How Dark Matter Works

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Introduction to How Dark Matter Works

NASA/Robert Williams and the
Hubble Deep Field Team
The Hubble Space Telescope
took this "deepest ever"
view of the universe.
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When you look up at night, you see myriads of stars spread across the sky. When astronomers look into the deepest reaches of the universe with powerful telescopes, they see myriads of galaxies, organized into large clusters and other structures. This might lead you to believe that the universe is composed mainly of galaxies, stars, gas and dust -- things that you can see. However, most astronomers believe that visible matter makes up only a small fraction of the mass of the universe. The majority of the universe is made of stuff we can't see -- so-called dark matter. Exactly what is dark matter? How can we detect it? What is its importance in the universe as a whole?

In this article, we'll examine these questions. We will look at the evidence for dark matter, how it can be detected and studied, the nature of dark matter, and how it helps define the structure and fate of the universe.

What is Dark Matter?
Simply put, dark matter cannot be seen by astronomers with telescopes. It doesn't emit or reflect enough light to detect, so it's not bright, like a star. Atoms, molecules and subatomic particles are dark matter. You and I are dark matter. Everything on Earth is dark matter. Planets, brown dwarf stars and black holes are dark matter. Basically, dark matter cannot be seen -- scientists can only estimate where it is based on gravitational effects on what they can see.

We can't see dark matter, but we can detect it by its effects on normal matter through gravity (rotation, gravitational-lensing) and by the X-rays emitted by hot, dark matter. So, what actually is dark matter? What is it made of? Let's take a look.

Video Gallery: Dark Matter
University of Chicago Professor Michael Turner clarifies how theoretical and experimental cosmologists challenge each other to unravel the deep mysteries of the universe.

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Dark Matter Composition

Let's be clear -- we don't know the exact nature of dark matter. But we can look at some possibilities.

�NASA/CXC/CfA/STScI/ESO
This X-ray telescope image shows that dark matter (shown in blue)
makes up most of the mass of this galaxy.

First, dark matter could be ordinary matter, which is made of protons, neutrons and electrons. This ordinary matter does not emit or absorb light, but it does exert gravitational effects. Some possibilities include the following:

Quiz Corner
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Brown dwarfs -- These large objects formed in the same way as stars but never accumulated enough gas and dust to reach the critical mass to start hydrogen fusion (see How Stars Work, How the Sun Works). Brown dwarfs are about 5 percent of the sun's mass, i.e. usually larger than a planet, but not as large as a star. Astronomers call these and similar objects MACHOs, which stands for Massive Compact Halo Objects. MACHOs can be detected by gravitational lensing. Astronomers think that brown dwarfs are not numerous enough to account for dark matter in the galaxy.
White Dwarfs -- These are the remnants of the cores of dead small to medium-size stars (see How Stars Work). Although large numbers of white dwarfs exist, there are not enough to account for dark matter (there should be large amounts of leftover helium from them, but this has not been observed).
Neutron stars/black holes -- These are the last remnants of the cores of large stars after supernovae explosions (see How Stars Work, How Black Holes Work). While they do �have large gravitational effects and are invisible because they can even prevent light from escaping (black holes), they are far too rare to account for dark matter.

Second, dark matter might be an entirely new type of matter, or extraordinary matter. Extraordinary matter probably consists of subatomic particles that interact weakly with ordinary matter and have been called WIMPs (for Weakly Interacting Massive Particles).

Neutrinos -- Subatomic particles that move near the speed of light, but have little mass. These particles probably make up little dark matter within galaxies because they move fast enough to escape even a galaxy’s pull of gravity. However, they may constitute some dark matter between galaxies. So, it is doubtful that they make up much dark matter.
New subatomic particles -- There could be many of these proposed particles. Many come from the theory of supersymmetry, which doubled the number of particles from the standard model (see �How Atom Smashers Work). They move relatively slowly and are relatively cold (i.e. undetectable by infrared and X-ray telescopes). Particle physicists are actively trying to find evidence for these theoretical particles to explain dark matter.
- Neutralinos (massive neutrinos) -- hypothetical particles that are similar to neutrinos, but heavier and slower. Although they have not been discovered, they are a leading candidate for extraordinary dark matter.
- Axions -- small, neutral, low-mass (less than a millionth of an electron) particles
- Photino -- similar to photons, but have a mass that is 10 to 100 times greater than a proton. Photinos are uncharged and interact weakly with matter.

Scientists estimate that ordinary matter might make up to 20 percent of the dark matter in the universe. �

Dark Matter Detection

�The problem of dark matter came about when astronomers began to study galaxies, like our own Milky Way. If we look at the structure of the galaxy as it would �appear from the outside, most of the Milky Way's 100 billion-plus stars lie in the galactic disk. Most of the stars are concentrated near the center of the dis�k around the nucleus and galactic bulge. Above and below the plane of the disk are a few hundred scattered globular clusters and a large, dim, round region called the halo.

