Dark Energy

The biggest part of our universe had been hidden for all of human history. Some fundamental aspect of our world had been missed and altogether unknown.

Scientists have long recognized that the universe has been growing larger since its origin 13.7 billion years ago in an extremely rapid expansion called the “Big Bang.” But we assumed that this expansion should be slowing down due to the pull of gravity. In 1998, however, two teams of astrophysicists discovered that the expansion is actually speeding up. They observed a mysterious form of “energy” that opposes gravity and is causing the galaxies throughout the universe to move apart faster and faster.

...the galaxies’ accelerated expansion signaled the presence of a previously unknown entity in the universe....“Dark Energy”...We call it “dark” because we don’t directly see it. “ Dark” is code for “we have absolutely no clue what it is!”

...Scientists have determined that Dark Energy makes up two-thirds of the universe. So, until we understand Dark Energy, we clearly cannot understand our universe. Also, Dark Energy was not predicted by standard theories of physics. That means our basic theories—our descriptions of elementary particles like electrons, neutrinos and quarks, and of the forces between them—need a major upgrade. Physics theories drive key parts of modern life, like electronics, engineering and medicine.

We’ve been in this position before. Around the turn of the 20th century, physicists thought they understood nearly everything. Starting with Sir Isaac Newton’s brilliant theory of gravity in the 17th century, scientists had successfully explained one natural phenomenon after another—from motion to optics to heat, sound and finally, by the late 19th century, electromagnetism. But then radioactivity and subatomic particles like the electron were discovered, and these phenomena could be explained only with the invention of quantum mechanics—a new theory governing the physics of very tiny distances. At the time, quantum theory appeared to have little immediate practical value. Yet quantum mechanics dominates our daily lives today. It’s at the heart of computers, medical devices, cell phones, cameras, iPods and every other modern electronic device.

Now history seems to be repeating itself. Until very recently, we thought we were on the brink of understanding all of cosmology—then came Dark Energy. It is far too soon to know whether Dark Energy will fuel the economic engine of the 21st century the way quantum mechanics drove that of the last 50 years. But if history is a guide, it’s clear we’re on the brink of dramatic changes.

...What’s next? NASA, the Department of Energy and the National Science Foundation have joined forces to plan space satellites and ground-based telescopes that will probe the properties of Dark Energy. What we observe about the history of Dark Energy light-years away will have huge implications for the future of the universe. If Dark Energy gets weaker, gravity could take over again and pull the universe back together. Scientists refer to this fate as the “Big Crunch.” If Dark Energy gets stronger with time, it eventually may pull apart the galaxies, stars and planets now held together by gravity. This is called the “Big Rip.” Or the universe could have an intermediate fate, expanding gradually to a vast, cold, empty place—called the "Big Chill.”