141 Dark Matter and Dark Energy

Introduction

One of the greatest surprises in modern astronomy is that most of the matter and energy in the universe are invisible. The stars, planets, gas, and dust that we can observe directly account for only about 5% of the total contents of the universe. The remaining 95% consists of two mysterious components: dark matter, which produces gravity but emits no detectable light, and dark energy, which appears to be driving the accelerated expansion of the universe.

The existence of dark matter was first inferred from the motions of galaxies and stars. Objects orbiting within galaxies move much faster than expected based on the amount of visible matter alone. Similarly, galaxies inside clusters remain gravitationally bound even though there appears to be far too little luminous matter to hold them together. These observations suggest that galaxies are embedded within enormous halos of invisible matter.

Although dark matter has never been observed directly, its gravitational effects have been measured in many independent ways, making its existence one of the strongest conclusions in modern astrophysics. Determining the nature of dark matter remains one of the major unsolved problems in physics.

Another major discovery of the past few decades is that the universe is not merely expanding—it is expanding at an accelerating rate. This unexpected acceleration is attributed to an unknown phenomenon called dark energy. Together, dark matter and dark energy dominate the evolution of the universe, yet neither is fully understood.

In this chapter, we examine the evidence for dark matter, explore the leading ideas about its composition, discuss the discovery of neutrino oscillations, and examine how dark energy influences the past, present, and future evolution of the universe.

Evidence for Dark Matter

The idea that most of the matter in the universe is invisible developed gradually over the twentieth century. In the 1930s, astronomer Fritz Zwicky studied the motions of galaxies within the Coma Cluster and found that the galaxies were moving too rapidly to be held together by the gravity of the visible matter alone. He proposed that the cluster contained large amounts of unseen mass, which he called dunkle Materie, or dark matter. At the time, his conclusion was considered controversial.

Several decades later, astronomer Vera Rubin and her collaborators provided much stronger evidence while studying the rotation of spiral galaxies. By measuring the Doppler shifts of light emitted by stars and gas, they determined how fast different parts of a galaxy were rotating.

If nearly all of a galaxy's mass were concentrated in its visible stars, Newton's law of gravitation predicts that stars farther from the galactic center should orbit more slowly. Instead, observations show that the orbital speeds remain nearly constant over a wide range of distances, producing what is known as a flat rotation curve. The simplest explanation is that galaxies are surrounded by enormous halos of invisible matter extending far beyond the visible stars.

Today, similar measurements have been made for thousands of galaxies, all leading to the same conclusion. Approximately 85% of the matter in the universe appears to be invisible and interacts primarily through gravity.

Additional Evidence

Galaxy rotation curves are only one line of evidence for dark matter. Independent observations from several different areas of astronomy all point to the same conclusion.

• Galaxy clusters: The motions of galaxies within clusters require much more mass than is visible in stars and gas.
• Gravitational lensing: The amount of bending of light around galaxies and galaxy clusters indicates significantly more mass than can be accounted for by luminous matter.
• The Bullet Cluster: Observations of two colliding galaxy clusters show that most of the mass, mapped using gravitational lensing, is separated from the hot gas observed in X-rays. This provides some of the strongest direct evidence that dark matter behaves differently from ordinary matter.
• Cosmic microwave background: Measurements of tiny temperature fluctuations in the cosmic microwave background allow astronomers to determine the total amount of ordinary matter and dark matter in the universe. These observations agree remarkably well with other independent measurements.

Although dark matter has never been detected directly in a laboratory, the agreement among these independent observations is remarkably strong. Today, the existence of dark matter is accepted by nearly all astronomers and cosmologists. The remaining mystery is not whether dark matter exists, but what it is made of.

What Could Dark Matter Be?

Although the existence of dark matter is supported by overwhelming observational evidence, its composition remains unknown. Because dark matter does not emit, absorb, or reflect detectable light, astronomers cannot observe it directly. Instead, they infer its presence from its gravitational effects on stars, galaxies, and light traveling through the universe.

