Read First Light: The Search for the Edge of the Universe for Free Online Page B

known as a Doppler shift. It is similar to the lengthening whistle of a departing train. According to Hubble’s law, the more distant a galaxy is, the faster it is movingaway from us, and therefore the more downwardly stretched—more redshifted—the galaxy’s light appears to be. Quasars are the most redshifted objects in the universe, which to most astronomers signifies that quasars are also the most distant objects that can be seen through a telescope. They inhabit the outer reaches of the optically explored universe: the edge of the universe.
    The word
redshifted
is misleading in the case of quasars, for a deeply redshifted quasar is not exactly red in color. Quasars disgorge opulent, multitudinous colors all at once—gamma rays, X rays, ultraviolets, blues, greens, yellows, reds, infrareds, microwaves, and, in the case of some quasars, radio waves, all of which are forms of light at different wavelengths. The trick of recognizing a redshift in a quasar by examining its light was at one time not easy to accomplish. Maarten Schmidt invented that trick in 1963. While reading the text of light from a quasar, Schmidt discovered that quasars are not nearby stars, as everyone had supposed, but monsters—objects on the backdrop of the sky, unimaginably far beyond the local galaxies. In effect, he showed that what looked like fireflies in our backyard were beacons near the horizon.
    As a telescope looks out at quasars it looks not only toward the edge of the universe, but also toward the beginning of time. Light consists of photons, which are inseparably both waves and particles. Light moves at a speed of 186,282 miles per second through space—a snail’s pace by the measure of cosmic distance. Nothing can move faster than a photon. A photon would require about fifty thousand years to traverse the length of the Milky Way galaxy. If a star were to explode on the other side of the Milky Way, astronomers would learn about it fifty thousand years later.
    An event cannot be seen, or known, until the photons emitted by that event reach a detector, such as photographic film or the retina of the human eye. A light-year is the distance that a photon can travel through a void in one year, which happens to be about six trillion miles. Photons produced by an event happening billions of light-years away from an observer will require billions of years to stream toward the observer. When a telescope makes a photograph of deep sky, it makes an image of the past; it displays events that took place in different periods of cosmic history, depending on how far away from the earth they are.
    Astronomers refer to the depth of astronomical vision as lookback time. Seeing outward is equivalent to looking backward in time, because the telescope’s mirror is capturing primeval light. The universe—as we see it—could be imagined as a series of concentric shells centered on the earth—shells of lookback time. The shells closest to the earth contain images of galaxies near us in time and space. Farther out are shells containing images of remote galaxies—galaxies as they existed before our time. Still farther out is the shell of the early universe. Some of the photons reaching a telescope’s mirror are nearly as old as the universe itself. The quasars are brilliant pinpoints of light that seem to surround the earth on all sides, shining out of deep time. Beyond the quasars, the observable universe has a horizon, which could be imagined as the inner wall of a shell. This horizon is the limit of lookback time, which is also an image of the beginning. As a mirror looks toward the edge, it looks toward the beginning. At the end of the sky lies the beginning.
    The sky could be imagined as a palimpsest containing stories written on top of one another going back to the origin of time. A telescope looking outward into lookback time strips layers from the palimpsest; it magnifies and reimages small, faint letters in the underlayers of the manuscript. The