Since 1970, I have been among the astronomers observing the exploding stars known as supernovae to learn what they are, how they work, and how they affect the chemistry of the universe. As a bonus, this investigation created ways to convert supernovae into the best cosmic yardsticks for measuring distances in the universe. One variety of supernova comes from the thermonuclear explosion of a dense stellar clinker left over from a burnt-out star like the sun. These "type supernovae" (SN la) make useful standard bombs whose distances can be accurately judged from their apparent brightness. Using SN la as a sailor might use a lighthouse to judge distances at sea, we can measure the distances to galaxies, the giant pinwheels and bloated zeppelins of stars in which supernovae explode.1 Remarkably, measuring the distances to supernovae has led to a dramatic new picture for the contents of the universe, dominated by a dark energy that springs from the properties of empty space itself.
Since 1912, astronomers have measured the motions of galaxies. Almost every one is moving away from our own Milky Way galaxy, the phenomenon known as the redshift. In 1929, Edwin Hubble connected the distances to galaxies with their redshifts, showing that distant galaxies recede more rapidly than our neighbors. This means we live in an expanding universe.
News of the expanding universe came as a big surprise to Albert Einstein. Back in 1917, when he consulted astronomers they had told him the universe was static. His newly invented theory of general relativity predicted either an expanding universe or a contracting one. But you can't fight the facts, even when they are wrong. Einstein sighed and stuck in a mathematical constant to fix this "problem" by inventing an expansive quality of space itself, which today we call "dark energy," to balance the inward pull of gravitating matter. Einstein's term, the cosmological constant, was introduced to make the universe stand still, balanced like a skilled cyclist at a stoplight. When, a decade later, Einstein Learned that Hubble's new astronomical observations showed that the universe was not standing still, he wasn't slow to throw the cosmological constant overboard. "It was theoretically unsatisfactory anyway," he said.2 The cosmological constant was banished from most serious discussions of cosmology. Who needed it?
By 1990, as astronomers slowly constructed an inventory for the contents of the universe, we ran into a problem, a puzzle, and a conundrum. The problem is that most of the gravitating material in the universe is invisible, the puzzle is that there is not enough of it, and the conundrum is that having enough of this dark matter would have the bad side effect of making the universe younger than its contents. Being invisible is not so bad—we can detect the effects of invisible mass even if it emits no light, just as a sailor knows an invisible puff of wind is coming by watching the riffles it makes on the water. Visible matter drains into the invisible web of cosmic troughs that cold dark matter forms. But the puzzle remains that the amount of matter in the universe is only about one-third of the amount that our favorite theories required to make the neatest universe. What's worse is the conundrum posed by cosmic timescales. The oldest stars in our galaxy appear to be about 12 billion years old. If the universe had its full load of gravitating matter, cosmic expansion should slow over time, and the universe would have clocked an elapsed time since the Big Bang of about 10 billion years. Having 12 billion year old stars in a 10 billion year old universe doesn't inspire confidence that this is a genuine history of the physical world. What's wrong with this picture? Are these small cracks in a beautiful fresco, do they show we have a serious conceptual problem with the Big Bang, or is something missing?
In the past several years, teams of scientists have been using new instruments and new telescopes, including the Hubble Space Telescope, to find distant supernovae. These let you measure directly the history of cosmic expansion. We expected to see how much the universe has been slowing down since the Big Bang. I have been involved with one of these teams, a cheerful, slightly anarchic band of brothers (with some sisters, too) that we call the high-z supernova search team. The letter is the astronomer's shorthand for redshift, so this means we've been looking for exploding stars at large redshifts and large distances.
In 1997, this work was well underway when I was invited to Princeton University to give a scries of lectures that became the foundation for this book. But, looking over my old notes, I see that we had almost no results to report in 1997: though we knew what the questions were, and saw how to get the answers, the surprising solution to these astronomical riddles has come together in a rush since then So I talked a lot about how supernovae explode and make new chemical elements and only a little about the way that supernovae would measure the history of cosmic expansion. Now the preliminary results are in and we have a new and surprising synthesis that solves the problems, puzzles, and conundrums of a decade ago.
The observations of distant supernovae show that we live in a universe that is not static as Einstein thought, and not just expanding as Hubble showed, but accelerating! We attribute this increase in expansion over time to a dark energy with a strange type of pressure. In its simplest form this might be Einstein's cosmologi-cal constant, which for 60 years was theoretical poison ivy. Dark energy makes up the missing component of mass-energy that theorists have sought, reconciles the ages of objects with the present expansion rate of the universe, and complements new measurements of the lingering glow of the Big Bang itself to make a neat and surprising picture for the contents of the universe.
The last five years have been a little like that moment in assembling a jigsaw puzzle when you complete the frame, pieces are dropping rapidly into place, and you can even see the shapes of the missing pieces. The missing piece may be the most important. A universe controlled by dark energy points to a deep gap in our understanding of submicroscopic aspects of empty space: the properties of the vacuum. No laboratory experiment measures and no physical theory predicts the amount of dark energy our observations imply. The next step forward in understanding the universe on the smallest scale will be to meld gravity with the other forces of nature. Perhaps when there is a new theoretical vision this ex travaganf universe, propelled by dark energy, will seem simple and inevitable. But for the moment, solving the mysteries of the accelerating universe has produced another delicious puzzle to investigate.
Our working picture of the universe today is extravagant; it has neutrinos as hot dark matter; something unknown as cold dark matter; inflation in the first 10"35 second after the Big Bang; and acceleration by dark energy now, when the universe is 1052 times older. This is wilder than anyone imagined, but it is based on evidence even though all of these things are invisible. We've built this picture by observing light from the Big Bang itself; from stars, steady, variable, and exploding; and from galaxies at the edge of the observable universe.
Seeing new aspects of the universe for the very first time is a pleasure experienced by the hard-working people who appear in this book. But why should we have all the fun? My aim is to help you share in this adventure where the thrill comes from understanding.
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