Galaxy clusters “are sort of like canaries in a coal mine in terms of structure formation,” Holzapfel says. If the density of dark matter or the properties of dark energy were to change, the abundance of clusters “would be the first thing to be altered.” The South Pole Telescope should be able to track galaxy clusters over time. “You can say, ‘At so many billion years ago, how many clusters were there, and how many are there now?’” says Holzapfel. “And then compare them to your predictions.”
Yet all these methods come with a caveat. They assume that we sufficiently understand gravity, which is not only the force opposing dark energy but has been the very foundation of physics for the past four centuries.
Twenty times a second, a laser high in the Sacramento Mountains of New Mexico aims a pulse of light at the Moon, 239,000 miles away. The beam’s target is one of three suitcase-size reflectors that Apollo astronauts planted on the lunar surface four decades ago. Photons from the beam bounce off the mirror and return to New Mexico. Total round-trip travel time: 2.5 seconds, more or less.
That “more or less” makes all the difference. By timing the speed-of-light journey, researchers at the Apache Point Observatory Lunar Laser-ranging Operation (APOLLO) can measure the Earth-Moon distance moment to moment and map the Moon’s orbit with exquisite precision. As in the apocryphal story of Galileo dropping balls from the Leaning Tower of Pisa to test the universality of free fall, APOLLO treats the Earth and Moon like two balls dropping in the gravitational field of the Sun. Mario Livio, an astrophysicist at the Space Telescope Science Institute in Baltimore, calls it an “absolutely incredible experiment.” If the orbit of the Moon exhibits even the slightest deviation from Einstein’s predictions, scientists might have to rethink his equations—and perhaps even the existence of dark matter and dark energy.
“So far, Einstein is holding,” says one of APOLLO’s lead observers, astronomer Russet McMillan, as her five-year project passes the halfway point.
Even if Einstein weren’t holding, researchers would first have to eliminate other possibilities, such as an error in the measure of the mass of the Earth, Moon or Sun, before conceding that general relativity requires a corrective. Even so, astronomers know that they take gravity for granted at their own peril. They have inferred the existence of dark matter due to its gravitational effects on galaxies, and the existence of dark energy due to its anti-gravitational effects on the expansion of the universe. What if the assumption underlying these twin inferences—that we know how gravity works—is wrong? Can a theory of the universe even more outlandish than one positing dark matter and dark energy account for the evidence? To find out, scientists are testing gravity not only across the universe but across the tabletop. Until recently, physicists hadn’t measured gravity at extremely close ranges.
“Astonishing, isn’t it?” says Eric Adelberger, the coordinator of several gravity experiments taking place in a laboratory at the University of Washington, Seattle. “But it wouldn’t be astonishing if you tried to do it”—if you tried to test gravity at distances shorter than a millimeter. Testing gravity isn’t simply a matter of putting two objects close to each other and measuring the attraction between them. All sorts of other things may be exerting a gravitational influence.
“There’s metal here,” Adelberger says, pointing to a nearby instrument. “There’s a hillside over here”—waving toward some point past the concrete wall that encircles the laboratory. “There’s a lake over there.” There’s also the groundwater level in the soil, which changes every time it rains. Then there’s the rotation of the Earth, the position of the Sun, the dark matter at the heart of our galaxy.
Over the past decade the Seattle team has measured the gravitational attraction between two objects at smaller and smaller distances, down to 56 microns (or 1/500 of an inch), just to make sure that Einstein’s equations for gravity hold true at the shortest distances, too. So far, they do.
But even Einstein recognized that his theory of general relativity didn’t entirely explain the universe. He spent the last 30 years of his life trying to reconcile his physics of the very big with the physics of the very small—quantum mechanics. He failed.