Nearly 100 years ago Edwin Hubble discovered that the universe is expanding: almost all galaxies are speeding away from our own Milky Way, and faraway galaxies are receding faster. That discovery was profound, but it was followed, in 1998, by an even more startling realization: the expansion is accelerating. For most of the 20th century, scientists had expected that over time gravity would pull galaxies toward one another, slowing the expansion. Instead two teams of astronomers studying supernovae—exploding stars that serve as markers for measuring cosmic distances—found that the growth of the universe is actually speeding up. This remarkable discovery, since confirmed by other observations, was awarded the Nobel Prize in Physics in 2011. But why is the universe accelerating? This enigma is one of the biggest unsolved mysteries in all of science.
To explain it, cosmologists have come up with two alternative ideas, either of which would revolutionize our understanding of the laws of nature. One is that Isaac Newton (and more recently, Albert Einstein) did not have the last word on gravity: although gravity is attractive on Earth and in the solar system, perhaps it acts differently, becoming a repulsive force, when it comes to the vast distances of intergalactic space. Maybe we need to modify the theory of how gravity operates on cosmic scales.
The other idea is that the universe is filled with some unseen stuff—now called dark energy—that counteracts the force of gravity, making objects repel instead of attract one another. Cosmological measurements indicate that dark energy, if it exists, currently makes up about 70 percent of the universe by mass or energy (mass and energy are equivalent, as Einstein showed with his equation E = mc2). Dark matter (no relation to dark energy), an invisible form of matter, makes up about 25 percent, and normal matter—things made of atoms, including stars, planets and people—contributes only about 5 percent. This picture has garnered more attention than the notion that gravity works differently on large scales because it neatly explains the formation of galaxies and larger structures in the universe and is consistent with all measurements to date.
But how can we know for sure if dark energy is to blame for cosmic acceleration? And if it is dark energy, what is the nature of this stuff? We recently launched an ambitious project called the Dark Energy Survey (DES) to better understand why the universe seems to be ripping apart.
The survey should provide answers by gathering a thorough record of the 14-billion-year history of cosmic expansion and the rate of growth of large-scale structure—the vast conglomerations of galaxies spread across the universe—with unprecedented precision. By studying how structures grouped together over time, we hope to distinguish among the various possibilities for why they are pulling apart now.
My colleagues and I at Fermi National Accelerator Laboratory and the University of Chicago, along with 300 other physicists and astronomers from 25 institutions in the U.S., Spain, the U.K., Brazil, Germany and Switzerland, make up the DES collaboration and have worked together to build, operate and analyze data from the Dark Energy Camera, the hardware heart of our project.
In 2012 we mounted this camera on a four-meter-diameter telescope at Cerro Tololo Inter-American Observatory, a U.S. facility high in the Andes Mountains of northern Chile. It took its first snapshots of the night sky that September and was commissioned over the following months. On August 31, 2013, the DES officially began surveying a large swath of the southern sky. The survey, now in its third season, will run from August to February every year for five years and ultimately produce a deep, high-resolution map of about 200 million galaxies spread over one eighth of the sky as well as a catalog of stellar explosions that can be used to track cosmic expansion. The survey has already collected a wealth of new data that is currently being analyzed and on the way toward unlocking the secret to the universe's expansion.
Fortunately for scientists, the same evidence that should distinguish between the modified gravity and dark energy hypotheses of acceleration should help clarify what dark energy is, if it exists. The survey will test two main ideas about dark energy. The simplest explanation for it may seem counterintuitive: that it is the energy of empty space. Suppose you took a box and emptied it of all matter—all the atoms, radiation, dark matter, and so on—and nothing could penetrate its walls. The inside of the box would be a perfect vacuum. According to classical physics, the vacuum—empty space—has no energy. But quantum theory says that even empty space carries energy. Physicists think of this energy as coming from “virtual” particles: at any time a particle and its antiparticle can appear spontaneously for a brief instant, then annihilate each other and disappear back into the vacuum. Virtual particles carry energy in exactly the form that would be needed to constitute dark energy and cause the expansion of the universe to speed up.
The only difficulty with this notion is that quantum physics predicts that the amount of vacuum energy in space should be 120 orders of magnitude (10120) larger than what it seems to be if it is responsible for dark energy. If you are working on a math problem, it is hard to make an error that big. Partly as a result of this discrepancy, cosmologists have proposed other explanations for dark energy aside from vacuum energy.
