I.

Introduction

Dark Energy, a mysterious form of energy that physicists believe is the single largest component of the universe. Dark energy is spread throughout the universe and appears to make up about 74 percent of its content. Dark energy possesses the important property of negative pressure, which means that the more space it occupies, the less energy it has. It therefore expands space in order to reduce its energy. This results in an acceleration in the rate of the universe’s expansion, an effect opposite to the gravitational attractive force of matter.

The discovery of dark energy is one of the most important recent findings in cosmology. Cosmology is the study of the universe as a whole: its contents, structure, and evolution. Astronomers have made two key observations that form the basis of modern cosmological understanding. The first key observation was that the universe is expanding. The second key finding was that the rate of expansion is accelerating (increasing) as the universe gets older.

Some form of energy, which scientists refer to as “dark energy,” must drive this acceleration. Perhaps even more surprising is the conclusion that only 4 percent of the content of the universe consists of ordinary matter, like the matter that we are familiar with here on Earth; the remaining 96 percent of the content of the universe consists of dark matter and dark energy. Astronomers can detect the effects of dark matter and dark energy on ordinary matter, but the nature of each remains mysterious.

II.

The Expanding Universe

In 1929 the American astronomer Edwin Hubble used a systematic set of measurements to show that the universe is expanding. If the universe is expanding at the same rate everywhere and if the distance between two nearby galaxies is observed to double, then the distance between all galaxies everywhere must double. That in turn means that more distant galaxies must be moving away more rapidly in order to double their distance from us in the same time as nearby galaxies double their much smaller distances from us. Mathematically, the velocity of a galaxy is directly proportional to its distance from an observer.

In 1929 Hubble and Milton Humason published data showing that the velocities of galaxies are indeed proportional to their distances. A galaxy that is twice as far away as another is receding from us at twice the velocity. In arriving at this result, these two astronomers measured the distances and velocities of a large number of galaxies. Measuring the velocities of galaxies is straightforward. When a light-emitting object is moving away from an observer, the frequency of the light received from it is observed to be shifted toward the red part of the electromagnetic spectrum—the range of radiant energies given off by light. Using large telescopes to obtain a spectrum of the faint light given off by a galaxy, astronomers measure this “redshift,” and they then calculate the galaxy’s velocity.

The challenge is measuring the distances to galaxies. What is required is a type of object that has the same intrinsic or true brightness in every galaxy. Astronomers call such an object a “standard candle.” One such standard candle is a type of star that varies in brightness and is called a Cepheid variable.

In 1912 American astronomer Henrietta Leavitt reported on her study of Cepheid variables in a nearby galaxy (the Small Magellanic Cloud). She found that Cepheids varied periodically in brightness and that the brighter Cepheids always had longer periods of brightness. Hubble used the largest telescope in existence in the 1920s, the 100-in (254-cm) Hooker telescope located at the Mount Wilson Observatory in California, to measure the periods of the Cepheids in the galaxies that he observed. From the periods, he could calculate their intrinsic or true brightness. By comparing their intrinsic brightnesses to how bright they appeared to be through the telescope, Hubble could deduce how distant they were because if two objects have the same intrinsic brightness, the more distant one will appear to be fainter.

By combining these velocity and distance measurements, Hubble showed that the distances of the galaxies were proportional to how fast they were moving away from us. This relationship between velocity and distance is called the Hubble Law, and the constant of proportionality is known as Hubble’s constant. This is just the relationship expected if the universe is expanding. In this expansion of the universe, it is the space between the galaxies that is expanding. The galaxies move apart for this reason, not because they are moving relative to each other in a fixed space. Instead of regarding redshift as a velocity measurement, it is more useful to think of it as a measurement of how much the space in which the light is moving has expanded during the light’s journey. Because it is space that is expanding, an astronomer in any galaxy anywhere in the universe would see exactly what we see—that all galaxies are moving farther away from him or her.

III.

