California’s Mount Wilson Observatory was the scene of one of astronomy’s greatest discoveries: In 1929, Edwin Hubble proved that the Universe is expanding.
From Static to Expanding Universe
The 1920s were a time of amazing astronomical progress: Scientists learned not only how to calculate precise distances to faraway stars, but also how to see light emitted by celestial bodies across the blue-green-yellow-red optical spectrum, a technique that gave them new information about each individual light source. Remarkable as each of these measurements was, however, it was the combination of the two that changed astronomy forever.
Astronomers had learned that when an object’s light shifted into the red part of the spectrum, it indicated the object was moving away from the observer. (The larger the shift, the faster the movement.) Based on that technique, they already knew that some stars and nebulae were moving away from Earth, but Edwin Hubble—by adding distance measurements to the mix—discovered a remarkable constant. The farther away a galaxy, he found, the faster it was moving away from us. Like a burst water balloon whose outermost drops travel faster than the rest of its contents, that pattern—the distance-to-speed ratio now called “Hubble’s constant”—was proof that the Universe was expanding.
The faster a galaxy recedes, the redder its motion vector. The phenomenon, known as “red shift,” led to the discovery of dark energy.
A Cosmic Shock: Expansion Isn’t Slowing Down
The discovery required a huge shift in cosmological thinking—and gave rise to the “Big Bang” theory, since patterns of expansion can be “rewound” backward to an original, exploding source. Seventy years later, scientists were poised to answer a big follow-up question: How quickly was that expansion, which had already been happening for billions of years, slowing down? Gravity, after all, demanded that all the mass in the Universe would act as a kind of brake on its outward momentum.
In 1998, two separate research teams set out to quantify the rate of deceleration, and they were armed with the ability to calculate the distance, speed, and age of celestial bodies more precisely than ever before. A specific type of exploding star, called a type Ia supernova, had been found to have a light so consistent it could be used as a “standard candle”—that is, the relative brightness of a supernova could tell astronomers exactly how near or distant that body is. Distance itself could be understood as a function of age; light from a nearby supernova might take only hundreds of thousands of years to reach us, while truly distant ones took billions. Add in redshift measurements to reveal how quickly various objects were traveling away, and the stage was set: By measuring the rate of expansion at different times in the past, the researchers could calculate the rate at which it was slowing down.
Then came the surprise. Both teams shocked the world—and themselves—by coming up with the same set of findings: the Universe’s expansion wasn’t slowing down at all; it was speeding up. Although individual galaxies were being held together by dark matter, a mysterious force—a force we didn’t even know existed—was dominating the dark spaces in between, pushing back against gravity with enough power to not only to negate but reverse its effects, causing expansion to accelerate.
Dark energy—the aspect of space we know least about—dominates the vast majority of our Universe.
The Questions—and the Answers—Are Only Getting Bigger
Scientists call this unknown force “dark energy,” and what we think we know about it amounts to three basic facts: It’s powerful enough to counteract gravity; it appears to be the largest component of our Universe (representing 70 percent of the total combined mass and energy); and it changes nearly everything we thought we knew about space, matter, and how our Universe evolved. Because when astronomers look back at some of the oldest and most distant supernovae known, they see a world in which gravity was once winning—when the momentum from the Big Bang was indeed slowing down. But as expansion pulled galaxies further apart (weakening the gravitational pull between them), dark energy took over, thus shifting the direction—and ultimate fate—of the Universe.
Efforts to figure out what dark energy actually is are underway all over the world, from observatories in the Chilean Andes to telescopes at the South Pole. Today, it’s widely considered the biggest mystery in the world, both literally—in comparison the mere 5 percent of the Universe we can actually see—and as a catalyst sure to drive the next century of big questions and discoveries.
All images © AMNH.