Introduction to The Dark Side of the Universe
The night sky is filled with light—tens of thousands of stars that we can see with the unaided eye, and billions more that become visible with the aid of a modest-sized telescope. With the Hubble Space Telescope, we can see hundreds of billions of other galaxies like our own Milky Way. Yet in all these dazzling views of the heavens, we are actually seeing only a tiny fraction of the material that makes up the universe—about 0.5 percent. The other 99.5 percent of the universe does not emit or absorb light, and almost all of this cosmic darkness is a complete mystery to scientists.
In 2005, physicists around the world were searching for information about the “dark side” of the universe. Understanding unseen forms of matter and energy, these scientists believe, is essential to understanding the fundamental nature of the cosmos. How do scientists know that there is more to the universe than meets the eye? What does this mysterious matter and energy—which scientists have named dark matter and dark energy—consist of? And how will forces associated with dark matter and dark energy determine the fate of the universe?
In the exploration of the dark side of the universe, images made by two telescopic surveys, both released in 2003, have played a crucial role. These images—produced by the Wilkinson Microwave Anisotropy Probe (WMAP) and the Sloan Digital Sky Survey (SDSS)—have allowed scientists to estimate accurately the percentages of dark matter, dark energy, and regular light-emitting matter in the universe.
WMAP, an orbiting telescope launched by the National Aeronautics and Space Administration (NASA), produced images of the universe when the cosmos was only 380,000 years old—before there were stars or galaxies. The light captured in these images was the oldest—and most distant—light ever seen. This light is the “echo of the big bang,” the enormous explosion that most scientists believe gave birth to the universe approximately 14 billion years ago. According to the big bang theory, the universe has been expanding ever since.
The WMAP images showed tiny ripples (variations) in the intensity of the microwave radiation left over from the big bang. The microwave ripples, explained the WMAP scientists, represented the gravitational effects of matter as it collected and condensed in certain regions of the hot, expanding gas cloud created by the big bang. Careful analyses of these ripples allowed the scientists to estimate how much matter and energy are in the universe.
The SDSS survey, created by a ground-based telescope at the Apache Point Observatory in New Mexico, revealed the distribution of hundreds of millions of galaxies out to a distance of 2 billion light-years from Earth. A light-year is the distance that light travels in one year—9.46 trillion kilometers (5.88 trillion miles). The survey revealed how gravity has caused galaxies to clump together in certain regions of space. By analyzing the distribution of these clumps, scientists were able to determine how gravity created the structure of the universe that we observe today.
Matter and Energy Estimates From Telescopes
Scientists have used the data from WMAP and SDSS—as well as supporting data from measurements of certain chemical elements in the universe—to make the following estimates of the composition of the cosmos. Approximately 4 percent of the universe consists of ordinary atoms of matter, such as hydrogen, helium, carbon, nitrogen, and oxygen. Only about 1/8 of this 4 percent—or about 0.5 percent of the total amount of matter and energy in the universe—exists in forms we can see with visible light, such as stars and planets. The remaining 7/8 of this material exists in forms that can be detected only with special telescopes. Among these forms are giant clouds of hot gas that emit X rays. In addition, visible light, X rays, and other forms of electromagnetic energy account for just 0.01 percent of the universe.
About 26 percent of the universe exists in the form of mysterious, invisible dark matter. Scientists believe that the gravitational effects of dark matter hold together our galaxy and all the other galaxies in the universe.
Roughly 70 percent of the universe exists in an unknown but weird form of energy called dark energy. The most remarkable feature of dark energy is that it has a gravitational force that is repulsive—that is, it causes galaxies to speed away from one another. In fact, scientists have found that dark energy is so repulsive that it is causing the universe to expand at an ever-faster rate.
How can scientists be sure that dark matter and dark energy exist if researchers cannot see them or even measure them directly? A number of scientific observations—some going back to the 1930's—support the reality of dark matter and dark energy.
