A new eye is now open to the cosmos. The Dark Energy Camera, which saw first light on September 12, 2012, is a spectacular new scientific facility with the grandest of goals: no less than understanding the evolution and fate of the entire universe.
For every telescope, “first light” is the moment when the optics and camera are assembled into a single instrument and turned to the night sky for the first time. But first light is just the beginning. While it often yields a spectacular photo or two, single photographs rarely lead to substantive results. Modern measurements require a subtle understanding of the equipment’s idiosyncrasies and the operators and scientists must spend a while familiarizing themselves with their instrument’s performance. After the facility has been put through its paces, real research begins. On January 9, Joshua Frieman, leader of the Dark Energy Survey (DES) collaboration, announced at the 221st meeting of the American Astronomical Society in Long Beach, California, that the team is wrapping up this getting-to-know-you phase, known as the commissioning period. They have already made interesting scientific observations, including discovering distant supernovae and clusters of galaxies.
The 570 megapixel Dark Energy Camera is hooked up to the venerable four-meter Blanco Telescope at the Cerro Tololo Inter-American Observatory, located in the Chilean Andes. Together, they will complete a study of the sky called the Dark Energy Survey, which may bring us closer to an answer to one of the deepest mysteries in cosmology: What is dark energy?
This question has been vexing astronomers since 1998, when astronomers discovered that, contrary to their expectations, the expansion of the universe wasn’t slowing down—it was speeding up! Cosmologists accounted for this surprising behavior by invoking a form of repulsive gravity first imagined by Einstein. But Einstein abandoned the idea when Hubble’s observation of the expanding universe made it seem unnecessary. Today, in the absence of a specific explanation, astronomers describe it with the generic term “dark energy.”
The Dark Energy Survey will help scientists probe the nature of dark energy. Over the course of 525 nights over five years, astronomers will survey a quarter of the southern sky to a depth of billions of light years, revealing the how the cosmic expansion rate has changed over nearly nine billions of years.
The Dark Energy Survey studies the universe in four distinct ways:
It looks for 4,000 distant supernovae. By comparing their distance (determined by simultaneously observing their brightness and their redshift, the change in their color due to the expansion of the universe, and comparing these two numbers), astronomers will get a good handle on the cosmic expansion history.
The camera will also study patterns in the spatial distribution of galaxies that are set by a phenomenon called baryonic acoustic oscillations. When the universe was smaller and hotter, the explosion of the Big Bang caused the universe to ring like a bell as the sound of the Big Bang rippled across the cosmos. About 370,000 years after the Big Bang, the universe cooled below a critical temperature, freezing these vibrations into patterns we can still see in microwave radiation and distribution of galaxies that are blazoned across the sky. This process is analogous to flash freezing the ripples on the surface of a pond. By comparing the apparent size of the ripples with their initial size, which can be calculated using information about the conditions that prevailed in the early cosmos, astronomers can provide crucial data on the shape of space itself: whether it is flat or curved and, if curved, exactly how.
The camera will also have the capacity to study the size and makeup of vast clusters of galaxies. Because the properties of dark energy help determine how and when these clusters formed, by studying their history, we can gain new insight into dark energy.
Finally, the Dark Energy Camera will see how light from distant clusters of galaxies is being bent by mass between those clusters and our telescopes. This information will tell us more about how dark energy has shaped the distribution of matter throughout the universe by studying the size and shape of clusters of galaxies over time. In total, the camera will be able to track three hundred million galaxies!
Through these four distinct strategies—each with different strengths and weaknesses—the survey will provide independent measurements of the dark energy of the universe.
The portion of the sky that the DES will study in detail is observable from Chile from September to February. Since first light, the collaboration has put their equipment through its paces. To get an early glimpse at a complete set of data, the DES collaboration will spend the rest of the 2012-2013 observation season studying a little under 5% of the region they will eventually explore. Using this strategy, they will have as good a measurement on a small portion of the sky after just a few nights of observation as they will over their entire target after five years. This will allow a relatively quick analysis of a subset of the sky and the caliber of this small study will already be world-class. The final shakedown is expected to be completed in February and in September of 2013, the survey will start in earnest, hopefully leading to new insights into the nature of dark energy.
Stay tuned. This is a very exciting time.
