One Universe: At Home in the Cosmos

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INTRODUCTION

Our Connection to the Universe

Millions of stars fill the sky near the center of our galaxy, the Milky Way. As we gaze at these dazzling but distant points of light, it's hard to imagine that many of them may be suns in planetary systems like our own.

We live in a universe filled with wonders. Comets hang like celestial torches before fading on their long journeys into space. The sun descends in a golden blaze on a summer evening, and countless stars spill from zenith to horizon through the dark night. At such moments, our cosmos inspires awe.

However, we rarely feel connected to the cosmos. We live at a hectic pace on a warm planet, insulated from the universe by the bright blue dome of sky. At night, when the heavens open up to us, we seldom cast more than a glance overhead. Even when we do notice the grandeur of the universe, it seems utterly separate from our lives. Planets, stars, and galaxies appear so small to our eyes that we cannot comprehend their enormous sizes, so far away that we cannot grasp the vast gulf of space between them, and so exotic that we cannot understand how they work. Our experiences on Earth seem so different from these wonders that nature surely must have followed another set of rules in creating them. Can we ever hope to divine those cosmic principles?

The answer is a resounding "Yes." A deep insight has emerged from astronomy and physics: The basic forces, quantities, and processes that govern our lives on Earth and that govern the workings of the universe are one and the same. In fact nature¹s laws are fewer in number, and often simpler, than the laws that human societies invent. We can study the natural laws on our planet and in our neighborhood in space, then use those laws to understand the behaviors of objects that lie forever out of reach. In so doing, we have learned that no wall separates our Earth and sky from the rest of the cosmos. We live in One Universe.

Some of those connections are easy to see. A crystal hanging in a window lights the room with bands of color on a sunny day. We use more elaborate crystals to break up light from stars and galaxies. Special instruments extract hidden details from those delicate rainbows, revealing what the objects are made of and how they move through space. Baseball fans watch the cosmos at work when they follow the arc of a home run soaring into the bleachers. The arc is a perfect illustration of the ever-present force of gravity, which pins us to the ground, keeps the Moon in orbit around Earth, and steers our Sun through our Milky Way galaxy. The Moon and the Sun also exert gravitational pulls on Earth, creating tides that we see as the twice-daily ebb and flow of the ocean. Stronger tides elsewhere in the universe turn the insides of moons to mush and stretch pairs of closely orbiting stars into egglike shapes.

Other connections come from watching things spin, a property that applies to nearly everything in space. The whirl of a gyroscope, as children know, prevents it from toppling on its side. Telescopes in space take advantage of that same principle by using three gyroscopes to keep a steady aim. On a larger scale, Earth's daily rotation on its axis stirs our atmosphere and stretches storms into spiral shapes. Other planets display similar stormy patterns, such as Jupiter's Great Red Spot.

Some of our links to the cosmos are more surprising, for they involve events too extreme to occur on Earth. For instance, the largest stars blow up in titanic blasts that seed the galaxy with heavy elements, such as iron, calcium, and silicon. These elements come only from stars; the universe has no other way to create them. They drift into clouds of gas and dust which collapse into a new generation of stars, planets, and--in our case--life. In other words, dying stars forged the elements that compose the blood in our veins, the bones in our bodies, and the chips in our computers. The stuff of stars is all around us even though the stars themselves seem so inaccessible.

Our awareness of these connections has grown as we have studied the natural world for thousands of years. The earliest natural philosophers--Plato, Aristotle, and Archimedes among them--tried to use their five senses, in combination with logic and reason, to explain the cosmos. However, their preconceptions got in the way. Earth sat unmoving at the center of the universe, they believed, and the celestial bodies moved around it in perfect patterns. These beliefs also affected their view of physical principles on Earth. For instance, Aristotle asserted that heavier objects fall faster than light objects, but he never bothered to put that claim to the test.

Our modern approach to gathering knowledge about the universe draws from traditions established by Galileo Galilei, Isaac Newton, Albert Einstein, and other great minds of the past several centuries. These physicists didn¹t care whether their results conformed to common-sense views about how the universe worked. Rather, they devised careful theories based on repeated experiments and mathematical analysis. Their theories strove to explain some of what was not understood, predict previously unknown phenomena, and consistently confirm their predictions by further tests. Describing nature as it was, not as the scientists supposed it to be, was at the heart of this scientific method.

In this way, for example, Newton assembled methodical descriptions of how objects move through the universe at everyday speeds. Much later, Einstein found more basic rules that explain how all objects move, even those that travel close to the speed of light. Newton's work was still correct, but it became a special part of Einstein's overall theory. This process is typical of science. Modern technology provides more penetrating insights about nature, leading to new theories that are more accurate but increasingly simpler at their cores. Rarely does a completely surprising phenomenon arise that forces us to overturn all aspects of an existing theory.

Today, we benefit from the creative use of technology to extend our vision far beyond Earth's surface and our solar system. Telescopes, spectrographs, electronic cameras, and other tools collect data every night from the farthest corners of the cosmos, revealing what our unaided eyes could never see. We also use computers to 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 move--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 haven¹t 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. It¹s 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.