Gravity Gives Clues to Dark Matter
Scientists concluded that dark matter exists by studying the way gravity affects the movement of visible matter in the universe. Gravity affects everything in the universe, from solar systems to galaxies. For example, the gravitational attraction of the sun holds Earth and the other planets of our solar system in orbit around the sun. The closer a planet is to the sun, the greater is the gravitational pull on that planet, and the faster that planet moves. Mercury, the innermost planet, orbits the sun at the breakneck speed of about 48 kilometers (30 miles) per second, while Pluto, the outermost planet, orbits at a comparatively sluggish 5 kilometers (3 miles) per second. So even if we could not see the sun, we could still infer its existence from its effects on the planets.
Gravitational effects of dark matter can also be observed in galaxies. In the 1930's, the Swiss-American astronomer Fritz Zwicky of the California Institute of Technology in Pasadena made many observations of galaxy clusters. These objects are large groups of hundreds or thousands of galaxies held close to one another by gravity. Zwicky measured the speeds at which galaxies within the clusters are moving. He made his measurements with a spectrograph, a device that spreads the light from galaxies (and other light sources) into a spectrum of its component colors—like the colors of a rainbow—ranging from red to blue. When objects in space that are moving away from Earth are observed with a spectrograph, their light is red-shifted—that is, their light displays more colors toward the red end of the spectrum. In contrast, the light of objects moving toward Earth is shifted toward the blue end of the spectrum.
Zwicky proposed that if the only gravity holding galaxies in place came from the visible stars within clusters, then the speedier galaxies in a cluster should have been able to escape the cluster's gravitational bonds. Zwicky deduced that because these galaxies remain in place, there must be a huge amount of gravity unaccounted for. He coined the term dark matter to describe the unseen material producing this gravitational influence. Today, scientists believe that dark matter accounts for almost 100 times as much mass (quantity of matter) as stars do.
At the time of Zwicky's observation, the idea that the universe contains large amounts of dark matter was too radical for most scientists to take seriously. It would take researchers several more decades to collect enough evidence to convince the scientific community that dark matter was real.
Beginning in the 1980's, X-ray telescopes revealed that some of Zwicky's dark matter actually consists of ordinary matter in the form of a hot gas that spreads throughout galaxies. However, this hot gas, which accounts for about seven times as much mass as the stars of the galaxies, still fails by a wide margin to produce enough gravity to hold galaxy clusters together.
Zwicky had suggested that some of the dark matter might consist of unusual stars and starlike objects that emit forms of electromagnetic radiation other than visible light, such as white dwarfs (small stars that have run out of fuel) or neutron stars (the spinning cores of exploded stars). Later, other scientists estimated the total mass of these objects, along with that of brown dwarfs (dim objects with more mass than a planet but less than a star) and black holes (regions where the gravitational force is so strong that nothing can escape). The researchers concluded that the total mass of all these strange objects does not amount to much—it is even less than that of ordinary stars. Therefore, there must be more to the dark matter mystery.
Scientists have discovered additional clues for solving this mystery since the 1980's. Some of these clues have come from the work of American astronomer Vera Rubin of the Carnegie Institution of Washington (D.C.) and others who have studied the speeds of stars within galaxies. By carefully measuring the motions of stars and clouds of gas within thousands of spiral galaxies like our own Milky Way, Rubin and the others demonstrated how dark matter holds the galaxies together. In spiral galaxies, stars move in almost circular orbits around the galactic center, much like a giant version of the solar system. However, in the solar system, the inner planets travel faster than the outer planets. In contrast, the stars and gas clouds far from the dense, star-rich center of a galaxy move at about the same speed as do the stars close in. This finding implied that the outlying stars and clouds must be influenced by gravity from unseen matter.
