Dark Energy

FIELDS OF STUDY: Cosmology; Astrophysics; Stellar Astronomy

ABSTRACT: It was long thought that the effects of gravity would slow the expansion of the galaxy. However, evidence has shown that it is instead speeding up. Scientists believe the explanation for this is dark energy, a force that counters gravity and allows the universe to continue expanding at a faster rate. Dark energy thus has a significant impact on the size and future of the universe.

A Force against Gravity

In the 1990s, researchers discovered that the universe was neither static nor expanding more and more slowly, as many had believed for decades. Instead, the pace of expansion was speeding up. This seemed to contradict gravity, a force that affects all normal matter and causes objects to be attracted to each other. For the universe to be expanding at an increasing rate, something would have to be working against gravity. Scientists differed on how this "something" came to be a force, but they agreed on one thing. Whatever was exerting the force to counter gravity and allow the universe to expand was something previously unknown. Researchers called it "dark energy."

A Changing Understanding of the Universe

In his theory of general relativity, German-born physicist Albert Einstein (1879–1955) proposed that time, space, duration, and distances are not absolutes but relative and depend upon the observer. This theory also found that time and space are affected by gravity and motion. It was the first significant change to gravitational theory since English physicist Isaac Newton (1642–1727) articulated his law of gravity. Einstein’s equations involving space and time relativity in a static universe had indicated that the universe would collapse in on itself. He resolved this by adding an extra term he called the cosmological constant to these equations. Represented by the Greek letter lambda (Λ), the cosmological constant posited that unoccupied areas of space are filled with a constant force that is dense enough and exerts enough pressure to prevent gravity from collapsing the universe.

Then American astronomer Edwin Powell Hubble (1889–1953) discovered that the rate at which a distant galaxy moves away from another increases the farther apart they are. This indicated that the universe was not static but was expanding. Thinking an expanding universe made the concept irrelevant, Einstein discarded the idea of a cosmological constant from his theory of relativity.

For many years after Hubble’s discovery, scientists believed that gravity would slow the rate at which the universe expands. All objects made of normal matter—objects made of protons, neutrons, and electrons—are pulled together by gravity, and this pull would limit and might even eventually stop the universe from growing, scientists believed. Then in 1998, a team of California-based astronomers using the Hubble Space Telescope and an Australian team at Mount Stromlo Observatory were studying a distant supernova. They made the remarkable discovery that the universe was in fact expanding faster than it had when the supernova had occurred. Additional research showed that the universe began expanding at a faster rate about four to seven billion years ago.

This left researchers in a quandary. If the universe were expanding as fast as the evidence showed, then Einstein’s theory of general relativity was wrong. Research indicated that the universe expanded at a steady rate for billions of years and then experienced a growth spurt beginning four to seven billion years ago that kept increasing. This was unexpected and contrary to all known ways the universe worked.

Scientists eventually came up with three possible answers to this dilemma. One theory was that Einstein’s cosmological constant concept was in fact in play and that something could not only counter the pressure of gravity but get faster at doing it over time. Other scientists suggested that space is filled with an unknown but very fluid energy able to suppress the effects of gravity. Still others thought Einstein’s theory of relativity might need to be revised to include calculations accounting for the acceleration. While the theories of what was wrong differed, all three pointed to dark energy as the solution.

The Mystery of Dark Energy

Scientists know little about dark energy. No one has ever seen it, and what is known about it is inferred from its effects on the things around it. Based on the degree to which it counters gravity as the universe expands, scientists estimate that about 68 to 74 percent of the entire universe is made of dark energy.

This becomes even more surprising when the estimated percentage of the universe that is made of dark matter is added. Dark matter is made up of unseen particles that have mass but emit no light. It was discovered when researchers determined that the visible portions of galaxies did not contain enough mass to prevent the galaxies from spinning apart. Like dark energy, dark matter cannot be studied directly, but researchers know how much there is based on its effect on galaxies. An estimated 22 to 27 percent of the universe is made of dark matter. When this is added to the 68 to 74 percent thought to be made of dark energy, it becomes clear that, for as large as stars and planets can be, all of them together with all other celestial bodies make up only a tiny fraction (just 4 to 5 percent) of the material of the universe.

Scientists have advanced several explanations for dark energy. Building on Einstein’s observation that empty space is not necessarily a void, some scientists suggest that dark energy is a property of space. Humankind’s ability to understand space is limited by available technology and by the fact that most observations are not made firsthand. Thus, it is likely there are many things about even empty space yet to be discovered. For example, Einstein theorized that space could have its own energy and that this energy could itself increase and grow. If this were the case, this higher energy production could explain why the growth of the universe was accelerating.

