ton waves "crowding" into a shorter distance. Such light is said to be "blueshifted." This picture "is true reasonably close to home," said Tyson. "When you get out to very, very large distances or large velocities, you have to make cosmological corrections.'' A redshift measurement of 1 extends about 8 billion years in look-back time, halfway back to creation. Quasars first emerged around 12 billion years ago, at a redshift of about 4. If we could see 13 billion years back to the time when astrophysicists think galaxy clustering began, the redshift (according to some theories) would be about 5. Galaxy formation probably extends over a wide range of redshifts, from less than 5 to more than 20. Theory suggests that redshift at the singularity would be infinite.

Thus early in the 20th century, astronomy—with the advent of spectrographs that allowed the measurement of redshifts—stood on the brink of a major breakthrough. The astronomer who took the dramatic step—which, boosted by relativity theory, quickly undermined then-current views of a static or stationary universe and provided the first strong observational evidence for the Big Bang paradigm—was Edwin Hubble. An accomplished boxer in college and a Rhodes scholar in classical law at Oxford University, who returned to his alma mater, the University of Chicago, to complete a doctorate in astronomy, Hubble eventually joined the staff of the Mount Wilson Observatory. His name has been memorialized in the partially-disabled space telescope launched in 1990, and also in what is probably the most central relationship in the theory of astrophysics.

Hubble's explorations with the 100-inch Hooker telescope on Mount Wilson were revolutionary. He demonstrated that many cloudlike nebulae in the celestial sky were in fact galaxies beyond the Milky Way, and that these galaxies contained millions of stars and were often grouped into clusters. From his work, the apparent size of the universe was dramatically expanded, and Hubble soon developed the use of redshift to indicate a galaxy's distance from us. He deduced that a redshift not only provided a measure of a galaxy's velocity but also indicated its distance. He used what cosmological distances were directly known—largely through the observations of Cepheid variable stars—and demonstrated that redshift was directly proportional to distance. That is, galaxies twice as far from his telescope were moving at twice the recessional speed.

The relationship between recessional velocity and distance is known as the Hubble constant, which measures the rate at which the universe is expanding. Tyson explained to the symposium that the expansion rate "was different in the past," because expansion slows down as the universe ages, due to deceleration by gravity. Agreeing

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