One idea—the second notion being tested by the survey—is that dark energy takes the form of a so far undetected particle that could be a distant cousin of the recently discovered Higgs boson: it would have some of the properties of the Higgs particle but would be 44 orders of magnitude lighter. This possibility is sometimes dubbed “quintessence.” One can think of such a particle as acting like a ball rolling down a hill at each point in space. The rolling ball carries both kinetic energy (because of its motion) and potential energy (because of the height of the hill it is rolling down); the higher an object is, the greater its potential energy is. As it rolls down, its potential energy declines, and its kinetic energy rises. If the quintessence particle is extremely light, with a mass less than about 10−33 electron volt (the electron, for example, has a comparatively huge mass of 511,000 electron volts), then it would be rolling very slowly today, with relatively little kinetic versus potential energy. In that case, its effect on cosmic expansion would be similar but not identical to that of vacuum energy and would lead to acceleration, although most versions of the quintessence idea predict that the acceleration would begin later in cosmic history than if vacuum energy were the culprit.
To distinguish among the possible causes of cosmic acceleration, the Dark Energy Survey—which is funded by the U.S. Department of Energy and the National Science Foundation, with additional support from the participating institutions and foreign funding agencies—is investigating four phenomena that are particularly sensitive to whatever is pulling the universe apart. And because each involves a different observable quantity, the four probes will not all be affected by the same measurement errors.
These four phenomena are supernovae, signatures of primordial sound waves, gravitational lensing (the bending of light by gravity) and galaxy clusters. Collectively they tell us how fast the universe has expanded and how much matter has clumped together to form large-scale structures at different epochs of cosmic history. At early times, up to about several billion years after the big bang, gravity fought against the expansion and enabled large-scale structures to form. But when the universe was around seven billion years old, matter became dilute enough that whatever was causing accelerated expansion—be it dark energy or modified gravity—became dominant over gravity and sped up the expansion, gradually shutting down the further formation of large structures. Vacuum energy, quintessence and modified gravity would each leave unique signatures in the history of the cosmic expansion rate and in the pattern of structure growth, imprints that we can tease out through these four probes.
A type Ia supernova is a unique category of stellar explosion that results when a small, dense object called a white dwarf star reaches a certain mass limit. These supernovae have a very special quality—they all reach a peak brightness that is nearly the same. Any differences in how bright they appear to us stem solely from their distance: those that look dimmer are farther away. This feature makes them so-called standard candles, or good cosmic yardsticks. We know, for instance, that a type Ia supernova 100 times as faint as another is 10 times farther away.
The DES will observe the same few patches of sky every few nights to measure accurate distances to a few thousand type Ia supernovae in the nearby and distant universe, nearly 100 times as many as were used in the 1998 discovery of cosmic acceleration. We are also using other telescopes to measure how much the light from these supernovae is shifted toward the red end of the visible spectrum. This redshift occurs for any object speeding away from an observer and tells us how much the wavelength of the light has been stretched by the expansion of the cosmos between the time it was emitted and now. The redshifts of the distant supernovae directly reveal the relative size of the universe then versus today. Taken together with the standard candle distance measurements to the same objects, the DES will be able to reconstruct the past 10 billion years of the expansion history of the universe with great precision.
Such a measurement can distinguish among different theories of cosmic acceleration because each would have produced a slightly different expansion history. If quintessence is at work, for example, accelerated expansion would have started somewhat later than in the vacuum energy scenario and built up more gradually, most models suggest. Thus, supernovae of a given redshift will appear brighter—will be closer—if the universe contains a Higgs-like quintessence particle than they will if vacuum energy is driving expansion. And if gravity, in turn, works differently than we think, the pattern among distant supernovae will again differ, although the details vary depending on the specific modifications investigators have proposed to the classic idea of how gravity works.
Very high precision in these measurements is necessary to distinguish among the different models because their predictions diverge only slightly. Therefore, we wish to know the distance versus redshift relation to roughly single-percent-level accuracy—a feat that the Dark Energy Camera, for the first time, should be able to manage.
Signatures of primordial sound waves
The DES will also use a relic from the beginning of the universe to study its expansion history. In the early universe, gravity was pulling matter together while the outward pressure of the electromagnetic radiation (light) in the cosmos resisted such compression. This competition created a series of sound waves. A few hundred thousand years after the big bang, when ordinary matter had cooled sufficiently from its initial hot state to transition from an ionized gas into atoms, the atoms and radiation went their separate ways (they effectively stopped interacting with one another), and this competition ceased. The distance traveled by the sound waves up to that point, which today corresponds to a scale of about 480 million light-years, ended up imprinted in the spatial distribution of galaxies as a slight tendency for pairs of galaxies to be separated by this distance compared with other distances.
This baryon acoustic oscillation (BAO) scale provides a standard ruler for measuring cosmic distances and the expansion history. That is, if you know the physical size of a ruler (the 480-million-light-year spread of many galaxies from one another) and can measure how big it appears (the angle of separation between those galaxies on the sky), then you can tell how far away it is. The DES will measure this BAO feature for about 200 million galaxies, enabling us to chart their distance versus their redshift as we do for supernovae. Galaxies of the same redshift would be closer if quintessence caused the universe to start accelerating later than if vacuum energy was responsible and acceleration began earlier. If there is no dark energy, we expect that the relation between distance and redshift will look different from either of those scenarios, although the particulars will again depend on how exactly gravity is altered.
