For close to 70 years following the discovery of Hubble's law, astronomers made assumptions about universal expansion that seemed reasonable and conformed to how the universe could be expected to behave. The expansion of the universe that began in the big bang would decrease in strength as the gravitational attraction of matter worked over time to slow expansion. Hubble's constant would be found to be decreasing over time. This fact was built into the standard relativistic models; the R-t graphs giving the scale factor R as a function of t were all concave downward, indicating the steady slowing of the universal expansion. (The Eddington and Lemaitre models from the early 1930s were never seriously pursued.) A basic constant of big bang cosmo-logical theories was the deceleration parameter, giving the rate at which the expansion of the universe is slowing down.

In the 1990s, separate teams of astronomers at Harvard University and the University of California at Berkeley embarked on an investigation of how Hubble's red shift-distance law holds up for more distant galaxies. In order to do this, it was necessary to obtain accurate data for the distance to these galaxies that was independent of the values given by the law itself. Since the red shifts were given directly by spectroscopic measurement, the recessional velocities of the galaxies were known. It was then simply a matter of matching the red shifts to the independently determined distances in order to see how well Hubble's law fit the data. The data for this research was provided by observations of distant galaxies made with the Hubble Space Telescope.

To estimate the distance to a galaxy, it is necessary to find a method for determining the intrinsic brightness of some of its stars. For galaxies fairly close to us it has been possible to use the Cepheid-variable method, a method greatly extended by observations with the Hubble Telescope and the new generation of powerful, Earth-based telescopes. To find the distances to more distant galaxies, astronomers have used an object known as a type 1A supernova. Such a supernova consists of a binary-star system, in which one member is a white dwarf and the other is a star excreting matter to the dwarf. The mass of the receiving star increases until it reaches a certain value, at which point the star explodes as a supernova. A key characteristic of this class of supernovae is that each possesses roughly the same intrinsic magnitude and exhibits a similar light curve of brightness variation during its short career as an exploding star. Such differences in intrinsic brightness that do exist are related to the star's period of variation and the shape of its light curveā€”the longer the period, the brighter the supernova.

Early in 1998 the astronomers involved in the supernova research made a major announcement. Suppose one begins with the Hubble constant as determined for galaxies in the neighborhood of the Milky Way galaxy. If one then takes a more distant galaxy and puts the value of its distance as given by the type 1A supernova yardstick method into the Hubble equation, one obtains a red shift that is larger than the one that is actually observed. The galaxy is receding more slowly than it should according to the red shift law. This implies that the expansion of the universe is accelerating as one moves forward in time and closer in space to the galaxy. It would be an understatement to say that this result came as a surprise.

Astronomers devoted much effort to verifying this result and to checking whether some other effect could account for it. If in fact there were obscuring dust or matter present in space, the supernovae would be closer than they appear to be, and the supernova yardstick method would be overestimating their distances. The reduced distances would then fit properly into the Hubble law, and the conventional picture would be confirmed. However, the optical effects of obscuring matter, such as a reddening of the light over and above the red shift of expansion, have not been observed.

The discovery of acceleration ranks with the Hubble relation and the cosmic background radiation as one of the fundamental findings of modern cosmology. With good reason, Science magazine called the discovery the scientific story of the year. One response to the supernovae studies has been to renew the study of relativistic world models, in which the cosmological constant X is positive. Cosmologists have speculated that space is filled with some kind of "dark energy" measured by X that propels the expansion of the universe. This idea makes sense if one supposes that expansion is subject to the retarding force of gravity and a propelling force corresponding to the dark energy. When the universe was younger and more dense, the force of gravity was strong relative to the dark energy force; as it has expanded and become less dense, the latter force has gained strength relative to gravity, and the result is the accelerated expansion observed by astronomers. This picture is supported by the most detailed data currently available on distant supernovae, which seem to indicate that the universe was decelerating until about six billion years ago, at which time, expansion entered its current phase of acceleration.

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