Dark Energy

I

Introduction

Dark Energy, hypothetical force that opposes the attraction of gravity throughout the universe and causes the expansion of the universe to accelerate. Dark energy is spread throughout the universe and is its single largest component at about 74 per cent of its content. As dark energy possesses the important property of negative pressure, which means that the more space it occupies, the less energy it has, it expands space in order to reduce its energy, thus accelerating the rate of the universe’s expansion.

II

Discovery of Dark Energy

In 1929 the American astronomer Edwin Hubble showed that the universe is expanding. Until the late 1990s it was assumed that this expansion was slowing down and would 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, as matter creates gravity, and the gravitational attraction between galaxies should slow the expansion over time.

However, in 1998 two international consortia of astronomers, the Supernovae Cosmology Project and the High Redshift Supernovae team, observed supernovae through the Hubble Space Telescope to show that the expansion of the universe may not be decelerating at all but accelerating. By looking at the intensity of distant supernovae, and comparing them with the intensity of supernovae within our own galaxy, it was possible to estimate their distances. In the same way, it was possible to find the speed at which they are receding away from us. The light from distant supernovae was found to be dimmer, and hence they were further away, than would be expected if the expansion of the universe was slowing down. This meant that the universe must have taken longer to reach its current size than was previously thought and hence its rate of expansion was smaller in the past. To account for this acceleration, a new and unexpected phenomenon, dubbed dark energy, was believed to be supplying the energy required to counteract the gravitational attraction of matter.

Observations made with the Wilkinson Microwave Anisotropy Probe, a satellite launched to map the tiny variations in the temperature of the cosmic background radiation, precisely determined the composition of the universe with this new phenomenon in mind. It found that only 4 per cent of the mass-energy density of the universe is made up of baryonic matter. Another 22 per cent is composed of non-baryonic dark matter, which is 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 per cent. Other observations of the behaviour of galaxies, clusters of galaxies, and the observed abundances of light elements such as hydrogen and helium, all supported this finding.

III

Nature of Dark Energy

The simplest candidate for dark energy is a constant form of energy known as the cosmological constant. Its existence was first postulated by Albert Einstein in an attempt to find a static universe in general relativity. In his cosmology, the cosmological constant would counterbalance the attractive gravitational force of ordinary matter, keeping the universe static. One possible origin for Einstein’s cosmological constant is the vacuum energy (or zero-point energy) of quarks and leptons. Unfortunately, the vacuum energy is vastly bigger than the cosmological constant, and instead of having the value needed to keep the universe static the cosmological constant would need to have the value required to make the expansion of the universe accelerate at the observed rate. Alternatively, the vacuum energy would have to be small enough to be consistent with measurements of the cosmological constant, and the mechanism that would be able to make this possible is still an open problem. Despite this, 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 (this dark matter is referred to as being “cold dark matter”, as cold particles travel much slower than the speed of light), is now the dominant model of the universe.

Yet as the model does not fully account for why the cosmological constant has the value that it does, alternative explanations of dark energy have been proposed. One is that the possible source of dark energy is a particle with a potential energy of 1 billionth of the rest energy of an electron. Such a potential energy would induce a tension in space, resulting in a repulsive force. An alternative theory to account for the acceleration of the expansion of the universe 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 already-known gravity, electromagnetism, and the strong and weak nuclear forces. This class of theories is often referred to as “quintessence” models, which predict that the nature of dark energy changes over the lifetime of the universe, whereas the cosmological constant is constant for all time. Precise measurements are now being planned to determine whether the properties of dark energy do change with time.

What are known to change with time, however, are the relative densities of matter and energy. 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. When the universe began, the radiation density was the highest of the three. It is now the lowest, and vacuum energy has taken over. That vacuum energy is now dominant in the universe at this precise moment is known as the cosmological constant’s “coincidence problem”.

Although the universe is currently expanding, its future is dependent on the form of energy that will dominate. If the universe is expanding at an accelerating rate, then it will gradually get emptier with time. At some point the dark energy that drives the acceleration will completely dominate and the universe will expand at an exponential rate.