Basic Measurements in Space

Beginning early in the 20th century, cosmology was revolutionized by astronomers making observations and developing theories that—in the context of first special and then general relativity—led in turn to more penetrating theories about the evolution of the universe. Such theories often required detection and measurement of stellar phenomena at greater and greater distances. These have been forthcoming, furthered by the development of larger optical telescopes to focus and harvest the light, spectrometers and more sensitive detectors to receive and discern it, and computer programs to analyze it. Improved measurements of the composition, distance, receding velocity, local motion, and luminosity of points of galactic light in the night sky provide the basis for fundamental ideas that underlie the Big Bang model and other conceptualizations about the nature of the universe. And radiation outside the optical spectrum that is detected on Earth and through probes sent into the solar system provides an alternative means of perceiving the universe.

With the recognition that the evolving universe may be the edifice constructed from blueprints conceived during the first few moments of creation has come the importance of particle physics as a context to test and explore conditions that are believed to have prevailed then. While many of those at the symposium refer to themselves still as astronomers (the original sky mappers), the term astrophysicist has become more common, perhaps for several reasons having to do with how central physics is to modern cosmology.

REDSHIFT PROVIDES THE THIRD DIMENSION

The development of the spectrograph in the 19th century brought about a significant change in the field of astronomy, for several reasons. First, since the starlight could be split into its constituent pieces (component wavelengths), the atomic materials generating it could at last be identified and a wealth of data about stellar processes collected. Second, spectrographic studies consist of two sorts of data. Bright emission lines radiate from the hot gases. Darker absorption lines are generated as the radiation passes through cooler gas on its journey outward from the core of a star. In the Sun, absorption occurs in the region known as the photosphere. Taken together, these lines reveal not only the composition of a source, but also the relative densities of its various atomic components. As an example, spectrographic studies of the Sun and other nearby stars indicate that 70 percent of the Sun is hydrogen and 28 percent helium, proportions of the two lightest elements that provide corroboration for the Big Bang model. Most stars reflect this proportion, and characteristic spectrographic signatures have also been developed for the major types of galaxies and clusters. But the third and most seminal contribution of spectroscopy to modern astrophysics was to provide a baseline for the fundamental measurement of cosmic distance, known as redshift.

The speed of light has been measured and is known. All radiation is essentially defined by its wavelength, and visible light is no exception. Light with a wavelength of around 4000 angstroms appears blue and that around 7000 angstroms appears red. Radiation with wavelengths just beyond these limits spills into (respectively) the ultraviolet and infrared regions, which our unaided eyes can no longer perceive. Starlight exhibits characteristic wavelengths because each type of atom in a star emits and absorbs specific wavelengths of radiation unique to it, creating a special spectral signature. Quantum physics permits scientists to postulate that the photons that constitute this light are traveling in waves from the star to an observer on Earth. If the source and observer are stationary, or are moving at equal velocities in the same direction, the waves will arrive at the receiving observer precisely as they were propagated from the source.

The Big Bang universe, however, does not meet this condition: the source of light is almost invariably moving away from the receiver. Thus, in any finite time when a given number of waves are emitted, the source will have moved farther from the observer, and that same number of waves will have to travel a greater distance to make the journey and arrive at their relatively receding destination than if the distance between source and observer were fixed. The wavelength expands, gets stretched, with the expanding universe. A longer wave is therefore perceived by the observer than was sent by the source. Again, since astrophysicists know the constituent nature of the light emitted by the stellar source as if it were at rest—from the unique pattern of spectral absorption and emission lines—they have input data for a crucial equation: Redshift = z = (λobs-λem)/λem.

By knowing the wavelength the galaxy's light had when it was emitted (λem), they can measure the different wavelength it has when it is received, or observed (λobs). The difference permits them to calculate the speed at which the source of light is "running away"—relatively speaking—from the measuring instrument. The light from a receding object is increased in wavelength by the Doppler effect, in exactly the same way that the pitch of a receding ambulance siren is lowered. Any change in wavelength produces a different color, as perceived by the eye or by spectrometers that can discriminate to within a fraction of an angstrom. When the wave is longer, the color shift moves toward the red end of the optical spectrum. Redshift thus becomes a measurement of the speed of recession.

Most objects in the universe are moving away from Earth. When astrophysicists refer to greater redshift, they also imply an emitting source that is moving faster, is farther away, and therefore was younger and hotter when the radiation was emitted. Occasionally a stellar object will be moving in space toward Earth—the Andromeda galaxy is an example. The whole process then works in reverse, with photon 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 on the value of the Hubble constant would allow cosmologists to ''run the film backwards" and deduce the age of the universe as the reciprocal of the Hubble constant. Throughout this chapter, this book, and all of the scientific literature, however, one encounters a range rather than a number for the age of the universe, because competing groups attempting to determine the Hubble constant make different assumptions in analyzing their data. Depending on the method used to calculate the constant, the time since the Big Bang can generally vary between 10 billion and 20 billion years. Reducing this uncertainty in the measurement of the Hubble constant is one of the primary goals of cosmology.

As Koo pointed out, three of the largest questions in cosmology are tied up together in two measures, the Hubble constant and Ω. "How big is the universe? How old is the universe? And what is its destiny?" Values for the Hubble constant, said Koo, range from 50 to 100 kilometers per second per megaparsec. This puts the age of the universe between 10 billion and 20 billion years; bringing in various postulated values for Ω, the range can shift to between 6.5 billion and 13 billion years. Said Koo, "If you are a physicist or a theorist who prefers a large amount of dark matter so that Ω will be close to the critical amount," then the discovery by Tyson and others of huge numbers of galaxies in tiny patches of sky should be rather disturbing, because the theoretical calculations predict far fewer for this particular cosmology. Koo called this the "cosmological count conundrum." Conceding that "we, as astronomers, understand stars, perhaps, the best," Koo pointed out another possible anomaly, whereby the faintest galaxies are found to be unusually blue, and thus he calls this result "the cosmological color conundrum.''

"We know that big, massive stars are usually in the very early stage of their evolution. They tend to be hot," explained Koo. And thus they appear very blue. Perhaps the faint galaxies are so blue because they have many more such massive stars in the distant past close to the epoch of their own birth. Resolving such puzzles, said Koo, will "push the limits of the age of the universe very tightly." If the factor of 2 in the uncertainty in the Hubble constant can be reduced, or the estimates of universal mass density refined, major theoretical breakthroughs could follow.