This method focuses on a feature of light predicted by Einstein's general theory of relativity. The paths of light rays, as they travel to Earth from distant galaxies, get bent by the gravitational field of the matter they pass. This bending leads to a distortion of the images of these galaxies, an effect known as gravitational lensing. When the bending effect is large, the resulting images can be dramatic: distant galaxies may appear as thin, very extended arcs of light, and one may even see multiple images of the same galaxy. The light rays from most galaxies, however, are bent only slightly, leading to very small distortions in their shapes that are not discernible by eye: this is the regime of weak gravitational lensing.
Light rays from equally distant galaxies near to one another on the sky get bent by nearly the same amount because they travel through roughly the same intervening matter. By measuring the shapes of many galaxies in a small patch of sky, we can infer how much the images have been distorted and thus the clumpiness of the intervening matter, even though each galaxy image is distorted only slightly. Repeating this measurement for galaxies in different parts of the sky thus reveals the general clumpiness of matter in the universe. The evolution of this clumpiness over time, because it reflects the competition between gravity and dark energy and is sensitive to any modification of gravity, can help tell us what is causing the universe to accelerate.
The DES will measure the shapes of 200 million galaxies to see this effect, covering over 20 times more galaxies and a greater area of sky than previous weak lensing studies. By making extremely precise measurements of the shapes of these galaxies across the sky at different distances from Earth, we can create the most precise map yet of the distribution of matter at various removes—that is, at various cosmic time periods because the farther something is from Earth, the longer its light takes to reach us.
The map will differ depending on what is pulling the universe apart. The effects of quintessence in hindering the growth of large-scale structure, for example, probably set in during an earlier cosmic epoch than those of vacuum energy. Because we know from measurements how clumpy the universe is today, if quintessence were at work, we would expect to see more clumpiness when the universe was younger than in the case of vacuum energy. That prediction may sound counterintuitive because dark energy would hinder clumps from forming, but for the universe to have its current structure after billions of years of expansion, it would have to have been relatively clumpy early on. If there were no dark energy, modified gravity would have led to yet a different pattern of clumpiness throughout time—although whether the clumpiness would be relatively more or less at early epochs would differ for different formulations of the laws of gravity.
Finally, the DES will also hunt for clusters of galaxies to trace cosmic clumpiness over time. Clusters, with masses of up to more than 1015 (1,000 trillion) times the mass of the sun, are the largest gravitationally bound objects in the universe, and they form against the pull of either dark energy or modified gravity. Unlike previous cosmological cluster surveys, which have been limited to smaller areas of sky, the DES aims to discover tens of thousands of clusters out to billions of light-years away.
Scientists will then compare the number of clusters they see close to Earth—corresponding to recent times—and far away in the past. Similarly to the effects on matter's clumpiness as shown by weak gravitational lensing, we expect to see more clusters in the early universe if quintessence is at work than if vacuum energy is shaping the cosmos (all other things being equal), and we would see a different and more complicated trend altogether if gravity behaves unusually.
The secret weapon for our project is the most powerful camera ever made for looking into this question. The Dark Energy Camera, mounted on the Victor M. Blanco Telescope, is designed to survey numerous objects, including galaxies, clusters and supernovae, in the shortest possible time. The ultrasensitive, 570-megapixel camera has a very large field of view, enabled by five large lenses, best for taking in large swaths of the universe at a go.
Since its official start in August 2013, the survey has covered nearly 5,000 square degrees of sky, obtaining color images of about 100 million galaxies. The supernova survey has discovered more than 1,000 type Ia supernovae so far. We are now analyzing these data to extract information about the supernovae's distances to compare with redshift. We are also measuring galaxy shapes to infer the weak lensing signal, identifying distant clusters of galaxies and measuring their properties, and measuring the spatial distribution of galaxies to hunt for the baryon acoustic oscillation signature. In about a year, the first phase of this analysis should be complete, and we can begin to look for clues that reveal the nature of the universe's expansion.
In the meantime, the experiment has made some interesting astrophysical findings, such as the discovery of 16 ultrafaint dwarf galaxy candidates in the Milky Way's backyard. These very nearby galaxies contain as little as a few tens of stars and are among the most dark matter–dominated objects known in the universe. Their darkness makes them very hard to detect, but they are of interest as the building blocks of larger galaxies like our own Milky Way and as potential sites for probing the nature of dark matter.
More DES data are coming in all the time. As you read this, scientists are analyzing these observations for clues to the nature of dark energy. We do not yet know whether the DES will provide definitive answers—dark energy or modified gravity? vacuum energy or quintessence?—but we do know it will take the next major step in the hunt for dark energy and for the root cause of our universe's mysteriously accelerating expansion.