The center of the Milky Way galaxy lends its awesome beauty to the skyline above the telescope domes at Cerro Tololo. The Greater and Lesser Magellanic Clouds grace the upper left corner of the photo. (Photo credit: Reidar Hahn)
Minute Physics: 2011 Nobel Prize: Dark Energy
In this video explainer, guest narrator Sean Carroll explains dark energy and cosmic expansion in honor of the 2011 Nobel Prize in physics.
NOVA scienceNOW: Cosmic Perspective: Dark Matter
In this short video, astrophysicist Neil deGrasse Tyson muses on just how much we don't know about the mysterious components of the universe, dark energy and dark matter.
If you were born on an isolated desert island in the middle of the ocean and had no communication with the outside world, your knowledge of geography would be limited. Peering through binoculars, gazing out in any direction, your view would be bounded by the sea’s horizon. Although you might speculate about what lies beyond the edge, you’d lack tangible evidence to support your hypothesis.
Confined to our planet and its environs, we face the same situation: We can see a portion of the universe, but we can only speculate about its full extent. We might surmise through its flat geometry that it continues indefinitely in all directions, like a prairie stretching out as far as the eye can see. (Flat in this context refers to a straight three-dimensional space, like an endless box.) However, our understanding of the actual universe is bounded by the edge of the observable universe. We cannot know for sure what lies beyond the enclave our instruments can detect.
Accordingly, we might wonder: How large is the part of the universe we’re potentially able to observe directly? At first glance, the answer might seem like a simple calculation. The speed of light is approximately 186,282 miles per second, or about 5.9 trillion miles per year. The time that has elapsed since the Big Bang is 13.75 billion years. Multiple the two figures and—voilà—we find that over the entire history of the universe, light could have travelled 13.75 billion light-years, or 81 billion trillion miles. But, in fact, that answer would be wrong.
Let’s think about when the light was produced. From the time of the Big Bang to the era of recombination (when neutral hydrogen atoms formed) some 380,000 years later, the universe was opaque to light. Photons bounced between charged particles and didn’t travel very far. The reason is that charged particles interact with photons—either absorbing or emitting them. Only after the era of recombination could light journey through space. That is because photons can pass through neutral hydrogen gas without being diverted. Therefore, any estimate of the size of the observable universe must assume that the farthest light we see was released after that pivotal era when space became transparent. (We may someday be able to detect neutrinos and other particles from before that era, pushing the timeline earlier and enlarging the realm of what is observable, but for now we are still limited.) The difference between the two times doesn’t change the calculation much, but is important to note.
Another adjustment is far more important. Since the primordial burst of creation, space has been stretching as the universe expands. A galaxy’s distance from us today is far greater than it was when it released the light. We can think, by analogy, of a relay race in which a girl tosses a ball to her teammate and then runs away from him. If the coach later asks the teammate what is the farthest throw he has caught he would give a very different answer than if he is asked where is the farthest player he has caught a ball from. Similarly, the distances traveled by the photons hurled by light sources do not reflect the much greater extent of the sources’ current positions. Thus, we could potentially observe light sources that are much farther out than 13.75 billion light-years, if their light was released when they were close enough to Earth.
Yet another factor that expands the limit of the observable universe is its acceleration. Not only is the universe expanding; its growth has been speeding up. Data from the Hubble Space Telescope, the WMAP (Wilkinson Microwave Anisotropy Probe) satellite and other instruments have been used to pin down the rate of acceleration, along with the current expansion rate, the age of the universe, and other important cosmological parameters.
Taking advantage of this wealth of information, in 2005 a team of astrophysicists led by J. Richard Gott of Princeton performed a detailed calculation of the radius of the observable universe. Their answer was 45.7 billion light-years—more than three times bigger than our first, naïve estimate! Within this sphere lie hundreds of billions of galaxies, each with hundreds of billions of stars.1
Image credit: Andrew Colvin
Gott’s team calculated this radius by figuring out how far away from us a source would be today if the light we now observe from it was emitted during the recombination era. In our relay race analogy, that’s determining where someone must have stood if she threw a ball and we caught it, and then using her running speed to figure out where she must be right now.
