On the plains of San Agustin, a vast new mexican valley filled with sagebrush and inhabited mostly by cattle, MIT radio astronomer Jacqueline Hewitt sits in the control room of the Very Large Array, a collection of 27 rail-mounted radio telescopes, each with a dish antenna more than 80 feet across. The antennas are positioned as far apart as possible, forming a Y with the control room at the center and the farthest telescope in each arm some 15 miles away. The dishes are all focused on a single point in the sky. Hewitt is tuning in on a quasar--a pointlike object billions of light-years away that radiates the energy of a million suns, mostly as radio waves. She is waiting for its signal to flicker.
At the four-meter optical telescope atop Cerro Tololo, in the dry mountains of northern Chile, another MIT astronomer, John Tonry, is looking at galaxies, trying to measure the subtle differences in brightness across their surfaces. And at Cerro Tololo’s sister telescope, the four-meter on Kitt Peak in southern Arizona, a team of observers from several institutions is scanning galaxies for planetary nebulas, the bright blobs of gas blown off by giant stars in their death throes.
The observations could hardly be more different, but each project is an attempt to answer two of the most fundamental questions about the universe: How big is it, and how old? The questions are equivalent. In asking how big the universe is, we are talking about the visible universe: the sphere, centered on Earth, whose radius is the distance light could have traveled since the Big Bang, when the universe came into being as a single point and began expanding rapidly outward. The distance to the edge of the visible universe, in light-years, is the same as the number of years since the whole universe began. The universe as a whole may well be bigger than the visible universe; in the first nanosecond of its existence it may have undergone a burst of faster-than-light inflation. But astronomers cannot hope to measure what they cannot see.
And they’ve been remarkably unsuccessful at measuring the size of what they can see. They have uncovered some of the secrets of black holes, of pulsars, and of colliding galaxies; they can follow the birth, life, and death of stars in great detail; they can even tell you what happened in the very first seconds after the Big Bang. But they don’t know when that Bang occurred, at least not very accurately. The universe is either 10 billion years old or 20 . . . or somewhere in between. The visible universe is either 20 billion light-years across or 40 billion . . . or somewhere in between.
