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 universe 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 descriptions 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 chapter? A discussion of how the constituents of the universe--light, stars, life--were created. Although -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 in-telligent beings in this vast space. Astronomers in the 21st century are uniquely positioned to answer these big questions.
What makes modern cosmology an empirical science is that we are literally able to peer into the past. When you look at your image reflected off a mirror one meter away, you see the way you looked six nanoseconds ago--the light's travel time to the mirror and back. Similarly, cosmologists do not need to guess how the universe evolved; we can watch its history through telescopes. Because the universe appears 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 predict formed at a cosmic age of hundreds of millions of years.
But that still leaves a tremendous gap. In between the release of the microwave background and the first rays of starlight was a period when the universe was dark and the microwave 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 background. Observations hint that the universe underwent a wrenching transition during the Dark Ages.