Interestingly, as the universe expands, the size of the observable portion will grow—but only up to a point. Gott and his colleagues showed that eventually there will be a limit to the observable universe’s radius: 62 billion light-years. Because of the accelerating expansion of the universe, galaxies are fleeing from us (and each other) at an ever-hastening pace. Consequently, over time, more and more galaxies will move beyond the observable horizon. Turning once again to our relay race analogy, we imagine that if the players get faster and faster as the race goes on, there will be more and more who were so far away when they first threw the ball that the light would never have had time to reach us.
Naturally not everything within the observable universe has been identified. It represents the spherical realm that contains all things that could potentially be known through their light signals. Or to draw from a famous comment by former Secretary of Defense Donald Rumsfeld, the observable universe contains “known unknowns,” such as dark matter, that could eventually be analyzed. Beyond the observable universe lie “unknown unknowns”: the subject of speculation rather than direct observation.
1The 45.7 billion light-year radius includes only light sources. If neutrinos and other particles that could penetrate the opaque conditions of the early universe are included the value becomes 46.6 billion light-years.
Yes, I know, we'll all be long gone by then. But if you could somehow stick around around to experience the universe ten thousand trillion trillion trillion trillion trillion trillion trillion trillion years from now, what would it be like?
Answering that question is a professional hobby for astronomers Fred Adams and Gregory Laughlin. They divide the life of the universe into five distinct stages, beginning with, well, the beginning—the Big Bang and the short period of explosive expansion that followed, all the way through to the formation of the very first stars about one million years later. That’s followed by the second stage, which Adams and Laughlin dub the “stelliferous era”—the era during which stars generate most of the universe’s energy. We are creatures of the stelliferous era; this is the universe we recognize as home.
But while the stars are hitting their stride during the stelliferous era, dark energy—the mysterious energy that is causing the expansion of the universe to accelerate—is well on its way to cosmic domination. If the acceleration continues at its present rate, in another hundred billion years or so, most of the visible universe will pass beyond our cosmic horizon. Future denizens of the Milky Way will turn their telescopes to the sky and see just one galaxy: their own.
As Lawrence Krauss and Robert Scherrer pointed out in a 2007 paper, these future astronomers will see no evidence of cosmic expansion or the Big Bang. They will probably conclude that their universe is static; that it is as it has always been and always will be. Ironically, the very force that sculpted their universe—dark energy—will have erased its own fingerprints.
This idea troubled Harvard astronomer Avi Loeb, who imagined a future in which astronomers would look back on today’s cosmology textbooks (which would then be 100 billion years old) with the same combination of reverence and skepticism with which we view biblical origin stories today. “You will have all these textbooks, but their claims will be unverifiable,” says Loeb.
Loeb went looking for a way in which future astronomers could tease out the history of their universe. He found the answer in hypervelocity stars, stars traveling so fast that they escape the gravity of their home galaxy. Using their advanced telescopes to monitor these stars, says Loeb, future astronomers just might be able to probe the universe beyond their galactic boundaries.
But even those galactic boundaries will be erased in the course of time. Astronomers estimate that the longest-lived stars will begin to burn out some ten trillion years from now, throwing our universe into an era of cosmic twilight. Here, the universe is lit only by the feeble embers of white dwarfs and neutrons stars, stellar corpses that will give off energy as they “hoover up” dark matter particles, says Adams. Though galaxies and galaxy clusters have managed to hold themselves together until now, a slow and steady stream of stars—the very same hypervelocity stars Loeb saw as cosmic ambassadors—will absent themselves from their galaxies until, over a period of about 1020 years, galaxies will “evaporate” entirely, Adams and Laughlin calculate. The finely-woven tapestry of the universe will come undone.
A computer simulation of the cosmic web of dark matter and ordinary matter. Image credit: NASA, ESA, and E. Hallman (University of Colorado, Boulder)
Given sufficient time, even the protons and neutrons that make up the stuff of universe will fall to pieces. How long will it take? That is still a mystery, though a combination of experiment and theory suggests that it will happen some time between 1033 and 1045 years after the Big Bang.
At that point, all that’s left of the stars and galaxies that once illuminated our universe will be a smattering of black holes. But even the reign of the black holes won’t last forever. As Stephen Hawking showed theoretically, black holes slowly leak out their contents via a process we now call Hawking radiation. Given enough time—as long at 10100 years—even the biggest black holes will evaporate away.