Will the Expansion Continue Forever?

After astronomers discovered that the universe is expanding, they then wanted to know whether the expansion would continue forever or whether it might slow down and eventually reverse direction so that the universe would collapse billions of years from now to an extremely dense state. A slowdown was reasonable to expect. Matter creates gravity, and the gravitational attraction between galaxies should slow the expansion over time.

A useful analogy is the launching of a rocket into space from the surface of Earth. Depending upon the launch speed, the rocket will either fall back to the Earth’s surface or escape the Earth entirely, continuing on with some final velocity. There is a borderline case, where the rocket only just manages to escape Earth’s gravity but gradually slows down until its velocity is nearly zero.

The rocket launch is closely analogous to the expansion of the universe as a whole. If there is very little mass and energy in the universe, then the expansion will continue forever. If the mass-energy density is high, then the expansion will slow and perhaps approach zero velocity or even recollapse. Both mass and energy have to be considered because, as the German-born American physicist Albert Einstein showed in his famous equation E = mc², mass and energy are equivalent.

A.

Supernovas and Acceleration

Imagine astronomers’ surprise when they discovered at the very end of the 20th century that in fact the expansion is not slowing down, or even remaining the same, but rather is speeding up—a totally unexpected result.

To determine how the expansion of the universe changes over time, it was important to measure the properties of very distant galaxies. Because it takes time (billions of years) for the light from distant galaxies to reach us, astronomers can measure the expansion rate as it was billions of years ago and compare it with the rate today. With the technology of the 1920s, Hubble could only measure galaxies within about 100 million light-years. (A light-year is the distance that light travels in one year in a vacuum, or about 9.46 trillion km [5.88 trillion mi].) Hubble detected no changes in the expansion rate over that amount of time.

To measure the expansion rate at greater distances, astronomers needed a standard candle that is so bright it can be detected at distances of billions of light-years. That standard candle is a certain type of supernova (exploding star) called a type Ia supernova. When this type of supernova explodes, it is extremely bright (sometimes even brighter than the galaxy that hosts it) for a short time.

The peak brightness for type Ia supernovas is always the same, and this makes them ideal standard candles. According to current theories, they all start out as the same type of object—white dwarf stars, which are superdense objects containing as much mass as our Sun in a volume as small as Earth. When white dwarf stars can no longer support themselves against the pull of gravity, they collapse and set off nuclear chain reactions that produce an enormous explosion and completely destroy the white dwarf. Since these supernovas all have the same intrinsic brightness, an observation of a type Ia supernova in a distant galaxy can be used to determine the supernova’s distance by comparing how bright it appears to be with its true brightness, which is known from astrophysical studies of supernovas in nearby galaxies. Spectroscopy is used to determine the redshift, which indicates by how much the space between us and the object has expanded.

In 1998, after systematic studies of many type Ia supernovas using the Hubble Space Telescope and several ground-based telescopes, two teams of astronomers—the Supernova Cosmology Project, led by Saul Perlmutter of the Lawrence Berkeley National Laboratory in California, and the High-Z Team, led by Brian Schmidt at the Australian National Laboratory—found that the supernovas at very large distances were much fainter than they were expected to be. This result can be explained if the expansion rate of the universe is faster now than when the supernovas exploded. If the rate of expansion is accelerating, then our motion away from distant supernovas has sped up since the light left them, sweeping us farther away from them. The light then has to travel a longer distance to reach us than if the expansion rate were constant. Because the amount of light that reaches us decreases with increasing distance, the supernovas appear to be fainter than they would if the universe were expanding at a constant rate.

IV.

Confirmation of Dark Energy

To account for this acceleration, a new and unexpected phenomenon must be supplying the energy required to counteract the gravitational attraction of matter. The energy responsible has been named dark energy. Using Einstein’s relationship between mass and energy, physicists can use the astronomical measurements for the acceleration rate to calculate how much dark energy there is. They calculated that dark energy contributes about 74 percent of the total density of mass and energy in the universe.

