Lesson： THE BIG BANG TO US - Explore what scientists know about the universe... so far.

My name is Alan Lightman. I'm a scientist and a writer. My family has a home on a small island in Maine. The island has no roads or bridges and there's no ferry service. One summer night in the wee hours, I was coming back to the island in my boat. I was alone on the water. I was captivated by the quiet and the stars overhead. I decided to turn off the engine. I laid down in the boat and looked up. After a few moments, my world dissolved into that star-littered sky. The boat disappeared, my body disappeared, and I found myself falling into infinity. I felt as if I were part of the stars. I was merging with something much larger than myself, and the vast expanse of time extending from the far distant past, long before I was born, and then into the far distant future, long after I'll be gone, seemed compressed to a dot. What was happening to me? As a scientist, I used to think that everything could be reduced to numbers, but at that moment in the boat, I believe you could have hooked up every neuron in my brain to a giant computer, and all of that data wouldn't have come close to explaining my experience. After a time, I sat up and started the engine again, and so began my personal journey to understand how these different worlds relate, the world of atoms and molecules, and the world of complex human experiences. In a material and impermanent cosmos in the age of science, what is it that has meaning, and how can we find it? One enduring and provocative question, where did it all come from? The planets, the stars, and of course, us. Where did it all come from? It's a long story. In 1929, American astronomer Edwin Hubble confirmed the astounding fact that all the galaxies are moving away from each other, like dots on the surface of a balloon that's being blown up. The universe is expanding. Before 1929, most scientists, including Einstein, thought that the universe was static, eternal and unchanging, with no beginning. But Hubble's discovery showed a universe in motion. Physicists worked out the math that described a universe that was changing in time. If the expansion of the universe is run backwards, the galaxies get closer and closer together. The universe grows more and more dense, and hotter and hotter, until a definite time in the past when all the matter of the observable universe was squeezed into a region smaller than a single atom, about 14 billion years ago, the Big Bang. We physicists believe that the infant universe consisted of rapidly fluctuating energy fields. Some of that energy turned into subatomic particles, like raindrops condensing out of a supersaturated cloud. At the same time, the universe was rapidly expanding and thinning out. In the first few minutes after the Big Bang, only the smallest and simplest atoms were created. The universe was thinning out and cooling too rapidly for any larger atoms to form. The creation of those atoms, like carbon and oxygen, would have to wait a couple of hundred million years for the first stars to form, and later the first galaxies. Inside vast clumps of gas, smaller regions collapsed under their own weight, forming dense hot cores. Eventually, these cores became hot and dense enough to start nuclear fusion reactions. The first stars were born. In the life of more massive stars, the fusion of small atoms continued to produce larger and heavier atoms. Three helium atoms could fuse to produce carbon, life's most essential element, followed by still heavier elements like oxygen. Within 10 billion years or so, most stars die a relatively quiet death, exhausting their nuclear fuel and eventually turning into cold, dark cinders in space. But a massive star, 10 times the mass of our sun and larger, ends its life in a violent explosion called a supernova. When a star explodes, it spews its gas, including its heavy atoms, into space. This image, based on data from NASA's Chandra X-ray Observatory, shows the atoms of sulfur, calcium, iron, and silicon. Here are the building blocks of planets and life. When many supernovas explode in roughly the same place and at roughly the same time, they produce a fountain of gas that shoots up into space and then rains back down on the galaxy, seeding it with large atoms. The gas in the Milky Way that created our solar system had been enriched by the heavy atoms produced in earlier supernovas. Every atom in our bodies, except for hydrogen and helium, was forged in a star. It's astonishing, but true, that if I could attach a small tag to each of the atoms in my body and travel with them backwards in time, when they were part of the soil and the air and the water, back to the formation of the Earth from a giant gas cloud in space, I'd find that those atoms, those exact atoms, originated in particular stars in the sky.