This result, as well as other observations, seems to suggest that our universe is of the open variety.

There are uncertainties in these estimates, mostly connected with uncertainties about cosmic distances. If the universe were precisely homogeneous and uniformly expanding, then the rate of expansion of the universe (the Hubble constant) could be determined by measuring the recessional speed and distance of any galaxy, near or far. Conversely, the distance to any galaxy could be determined from its redshift and the application of Hubble's law. The average density of matter could be figured by estimating the mass of a group of galaxies and then dividing by the volume of space that it occupies. However, the universe is not at all homogeneous; structure is apparent on every scale we have studied. Because of local inhomogeneities, the rate of expansion of the universe and the average density of matter need to be measured over as large a region as possible. Accurate determinations of distances to galaxies are needed for both of these measurements.

One possible means of accurately determining distance involves the scattering of the cosmic background radiation by hot gas in clusters of galaxies by a process called the Sunyaev-Zeldovich effect. The hot gas gives a slight energy boost to the radio waves as they pass through it on their way to the earth. As a result of measuring both the change in energy of the radio waves and the x-ray emission from the hot gas, the distance to the cluster of galaxies will be well determined. With the MMA, AXAF, and other instruments, astronomers hope to make these measurements in the coming decade. Such measurements repeated for a large number of galaxy clusters would permit a more accurate determination of the rate of expansion of the universe.

Likewise, studies of the velocities and distances to a large number of galaxies using new 4-m telescopes equipped with spectrographs of novel design are capable of measuring the distances to hundreds of galaxies at a time. These data could pin down the local values of both omega and the Hubble constant. Peculiar velocities of galaxies depend on the extra amount of matter concentrated in a region, over and above the average density of cosmic matter. Measurements of peculiar velocities, together with a knowledge of how much matter there is above the average, lead to an estimate for omega.

Despite these difficulties, cosmologists are almost positive that the value of omega lies between 0.1 and 2. Enough matter has been identified so that omega cannot be less than about 0.1. On the upper end, an omega larger than 2, together with the current rate of expansion, would translate to an age of the universe that is less than the age of the earth as determined by radioactive dating.

The inflationary universe model firmly predicts that omega should be equal to 1, exactly. On this basis, the model can in principle be either ruled out or supported from observational evidence. Since current observations suggest a value of omega closer to 0.1, scientists who believe on theoretical grounds that



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