Dark energy is the single most important element in the universe. It influenced how the cosmos was born, how it is evolving today, and how it all will end trillions of years in the future. Right now, this energy is causing the universe to expand faster and faster; in the far future, the expansion may become so rapid that space itself will be torn apart. And yet we know next to nothing about what dark energy is. We don’t even have a proper name for it—the very term “dark energy” is little more than a scientific shrug.
OUR COSMIC FATE hangs in the balance, depending on the behavior of dark energy. If dark energy increases, everything will be torn apart; if it changes direction, the cosmos could end in a big crunch. (Credit: NASA/CXC/M.Weiss) Small wonder, then, that our recent DISCOVER magazine cover story about the mystery of dark energy (Confronting the Dark by Zeeya Merali) produced such an outpouring of curious reader mail. In a previous post, I addressed some of the key cosmological questions submitted by our readers. But really, that first set of responses only scratched the surface. For every letter writer who asked broadly about the nature of the Big Bang, someone else who wanted to know more about dark energy itself. So as promised, here is a second installment addressing how scientists came to realize that energy, not matter, rules the universe. As before, I need to put out a disclaimer up front. The answers I give here are not just my own. They are distilled from the dedicated efforts of astronomers and physicists around the world—although some of the more philosophical questions inevitably trigger more personal answers. I should also acknowledge that there is a lot we still do not know about the universe. But over the past 15 years, scientists have put together a convincing case that dark energy is real. At this point, it would require revolutionary new discoveries to disprove that. That said, let’s move on to Cosmic Questions II: The Dark Universe. What the scientists studying dark energy are doing is just speculation that leads nowhere. It is obvious that they have no idea what it's all about. --Dick and Linda C. There is a powerful question embedded in that statement. Cosmologists talk all the time about how little they understand about dark energy, about how enigmatic it is. Can they be sure that it exists at all? To paraphrase a certain president: Yes they can. Let me back that answer with a quick review of how dark energy was discovered in the first place. Ever since the Big Bang theory became widely accepted in the 1960s, researchers have been trying to measure a number called the “deceleration parameter.” That number describes the rate at which the universe is slowing down due to the mutual attraction of all the matter in the universe. The rate of deceleration is significant for several reasons. It tells you the total mass of the observable universe. It tells you the fate of the universe, by showing if things are slowing down enough that they will eventually come to a halt and reverse. And it is crucial for determining the age of the universe, since it tells you how the rate of cosmic expansion has been changing since the time of the Big Bang. To measure the deceleration, you need to compare how the universe is expanding now with the way it was expanding in the distant past. Fortunately, the finite speed of light acts like a visual time machine. A galaxy located one million light years away appears to us as it was a million years ago. A galaxy a billion light years ago appears as it was a billion years ago, and so on.
