century ago the universe was thought to be about 30 million years old, encompassing only our own galaxy. But in 10 years during the early 20th century, the physicist Albert Einstein and the astronomer Edwin Hubble changed that forever. Hubble showed that cloudy light patches in the sky, called nebulae, are actually other galaxies and that each is moving away from the others, suggesting a theory of the origins of the universe that eventually became known as the Big Bang. His discovery fit neatly with the mathematics of Einstein's general relativity theory: space expands, carrying with it galaxies and everything else.

Big scientific instruments and big ideas have expanded knowledge a great deal since then. In the 1960's quasars were discovered -- the most powerful beacons of the universe, young galaxies brightened by the gobbling up of stars by their central black holes. That discovery spelled the end of the unevolving Steady State model of the universe that had been a competitor to the Big Bang theory. In 1964 two physicists at Bell Laboratories serendipitously discovered a cosmic sea of microwaves. Two decades earlier, the physicist George Gamow had predicted a faint microwave afterglow from a Big Bang beginning of the universe, but no one took it seriously enough to search for it. After 1964, cosmology, the study of the origin and development of the universe, which physicists had been very wary of, was suddenly a legitimate pursuit. Since then astronomers and physicists, working together, have developed what is known as the standard hot Big Bang model of the universe, a major intellectual achievement of the 20th century. It describes in detail the 14-billion-year evolution of the universe, from the primordial soup of elementary particles when it was less than a second old, to the galaxies, and clusters and superclusters of galaxies, that define it today.

Ken Croswell, who was a popular science writer even before he received a doctorate in astronomy in 1990, chronicles the rise of the standard model in ''The Universe at Midnight.'' As his title suggests, he approaches cosmology from the perspective of astronomy and conveys the romance of the profession, and the science. The drama of pinning down the elusive Hubble constant (the rate at which the universe expands) plays out in interviews, including one with the eminent astronomer Allan Sandage, a protege of Hubble, and one with Wendy Freedman, the young scientist who led the modern assault on the problem with the Hubble Space Telescope. They occupy offices within a few yards of one another, but their views are light-years apart.

Today's cosmology is a delicate choreography of physics and astronomy. Telescopes, looking back in time, let astronomers peer into the inner space of the elementary particles. Particle accelerators strive to recreate the quark soup of the beginning. Croswell only alludes to the breakthrough behind the current excitement, the coming together of the physics of elementary particles and the birth of the universe.

Around 1980, the fundamental building blocks were identified: quarks, leptons, gluons and the like. They were all present in the primordial soup. Astronomers have long known that the gravity of stars is insufficient to hold galaxies and other structures in the universe together. They reasoned that the gravity of something not visible -- dark matter -- was responsible. The surprise came in the 1980's when David Schramm, an inner space-outer space pioneer, argued that there wasn't enough ordinary matter to account for the effect and that a new form of matter, produced just after the Big Bang and not made of quarks or leptons, must be the primary stuff of the cosmos. While stars and their byproducts (including us) are made of quarks, the universe is probably not. Today, the neutralino, a hypothetical particle predicted by superstring theory, is the leading candidate for the stuff of the cosmos. Particle accelerators struggle to produce the first ones made since the Big Bang, while ultrasensitive detectors in underground laboratories try to detect the neutralinos holding our galaxy together.

Even ordinary matter comes from particle interactions during the first microsecond after the Big Bang, which led to a tiny excess of quarks over antiquarks in the primordial soup. As the universe cooled, the quarks annihilated all the antiquarks. The few remaining quarks became the matter of which we are made.

Even weirder than quarks or dark matter is dark energy. There is twice as much dark energy as dark matter; it has repulsive gravity, and it is causing the universal expansion to speed up. Its presence was discovered recently by competing teams, one led by a physicist and the other by an astronomer. Both were trying to measure a slowing down in the expansion of the universe. Instead, they found it is accelerating. Dark energy has been called the deepest mystery in all of physics and astronomy. It could simply be the quantum energy of nothingness (something almost mundane to physicists), or something truly exotic -- the influence of the additional spatial dimensions predicted by string theory. It will tell us the destiny of the universe.

The cosmic communion of physics and astronomy produced the most important idea in cosmology since the Big Bang: inflation theory. It posits that there was a period of tremendous expansion, driven by the forces of particle physics during the earliest moments. This transformed the quantum fuzziness of the subatomic world to cosmic-size lumpiness in the distribution of matter, seeding the cosmic structures we see today, from galaxies to superclusters.

Einstein's theory permits three kinds of Big Bang universes: one curves back on itself like the surface of a ball, another curves away like the surface of a saddle, and the third, preferred by inflation, is uncurved. Measurements of the tiny variations in the intensity of the cosmic microwave background recently gave inflation a big boost by confirming that the universe is not spatially curved and that the lumpiness of matter probably arose from quantum fuzziness.

There is more to come. We are in the midst of the most exciting period of cosmic discovery yet. We now have to make sense of what we've found, an absurd combination of quarks, dark matter, dark energy and cosmic speed-up. If physicists and astronomers succeed in that quest, this will be remembered as the Golden Age of cosmology.

Michael S. Turner is the Rauner professor and chairman of the department of astronomy and astrophysics at the University of Chicago.