What goes up must come down. Few on Earth would argue with the fundamental law of gravity.
But ten years ago this month the Astronomical Journal accepted a paper for publication that revealed there is a dark side of the force.
For decades physicists were convinced that gravity should be causing the expansion rate of the universe to slow.
"When I throw my keys up in the air, the gravity of the Earth makes them slow down and return to me," said Mario Livio, a theoretical physicist at the Space Telescope Science Institute (STScI) in Baltimore, Maryland.
But the study, along with an independent work released later the same year, showed that the expansion rate is actually speeding up.
This observation, Livio said, is as if "the keys suddenly went straight up toward the ceiling."
Scientists attribute the phenomenon to dark energy, a force that repels gravity. Even more surprising, measurements show that dark energy accounts for about 74 percent of the substance of the universe.
A decade later, a new suite of experiments may pin down the properties of dark energy and solve what some experts are calling "the most profound problem" in modern physics.
"This is game-changing science," Michael Turner, a cosmologist at the University of Chicago, told a packed auditorium during the Decade of Dark Energy Symposium held last week at STScI.
"We've gone from establishing the phenomenon to probing the underlying cause," he said. "We're not anywhere near the point where it's time to give dark energy a rest."
Energy in a Vacuum
So far, one of the biggest challenges for dark energy researchers is marrying observations to theory.
"We have two known, totally unsatisfactory explanations," Turner said.
One possibility is there is no dark energy, and gravity works differently than scientists think.
But "physicists are conservative. We don't want to throw away our theory of gravity when we might be able to patch it up," Adam Riess, an STScI cosmologist and lead author on one of the dark-energy discovery papers, told National Geographic News.
"Basically it all comes down to the fact that there's one relatively simple equation we work with to describe the universe," Riess said.
"Because we see this extra effect, we can either blame it on the left-hand side of the equation and say we don't understand gravity, or we can blame it on the right-hand side and say there's this extra stuff."
The extra stuff—and the leading contender for explaining dark energy—is quantum vacuum energy.
The idea is tied to quantum mechanics, which predicts that even in the vacuum of space, particles are constantly winking in and out of existence, generating energy.
The trick is that no one has been able to unify the math used in quantum mechanics, which describes the physics of the very small, with the equations in general relativity, which deal with large-scale interactions.
"The two theories use two different sets of rule books [and] we've always known that these two books are incompatible," Riess said. "Dark energy is one of the few cases in nature that really requires us to use both sets of rules."
Quantum calculations, however, predict that the amount of vacuum energy in the universe should be more than a hundred orders of magnitude greater than has been observed.
To help solve the riddle, NASA and the U.S. Department of Energy will soon announce the flagship of the Joint Dark Energy Mission (JDEM), the first program specifically designed to study dark energy.
A request for proposed probes will come out later this year and a decision will be made by 2009, Michael Salamon, program scientist for NASA's Physics of the Cosmos program, told National Geographic News.
Salamon also stressed that current NASA missions have already played a key role in measuring dark energy.
"For one, the Hubble Space Telescope has weighed in on dark energy by virtue of the measurements of supernovae," he said.
Researchers first observed accelerated expansion by studying Type Ia supernovae—the explosive deaths of white dwarf stars.
Astronomers know that each Type Ia explosion has about the same brightness.
As light from the most distant explosions travels toward Earth, it is stretched by the universe's expansion so that it appears red, a phenomenon known as redshift. The higher the redshift, the longer light has been traveling and the further back in time the supernova occurred.
Examining as many supernovae as possible can help researchers measure how fast galaxies are moving away from one another.
Supernovae studies have allowed scientists to see that dark energy has been impacting galaxies since as far back as nine billion years ago.
Other groups are looking for even earlier clues in the cosmic microwave background, the leftover radiation from the Big Bang about 13.7 billion years ago.
In 2003 NASA's Wilkinson Microwave Anisotropy Probe produced the first full map of the early microwave sky in unprecedented detail.
WMAP revealed tiny ripples in density that are the seeds of today's galaxies, Licia Verde, an astrophysicist at the Institute of Space Sciences in Bellaterra, Spain, said during the symposium.
"This is a cosmic symphony. You are really seeing sound, [and] the sound can help you understand how the instrument was made," Verde said. (Related: "Is This What the Big Bang Sounded Like?" [March 22, 2005].)
And in 2005 astronomers found that sound waves rippling through the primordial plasma 400,000 years after the Big Bang left imprints in modern nearby galaxies.
These so-called baryon acoustic oscillations offer another yardstick for measuring the expansion rate of the universe over time and putting limits on the value of dark energy.
Ultimately it will take data from a combination of methods to help unravel the mystery, the experts said.
"The name of the game is to take more measurements over the expansion history of the universe, make each of them more precise, and tighten the model for understanding how dark energy works," STScI's Riess said.
A key goal of experiments is to measure the ratio of energy density to pressure in the universe, denoted by the letter "w."
This value tells physicists "what kind of gravity a material has—whether it's repulsive or attractive—and how strong it is," Riess said.
"If [dark energy] is vacuum energy, then w will be -1 always and precisely," a find that would match quantum predictions with general relativity.
Otherwise, it might be time to re-write the rules.
Lawrence Krauss, a theoretical physicist at Case Western Reserve University, noted at the symposium that most observations currently show that the value for w is pretty close to -1.
For theorists, he quipped, "measuring w … is therefore not going to tell us anything we don't know already."
But "new windows show us new surprises. You have to do what you can do, because you don't know where the answer's going to come from."