Figure 4. Map of the density distribution of mass inferred from studies oflarge-scale flow (upper panel) compared to the density distributionof galaxies observed in a largeredshift survey (lower panel). (Courtesyof Avishai Dekel et al., Sept. 1993, and Amos Yahil et al., Sept.1993.)

same region in the lower panel. The two roughly agree, suggestingthat galaxies do trace mass, at least approximately. This resultis important evidence that the gravitational instability pictureis basically correct.

Because cosmic flows can measure the clustering of matter on evenvery large scales, they are the best indicator of the absolute levelof density fluctuations in the universe today. This indicator ofdensity fluctuations can be compared to the strength of CMBR fluctuationson larger scales at earlier times. Close to Earth, within 250 millionlight-years where flows are well measured, flow velocities have approximatelythe magnitude predicted if standard dark-matter theories employ theCOBE measurements. This local agreement suggests that our basic modelfor structure formation, spanning many decades of length scale anddepending on details of the nature of dark matter, is approximatelycorrect.

By measuring the size of the flow motions around particular clumpsof galaxies, astronomers can estimate the total amount of matterin each one. If galaxies trace the distribution of matter, or evenif not, as long as their distribution is biased in a consistent andpredictable way, astronomers can generalize from the galaxy massesto estimate the total matter density in the universe. In short, astronomerscan “weigh the universe” and measure the elusive parameter Ω the ratio of the mean mass density to that required to closethe universe and eventually stop its expansion.

Our present knowledge of galaxy formation and biasing is still poor.Nevertheless, cosmologists can draw two conclusions. First, measurementsof cosmic flows on all scales are an important test of competingtheories of structure formation. And second, the high values observedfor cosmic velocity flows are the strongest indicator so far thatΩ might actually be 1, a value favored by theoretical considerations,as explained below in section V.

Measuring cosmic flows

Since cosmic flow is a deviation from the Hubble-law motion of agalaxy, measuring it requires two types of observations: First, theobserved redshift of the galaxy must be measured from its spectrum.Second, an independent estimate of the distance of the galaxy mustbe made, which is much more

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