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.