This result is so surprising that astronomers would question the result if the supernova observations were the only evidence for acceleration. After all, what is observed is that distant supernovas are about 20 percent fainter than those nearby. It might be that supernovas that occurred a few billion years ago are somehow different (intrinsically fainter) than more recent ones. However, there is strong independent confirmation that dark energy exists.

The total density of mass plus energy can be directly measured. This is because the density of the universe determines the evolution of the properties of matter and electromagnetic radiation as the universe ages. Initially, shortly after the expansion began, the universe was extremely hot and dense. Matter and radiation interacted strongly with each other at this time, and the universe was opaque.

As the universe expanded, it grew larger and less dense. When it was a few hundred thousand years old, it had cooled to the point where protons and electrons were moving slowly enough to bind to each other and form hydrogen atoms, allowing the universe to become transparent and the radiation to fly freely. This light is seen today as very uniform long-wavelength radiation coming from outer space, and it has been named the cosmic microwave background radiation. This cosmic microwave background is the radiation left over from very early times in the universe’s history, shortly after the expansion began.

The temperature of this radiation is approximately 2.725 Kelvin, and this is true no matter what direction one looks in the sky. However, tiny deviations (about 1 part in 100,000) from this uniformity can be detected with accurate instruments. The appearance of the size and distribution of these variations is determined in part by the density of the universe. From observations made with the Wilkinson Microwave Anisotropy Probe (WMAP), a satellite launched to map the tiny variations in the temperature of the cosmic background radiation, scientists have determined that only 4 percent of the mass-energy density of the universe is made up of baryonic matter—that is, essentially the protons and neutrons that make up our world here on Earth, including our bodies. Another 22 percent is composed of dark matter, matter that emits no light or other electromagnetic radiation but that can be detected because it exerts a gravitational force. Dark energy contributes the remaining 74 percent.

Other observations of the large-scale structure of the universe—such as the behavior of galaxies, clusters of galaxies, and the observed abundances of light elements such as hydrogen and helium—are all consistent with the conclusion that the density of matter, including dark matter, in our universe is only about 26 percent of the total mass-energy density, with dark energy making up the rest.

V.

A Cosmological Constant and Dark Energy

A possible explanation of dark energy that fits very simply within the framework of Einstein’s theory of general relativity is the existence of a cosmological constant. Einstein originally introduced the cosmological constant into his equations in an attempt to render the universe static (neither expanding nor contracting). Einstein’s equations for general relativity predicted that the universe could not be static, but at the time Einstein formulated them, he and other scientists believed that the universe was unchanging. So Einstein introduced the cosmological constant to balance gravity. When Hubble later discovered that the universe is expanding, Einstein called the introduction of the cosmological constant “the biggest blunder” of his life, according to American physicist George Gamov, writing in 1971.

The possibility that the equations of general relativity should include a cosmological constant is now being seriously reconsidered because of the discovery of dark energy. Instead of having the value needed to keep the universe static, however, the cosmological constant would now have the value required to make the expansion of the universe accelerate at the observed rate.

The model of the universe that includes the cosmological constant, combined with other strong evidence that the dark matter in the universe is slow-moving as compared to the speed of light (a property called “cold”), is now the dominant model of the universe. Because all observations to date are consistent with this model, astronomers call it the “concordance” model or the “Lambda-CDM” model. (Lambda is the symbol used for the cosmological constant, and CDM stands for Cold Dark Matter.)

A.

