Until recently, most astronomers believed that the universe had entered a very boring middle age. According to this paradigm, the early history of the universe--that is, until about six billion years after the big bang--was an era of cosmic fireworks: galaxies collided and merged, powerful black holes sucked in huge whirlpools of gas, and stars were born in unrivaled profusion. In the following eight billion years, in contrast, galactic mergers became much less common, the gargantuan black holes went dormant, and star formation slowed to a flicker. Many astronomers were convinced that they were witnessing the end of cosmic history and that the future held nothing but the relentless expansion of a becalmed and senescent universe.
In the past few years, however, new observations have made it clear that the reports of the universe's demise have been greatly exaggerated. With the advent of new space observatories and new instruments on ground-based telescopes, astronomers have detected violent activity occurring in nearby galaxies during the recent past. (The light from more distant galaxies takes longer to reach us, so we observe these structures in an earlier stage of development.) By examining the x-rays emitted by the cores of these relatively close galaxies, researchers have discovered many tremendously massive black holes still devouring the surrounding gas and dust. Furthermore, a more thorough study of the light emitted by galaxies of different ages has shown that the star formation rate has not declined as steeply as once believed.
The emerging consensus is that the early universe was dominated by a small number of giant galaxies containing colossal black holes and prodigious bursts of star formation, whereas the present universe has a more dispersed nature--the creation of stars and the accretion of material into black holes are now occurring in a large number of medium-size and small galaxies. Essentially, we are in the midst of a vast downsizing that is redistributing cosmic activity.
TO PIECE TOGETHER the history of the cosmos, astronomers must first make sense of the astounding multitude of objects they observe. Our most sensitive optical views of the universe come from the Hubble Space Telescope. In the Hubble Deep Field studies--10-day exposures of two tiny regions of the sky observed through four different wavelength filters--researchers have found thousands of distant galaxies, with the oldest dating back to about one billion years after the big bang. A more recent study, called the Hubble Ultra Deep Field, has revealed even older galaxies.
Obtaining these deep-field images is only the beginning, however. Astronomers are seeking to understand how the oldest and most distant objects evolved into present-day galaxies. It is somewhat like learning how a human baby grows to be an adult. Connecting the present with the past has become one of the dominant themes of modern astronomy.
A major step in this direction is to determine the cosmic stratigraphy--which objects are in front and which are more distant--among the thousands of galaxies in a typical deep-field image. The standard way to perform this task is to obtain a spectrum of each galaxy in the image and measure its redshift. Because of the universe's expansion, the light from distant sources has been stretched, shifting its wavelength toward the red end of the spectrum. The more the light is shifted to the red, the farther away the source is and thus the older it is. For example, a redshift of one means that the wavelength has been stretched by 100 percent--that is, to twice its original size. Light from an object with this redshift was emitted about six billion years after the big bang, which is less than half the current age of the universe. In fact, astronomers usually talk in terms of redshift rather than years, because redshift is what we measure directly.
Obtaining redshifts is a practically foolproof technique for reconstructing cosmic history, but in the deepest of the deep-field images it is almost impossible to measure redshifts for all the galaxies. One reason is the sheer number of galaxies in the image, but a more fundamental problem is the intrinsic faintness of some of the galaxies. The light from these dim objects arrives at a trickle of only one photon per minute in each square centimeter. And when observers take a spectrum of the galaxy, the diffraction grating of the spectrograph disperses the light over a large area on the detector, rendering the signal even fainter at each wavelength.