In the 1990's, more evidence for dark matter came from research into a phenomenon called gravitational lensing, which affects some light waves traveling to Earth from such distant objects as quasars (objects at the center of some galaxies that give off enormous amounts of radiation). The gravity of nearer massive objects, such as galaxies or galaxy clusters, causes these light rays to bend. As a result, the distant object appears as an arc, a ring, or even as multiple images. A team led by American astronomer J. Anthony Tyson, then of Lucent Technology's Bell Labs in Murray Hill, New Jersey, used gravitational lensing to map the distribution of dark matter in galaxy clusters. Tyson demonstrated how dark matter is responsible for the large amount of light bending observed through telescopes. Other astronomers have reported similar conclusions.
By 2000, the evidence was overwhelming that gravity from some form of mysterious dark matter holds galaxies and clusters of galaxies together. However, scientists still do not know what this mysterious matter is.
Exotic Leftovers From the Big Bang
In 2005, the leading ideas for the makeup of dark matter involved certain exotic forms of matter that were created in the high temperatures and enormous energies that existed shortly after the big bang. Physicists who study the elementary particles of the universe believe that not all of these particles have been observed in the laboratory. And scientists think that one or more of the particles may turn out to be the stuff of dark matter.
Two of the main candidates for dark matter particles are axions and neutralinos. Axions are theoretical particles that have a mass a trillion times as small as that of an electron. Electrons are negatively charged particles that orbit the nucleus (center part) of an atom; they are the lightest known particle with an electric charge. Neutralinos are theoretical particles predicted to be hundreds of times as massive as protons, the positively charged particles inside the nucleus. Neutralinos belong to a general class of theoretical particles known as WIMPs (weakly interacting massive particles). Despite their great mass, neutralinos and other WIMPs interact very weakly with the atoms of ordinary matter. As a result, scientists believe that out of billions of WIMPs passing through your body every few seconds, only one interacts with any of the atoms that make up your body.
The tiny mass of axions and the weakly interacting nature of WIMPs make these particles difficult to detect. In fact, as of 2005, scientists had observed neither WIMPs nor axions—though they were carrying out ever more sensitive experiments to find these elusive particles.
One type of exotic particle that has been observed and identified as a small component of dark matter is the neutrino. Neutrinos—which are different from neutralinos—are electrically neutral particles produced in nuclear reactions inside stars, in the interaction of cosmic rays with the atmosphere, and in the decay of radioactive elements. Special detectors on Earth can sometimes measure the effects of neutrinos. In 1998, scientists using the Super-Kamiokande (Super-K) neutrino detector in Japan studied neutrinos produced in the atmosphere to show that neutrinos have a tiny mass—less than one-millionth that of an electron. This finding suggests that neutrinos left over from the big bang could account for about the same amount of cosmic mass as stars do. The Super-K experiment did not solve the riddle of dark matter, but it established that at least some dark matter consists of exotic particles rather than ordinary atoms.
Dark Energy and Accelerating Expansion
Although much remains unknown about dark matter, dark energy is an even deeper mystery. Research that would eventually lead to the idea of dark energy had its roots in 1929, when American astronomer Edwin Hubble, making measurements at Mount Wilson Observatory in California, observed that other galaxies are moving away from ours. This observation provided the first evidence that the universe is expanding. Before Hubble's discovery, most scientists had believed that the size of the universe was unchanging. After the discovery, astronomers began to wonder if the universe would keep expanding forever.
The answer to this question, astronomers believed, lay in the general theory of relativity, the theory of gravity proposed in 1915 by the German-born scientist Albert Einstein. Most scientists familiar with Einstein's theory assumed that the gravitational attraction of all the matter in the universe would cause the expansion of the universe to slow down over time. They theorized that the future of the universe would depend only on the amount of matter it contained. If the amount of matter was great, the universe might eventually stop expanding—and then it could begin to contract. If the amount of matter was not so great, the expansion could continue forever. For almost 70 years after Hubble's discovery, these appeared to be the only two possibilities.
