image: Library of Congress

A century ago, the prevailing opinion about the universe was that it consisted only of our Milky Way galaxy, and was static and unchanging. This puzzled Albert Einstein, because he understood that gravity was a force that pulled matter together. If the universe was static, and gravity was exerting its pull, making everything move closer and closer together, why hadn’t the universe collapsed on itself?

Einstein reasoned that there must be some force in the universe that repels gravity, something that pushed matter apart and kept the universe static. He named this mysterious anti-gravitational force the “cosmological constant.”

Michael Turner, chairman of the Astronomy & Astrophysics department at the University of Chicago, explains, “Einstein introduced the cosmological constant as a fudge factor. He thought he knew the answer at the back of the book. He thought the universe was static so he put in this fudge factor.”

As shown on PBS’s "NOVA", by 1929, scientists had discovered that the universe was not static at all—it was expanding. This led to the idea of the Big Bang, a gigantic blast that generated so much momentum that the universe is still expanding from it. Many astronomers believed that all matter in the universe, and all the gravity it exerts, would eventually slow this post-Big Bang expansion down, and maybe even stop it completely, causing a kind of Big Crunch, or implosion, at some point in the future.

Still, because Einstein’s “fudge factor” came out of his belief that the universe was static, these new discoveries about the non-static nature of the cosmos caused him to call his cosmological constant the greatest blunder of his career. But it turns out that he spoke too soon.

image: NASA/WMAP Science Team

In February, NASA released images from a satellite called the Wilkinson Microwave Anisotropy Probe (WMAP), which observed the universe for 12 months. The WMAP, the result of a partnership between the Goddard Space Flight Center and Princeton University, captured “cosmic microwave background” radiation, the afterglow of the Big Bang that bathes the entire universe. By measuring small variations in the temperature of the cosmic microwave background radiation, the WMAP created a “baby picture” of the cosmos that reveals a lot about its nature and content.

The light in this “cosmic portrait” taken by the satellite is from 380,000 years after the Big Bang. It reveals that the universe is 13.7 billion years old (with a 1 percent margin of error), and that the first stars ignited about 200 million years after the Big Bang.

According to the WMAP, the content of the universe is only 4 percent atoms, which includes the matter that makes up all of the planets, stars, and us. Twenty-three percent of the universe is a mysterious type of material known as "cold dark matter,” and 73 percent is something even more mysterious—something scientists call “dark energy.” This dark energy acts as a sort of anti-gravity, forcing the universe not only to keep expanding, but also to accelerate in its expansion, which contradicts the Big Crunch theory. Although the nature of the dark energy is still a mystery, the information gathered by the WMAP seems to indicate that it will impel the universe to expand forever.

So there is indeed an anti-gravitational force in the universe that keeps it from closing in on itself. Einstein was right after all.

“If Einstein heard these results today, he would say, ‘Yahoo!’” says Alex Filippenko, professor of astronomy at the University of California at Berkeley. “It would be such a thrill for him, I think, to see that his original prediction that such a weird ‘stuff’ might exist in the universe turned out to be actually true.”