bright, variable stars undergoing regular pulsation. The period-luminosity relation for these stars has a small scatter, which makes them attractive as calibrators. Furthermore they are abundant in spiral galaxies, and can be seen out to about 30 Mpc with HST. The Key Project has already published 29 papers on distances to individual galaxies based on the Cepheid method.

Paper #7 is effectively a Final Report, which revises some earlier data, and places all the data on a common zero point. The results of the revised calibration are then applied to secondary distance indicators in galaxies at distances of 60 – 400 Mpc. And the final answer, from the enormous effort that has been made, is that H₀ = 72 ± 8 km s^-1 Mpc-1. Without question, the professional astronomical community will regard this outcome as a triumph, solidly based, with meticulous attention to accounting for every conceivable source of error. Because H₀ is a ubiquitous parameter in extragalactic astronomy and cosmology, a large number of papers can be expected to cite this result. And arguments should cease.

Newcomer #9 reports an experiment to trap, store, and release a light pulse. Ron Walsworth and Mikhail Lukin and their colleagues at the Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts, have brought light to a standstill in a vapor of Rb atoms. Physicists had already found out how to stop light in Bose-Einstein condensate at microkelvin temperatures. Walsworth and Lukin demonstrate the same effect in an atomic vapor. The gas used in these experiments is normally opaque, but it can be made transparent near to an atomic resonance by shining a laser beam called the "coupling field." This makes it selectively transparent to light of a certain color. Then the "signal field," a pulse of laser light is introduced, and it slows to 60 km s^-1, in a dramatic reduction in group velocity caused by coupled light and atomic excitations. The coupling field is gradually reduced to zero, leading to a dark state in which the signal light pulse is effectively trapped inside the cell holding the vapour. Paper #9 reports that the process is reversible, with the storage time limited only by the atomic coherence time. This avenue of research may lead to applications in quantum computation if techniques can be found to control the quantum evolution of photon-atom interacting system.

Physicists’ quest for quark matter is the backdrop to Hot Paper #4, which describes results from the Relativistic Heavy-Ion Collider at Brookhaven National Laboratory. The first collisions between Au nuclei have resulted in the highest center-of-mass energies achieved at the Laboratory to date. The initial results at 56 and 130 GeV are a first step in being able to build a full understanding of nucleus-nucleus collisions at these energies. A prediction of quantum chromodynamics is that at sufficiently high energies, quarks and gluons become deconfined. A phase change occurs in which the low energy phase of quarks and gluons imprisoned in a nucleus is replaced by a quark-gluon plasma extending through the bulk of nuclear matter. Astronomers may have come close to seeing strange quark matter, with the discovery of the first quark stars by NASA’s Chandra Observatory.