Scientists have proposed many possible explanations for dark matter. Some involve familiar forms of matter that are simply difficult to observe, while others require entirely new particles that have never been detected experimentally.

Ordinary Matter Candidates

The simplest possibility is that dark matter consists of ordinary matter that is too dim to see. Early candidates included faint stars, brown dwarfs, white dwarfs, isolated planets, and small black holes. Collectively, many of these objects were called massive compact halo objects (MACHOs).

One way to search for MACHOs is through gravitational microlensing. When a compact object passes between Earth and a distant star, its gravity bends and focuses the star's light, causing the star to brighten temporarily in a characteristic way.

Large microlensing surveys have detected some MACHOs, demonstrating that these objects exist. However, they are far too rare to account for most of the dark matter in the universe. Today, astronomers conclude that ordinary matter—including stars, planets, gas, dust, and compact stellar remnants—accounts for only a small fraction of the total matter present.

New Particle Candidates

If dark matter is not composed primarily of ordinary matter, it may consist of previously unknown elementary particles. These particles would interact only very weakly with ordinary matter while still exerting gravitational attraction.

One long-standing class of candidates is known as weakly interacting massive particles (WIMPs). WIMPs arise naturally in several proposed extensions of the Standard Model of particle physics. Because they rarely interact with ordinary matter, they would be extremely difficult to detect directly, explaining why they have remained hidden despite making up much of the matter in the universe.

Another well-known candidate is the axion, a hypothetical particle originally proposed to solve a problem in quantum chromodynamics (QCD). Although axions would be extraordinarily light, enormous numbers of them could collectively account for the observed dark matter.

Numerous experiments around the world continue to search for WIMPs, axions, and other proposed particles using underground detectors, particle accelerators, and sensitive astronomical observations. To date, no dark matter particle has been conclusively identified.

Whether dark matter consists of undiscovered particles, primordial black holes, or an entirely different form of matter remains one of the most important unanswered questions in modern physics. Identifying its nature is a major goal of current research in astrophysics, cosmology, and particle physics.

Neutrino Oscillations

Neutrinos are among the most abundant particles in the universe. Every second, trillions of neutrinos produced by the Sun pass through your body without interacting. Because neutrinos carry no electric charge and interact only through the weak nuclear force and gravity, they are extraordinarily difficult to detect.

The Standard Model originally assumed that neutrinos were massless. However, experiments performed during the late twentieth and early twenty-first centuries demonstrated that neutrinos can spontaneously change from one type, or flavor, into another as they travel through space. This phenomenon is known as neutrino oscillation.

There are three known neutrino flavors:

• electron neutrinos ([latex]\nu_e[/latex]),
• muon neutrinos ([latex]\nu_\mu[/latex]), and
• tau neutrinos ([latex]\nu_\tau[/latex]).

A neutrino produced as one flavor can later be detected as another. This behavior is possible only if neutrinos possess mass. Although the individual neutrino masses are extremely small, the discovery of neutrino oscillations demonstrated conclusively that they are not zero.

The first convincing evidence for neutrino oscillations came from studies of neutrinos produced in Earth's atmosphere and in the Sun. In particular, the Super-Kamiokande experiment in Japan and the Sudbury Neutrino Observatory (SNO) in Canada showed that electron neutrinos generated by nuclear fusion in the Sun transform into other flavors before reaching Earth. These discoveries solved the long-standing solar neutrino problem, in which experiments detected fewer solar neutrinos than theoretical models predicted.

The discovery of neutrino oscillations was recognized with the 2015 Nobel Prize in Physics, awarded jointly to Takaaki Kajita and Arthur B. McDonald for demonstrating that neutrinos have mass through observations of neutrino flavor change.

Although neutrinos contribute a small amount to the total mass of the universe, modern cosmological observations show that they cannot account for all of the dark matter. Their masses are simply too small. As a result, physicists continue to search for additional particles or other explanations for the dominant component of dark matter.

