simulate processes that we cannot duplicate in laboratories on Earth. For instance, computer models shed light on the pervasive influence of gravity, which extends invisible tendrils across the entire cosmos. The programs calculate billions of years of gravitational interactions among galaxies to show why the universe looks the way it does today.

These scientific pursuits rely upon studies of three fundamental aspects of nature: motion, matter, and energy. Motion is a logical starting point, since everything moves--from the atoms in stationary objects to the most distant galaxies. Ancient observers founded the science of astronomy by charting the motions of the Sun, Moon, stars, and planets in painstaking detail. Today, our telescopes and observing tools are sensitive enough to detect planets around other stars. But we have learned that the motions of celestial objects are ever-changing. Just slight alterations in their paths through space can have dramatic consequences. For that reason we keep a wary eye on space, watching for comets and asteroids that could be headed our way.

Matter comes in many forms, from the familiar objects in our homes to exotic varieties in space. These diverse substances share a list of ingredients: about 100 unique elements. Most are in short supply--our universe consists almost entirely of hydrogen and helium, with just a dash of heavier elements thrown in. On Earth we are accustomed to seeing matter within the narrow range of temperatures and pressures that make life possible. But such conditions are rare elsewhere. Just a few atoms drift here and there in the cold spaces between stars and galaxies. Within a star, it's hot and dense enough to ignite nuclear fusion--an energy bonanza we haven't yet harnessed. The strangest objects in the universe are forms of matter we will never create here: neutron stars and black holes.

When matter is put in motion, it emits energy. Energetic outbursts throughout the cosmos give us insights into objects that we otherwise would never detect. A star explodes somewhere once every second, blasting light and ghostly particles called neutrinos into space. Gas plunges into black holes at the centers of galaxies, releasing waves of x-rays. The Sun is a constantly churning ball of charged gas laced with magnetic fields that writhe and snap, propelling dangerous flares toward Earth. Our eyes are tuned to a tiny part of this rich display of energy, but the rest bombards us and our planet constantly. We have devised clever ways to see those elusive waves, from giant radio receivers on the ground to x-ray and gamma-ray telescopes in orbit.

Beyond these ongoing studies, we face steeper challenges ahead. Some of the questions at the frontiers of cosmological science today seem extraordinarily hard to address: Have matter and energy combined to create life elsewhere? What are the essential ingredients of matter? Does a single theory of physics describe the behaviors of all forces and particles in the universe? What sparked the birth of the universe? What is its ultimate fate, after all the suns have burned out?

We will explore these questions with the same scientific tools that have revealed the universal laws of nature so far. For instance, searches for life on other planets are planned or under way with space probes and observations from Earth. Particle colliders probe ever more deeply into the nesting Russian doll of the atom. The bizarre consequences of modern physics suggest that the tiniest components of matter, which dwell in a Wonderland that we are straining to comprehend, may have sown the seeds of the universe itself. As for the future, we have found hints that an eerie force of repulsion permeates the universe, forcing it to expand more quickly as time goes on.

Our studies of the distant universe move forward because we are confident that the principles of physics governing nature on Earth also apply throughout the cosmos. Basic quantities such as the strength of gravity or the charge of an electron remain the same--within the limits of our abilities to measure them--no matter where one goes. Atoms shine or decay radioactively in a laboratory on Earth in the same way as they do billions of light-years across space. Magnetic fields exist everywhere and affect charged particles in the same way.

What's more, our Sun is an ordinary star, like billions of others in the Milky Way. Our galaxy is much like other spiral galaxies in the universe. It's quite likely that our planet is just one of countless rocky planets orbiting stars at hospitable distances--not too hot, not too cold. Five hundred years ago, Nicolaus Copernicus voiced the notion that there is nothing special about our place in the cosmos or the time in which we live. The Copernican principle still holds sway. It gives us the freedom to apply what we know about Earth, the Sun, and the Milky Way to any other location in the cosmos because we assume the laws of nature here are quite ordinary.

On the largest scales of all, we are finding that the universe looks the same in every direction. Any big chunk of space contains galaxies arrayed in similar patterns as any other big chunk. The faint remnants of heat left over from the explosive origin of the universe are smooth across the entire sky to within one part in 100,000. We refer to this large-scale uniformity of the universe as the cosmological principle. It makes it even more likely that the natural laws on our cosmic city block are the same as those elsewhere.

Indeed, as we tour the cosmos, we will find that the behaviors of the largest and smallest objects spring from the same physical principles. Between these extreme scales lies the universe as we know it: grains of sand, babies, jumbo jets, our planet and its neighbors in space. The physics of this comfortable world offers us a template to understand the mysteries of our One Universe.