Only now will we enter what Adams and Laughlin dub the “dark” era. The dark era isn’t just very, very dark; it is also very, very boring. Next to nothing actually happens in the dark era. Thanks to the accelerating expansion of the universe, even humdrum particle collisions will become rarities.
Will the lonely monotony of the dark era ever end? Maybe. The same energy that has been driving the accelerating expansion of the universe could suddenly change character, a phenomenon theorists call vacuum energy decay. It happened once before—when the era of inflation ground to a halt soon after the Big Bang—and theorists believe that it should happen again.
“You could imagine a new start” for the universe, says Adams, in which matter gets a second chance to coalesce into stars, planets, even people. Or, the vacuum energy could decay before the universe ever makes it to the dark era. “If that happens,” says Loeb, “we’re back to a situation where once again we can see all those galaxies that we lost.”
Of course, these scenarios are a strong cocktail of science and speculation—and the further we look into the future, the more speculation is poured into the mix. So why study a universe that even our most distant descendants will never live to see?
The numerical models scientists use to project into the distant future can yield new insights into stellar life cycles—like how small, long-lived stars evolve into red giants—that we can’t observe progressing over the course of one (or many) lifetimes, says Adams. It also gives us a way “to gauge the cosmic importance of various aspects of the standard model,” says Loeb, by watching how they play out over time.
“It is part of our worldview to want to know what will happen,” adds Loeb. Yet I don’t think I’m alone in enjoying the fact that the next plot twist is, ultimately, a mystery.
FQXi: Predicting the End
Science writer Govert Schilling talks with Fred Adams and Greg Laughlin about how they became the authors of the future-biography of our universe.
The Five Ages of the Universe: Inside the Physics of Eternity
Fred Adams and Greg Laughlin had the bad fortune to publish this book just around the time that dark energy was discovered; their predictions therefore don't account for dark energy. Most of their conclusions about the distant future remain valid, though.
First, the good news: Despite earlier doomsday prognoses, the cosmos is not fated to implode. If you stay up late at night worrying that the entirety of creation will cave in on you in a universal Big Crunch, rest assured. Astronomical evidence suggests that the Big Bang expansion will never reverse itself; the universe will not collapse back down to a point. Rather, the universe, like ever-sprawling suburbs, will burgeon forever, its galaxies receding farther and farther away from each other.
Now, the bad news: Cosmic expansion, discovered in 1998 through supernova observations conducted by Nobel Prize-winning physicists Saul Perlmutter, Brian Schmidt, and Adam Riess and their research teams, is not only continuing, it is picking up its pace. Though we don’t know what is causing the acceleration, one leading idea, called “phantom energy,” has ominous implications. If phantom energy continues to drive the universe faster and faster outward, it could literally tear space into shreds—a doomsday scenario called the “Big Rip.”
Phantom energy, proposed by Dartmouth physicist Robert Caldwell in 1999, is one possible variety of the mysterious stuff researchers call dark energy. Dark energy is the catchall phrase for the hidden agent of cosmic acceleration, and encompasses many different approaches. The simplest proposal adds an extra “antigravity” term, called a “cosmological constant,” to Einstein’s equations of general relativity. Einstein himself had proposed the term to stabilize his equations, but then hastily removed it after Edwin Hubble showed in 1929 that the universe was expanding—and not stable at all. Adding the term back into the equations, while assuming that the geometry of the universe is flat, leads to a prediction that the cosmic growth rate will gradually and steadily increase.
While a cosmological constant seems to model the accelerating universe nicely, at least according to current astronomical observations, many researchers are seeking a more tangible explanation. Instead of an extra term arbitrarily added to the gravitational equations, they are looking for an actual physical substance with repulsive properties. This substance must have a bizarre property called negative pressure. Though we never encounter negative pressure in everyday life, you can picture how it works by imagining poking a soap bubble and seeing it inflate rather than pop.
Gravitational researchers model the behavior of substances using an equation, called an equation of state, that relates the material’s pressure to its density. For conventional substances such as fluids and gases, both pressure and density are positive, so their ratio, usually called “w,” is positive as well. However, if a substance were to mimic a cosmological constant, its w ratio would be negative one. That is, if you try to squeeze it, it will get bigger, not smaller.
