Long, long ago, when the universe was only about 100,000 years old — a buzzing, expanding mass of particles and radiation — a strange new energy field switched on. That energy suffused space with a kind of cosmic antigravity, delivering a not-so-gentle boost to the expansion of the universe.
After another 100,000 years or so, the new field simply winked off, leaving no trace other than a speeded-up universe.
So goes the story being promulgated by a handful of astronomers from Johns Hopkins University. In a bold and speculative leap, the team has posited the existence of this field to explain an astronomical puzzle: The universe seems to be expanding faster than it should be.
The cosmos is expanding only about 9 percent more quickly than theory prescribes. But this discrepancy has intrigued astronomers, who think it might be revealing something new.
“If we’re going to be serious about cosmology, this is the kind of thing we have to be able to take seriously,” said Lisa Randall, a Harvard theorist.
Some researchers say the problem could be solved by inferring the existence of previously unknown subatomic particles. Others, such as the Johns Hopkins group, are invoking new kinds of energy fields. Or it could all be a mistake.
At issue is a number called the Hubble constant, named after astronomer Edwin Hubble, who in 1929 discovered that the universe is expanding. As space expands, it carries galaxies away from each other. The farther apart two galaxies are, the faster they will fly away from each other. The Hubble constant simply says by how much.
Until a few decades ago, astronomers could not agree on the value of the Hubble constant. But in 2001, a team using the Hubble Space Telescope and led by Wendy Freedman, reported a value of 72. For every megaparsec farther away from us that a galaxy is, it is moving 72 kilometers per second faster. (A megaparsec is 3.26 million light-years.)
More recent efforts by Adam G. Riess, of Johns Hopkins and the Space Telescope Science Institute, and others have obtained similar numbers.
These results are so good that they now disagree with results from the European Planck spacecraft, which predict a Hubble constant of 67.
In short, Planck’s Hubble constant is based on a cosmic baby picture. In contrast, the classical astronomical value is derived from what cosmologists modestly call “local measurements,” a few billion light-years deep into a middle-aged universe. What if that baby picture left out or obscured some important feature of the universe?
Early dark energy hints at a link to, or between, two mysterious episodes in the history of the universe. As Riess said, “This is not the first time the universe has been expanding too fast.”
The first episode occurred when the universe was less than a trillionth of a trillionth of a second old, when a violent ballooning propelled the Big Bang, cosmolegists surmise.
The second is unfolding today: Cosmic expansion is speeding up. The issue came to light in 1998, when two teams of astronomers asked whether the collective gravity of the galaxies might be slowing the expansion. They discovered the opposite: The expansion was accelerating under the influence of an antigravitational force later called dark energy. The teams won a Nobel Prize.
Dark energy comprises 70 percent of the mass energy of the universe. And, spookily, it behaves very much like a fudge factor known as the cosmological constant, a cosmic repulsive force that Albert Einstein inserted in his equations a century ago thinking it would keep the universe from collapsing under its own weight. Under the influence of dark energy, the cosmos is now doubling in size every 10 billion years — to what end, nobody knows.