Quantum Problems for a Cosmological Constant

Dark energy is not associated with matter or with electromagnetic radiation. Rather it is a property of space itself. Calculations based on quantum mechanics indicate that even in empty space, there is a probability that a particle and its antiparticle can appear for a short amount of time, before recombining with each other and disappearing. The combined energies of these virtual particles could account for the dark energy. One interpretation is that the tiny elementary particles that are flickering in and out of existence give space a kind of springiness that pushes it apart. Unfortunately, theoretical attempts to calculate the energy associated with this process derive a value for the cosmological constant that is about 10¹²⁰ times too large to account for the measured acceleration of the expansion of the universe. (To imagine this large number, think of ten, followed by 120 zeros.) This huge mismatch has been referred to as the most unsuccessful computation in the history of theoretical physics. It is a sign that there is some new fundamental physics about our universe that remains to be understood.

Prior to the discovery that the universe is accelerating, physicists believed that in fact the cosmological constant would turn out to be zero, and that some still-to-be discovered physical processes or particles would be found to cancel the energy associated with empty space that was predicted by conventional quantum mechanics. Perhaps such a mechanism does exist, but if so, it must provide what seems to be an unlikely answer, one that is very large and that almost but not quite compensates for the calculated very large energy of the vacuum.

B.

The Coincidence Problem

Another observation that current theory cannot explain is the fact that we seem to live at a special time in the universe’s history when the dark energy density is of the same order of magnitude as the matter density. The matter density in the universe (26 percent) is only about a factor of 3 smaller than the energy density (74 percent). This similarity is a temporary phenomenon, astronomically speaking. In fact, there is only about a 1 percent chance that an observer living at a randomly chosen time since the beginning of the expansion would observe the matter and energy densities to be so nearly the same. The fact that we happen to be alive at this special time is called the “coincidence problem.”

It is easy to understand why the relative densities of matter and energy change with time. As the universe expands, its volume increases. But according to the law of conservation of energy, the total energy remains the same. That means that the densities of matter, radiation, and vacuum energy must all decrease with time. These densities do not, however, all decrease at the same rate. The density of radiation decreases fastest, followed by that of matter, and then vacuum energy. Initially, the universe was very, very hot, and the radiation-energy density was the highest of the three; it is now the lowest. For a time, the matter density was highest, and then was diluted by expansion. Now the vacuum energy is beginning to take over. About 10 billion years ago, the density of energy in the vacuum was about ten times less than the density of matter, and billions of years from now, the vacuum-energy density will be ten times higher than the density of matter. If there is a physical explanation for why we are at a stage in the universe’s history when the matter and energy densities are roughly the same, we do not yet know what it is.

VI.

Alternatives to a Cosmological Constant

Because theory cannot yet offer an explanation of why the cosmological constant has the value that it does, physicists have proposed alternative explanations, yet to be tested, to account for the acceleration of the expansion. These mechanisms make specific predictions that can be checked against data from current and future observations and experiments.

One such theory is that there may be a previously unknown type of force in the universe that produces the observed acceleration. This would constitute a fifth fundamental force to go with the four already known—gravity, electromagnetism, and the strong and weak nuclear forces. This class of theories is often referred to as “quintessence” models, a name that refers to ancient ideas of a “fifth essence.” These models predict that the nature of dark energy changes over the lifetime of the universe, whereas the cosmological constant is exactly that—constant for all time. Precise measurements are now being planned to determine whether the properties of dark energy do change with time.

Another class of theories to explain the acceleration of our universe is based on the hypothesis that it is necessary to modify our current understanding of how gravity works at the very largest scales or at earlier times in the universe. But there are no new theories of gravity that have yet come close to being successful. The current theory of gravity, which is described by the equations in Einstein’s theory of general relativity, has been verified by numerous rigorous tests. So any model that includes new behavior to help explain the universe’s acceleration must also be consistent with all of the successful tests of general relativity. A complete new model of gravity has yet to be found.

The acceleration of the universe and the existence of dark energy seem well established. However, these remarkable and totally unexpected discoveries only raise new questions. Neither discovery was predicted in advance by theory, and so our current understanding of basic physics—elementary particles, gravity, fundamental forces—must be in some way incomplete. Over the next decade, many new experiments will be required to determine the true nature of dark energy.