Then, in the late 1990's, a startling new understanding of cosmic expansion arose from the work of two groups of astronomers. One group was led by Saul Perlmutter of the Lawrence Berkeley National Laboratory in Berkeley, California. The other group was directed by Brian Schmidt of the Mount Stromlo and Siding Spring observatories in Australia. Each of these groups independently reached the totally unexpected conclusion that the universe is expanding at a rate faster than it had been in the past. Additional observations by these and other researchers confirmed this discovery and placed the beginning of the speed-up at approximately 5 billion years ago. Observations by astronomer Adam Riess of the Space Telescope Science Institute in Baltimore, Maryland, showed that before that time, the expansion rate of the universe had been slowing down.
The surprising discovery of an accelerating expansion rate was based on careful analyses of the brightness levels of exploding stars called type Ia supernovae in galaxies between 4 billion and 7 billion light-years away. Soon after they explode, these supernovae become as bright as an entire galaxy and can be seen over vast cosmic distances. And because they are all similar to one another, they serve as cosmic “yardsticks” for astronomers attempting precise measurements of distance. The dimmer the supernova appears, the farther away it is. The astronomers measured the distances to numerous supernovae, as well as the rates at which their galaxies are moving away from us. They discovered that supernovae roughly 4 billion to 7 billion light-years away are dimmer—and thus farther away—than would be expected if the universe had been slowing down. Therefore, the researchers concluded, the universe has been expanding at an accelerating pace over the past 5 billion or so years.
A Repulsive Force of Gravity?
However, if gravity is an attractive force, how can the universe's expansion be accelerating? To answer this question, astronomers turned again to the general theory of relativity. General relativity actually allows for the possibility of a repulsive form of gravity operating in the cosmos. Dark energy is the unusual form of energy whose gravitational force is repulsive.
The nature of dark energy is one of the most profound mysteries in science. Some physicists note that the simplest explanation for dark energy is that it consists of the energy associated with the vacuum (space with no matter in it)—a type of energy predicted by the theory of quantum mechanics. Physicists use the theory of quantum mechanics, also called quantum theory, to describe the often-bizarre behavior of the microscopic world of atoms and subatomic particles.
According to the laws of quantum mechanics, even a vacuum is not entirely empty. It is full of “virtual” particles that suddenly pop into existence and then rapidly disappear. The existence of such particles in a vacuum was demonstrated in the 1940's by physicist Willis Lamb at Columbia University in New York City. Lamb measured slight shifts in the radiation that hydrogen atoms give off when they are heated. He attributed these shifts to the effects of invisible particles that materialize for brief periods. According to the general theory of relativity, the gravity associated with such virtual vacuum particles is repulsive.
Also according to quantum theory, the degree to which this vacuum force repels objects in the universe is directly proportional to the energy of the virtual particles in the vacuum. In other words, the greater the energy of the vacuum particles, the more repellent is the vacuum force. Since the birth of quantum mechanics in the early 1900's, theoretical physicists have tried to use mathematical calculations to estimate the energy of these particles. But these energy estimates, based on quantum theory, are absurdly large. If the vacuum of space weighed as much as the estimates imply, the universe would be expanding billions and billions of times as fast as it appears to be. In 2005, this problem continued to bedevil theorists who favored the energy of the vacuum as an explanation for dark energy.
Some theoretical physicists are pursuing other ideas about the nature of dark energy. Among these ideas is the possibility that dark energy is a tangle of invisible, elastic “sheets” of energy that fill the universe. Another idea is that dark energy is an indirect influence on the universe caused by hidden, extra dimensions of space—dimensions beyond the three familiar spatial dimensions of height, width, and depth. These and other ideas are based on the mathematical calculations of theorists. As of 2005, physicists had discovered no hard evidence for any of these ideas.
A Great Cosmic Battle
The true nature of dark energy will determine the outcome of the continuing battle between the attractive gravitational force of dark matter and the repulsive gravitational force of dark energy. For approximately the universe's first 9 billion years, the gravity of dark matter dominated the struggle. During this period, the expansion of the universe gradually slowed, and as dark matter came together, its gravity swept up atoms into galaxies. Then, approximately 5 billion years ago, there was a big change. The repulsive gravity of dark energy overcame attractive gravity, and the expansion of the cosmos began to speed up.
