Dark Energy

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.

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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.

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