Dark energy crisis: a new NASA mission may shed light on dark energy, but at the cost of Einstein's theory of general relativity

Natural History, Dec, 2008 by Donald Goldsmith

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Einstein's famously called it "his greatest blunder." In 1917, to reconcile his equations for general relativity with the then-prevailing notion of a static universe, he added a fudge factor, later named the "cosmological constant." Einstein abandoned the idea after the astronomer Edwin Hubble demonstrated in 1929 that the universe is expanding. But recent observations imply that Einstein may have been right after all. Given the complexity of cosmology and the profound confusion that reigns in the filed today, however, any vindication will surely entail further twists.

Ever since the big bang, space has expanded, carrying galaxies along with it; but astronomers expected to find the galaxies' outward rush slowing down owing to the attractive force of gravity. Instead, the past decade has revealed that not only is the expansion of the universe not slowing down, but in fact it is accelerating: the expansion is proceeding at an ever-greater rate. That discovery implies an eventually lifeless cosmic future of cold, near-total emptiness.

The culprit is a force called "dark energy," a kind of energy unlike any already known, which pushes galaxies farther apart, counteracting gravity's effect of pulling them together. Physicists estimate that visible matter accounts for only 4 percent of the mass of the universe, while another 22 percent consists of "dark matter," a ghostly substance that never interacts significantly with ordinary matter. Dark energy makes up the vast remainder of the universe. The mysterious energy could be the cosmic component represented by Einstein's cosmological constant--but only if the amount of dark energy within a given amount of space does not vary over time.

NASA and the U.S. Department of Energy have invited proposals for a $600 million space probe to determine whether that is true. Here's one twist: if it turns out that the density of dark energy in the universe evolves over time, then Einstein was not only wrong about the constant--which would be an "inconstant"--but also mistaken in the equations he derived from his theory of general relativity to describe the universe. In that case, physicists will have to rethink major assumptions about the universe and the fundamental laws that govern it.

HOW DID WE REACH THIS critical juncture m cosmology? Tremendously successful astronomical observations over the past decade have revolutionized our understanding of the cosmos. First, astronomers made great strides in measuring how rapidly galaxies are moving toward or away from us, thanks to more precise measurements of how much the Doppler effect shifts light toward shorter or longer wavelengths. The radiation from the most distant stars and galaxies, for example consists primarily of infrared, which has a longer wavelength than visible light, because the expanding universe has stretched (or red-shifted) the visible light waves into the infrared portion of the electromagnetic spectrum.

It was the redshift of distant specimens of a rare class of exploding stars called Type Ia supernovas that: revealed the existence of dark energy. Type Ia supernovas all have essentially equal luminosities, so if one looks fainter to us than another, it must be farther away. That makes them handy for estimating the distances to faraway galaxies that host such supernovas. In 1998, after searching through thousands of remote galaxies to compile a sampling of more than four dozen Type Ia supernovas, two teams of astronomers made improved estimates of the distances to the galaxies and correlated them with the galaxies' and supernovas' redshifts. Most of the astronomers expected that their more precise measurements would provide evidence of a slowdown in the universe's expansion. Instead, they discovered that the expansion is accelerating. What was driving this acceleration was unknown; eventually it was attributed to what was called "dark energy."

Cosmologists confronted with the surprising evidence of accelerating expansion turned to Einstein's formerly rejected calculations for a possible solution. Although Einstein himself offered no physical description of his cosmological constant, it can be said to represent a weird tension, intrinsic to empty space, that counteracts gravity--in effect a sort of universal negative pressure that makes space expand. The exact value that Einstein assigned to his cosmological constant has long since been ruled out by observational data compiled during the six decades after 1917, yet theoreticians have occasionally tried other values in hopes of finding a better fit to those observations.

Even if we set the value of the cosmological constant to account for the accelerating expansion of the universe, the implications of a nonzero constant boggle the mind. The constant's mathematical value corresponds to the amount of dark energy that lurks in every cubic centimeter of space. It follows that, as the cosmos expands and new space comes into existence, every cubic centimeter of new space will contain just as much dark energy as every old cubic centimeter does. Accordingly, the universe not only expands at an accelerating rate, but also produces a correspondingly growing amount of dark energy. Here we find the ultimate free lunch--energy produced without any investment beyond the original creation of the universe. But doesn't this violate the law of conservation of energy, which states that the total amount of energy remains constant, though it may change its form (matter being a form of energy, with a value expressed by Einstein's famous equation, E = [mc.sup.2])? No, because this law applies to any isolated, closed system--and the universe, which continuously creates new space, does not satisfy that condition.