In 1998, shortly after the discovery of cosmic acceleration, Caldwell, along with astrophysicists Rahul Dave, then at Penn, and Paul Steinhardt of Princeton, argued for the greatest amount of flexibility in trying to describe dark energy. Borrowing the ancient Greek term for the “fifth essence,” they dubbed it “quintessence” and asserted that its equation of state could vary over time and space. Rather than assuming that w is negative one, they urged that its actual value be pinned down through astronomical observation.
This flexibility spurred Caldwell to think about the worst-case scenario imaginable for dark energy. He wondered what would happen if the amount of negative pressure exceeded the density—that is, if w was less than negative one. In our soap bubble analogy, larger negative pressure would mean that pressing on its exterior would cause it to puff up even faster. In the far future, he realized, gravity and all other forces would eventually lose potency, overwhelmed by the hulking menace of what he dubbed phantom energy. The reason is that unlike the steady density of the energy associated with a cosmological constant, the density of phantom energy would grow greater and greater, rising along with its negative pressure, building to a lethal crescendo—a Big Rip.
In Caldwell’s end game scenario, our distant descendants (relocated to other planets, perhaps) will notice the first sign of trouble billions of years from now as other galaxies recede beyond detection and disappear from the sky. (That would happen under the cosmological constant picture too, but is exacerbated in the case of phantom energy.) Eventually our local group of galaxies, including Andromeda, will become cosmic hermits. Then, tens of millions of years before the ultimate doomsday, the local group and then the Milky Way itself will break up like a dropped box of peanut brittle.
In the cosmic twilight times, all planetary systems will be torn apart and the stars and planets will explode. About thirty minutes later, all atoms will burst like fireworks. Then, in a mere fraction of a second, space itself will tear to shreds in the all-destroying Big Rip.1
If the devouring of reality by a ravenous shredder sounds revolting, you can console yourself with an alternative developed by physicist Pedro Gonzalez-Diaz of the Institute of Mathematics and Fundamental Physics in Madrid. He conjectures that phantom energy could feed one of the many wormholes that could exist in the fabric of spacetime. Over billions of years, the wormhole would swell up faster than the cosmic expansion rate and eventually engulf the material content of whole universe—galaxies and such—sparing it from a Big Rip. Theoretically, then, everything could pass unscathed out the other end of the wormhole, emerging in another sector of reality, wherever that would be. Not exactly a joy ride, but at least it wouldn’t leave you in tatters!
The jury is still out on what causes cosmic acceleration. It could well be the case that the culprit is not phantom energy, but rather a gentler form of dark energy—such as a type of quintessence with a w value greater than or equal to negative one. If so, the expansion of space would increase more gradually, leaving the local group and the Milky Way gravitationally bound together as other galaxies eventually recede from view. While our enclave of space would be lonely, it would remain intact. In short, we’d experience a “Big Stretch” rather than a “Big Rip.”
1A recent calculation by a team of researchers led by Chinese physicist XiaoDong Li uses current data to estimate that the Big Rip will take place 16.7 billion years from now. The dissolution of atoms would take place a mere 30 quintillionth of a second before the Big Rip.
Simulation of the sky as viewed by the Bell Labs microwave receiver. Credit: NASA / WMAP Science Team
This is the cosmic background radiation as detected with a Bell Labs radio telescope in 1964. The band across the middle is the center of our galaxy. The rest is the humming echo of the Big Bang, uniform in every direction—just as theorists had been predicting.
“Which is an amazing thing,” P. James E. Peebles—one of the very same cosmologists who helped predict it—recalls thinking. “But there it is: The universe is simple.”
As Einstein once famously said, “The most incomprehensible thing about the universe is that it is comprehensible.” But why should it be? Why would something so vast and complex and old be within the comprehension of a species that spent millennia believing it occupied the center of existence? Yet century after century cosmologists have operated under the assumption that the universe is simple, and it appears to have worked—at least so far.
