The Greatest Story Ever Told

Natural History Magazine

by Neil deGrasse Tyson

From Natural History Magazine, March 1998

The world has persisted many a long year, having once been set going in the appropriate motions. From these everything else follows.

Lucretius

In the beginning, sometime between 12 and 16 billion year ago, all the space and all the matter and all the energy of the known universe was contained in a volume less than one-trillionth the size of the point of a pin. Conditions were so hot, the basic forces of nature that collectively describe the universe were unified. For reasons unknown, this sub-pin-point-size cosmos began to expand. When the universe was a piping-hot 1030 degrees and a youthful 10-43 seconds old—before which all of our theories of matter and space break down and have no meaning—black holes spontaneously formed, disappeared, and formed again out of the energy contained within the unified field. Under these extreme conditions, in what is admittedly speculative physics, the structure of space and time became severely curved as it gurgled into a spongy, foamlike structure. During this epoch, phenomena described by Einstein's general theory of relativity (the modern theory of gravity) and quantum mechanics (the description of matter on its smallest scales) were indistinguishable. As the universe continued to expand and cool, gravity split from the other forces. Quickly thereafter, the strong nuclear force and the electro-weak force split from each other, which was accompanied by an enormous release of stored energy that induced a rapid, thirty-power-of-ten increase in the size of the universe. This release of stored energy is loosely analogous to the release of a substance's latent heat upon cooling to its own freezing point. For example, the thermal energy stored in one gram of water at zero degrees exceeds that stored in one gram of ice at the same temperature. The energy difference represents the latent heat of water. The rapid expansion of the universe, known as the epoch of inflation, stretched and smoothed out the cosmic distribution matter and energy so that any regional variation in density became less than one part in 100,000. Continuing onward with what is now laboratory-confirmed physics, the universe was hot enough for photons to spontaneously convert their energy into matter-antimatter particle pairs, which immediately thereafter annihilated each other, returning their energy back to photons. For reasons unknown, this symmetry between matter and antimatter had been broken at the previous force splitting, which led to a slight excess of matter over antimatter. This asymmetry was small but really, really, important for the future evolution of the universe: for every billion antimatter particles, a billion +1 matter particles were born. As the universe continued to cool, the electro-weak force split into the electromagnetic force and the weak nuclear force, completing the four distinct and familiar forces of nature. While the energy of the photon bath continued to drop, pairs of matter-antimatter particles could no longer be created spontaneously from the available photons. All remaining pairs of matter-antimatter particles swiftly annihilated, leaving behind a universe with one particle of ordinary matter for every billion photons—and no antimatter. Had this matter-over-antimatter asymmetry not emerged, the expanding universe would forever be composed of light and nothing else, not even astrophysicists. Over a roughly three-minute period, protons and neutrons assembled from the annihilations to become the simplest atomic nuclei. Meanwhile, free-roving electrons thoroughly scattered the photons to and fro, creating an opaque soup of matter and energy. When the universe cooled below a few thousand degrees kelvin—about the temperature of fireplace embers—the loose electrons moved slowly enough to get snatched from the soup by the roving nuclei to make complete atoms of hydrogen, helium, and lithium the three lightest elements. The universe is now (for the first time) transparent to visible light, and these free-flying photons are visible today as the cosmic microwave background. Over the first billion years, the universe continued to expand and cool as matter gravitated into the massive concentrations we call galaxies. Between fifty and a hundred billion of them formed, each containing hundreds of billions of stars that undergo thermonuclear fusion in their cores. Those stars with more than about ten times the mass of the Sun achieve sufficient pressure and temperature in their cores to manufacture dozens of elements heavier than hydrogen, including the elements that compose planets and the life upon them. These elements would be embarrassingly useless were they to remain locked inside the star. But high-mass stars fortuitously explode, scattering their chemically enriched guts throughout the galaxy. After seven or eight billion years of such enrichment, an undistinguished star (the Sun) was born in an undistinguished region (the Orion Arm) of an undistinguished galaxy (the Milky Way) in an undistinguished part of the universe (the outskirts of the Virgo super cluster). The gas cloud from which the Sun formed contained a sufficient supply of heavy elements to spawn a system of nine planets, thousands of asteroids, and billions of comets. During the formation of this star system, matter condensed and accreted out of the parent cloud of gas while circling the Sun. For several hundred million years, the persistent impacts of high-velocity comets and other leftover debris rendered molten the surfaces of the rocky planets, preventing the formation of complex molecules. As less and less accretable matter remained in the solar system, the planet surfaces began to cool. The one we call Earth formed in a zone around the Sun where oceans remain largely in liquid form. Had Earth been much closer to the Sun, the oceans would have vaporized. Had Earth been much farther, the oceans would have frozen. In either case, life as we know it would not have evolved. Within the chemically rich liquid oceans, by a mechanism unknown, there emerged simple anaerobic bacteria that unwittingly transformed Earth's carbon dioxide-rich atmosphere into one with sufficient oxygen to allow aerobic organisms to emerge and dominate the oceans and land. These same oxygen atoms, normally found in pairs (O2), also combined in threes to form ozone (O3) in the upper atmosphere, which served (and continues to serve) as a shield that protects Earth's surface from most of the Sun's molecule-hostile ultraviolet photons. The remarkable diversity of life on Earth, and we presume elsewhere in the universe, is owed to the cosmic abundance of carbon and the countless number of molecules (simple and complex) made from it. How can you argue when there are more varieties of carbon-based molecules than all other molecules combined. But life is fragile. Earth's encounters with large, leftover meteors, a formerly common event, wreak intermittent havoc upon the ecosystem. A mere sixty-five million years ago (less than two percent of Earth's past), a ten-trillion-ton asteroid hit what is now the Yucatan Peninsula and obliterated over 90 percent of Earth's flora and fauna—including dinosaurs, the dominant land animals. This ecological tragedy pried open an opportunity for small, surviving mammals to fill freshly vacant niches. One big-brained branch of these mammals, that which we call primates, evolved a genus and species (Homo sapiens) to a level of intelligence that enabled them to invent methods and tools of science; to invent astrophysics; and to deduce the origin and evolution of the universe.

Yes, the universe had a beginning. Yes, the universe continues to evolve. And yes, every one of our body's atoms is traceable to the big bang and to the thermonuclear furnace within high-mass stars. We are not simply in the universe, we are part of it. We are born from it. One might even say we have been empowered by the universe to figure itself out—and we have only just begun.

Neil deGrasse Tyson, an astrophysicist, is the Frederick P. Rose Director of New York City’s Hayden Planetarium and a Visiting Research Scientist at Princeton University.