NASA/Photo courtesy Ned Wright
Our home galaxy, the Milky Way

�In studying the Milky Way, astronomers wanted to measure the masses and distributions of masses within the galaxy and star clusters. But you can't just weigh something the size of a galaxy -- you have to find its mass by other methods. One method is to measure the light intensity, or luminosity. Luminosity is related to a star's mass (the more luminous, the more mass -- see How Stars Work). From luminosity measurements, we know that there are about 15 billion solar luminosities (equivalent of sun-masses) between the sun's orbit and the center of the Milky Way.

Another approach to measuring galactic mass is by the rotation of the galactic disk. Imagine that the galaxy is spinning, like a CD or merry-go-round, and that you are looking at it edge-on. Within the galaxy, stars lie at different distances from the center. Some of these stars are moving away from us, while others are moving toward us. We can measure the speed and direction at which stars are moving by measuring the light that comes from them using the Doppler Effect. We can then graph the velocity of stars at different distances from the galaxy's center to get a galactic rotation curve.

Doppler Effect
Much like the high-pitched sound from a fire-truck siren gets lower as the truck moves away, the movement of stars affects the wavelengths of light that we receive from them. This phenomenon is called the Doppler Effect. We can measure the Doppler Effect by measuring lines in a star's spectrum (see How Light Works and How Stars Work) and comparing them to the spectrum of a standard lamp. The amount of the Doppler shift tells us how fast the star is moving relative to us. In addition, the direction of the Doppler shift can tell us the direction of the star's movement. If the spectrum of a star is shifted to the blue end, the star is moving toward us; if the spectrum is shifted to the red end, the star is moving away from us.

The rotation curve tells us about the distribution of mass within the galaxy. If the galaxy is like our solar system, where mass is concentrated in the center, the force of gravity will be greater near the center (the force of gravity decreases with distance). Therefore, objects close to the center orbit faster than those farther away, much like a spinning ice skater who rotates fastest when her arms are tucked in, or closer to her center. So, we would expect stars close to the galactic center to have higher rotational velocities than those farther out and that the galactic rotation curve would decrease exponentially as a function of distance.

But as we'll see on the next page, astronomers discovered that things were not exactly as they expected.

Mass-to-Light Discoveries

When astronomers measured the galactic rotation curve for the Milky Way, the rotational velocity did not decrease exponentially with distance -- it actually increased, then settled to a near-constant value. So they concluded that most of the galactic mass was located at the edges of the galaxy (outside the sun's orbit of 28,000 light years from the center), or in the halo portion. The outer regions and the halo portions of the galaxy emit very little light. Therefore, whatever mass is in these regions (and there's lots of it) is dark, hence the term "dark matter." In fact, there is six times more dark matter than light matter in the Milky Way.

NASA/WMAP Science Team
The composition of the universe

This discovery of a high mass-to-light ratio for the Milky Way was not altogether new. In 1933, astronomer Fritz Zwicky used similar methods to measure luminous mass and rotational mass in clusters of galaxies, (large swarms of galaxies that orbit each other). He found mass-to-light ratios that were greater than 100. Zwicky suggested that the differences between total mass and luminous mass were due to dark matter. His findings were not well received by most astronomers, but today the idea that dark matter exists is generally accepted.

In the 1960s, astronomer Vera Rubin made a rotational curve for the Andromeda galaxy (M31) and found a similar pattern to that observed in the Milky Way. She and her colleague, Kent Ford, made rotational curves for several spiral galaxies and found curves similar to that of the Milky Way. The implication of all of these results pointed to two possibilities:

1. Something was fundamentally wrong with our understanding of gravity and rotation. This was not likely because Newton’s laws have withstood many tests for centuries -- they apply to most situations except objects traveling near the speed of light or in extreme gravity, in which case Einstein’s theories of special and general relativity apply (see How Special Relativity Works).

2. Galaxies and galactic clusters must contain far more dark matter than light matter.

Astronomers can detect dark matter by examining the X-rays it emits. In the next section, we’ll find out how dark matter can also bend light.

X-rays and Light-bending

��In� addition to rotation curves, astronomers have used X-ray observations to confirm the large masses of galaxies and galactic clusters. When heated to high temperatures (millions of degrees Celsius), gases emit X-rays. The hotter the matter, the more X-rays emitted. So, when astronomers looked at intra-cluster medium (the spaces between galactic clusters) with X-ray telescopes, they found large gas clouds at tens to hundreds of millions of degrees Celsius. These gas clouds were invisible to optical telescopes. When astronomers estimated the mass from temperature measurements, they confirmed mass-to-light ratios of 100 or more, which provided more evidence for dark matter.

NASA/CXC/E. O'Sullivan et al.
This X-ray telescope image shows that galaxy NGC455 is
surrounded by a cloud of gas that is 10 million degrees Celsius.