Depending on how this battle between dark matter and dark energy plays out, the universe will have one of at least three possible—and very different—futures. If the density of dark energy—that is, if the amount of dark energy in a given area of space—remains constant the expansion of the universe will accelerate at a steady rate. The expansion would eventually push almost all galaxies so far away that each galaxy would become a far-flung, isolated island. Today, hundreds of billions of galaxies can be seen from our Milky Way, but in another 100 billion years, only a few hundred galaxies would be visible from the Milky Way.
If the density of dark energy decreases sufficiently, the expansion could begin to slow, and then reverse, with galaxies moving closer and closer over time. In 100 billion years, all the galaxies could squeeze together in a “big crunch.” This crunch might be followed by another big bang—and a brand-new universe. If the density of dark energy increases over time, the expansion of the universe will accelerate at an ever-increasing rate. Galaxies, stars, and planets will spread farther apart at a rapidly accelerating pace. In approximately 50 billion years, they will be torn apart in a violent “big rip.” Even individual atoms will be torn apart under this scenario.
Daring to Explore the Dark Side
Although scientists may still be in the dark regarding many questions about the universe's dark side, they are not without the means to solve some of the remaining riddles. Many ongoing and planned projects around the world are exploring the nature of dark matter and dark energy. Powerful particle accelerators as well as various kinds of specialized detectors and new types of telescopes are all pointing the way to a better understanding of dark matter and dark energy.
Particle accelerators are large machines that allow physicists to briefly duplicate the intense temperatures, pressures, and energies of the universe shortly after the big bang. These machines force subatomic particles to collide at high speeds, causing them to either break apart into new particles or join together to form other types of particles. Some of the resulting particles may be similar to the exotic dark matter that was created in the early universe. At Fermi National Accelerator Laboratory (Fermilab) near Batavia, Illinois, physicists were using the world's most powerful accelerator in 2005 to try to create such theoretical dark matter particles as the neutralino. An even more powerful accelerator—the Large Hadron Collider near Geneva, Switzerland—was scheduled to begin operation in 2007. The detection of dark matter particles would go a long way toward answering questions about the nature of dark matter.
Another dark matter project is the Cryogenic Dark Matter Search (CDMS II), a collaboration involving several institutions, including Fermilab and Brown University in Providence, Rhode Island. This project, which seeks to detect WIMPs, consists of special crystal detectors buried about 0.8 kilometer (0.5 mile) underground in an inactive iron mine in Minnesota. The depth of the detectors as well as protective shielding around them block cosmic rays and most other unwanted particles from reaching the detectors. By contrast, WIMPs would be able to penetrate the layers of earth and shielding and make it to the detectors because they interact so weakly with ordinary matter. In the detectors, they would produce telltale electronic signals that scientists could identify to verify the existence of these theoretical bits of dark matter.
The Joint Dark Energy Mission (JDEM) is a plan for an orbiting telescope that NASA and the Department of Energy hope to launch on a three-year mission to learn more about dark energy. The telescope, which would contain the largest digital camera ever constructed, would be able to find thousands of type Ia supernovae by looking back in time some 10 billion years. Precise measurements of the distances to these stellar explosions and the rates at which they are moving away from us would help scientists clarify the nature of dark energy.
Another telescope in the works is the Large Synoptic Survey Telescope, a ground-based telescope that might begin operating by 2012. This project, directed by J. Anthony Tyson of the University of California at Davis, is being designed to map and measure dark energy and dark matter through various kinds of detailed sky surveys.
These are just a few of the many projects that astronomers, physicists, and other scientists hope will increase their understanding of the mysterious, invisible matter and energy that make up most of the universe. In 2005, it was hard to believe that scientists once thought the universe consisted of only the stars and other shining objects they could see in the night sky. Now we know that an unseen dark side holds more secrets than those scientists ever imagined.