Darkness on the edge of the universe

In billions of years, the only stars that shine in the night sky will be those in Earth_s immediate neighborhood.

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In billions of years, the only stars that shine in the night sky will be those in Earth_s immediate neighborhood.

IN a great many fields, researchers would give their eyeteeth to have a direct glimpse of the past. Instead, they generally have to piece together remote conditions using remnants like weathered fossils, decaying parchments or mummified remains. Cosmology, the study of the origin and evolution of the universe, is different. It is the one arena in which we can actually witness history.

The pinpoints of starlight we see with the naked eye are photons that have been streaming toward us for a few years or a few thousand. The light from more distant objects, captured by powerful telescopes, has been traveling toward us far longer than that, sometimes for billions of years. When we look at such ancient light, we are seeing _ literally _ ancient times.

During the past decade, as observations of such ancient starlight have provided deep insight into the universe_s past, they have also, surprisingly, provided deep insight into the nature of the future. And the future that the data suggest is particularly disquieting _ because of something called dark energy.

This story of discovery begins a century ago with Albert Einstein, who realized that space is not an immutable stage on which events play out, as Isaac Newton had envisioned. Instead, through his general theory of relativity, Einstein found that space, and time too, can bend, twist and warp, responding much as a trampoline does to a jumping child. In fact, so malleable is space that, according to the math, the size of the universe necessarily changes over time: the fabric of space must expand or contract _ it can_t stay put.

For Einstein, this was an unacceptable conclusion. He_d spent 10 grueling years developing the general theory of relativity, seeking a better understanding of gravity, but to him the notion of an expanding or contracting cosmos seemed blatantly erroneous. It flew in the face of the prevailing wisdom that, over the largest of scales, the universe was fixed and unchanging.

Einstein responded swiftly. He modified the equations of general relativity so that the mathematics would yield an unchanging cosmos. A static situation, like a stalemate in a tug of war, requires equal but opposite forces that cancel each other. Across large distances, the force that shapes the cosmos is the attractive pull of gravity. And so, Einstein reasoned, a counterbalancing force would need to provide a repulsive push. But what force could that be?

Remarkably, he found that a simple modification of general relativity_s equations entailed something that would have, well, blown Newton_s mind: antigravity _ a gravitational force that pushes instead of pulls. Ordinary matter, like the Earth or Sun, can generate only attractive gravity, but the math revealed that a more exotic source _ an energy that uniformly fills space, much as steam fills a sauna, only invisibly _ would generate gravity_s repulsive version. Einstein called this space-filling energy the cosmological constant, and he found that by finely adjusting its value, the repulsive gravity it produced would precisely cancel the usual attractive gravity coming from stars and galaxies, yielding a static cosmos. He breathed a sigh of relief.

A dozen years later, however, Einstein rued the day he introduced the cosmological constant. In 1929, the American astronomer Edwin Hubble discovered that distant galaxies are all rushing away from us. And the best explanation for this cosmic exodus came directly from general relativity: much as poppy seeds in a muffin that_s baking move apart as the dough swells, galaxies move apart as the space in which they_re embedded expands. Hubble_s observations thus established that there was no need for a cosmological constant; the universe is not static.

Had Einstein only trusted the original mathematics of general relativity, he would have made one of the most spectacular predictions of all time _ that the universe is expanding _ more than a decade before it was discovered. Instead, he was left to lick his wounds, summarily removing the cosmological constant from the equations of general relativity and, according to one of his trusted colleagues, calling it his greatest blunder.

But the story of the cosmological constant was far from over.

Fast forward to the 1990s, when we find two teams of astronomers undertaking painstakingly precise observations of distant supernovae _ exploding stars so brilliant they can be seen clear across the cosmos _ to determine how the expansion rate of space has changed over the history of the universe. These researchers anticipated that the gravitational attraction of matter dotting the night_s sky would slow the expansion, much as Earth_s gravity slows the speed of a ball tossed upward. By bearing witness to distant supernovae, cosmic beacons that trace the universe_s expansion rate at various moments in the past, the teams sought to make this quantitative. Shockingly, however, when the data were analyzed, the teams found that the expansion rate has not been slowing down. It_s been speeding up.

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