That assumption goes back to Copernicus. The picture of the heavens he inherited from the ancients was crowded with invisible spheres that carried the moon, sun, planets, and stars. The geometry to explain those motions was embroidered with epicycles and deferents—circles, and circles within circles, and circles adjacent to circles, all fabricated by astronomers over the course of a couple of millennia in an attempt to make sense of the motions of the celestial bodies around a stationary Earth. The problem with this picture, Copernicus realized, was that it divided the universe into two realms, the terrestrial and the celestial. What if the universe instead was one big happy realm? Once Copernicus removed the Earth from its place of privilege and set it in orbit around the sun, he arrived at equations that predicted the motions of the heavens with far greater accuracy. A century and a half later, Isaac Newton used the sun-centered model to create his law of universal gravitation—emphasis on “universal.” By uniting the physics of the terrestrial with the physics of the celestial, he showed that Copernicus was right: The universe is simple.
For the next three centuries, the discoveries of moons and planets and comets corroborated Newton’s idea, with one exception: an aberration in the orbit of Mercury. In 1915 Einstein fixed that problem, via the general theory of relativity, by reconceiving gravity not as a force that acts across space but as a property of space itself. Two years later, he published a paper exploring the “cosmological considerations” of this new view of gravity. What might this tweaked law of universal gravitation have to say about the history and structure of the universe? To keep the math simple, Einstein and then other theorists had to assume the universe was simple, too. So they returned to Copernicus’s assumption: The Earth doesn’t have a privileged position in the universe. On the largest scale, the cosmos would look the same in every direction no matter where you are in it.
Which was what the 1964 vision of the cosmic background radiation revealed. This picture of the universe, however, was almost too simple. Where were the subtle fluctuations in temperature that would represent the seeds of the galaxies, clusters of galaxies, and superclusters of galaxies—everything that would grow into the universe as we know it?
To answer that question, NASA set to work designing a satellite to look for those fluctuations. In 1991 and 1992, that satellite, the Cosmic Background Explorer (COBE), found them—differences in the temperature at a level of one part in 100,000:
The sky as seen by COBE. Credit: NASA Legacy Archive for Microwave Background Data Analysis (LAMBDA)
I met the the co-principal investigator of that project, George Smoot, in his office at the University of California, Berkeley, just days after he won the Nobel Prize in physics. Never a particularly calm presence, he was even more animated on this occasion. Underslept and overadrenalized, he shouted, “Time and time again the universe has turned out to be really simple!”
Sitting across from him, nodding emphatically, was fellow physicist Saul Perlmutter of Lawrence Berkeley National Laboratory. “It’s like, why are we able to understand the universe at our level?” he said, echoing Einstein.
Yet Perlmutter himself is among the scientists whose work has most threatened the notion that the universe will be ultimately comprehensible. In 1998, he was the leader of one of the two teams that found the expansion of the universe is not slowing down, as you might naively expect, but speeding up. (He would share the Nobel for that discovery in 2011.) At first physicists considered the discovery of “dark energy” difficult to accept—a force more powerful than gravity on a cosmic scale?—but in 2003 came the first results from the successor to COBE, the Wilkinson Microwave Anistropy Probe (WMAP):
The sky as seen by WMAP. Credit: WMAP Science Team, NASA
By reading the patterns in those even finer fluctuations, cosmologists could calculate the portion of the universe that takes the form of dark energy: 72.8 percent. So what is it?
Yet before theorists can begin to answer that question, they need to know how dark energy behaves. Does it vary across space and over time, or is it constant? The successor to WMAP, the Planck satellite, should provide a strong clue when its results are released early next year. So far, though, all the data from less precise experiments are pointing toward dark energy being constant. In that case, theorists agree, the answer to “What is dark energy?” will require them to unite the physics of the very big (relativity) with the physics of the very small (quantum mechanics), just as Newton had united the physics of the terrestrial with the physics of the celestial.
“We shouldn’t be shocked that we’re finding a few surprises,” Perlmutter later told me. “Based on just some fragment of information, and a very interesting theory of Einstein’s, people were able to try out the simplest possible model of the universe. ‘We don’t know anything but let’s imagine that it’s as simple as it could possibly be, because we have no other information to go on.’ And then they said, ‘Let’s take a few more pieces of information,’ and those pieces of information fit, and they fit well into this ridiculously simple, intentionally cartoonish picture.”
But now? We don’t know what the vast majority of the universe is. And, physicists acknowledge, we might never know. The universe just might be incomprehensible after all. But assuming the solution exists, Perlmutter at least has faith as to what it will look like: Copernicus’ solution, and Newton’s, and Einstein’s.