Dark Energy and the Fate of the Universe

For much of the twentieth century, astronomers debated whether the universe would continue expanding forever or eventually stop and collapse under its own gravity. The answer seemed to depend on the average density of matter in the universe. If enough matter were present, gravity could eventually halt the expansion. If too little matter existed, the universe would expand forever.

To describe this idea, cosmologists defined the critical density, the average density of matter required for gravity to exactly balance the expansion of the universe. It is approximately

This density corresponds to only a few hydrogen atoms per cubic meter when averaged over the enormous volume of the universe, illustrating just how empty intergalactic space is.

For many years, astronomers attempted to determine whether the actual density of the universe was greater than, equal to, or less than the critical density. However, an unexpected discovery in the late 1990s fundamentally changed this picture.

The Discovery of Dark Energy

Observations of distant Type Ia supernovae revealed that the expansion of the universe is not slowing down. Instead, it is accelerating. This surprising result cannot be explained by gravity acting on ordinary matter and dark matter alone. Instead, astronomers infer the existence of a previously unknown form of energy called dark energy, which appears to oppose the attractive effects of gravity on the largest scales.

The simplest explanation for dark energy is Einstein's cosmological constant, a term that can be included in the equations of general relativity. Einstein originally introduced the cosmological constant in an attempt to describe a static universe before the expansion of the universe was discovered. After Edwin Hubble demonstrated that the universe is expanding, Einstein reportedly regarded the idea as unnecessary. Today, however, a cosmological constant is once again consistent with many astronomical observations and provides the simplest explanation for the observed accelerated expansion.

The Composition of the Universe

Measurements of the cosmic microwave background, distant supernovae, and the large-scale distribution of galaxies indicate that the universe is remarkably close to spatially flat. These observations also reveal the approximate composition of the universe:

• about 5% ordinary (baryonic) matter,
• about 27% dark matter, and
• about 68% dark energy.

Although ordinary matter—including stars, planets, gas, and dust—is the portion we observe directly, it represents only a small fraction of the universe. The remaining 95% consists of dark matter and dark energy, whose fundamental nature remains one of the greatest mysteries in modern physics.

The Future of the Universe

Current observations indicate that the universe will most likely continue expanding indefinitely. Because dark energy dominates the large-scale evolution of the universe, the expansion is expected to remain accelerated. Galaxies that are not gravitationally bound to the Milky Way will gradually move farther away, and over extremely long timescales, the observable universe will become increasingly empty as distant galaxies disappear beyond our cosmic horizon.

Although the long-term behavior of dark energy remains uncertain, modern observations strongly support a universe that is nearly flat and whose expansion will continue for the foreseeable future. Determining the physical nature of dark energy is now one of the central challenges in cosmology.

Section Summary

• Dark matter is invisible matter that interacts primarily through gravity. Although it cannot be observed directly, its presence is inferred from galaxy rotation curves, galaxy clusters, gravitational lensing, and measurements of the cosmic microwave background.
• Studies of spiral galaxies by Fritz Zwicky, Vera Rubin, and many other astronomers demonstrated that galaxies contain much more mass than can be accounted for by their visible stars, gas, and dust.
• Ordinary matter accounts for only about 5% of the total mass-energy content of the universe. Approximately 27% is dark matter, while about 68% is dark energy.
• Several candidates have been proposed for dark matter, including MACHOs, WIMPs, and axions. Current observations indicate that ordinary matter such as faint stars and planets cannot account for most of the dark matter.
• Neutrino oscillations demonstrate that neutrinos have nonzero mass by allowing neutrinos to change flavor as they travel through space. Although neutrinos contribute to the total mass of the universe, they cannot account for all of the observed dark matter.
• The critical density is the average density required for gravity alone to produce a spatially flat universe. It is approximately

• Observations of distant supernovae show that the expansion of the universe is accelerating. The simplest explanation is the existence of dark energy, often modeled by Einstein's cosmological constant.
• Current cosmological observations indicate that the universe is remarkably close to spatially flat and will most likely continue expanding indefinitely.
• Determining the nature of both dark matter and dark energy remains one of the most important challenges in modern astronomy, cosmology, and particle physics.