Introduction to special issueBREAKTHROUGH OF THE YEAR

ASTRONOMY: Cosmic Motion Revealed

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Science  18 Dec 1998:
Vol. 282, Issue 5397, pp. 2156-2157
DOI: 10.1126/science.282.5397.2156a

Astronomers peered deep into the universe and found that it is flying apart ever faster, suggesting that Einstein was right when he posited a mysterious energy that fills “empty” space

The nature and ultimate fate of the universe have preoccupied philosophers and scientists for centuries. Scientists discovered decades ago that the universe is now expanding, with its galaxies rushing apart in all directions. But the pull of gravity could slow that expansion, and so researchers have tried to work out the final destiny of the cosmos: whether there is enough matter to cause it to one day collapse on itself, or whether it will expand forever. In 1998 two teams of astronomers peered across an enormous gulf of time and space to answer that fundamental question—and amazed even themselves with what they found.

Cosmic Speed Trap.

The brightness of a fading supernova showed how fast the universe is expanding.


Not only is there too little matter in the universe to ever halt the expansion on its own, but the outward motion appears to be speeding up, not slowing down. At the same time, the finding raises such profound questions about the nature of space that cosmologists are wondering whether the ultimate fate of the universe can ever be known for certain.

In a triumph for astronomers' ability to look deep into the past, the independent teams came to their conclusions by observing far-off exploding stars called supernovae that turn out to be surprisingly dim, revealing an acceleration that has swept them to unexpectedly large distances from Earth. With these results, reaching billions of light-years into space, astronomers have gained a secure foothold in the deepest and most mysterious reaches of the cosmic past. We name their findings, which transform our view of the universe and pose fundamental new questions for physics, as Breakthrough of the Year for 1998.

Adding to the drama of the find, the simplest explanation for the accelerating expansion is a bizarre energy that on large scales counteracts gravity, pushing matter apart, an idea that Albert Einstein posited in 1917 and later rejected. This year's discoveries suggest that most of the energy of the universe is in this form, which Einstein called the cosmological constant, or lambda. Because matter and energy are interchangeable, this huge energy reservoir means that the universe of matter, from tables and chairs to stars and clusters of galaxies, may be but the minor portion of creation.

These implications are so profound and unsettling that astronomers around the world are still trying to disprove the finding, to uncover anything that could create a false impression of cosmic acceleration. To date they have been unsuccessful, and so physicists are rushing to explain the origin of the cosmic energy. The lambda symbol, λ, is once again sprinkled throughout equations in the astronomy and physics journals, and the new results have inaugurated a small industry of theoretical searches for even quirkier possibilities for boosting the expansion.

Back in 1917, when Einstein proposed the constant, he and other scientists thought that the universe was static, neither expanding nor collapsing. He put the cosmic repulsion into his equations to prevent the universe from collapsing on itself from the gravitational pull of the matter inside it.

But by 1929, astronomer Edwin Hubble had peered into the heavens and startled the scientific world of his day by discovering that the universe is in fact expanding. Born in a hot, dense state called the big bang, the cosmos has been likened to a display of fireworks whose most brilliant moments are behind it. Like fading cinders in the fireworks, galaxies that were initially close to each other are today drifting apart slowly, while those that began slightly further apart are flying away from each other at higher speeds. From our vantage point in the Milky Way, the speed at which any other galaxy is moving away can be clocked using the “redshift” of its light—a drop in frequency and increase in wavelength akin to the dip in pitch of a receding train's whistle.

But gauging a galaxy's actual distance is difficult. Hubble managed it by observing the apparent brightness of stars called Cepheid variables, whose intrinsic brightness is known; these stars can thus be used as “standard candles” to measure distance, as more distant Cepheids appear dimmer. Hubble compared the redshifts with the distances and discovered the expansion.

Einstein accepted Hubble's find. But he reasoned that if the expansion was a relic of a primeval explosion, the cosmological constant—which he felt made the equations unaesthetic—wasn't needed. He withdrew the idea and called it his “biggest blunder.”

As cosmologists continued to work with the notion of an expanding cosmos, they concluded that over the 12- to 15-billion-year life of the universe, the expansion would slow slightly, thanks to the pull of gravity that every galaxy exerts on every other. But spotting such a change requires probing deep into the past by looking at stars glittering billions of light-years away—too far away for Cepheids to be seen.

So for the past 20 years, astronomers have turned to a new kind of standard candle: the brightest kind of supernova, which happens nearly the same way each time. But these bright, massive explosions are rare—only two or three erupt in a typical spiral galaxy per millennium. To find enough of them, astronomers make electronic images of large swaths of sky in a single night, capturing tens of thousands of distant galaxies, and then image the same areas a few weeks later. Once the images are overlaid and subtracted on a computer, any new supernovae leap out and can be observed until they fade away.

The two teams, both of which have members in Europe, Latin America, Australia, and the United States, collected their supernova data with increasing efficiency over the last few years, expecting to find out by how much gravity was slowing cosmic expansion. Early this year, both teams announced that their expectations had been turned upside down: The relative dimness of the supernovae showed that they are 10% to 15% farther out than expected even in a universe with little matter, indicating that the expansion has accelerated over billions of years. At year's end, with dozens of supernovae analyzed, published, or in press, those conclusions stand.

That finding resurrects a mysterious repulsion that counteracts gravity, with lambda as the most likely candidate. There were earlier hints, from theories of cosmic evolution and observations of the large-scale structure of the universe, that the cosmos holds little mass and that there might be a lambda, but the idea was generally considered outlandish. Now lambda is respectable once more, and Einstein is proved right, albeit for reasons he could not have foreseen. In fact lambda appears to be dominant in the universe: In the simplest theoretical picture, the supernova data imply that 70% of the universe's energy is in the form of lambda and only 30% is matter.

Physicists have since interpreted lambda as a quantum-mechanical effect: that the evanescent particles that flicker in and out of existence in “empty” space provide a well of energy that gives space its springiness, shoving it apart. But so far, calculations suggest that such a lambda should be many orders of magnitude larger than the supernova groups have seen. That puzzle has launched a search for new physics principles, such as symmetries in the fabric of space, that might help cancel out huge terms in the equations. Other candidates for this strange energy, which go by names like quintessence and X-matter, have also been put forth, as physicists vie for the prize of explaining what most of the universe is made of. And because some of those forms of energy may change over time, cosmologists have become less confident about declaring the fate of the universe hundreds of billions of years hence.

Indeed, at this point the cosmological constant remains in the realm of theory; no one yet knows the precise nature of the energy causing the universe to fly apart ever faster. Astronomers continue to gather data and to search for any effect other than acceleration that could explain their findings. But despite their efforts, they have found no reason to doubt their work. Although the nature of the universe was once chiefly the realm of philosophers, in 1998 it seems that cosmology is grounded in data, as visions of distant supernovae revealed the true nature—and perhaps the future—of the cosmos. Scientists and philosophers both will be grappling with the implications for years to come.

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