THE DARK AGES of the Universehen I look up into the sky at night, I often wonder whether we humans are too preoccupied with ourselves. There is much more to the universe than meets the eye on earth. As an astrophysicist I have the privilege of being paid to think about it, and it puts things in perspective for me. There are things that I would otherwise be bothered by—my own death, for example. Everyone will die sometime, but when I see the universe as a whole, it gives me a sense of longevity. I do not care so much about myself as I would otherwise, because of the big picture.Cosmologists are addressing some of the fundamental questions that people attempted to resolve over the centuries through philosophical thinking, but we are doing so based on systematic observation and a quantitative methodology. Perhaps the greatest triumph of the past century has been a model of the uni-verse that is supported by a large body of data. The value of such a model to our society is sometimes underappreciated. When I open the daily newspaper as part of my morning routine, I often see lengthy de-scriptions of conflicts between people about borders, possessions or liberties. Today’s news is often forgotten a few days later. But when one opens ancient texts that have appealed to a broad audience over a longer period of time, such as the Bible, what does one often find in the opening chap-ter? A discussion of how the constituents of the universe—light, stars, life—were created. Al-though humans are often caught up with mundane problems, they are curious about the big picture. As citizens of the universe we cannot help but wonder how the first sources of light formed, how life came into existence and whether we are alone as intelligent beings in this vast space. Astrono-mers in the 21st century are uniquely positioned to answer these big questions.What makes modern cosmology an em-pirical science is that we are literally able to peer into the past. When you look at your image reflected off a mirror one meter Astronomers are trying to fill in the blank pages in our photo album of the infant universe WTHE DARK AGES of the UniverseBy Abraham Loeb S C IE N T IF IC A ME R I C A N 4748 S C IE N T I F IC A ME R I C A N NO V EMB ER 2 0 06away, you see the way you looked six nanoseconds ago—the light’s travel time to the mirror and back. Similarly, cosmolo-gists do not need to guess how the universe evolved; we can watch its history through telescopes. Because the universe ap-pears to be statistically identical in every direction, what we see billions of light-years away is probably a fair representation of what our own patch of space looked like billions of years ago.The ultimate goal of observational cosmology is to capture the entire history of the universe, providing a seamless picture of our descent from a shapeless gas of subatomic particles. We have a snapshot of the universe as it was 400,000 years after the big bang—the cosmic microwave background radiation—as well as pictures of individual galaxies a billion years later. By the middle of the next decade, NASA plans to launch a new space telescope, named the James Webb Space Telescope (JWST), that should be able to pick up the first galaxies, which theorists pre-dict formed at a cosmic age of hundreds of millions of years.But that still leaves a tremendous gap. In between the re-lease of the microwave background and the first rays of star-light was a period when the universe was dark and the micro-wave background no longer traced the distribution of matter. It might sound like a languid, gloomy time, a boring interlude between the immediate aftermath of the big bang and the bustling cosmos of the present day. Yet a great deal happened in these Dark Ages: the primordial soup evolved into the rich zoo of celestial bodies we now see. Within the inky blackness, gravitational forces were assembling objects in the cosmos.The situation that astronomers face is similar to having a photo album of a person that contains the first ultrasound image of him or her as an unborn baby and some additional photos as a teenager and an adult. If you tried to guess from these pictures what happened in the interim, you could be seriously wrong. A child is not simply a scaled-up fetus or scaled-down adult. The same is true with galaxies. They did not follow a straightforward path of development from the incipient matter clumping evident in the microwave back-ground. Observations hint that the universe underwent a wrenching transition during the Dark Ages.Astronomers are currently searching for the missing pages of the cosmic photo album, which will show how the universe evolved during its infancy and made the building blocks of galaxies like our own Milky Way. A decade ago, when I start-ed to work on this effort, only a handful of researchers were interested in it. Now it motivates a major fraction of future observational projects and promises to be one of the most exciting frontiers in cosmology over the next decade. From Ions to Ionsac c or di n g t o the big bang theory, the early universe was filled with hot plasma—a cauldron of protons, electrons and photons, with a smattering of other particles. The freely mov-ing electrons interacted with photons through a process known as Thomson scattering, which coupled matter and • Much of the attention in cosmology over the past several years has focused on the cosmic microwave background radiation, which provides a snapshot of the universe at an age of 400,000 years. But between this moment and the appearance of the first galaxies was a period of almost total darkness, broken by not so much as a glimmer of starlight. Hidden in the shadows of this era are the secrets of how galaxies took shape.• Clearly, it is hard to probe a period that is by its very nature practically invisible. The key is to look for the feeble radio waves emitted by electrically neutral hydrogen gas as it interacts with the background radiation. Observers are now starting to do so.• The result should be an even more interesting map than that of the microwave background. It will be fully three-dimensional and will show, step by step, how form emerged from formlessness. Overview/Epoch of ReionizationJ E AN -F R ANC OI S P OD E V IN ( pr e c e di ng p a g e s and s ta r t o f b i g b a ng , th is p ag e) ; M A X T EG M AR K M a ss ac h u s et ts I ns ti tu te o f Te c h no lo g y (WM AP)10 –43