Dark Matter Gravitationally Bends Light

�In his general theory of relativity, Albert Einstein showed that massive objects can distort space-time with their gravity. Let's look at this phenomenon with a massive galactic super-cluster. The super-cluster distorts the space-time around it. Light rays emanating from a distant object behind the super-cluster pass through the distorted space-time. As they do, the light rays bend and converge upon the observer. Therefore, the super-cluster acts as a large gravitational lens, much like an optical lens (see How Light Works).

The distorted image of the distant object can appear in three possible ways depending upon the shape of the lens:

1. Sphere -- image appears as a ring of light known as an Einstein ring

2. Oblong or elliptical -- image gets split into four images and appears as a cross known as an Einstein cross

3. Cluster -- image appears as a series of banana-shaped arc and arclets

By measuring the angle of bending, astronomers can calculate the mass of the gravitational lens (the greater the bend, the more massive the lens). Using this method, astronomers have confirmed that galactic clusters indeed have high masses, as indicated by rotation curves and X-ray images. The high masses exceed the masses measured by luminous matter (i.e. high mass-to-light ratio) and provide evidence of dark matter.

MOND: Modified Newtonian Dynamics
In the 1980s, physicist Mordecai Milgrom suggested that perhaps dark matter did not exist. He thought that Newton's second law of motion (force = mass x acceleration, f = ma), a fundamental law of physics, should be re-examined in the cases of galactic motions. This would be a major change to the way we understand the universe, because Newton's second law forms the basis of many laws of physics.

Milgrom suggested a modification to Newton's second law called MOND, or Modified Newtonian Dynamics. This modification involves adding a new mathematical constant into Newton's second law. MOND has met skepticism from many astronomers and physicists because it was not introduced as a fundamental physics principle, but as a solution to a specific problem.

Also, MOND cannot account for evidence of dark matter discovered by other techniques that do not involve Newton's second law, such as X-ray astronomy and gravitational lenses. Furthermore, physicists recently tested Newton's second law down to accelerations as low as 5x10^-14 m/s^2� and reported that f = ma holds true with no necessary modifications (see American Institute of Physics News Update: "Newton's Second Law of Motion," April 11, 2007). The fate of MOND is still questionable and being explored.

Dark Matter and the Fate of the Universe

When astronomers Margaret Geller and Emilio E. Falco plotted the positions of galaxies and galactic clusters in the universe, it became clear that these objects were not randomly distributed. Instead, they were clumped together in long filaments (walls) interspersed with empty spaces (voids), thereby giving the universe a cobweblike structure. How did such a structure form? What holds it together?

NASA/WMAP Science Team
According to this timeline, the expansion of the universe is accelerating.

The Big Bang theory of the formation of the universe states that the early universe underwent an enormous expansion and that it is still expanding today. The only explanation for this type of structure is that gravity is causing some of these galaxies to clump together into walls or filaments. For gravity to clump these galaxies together, there must be large amounts of mass left over from the Big Bang, particularly unseen mass (i.e. dark matter). In fact, supercomputer simulations of the formation of the universe show that galaxies, galactic clusters and larger structures can eventually form over time from aggregations of dark matter in the early universe. So, dark matter may be an important "glue" that holds this universal structure together. A question for future research is whether dark matter fills the entire universe, all the way to the galactic walls.

Besides giving the universe structure, dark matter may play a role in its fate. The universe is expanding, but will it expand forever? Gravity will ultimately determine the fate of the expansion, and gravity is dependent upon the mass of the universe; specifically, there is a critical density of mass in the universe of 10^-29g/cm^3� (equivalent to a few hydrogen atoms in a phone booth) that determines what might happen.

Closed universe -- If actual mass density is greater then critical mass density, the universe will expand, slow, stop and collapse back on itself into a "Big Crunch."
Critical or flat universe -- If actual mass density equals critical mass density, the universe will continue to expand forever, but the rate of expansion will slow more and more as time progresses. Everything in the universe will eventually become cold.
Coasting or open universe -- If actual mass density is less than critical mass density, the universe will continue to expand with no change in its rate of expansion.

Measurements of mass density must include both light and dark matter. So, it is important to know how much dark matter exists in the universe.

Recent observations of the motions of distant supernovae suggest that the universe's rate of expansion is actually accelerating. This opens up a fourth possibility, an accelerating universe, in which the all galaxies will move away from each other relatively rapidly and the universe will become cold and dark (faster than in the open universe, but still on the order of tens of billions of years). What causes this acceleration is unknown, but it has been called dark energy. Dark energy is even more mysterious than dark matter; however, there must be lots of it to account for the acceleration of the universe.

Current research in cosmology centers on resolving these questions:

What is the nature of dark matter?
How much dark matter actually exists?
What is the exact distribution of dark matter in the universe?
What is dark energy?

Answers to these questions will improve our understanding of the origins, structure and fate of the universe.

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