Dark Energy

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.

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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.

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