Quantum physics plays a role in another explanation of dark energy. Under quantum theories, the tiniest particles of matter and energy do not behave the same way that larger particles do. If the seemingly empty portions of space are in fact filled with tiny particles acting in unexpected ways, this could account for the effects of dark energy. However, when scientists attempted to determine how much energy could be produced this way, their answers seemed mathematically impossible.

Some researchers are investigating the possibility that the effects of dark energy mean that Einstein’s theory of gravity is wrong. Scientists are studying how galaxies are pulled together to form clusters as a way of testing this hypothesis. To prove that Einstein was wrong would require the development of an alternate theory of gravity that explains the accelerated growth of the universe while still adequately accounting for the way the solar system and galaxy clusters hold together, as Einstein’s long-standing theory does.

In a study published in the journal Physical Review Letters in October 2014, researchers from the University of Portsmouth and the University of Rome proposed a new model of dark energy, based on data from the European Space Agency's (ESA's) Planck satellite observatory and from various other surveys, including the Sloan Digital Sky Survey (SDSS) at Apache Point Observatory in New Mexico. In this model, dark energy is posited to interact with dark matter in a way that causes the former to increase while the growth of the latter decreases; essentially, in simplified terms, the model suggests that dark energy is gradually “swallow[ing]” dark matter. Coauthor David Wands, director of Portsmouth's Institute of Cosmology and Gravitation, explained, “Dark matter provides a framework for structures to grow in the Universe. The galaxies we see are built on that scaffolding and what we are seeing here, in these findings, suggests that dark matter is evaporating, slowing that growth of structure.” In the study, the researchers argued that this type of interaction between dark energy and dark matter “is favored by current cosmological data sets.”

While scientists do not know what dark energy and dark matter are, they have many theories. Astronomers have evidence that dark matter exists and know it does not absorb, emit, or reflect light, and dark energy is a force that repels and is active in the expansion of the universe. Some theoretical physicists suggest other dark particles and forces may yet be discovered.

Dark Energy Exploration

On August 31, 2013, more than one hundred American, Spanish, German, Brazilian, and British researchers began work on the Dark Energy Survey (DES). Using the most powerful camera available, the team launched a multi-year effort to survey a wide expanse of the southern sky. They sought to measure just how fast the universe has been growing throughout each part of its 13.7-billion-year history.

The camera used for this project is called DECam, short for Dark Energy Camera. It is a 570-megapixel camera mounted on a 4-meter (13.1-foot) telescope at the Cerro Tololo Inter-American Observatory in the Chilean Andes. With lenses up to 1 meter (3.3 feet) across and a mass of over four tons, the camera was the most powerful then known. Even so, researchers would not be able to see dark energy. Instead, they used these devices to photograph the far-distant galaxies moving apart. This data can help researchers determine how fast the galaxies are moving and how the expansion rate has changed over time.

The DES team has imaged hundreds of millions of objects. These images are analyzed by powerful computers and artificial intelligence. Scientists compare their results against data from the European Space Agency's Planck observatory.

The work of the DES team and other ongoing research will help unravel the mysteries of cosmological acceleration and provide important glimpses into aspects of the universe that have remained hidden. By learning how fast the growth of the universe has been increasing and observing the effects dark energy is having on faraway galaxies, scientists hope to better understand the mysterious force that makes up more than two-thirds of the universe.

In early 2024, an international team of scientists known as the Dark Energy Spectroscopic Instrument (DESI) published data after embarking on a five-year study of the universe that suggested dark energy was not a constant force as previously understood but instead something that was in a constant state of flux. If proven sound, this finding would cast a shadow of a doubt on the preexisting theory that the universe expands at a constant rate.

Principal Terms

• cosmological constant: developed by Einstein, the concept that empty space has sufficient density and pressure to prevent the universe, once believed to be static, from collapsing under the pressure of its own gravity.
• gravity: a force of mutual attraction between two masses that increases with increased proximity.
• quantum physics: the subfield of physics that studies the often contradictory and counterintuitive behavior of the smallest units of matter and energy, such as subatomic particles, atoms, and molecules.
• theory: an explanation of some aspect of the natural universe that has been developed and thoroughly tested by means of the scientific process of observation, development of a hypothesis or reasoned explanation, experimentation, and conclusion.

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