This week, we’ll investigate whether there are viable alternatives to the idea of dark energy, the mysterious stuff that astrophysicists believe is pushing our universe apart.
In every direction we look, galaxies are hurtling away from us. That isn’t surprising in itself—after all, the Big Bang sent space and everything in it flying apart. One would expect that the gravitational pull of all the “stuff” in the cosmos would gradually slow down this expansion, bringing it to a dead stop or even collapsing everything back together in a "Big Crunch." Yet instead, astronomers see that the galaxies in our universe are rushing apart faster and faster.
What could be causing this acceleration? Physicists call it dark energy, and it could make up more than 70 percent of the cosmos. But so much remains unknown about dark energy that some scientists are asking whether it exists at all.
What if, instead of a mysterious unseen energy, "there is something wrong with gravity?" asks Sean Carroll, a theoretical physicist at the California Institute of Technology.
Einstein's theory of general relativity represents gravity as the curvature of space and time. Perhaps this idea "is still right, but we're not solving the equations correctly," suggests Carroll. "We're used to thinking of the universe expanding perfectly smoothly, and we know it isn't, and maybe these deviations are important." If we accounted for how the universe is clumpy instead of smooth, it might turn out that the gravitational pull of clusters of galaxies and other large agglomerations of matter alter spacetime more than previously appreciated. Distant objects would thus appear to be farther away than they actually are, leading to the false conclusion that the universe's expansion is accelerating.
The problem with this kind of model, Carroll says, is that while it suggests that these clumped-up astronomical bodies might distort our view of the universe more than suspected, gravity still remains the weakest of the known fundamental forces of nature. Also, these astronomical clumps would evolve in size and gravitational strength over time. In contrast, the mysteries that dark energy was invoked to solve require something with a lot of energy that changes less over time.
Another approach is to modify the laws of gravity to do away with dark energy. This tack suggests that "the laws of gravity as we know them work better on relatively small scales such as our solar system,” says Carroll, but perhaps they need “tweaks” to work on cosmic scales. Carroll and other theorists have developed alternative descriptions of gravity that could explain why the universe evolved as it did. One set of scenarios suggests that the strength of gravity increases over time and has different values depending on the distances involved. But critics argue that, to avoid contradicting well-established features of general relativity, these models are unacceptably contrived.
Another family of alternative gravity models analyzes how gravity behaves if there are extra dimensions of reality, as suggested by string theory. But this approach has problems of its own: It leads to empty space "decaying" into particles in potentially detectable ways, Carroll says.
To avoid modifying gravity, some theorists have suggested that our galaxy and its neighborhood might lie within a giant void, an emptier-than-average region of space roughly 8 billion light years across. With so little matter to slow down its expansion, the void would expand faster than the rest of the universe. If we lived near the heart of this void, our observations of accelerating cosmic expansion would be an illusion.
"The advantage of giant void models is that they don't require any new physics to explain the apparent acceleration of the universe, like the existence of some weird dark energy or a modified theory of gravity," says theoretical cosmologist Phil Bull at the University of Oxford.
Still, "there are lots and lots of problems with void scenarios," says theoretical physicist Malcolm Fairbairn at King's College London. "It's very difficult to get them to fit existing data—for instance, the cosmic microwave background (CMB) radiation usually gets distorted in these models compared to what we actually see." For the void model to match observations of CMB radiation, we would need to be very close to the center of the void, to within one part in 100 million. That "seems like an unacceptable 'fine-tuning' to some people,” says Bull. “Why should we find ourselves so close to the center?"
In addition, astronomers using NASA's Hubble Space Telescope recently found evidence against the existence of such a void. After refining their measurements of the rate at which the universe is expanding, they all but ruled out the possibility that the accelerating expansion is an illusion created by a void. In addition, if we are living inside a void, Bull and his colleagues argue, we should see very strong fluctuations of cosmic microwave background radiation reflected off hot gas in the clusters of galaxies surrounding the void. Yet we do not see any reflections that strong. "This was pretty much the final nail in the coffin for void models," Bull says.
To support the existence of dark energy—or vindicate one of these alternatives—we need giant sky surveys which will clock the speeds of even more galaxies, Fairbairn says. The colorful scenarios that theorists are dreaming up "ultimately show what an interesting and weird universe we live in," Carroll says. "It's one where we must keep an open mind as to what the answers may be."