Both of these possibilities are lumped together in 'dark energy.' "Another possibility is that there is an evolving scalar field that fills space (like the Higgs field or the inflaton field that drove the rapid early expansion of the universe). "Why is the universe accelerating? The most studied possibility is that the cosmological constant (or equivalently, the vacuum energy of empty space) is driving cosmic acceleration," Spergel writes in the Science article. After astronomer Edwin Hubble's observations in 1929 famously showed that the universe is actually expanding, Einstein dropped the cosmological constant, allegedly deeming it the "biggest blunder" of his life.īut the constant looks prescient now that astronomers are grappling with the nature of dark energy. In 1917, Einstein inserted a term called the " cosmological constant" into general relativity, as a repulsive force that would counteract gravity and achieve a static universe (which was the prevailing view of the universe's nature at the time). Dark energy, meanwhile, is a mysterious force that's associated with empty space and is thought to be responsible for the accelerating expansion of the universe. Indeed, observations suggest that just 5 percent of the universe is familiar atomic matter, while 25 percent is dark matter and about 70 percent is dark energy.ĭark matter neither emits nor absorbs light, betraying its existence only through its gravitational effects. General relativity also implies that the vast majority of the universe is composed of stuff that humans cannot detect directly or (at this point) even understand, David Spergel of Princeton University writes in another review article in the same issue of Science.Ĭareful study of the motion of matter and light throughout the universe has revealed that "normal" matter alone cannot explain space-time's curvature patterns, Spergel notes. "I think it's fair to say that there's no credible measurement or observation that causes one to doubt it within its domain of applicability." A dark universeĪlbert Einstein at the blackboard. "We've tested it in many, many different ways," he said. Other types of observational evidence have also helped put general relativity on firm footing, Blandford said. The agreement today is better than 10 −4." "Einstein used general relativity to explain a ~10 percent discrepancy in the precession attributable to the gravitational pulls of the other planets, ~43 arc sec per century. "It explained the anomalous precession of Mercury's perihelion, or the rotation of the line joining the sun to the point of closest approach of the planet," Blandford writes in the Science review article. Peculiarities in the orbit of Mercury around the sun also back up general relativity. ( Exoplanets can sometimes be detected by studying how their star systems bend light from background objects.) In fact, on a smaller scale, the phenomenon even helps planet hunters search for worlds beyond Earth's solar system. This has indeed been observed astronomers routinely use "gravitational lenses" to study faraway light sources. General relativity predicts that light will take a curved path around a massive object such as a galaxy cluster, which warps the fabric of space-time significantly. It's a powerful, radical idea - and it has stood up to intense scrutiny for a century now, Blandford writes in a special review article published online today (March 5) in the journal Science. (Imagine a marble rolling down the incline created by the bowling ball on the bed.) General relativity characterizes gravity not as an innate force acting on objects but rather the consequence of space-time's curvature. He was guided first and foremost by physics intuition that extraordinarily powerful physics intuition which had served him so well in the past did not let him down here." "He was guided not by mathematical ideas or mathematical techniques. He proposed things which he subsequently retracted. This monumental insight did not come to Albert Einstein easily he earned his way to it, over a decade of intense thought and hard work. General relativity expanded on this latter idea, explaining that matter warps space-time, much as a bowling ball set down on a bed creates a depression in the sheets.
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