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.
It’s as if that tossed ball shot away from your hand, racing upward faster and faster. You’d conclude that something must be driving the ball away. Similarly, the astronomers concluded that something in space must be pushing galaxies apart ever more quickly. And after scrutinizing the situation, they have found that the push is most likely the repulsive gravity produced by a cosmological constant.
When Einstein introduced the cosmological constant, he envisioned its value being finely adjusted to exactly balance ordinary attractive gravity. But for other values the cosmological constant’s repulsive gravity can beat out attractive gravity, and yield the observed accelerated spatial expansion, spot on. Were Einstein still with us, his discovery that repulsive gravity lies within nature’s repertoire would have likely garnered him another Nobel prize.
As remarkable as it is that even one of Einstein’s “bad” ideas has proven prophetic, many puzzles still surround the cosmological constant: If there is a diffuse, invisible energy permeating space, where did it come from? Is this dark energy (to use modern parlance) a permanent fixture of space, or might its strength change over time? Perhaps most perplexing of all is a question of quantitative detail. The most refined attempts to calculate the amount of dark energy suffusing space miss the measured value by a gargantuan factor of 10123 (that is, a 1 followed by 123 zeroes) — the single greatest mismatch between theory and observation in the history of science.
THESE are vital questions that rank among today’s deepest mysteries. But standing beside them is an unassailable conclusion, one that’s particularly unnerving. If the dark energy doesn’t degrade over time, then the accelerated expansion of space will continue unabated, dragging away distant galaxies ever farther and ever faster. A hundred billion years from now, any galaxy that’s not resident in our neighborhood will have been swept away by swelling space for so long that it will be racing from us at faster than the speed of light. (Although nothing can move through space faster than the speed of light, there’s no limit on how fast space itself can expand.)
Light emitted by such galaxies will therefore fight a losing battle to traverse the rapidly widening gulf that separates us. The light will never reach Earth and so the galaxies will slip permanently beyond our capacity to see, regardless of how powerful our telescopes may become.
Because of this, when future astronomers look to the sky, they will no longer witness the past. The past will have drifted beyond the cliffs of space. Observations will reveal nothing but an endless stretch of inky black stillness.
If astronomers in the far future have records handed down from our era, attesting to an expanding cosmos filled with galaxies, they will face a peculiar choice: Should they believe “primitive” knowledge that speaks of a cosmos very much at odds with what anyone has seen for billions and billions of years? Or should they focus on their own observations and valiantly seek explanations for an island universe containing a small cluster of galaxies floating within an unchanging sea of darkness — a conception of the cosmos that we know definitively to be wrong?
And what if future astronomers have no such records, perhaps because on their planet scientific acumen developed long after the deep night sky faded to black? For them, the notion of an expanding universe teeming with galaxies would be a wholly theoretical construct, bereft of empirical evidence.
We’ve grown accustomed to the idea that with sufficient hard work and dedication, there’s no barrier to how fully we can both grasp reality and confirm our understanding. But by gazing far into space we’ve captured a handful of starkly informative photons, a cosmic telegram billions of years in transit. And the message, echoing across the ages, is clear. Sometimes nature guards her secrets with the unbreakable grip of physical law. Sometimes the true nature of reality beckons from just beyond the horizon.
Brian Greene, a professor of physics and mathematics at Columbia and the author of the forthcoming book The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmo.