Cosmic Hide and Seek: the Searchfor the Missing Mass
by Chris Miller
Copyright © 1995 by Chris Miller, all rights reserved. This text may be freely redistributed among individuals in any medium so long as it remains unedited and appears with this notice. Any commercial or republication requires the written permission of the author.
Scientists using different methods to determine the mass of galaxies have found a discrepancy that suggests ninety percent of the universe is matter in a form that cannot be seen. Some scientists think dark matter is in the form of massive objects, such as black holes, that hang out around galaxies unseen. Other scientists believe dark matter to be subatomic particles that rarely interact with ordinary matter. This paper is a review of current literature. I look at how scientists have determined the mass discrepancy, what they think dark matter is and how they are looking for it, and how dark matter fits into current theories about the origin and the fate of the universe.
In 1933, the astronomer Fritz Zwicky was studying the motions of distant galaxies. Zwicky estimated the total mass of a group of galaxies by measuring their brightness. When he used a different method to compute the mass of the same cluster of galaxies, he came up with a number that was 400 times his original estimate (1). This discrepancy in the observed and computed masses is now known as "the missing mass problem." Nobody did much with Zwicky's finding until the 1970's, when scientists began to realize that only large amounts of hidden mass could explain many of their observations (2). Scientists also realize that the existence of some unseen mass would also support theories regarding the structure of the universe (3). Today, scientists are searching for the mysterious dark matter not only to explain the gravitational motions of galaxies, but also to validate current theories about the origin and the fate of the universe.
Mass and Weight. What exactly is mass? Most people would say that mass is what you weigh. But to scientists, mass and weight are different things. Mass is the measure of a quantity of matter--how much stuff there is. Weight, on the other hand, is the effect that gravity has on that stuff. Weight is dependent on mass--the more mass you have, the more gravity pulls you down, and the more you weigh. When an astronaut floats in space, we say that the astronaut is weightless. But the astronaut still has a body, and so has mass.
Hide and Seek. Scientists estimate that 90 to 99 percent of the total mass of the universe is missing matter (4). Actually, "missing matter" may be misleading--it's really the light that is missing (5). Scientists can tell that the dark matter is there, but they cannot see it. Bruce H. Margon, chairman of the astronomy department at the University of Washington, told the New York Times, "It's a fairly embarrassing situation to admit that we can't find 90 percent of the universe" (6). This problem has scientists scrambling to try and find where and what this dark matter is. "What it is, is any body's guess," adds Dr. Margon. "Mother Nature is having a double laugh. She's hidden most of the matter in the universe, and hidden it in a form that can't be seen" (5).
Determining the Mass of Galaxies
How do we measure the mass of the universe? Since the boundaries (if there are any) of the universe are unknown, the actual mass of the universe is also unknown. But scientists talk of the missing mass of the universe in percentages, not real numbers. Since the majority of the matter that we can see is clumped together into galaxies, the total mass of all the galaxies should be a good indication of the mass of the universe. Although it isn't possible to add up an infinite number of galaxies, scientists can infer the percentage of the universe's missing mass from estimates of the missing mass in galaxies and clusters of galaxies (7). And because scientists (like Fritz Zwicky) use different techniques to determine the masses of galaxies, they can perceive mass that they cannot see.
The Doppler Shift. One of the tools that scientists use to detect the motions of galaxies is the Doppler Shift. The Doppler Shift was discovered in the 1800's by Christian Doppler when he noticed that sound travels in waves much like waves on the surface of the ocean (7). Doppler also noticed that when the source of the sound is moving, the pitch of the sound is different, depending on whether the source is moving toward or away from the observer. Take, for example, the horn on a train. As the speeding train passes by you, the sound of the horn changes to a lower pitch. This is the Doppler Shift. When the train approaches, the sound waves get pushed together by the motion of the train. As the train speeds away, the sound waves get stretched out.
The Doppler Shift also works with light. When a light source is moving toward you, the light becomes bluer (called a blue shift). When a light source is moving away from you, the light becomes redder (called a red shift). And the faster something is moving, the farther the light is shifted. But the Doppler shift for light is very subtle and cannot be detected with the naked eye. Scientists use a device called a spectroscope to measure Doppler Shift and determine how fast stars and galaxies are moving (7).
Rotational Velocity. Using the power of the Doppler Shift, scientists can learn much about the motions of galaxies. They know that galaxies rotate because, when viewed edge-on, the light from one side of the galaxy is blue shifted and the light from the other side is red shifted. One side is moving toward the Earth, the other is moving away. They can also determine the speed at which the galaxy is rotating from how far the light is shifted (7). Knowing how fast the galaxy is rotating, they can then figure out the mass of the galaxy mathematically.
As scientists look closer at the speeds of galactic rotation, they find something strange. The individual stars in a galaxy should act like the planets in our solar system--the farther away from the center, the slower they should move. But the Doppler Shift reveals that the stars in many galaxies do not slow down at farther distances. And on top of that, the stars move at speeds that should rip the galaxy apart; there is not enough measured mass to supply the gravity needed to hold the galaxy together (7).
These high rotational speeds suggest that the galaxy contains more mass than was calculated. Scientists theorize that, if the galaxy was surrounded by a halo of unseen matter, the galaxy could remain stable at such high rotational speeds.
Seeing the Light. Another method astronomers use to determine the mass of a galaxy (or cluster of galaxies) is simply to look at how much light there is. By measuring the amount of light reaching the earth, the scientists can estimate the number of stars in the galaxy. Knowing the number of stars in the galaxy, the scientists can then mathematically determine the mass of the galaxy(1).
Fritz Zwicky used both methods described here to determine the mass of the Coma cluster of galaxies over half a century ago. When he compared his data, he brought to light the missing mass problem. The high rotational speeds that suggest a halo reinforce Zwicky's findings. The data suggest that less than 10% of what we call the universe is in a form that we can see (8). Now scientists are diligently searching for the elusive dark matter--the other 90% of the universe.
Dark Matter
What do scientists look for when they search for dark matter? We cannot see or touch it: its existence is implied. Possibilities for dark matter range from tiny subatomic particles weighing 100,000 times less than an electron to black holes with masses millions of times that of the sun (9). The two main categories that scientists consider as possible candidates for dark matter have been dubbed MACHOs (Massive Astrophysical Compact Halo Objects), and WIMPs (Weakly Interacting Massive Particles). Although these acronyms are amusing, they can help you remember which is which. MACHOs are the big, strong dark matter objects ranging in size from small stars to super massive black holes (1). MACHOs are made of 'ordinary' matter, which is called baryonic matter. WIMPs, on the other hand, are the little weak subatomic dark matter candidates, which are thought to be made of stuff other than ordinary matter, called non-baryonic matter. Astronomers search for MACHOs and particle physicists look for WIMPs.
Astronomers and particle physicists disagree about what they think dark matter is. Walter Stockwell, of the dark matter team at the Center for Particle Astrophysics at U.C. Berkeley, describes this difference. "The nature of what we find to be the dark matter will have a great effect on particle physics and astronomy. The controversy starts when people made theories of what this matter could be--and the first split is between ordinary baryonic matter and non-baryonic matter" (10). Since MACHOs are too far away and WIMPs are too small to be seen, astronomers and particle physicists have devised ways of trying to infer their existence.
MACHOs
Massive Compact Halo Objects are non-luminous objects that make up the halos around galaxies. Machos are thought to be primarily brown dwarf stars and black holes (2). Like many astronomical objects, their existence had been predicted by theory long before there was any proof. The existence of brown dwarfs was predicted by theories that describe star formation (7). Black holes were predicted by Albert Einstein's General Theory of Relativity (11).
Brown Dwarfs. Brown dwarfs are made out of hydrogen--the same as our sun but they are typically much smaller. Stars like our sun form when a mass of hydrogen collapses under its own gravity and the intense pressure initiates a nuclear reaction, emitting light and energy. Brown dwarfs are different from normal stars. Because of their relatively low mass, brown dwarfs do not have enough gravity to ignite when they form (7). Thus, a brown dwarf is not a "real" star; it is an accumulation of hydrogen gas held together by gravity. Brown dwarfs give off some heat and a small amount of light (7).
Black Holes. Black holes, unlike brown dwarfs, have an over-abundance of matter. All that matter "collapses" under its own enormous gravity into a relatively small area. The black hole is so dense that anything that comes too close to it, even light, cannot escape the pull of its gravitational field (11). Stars at safe distance will circle around the black hole, much like the motion of the planets around the sun (7). Black holes emit no light; they are truly black.
Detecting MACHOs
Astronomers are faced with quite a challenge with detecting MACHOs. They must detect, over astronomical distances, things that give off little or no light. But the task is becoming easier as astronomers create more refined telescopes and techniques for detecting MACHOs.
Searching with Hubble. With the repair of the Hubble Space Telescope, astronomers can detect brown dwarfs in the halos of our own and nearby galaxies. Images produced by the Hubble Telescope, however, do not reveal the large numbers of brown dwarfs that astronomers hoped to find. "We expected [the Hubble images] to be covered wall to wall by faint, red stars," reported Francesco Paresce of the Johns Hopkins University Space Telescope Science Institute in the Chronicle of Higher Education (5). Research results are disappointing--calculations based on the Hubble research estimate that brown dwarfs constitute only 6% of galactic halo matter (12).
Gravitational Lensing. Astronomers use a technique called gravitational lensing in the search for dark matter halo objects. Gravitational lensing occurs when a brown dwarf or a black hole passes between a light source, such as a star or a galaxy, and an observer on the Earth. The object focuses the light rays, causing the light source to brighten (13). Astronomers diligently search photographs of the night sky for the telltale brightening that indicates the presence of a MACHO.
Wouldn't a MACHO block the light? How can dark matter act like a lens? The answer is gravity. Albert Einstein proved in 1919 that gravity bends light rays (13). He predicted that a star, which was positioned behind the sun, would be visible during a total eclipse. Einstein was right--the gravity of the sun bent the light rays coming from the star and made it appear next to the sun.
Not only can astronomers detect MACHOs with the gravitational lens technique, but they can also calculate the mass of the MACHO by determining distances and the duration of the lens effect (13). Although gravitational lensing has been known since Einstein's demonstration, astronomers have only begun to use the technique to look for MACHOs in the past two or three years.
Gravitational Lensing projects include the MACHO project (America and Australia), the EROS project (France), and the OGLE project (America and Poland). Preliminary data from these projects suggest the existence of lens objects with masses between that of Jupiter and the sun (9).
Circling Stars. Another way to detect a black hole is to notice the gravitational effect that it has on objects around it. When astronomers see stars circling around something, but cannot see what that something is, they suspect a black hole. And by observing the circling objects, the astronomers can conclude that, indeed, a black hole does exist.
In January of 1995, a team of American and Japanese scientists announced "compelling evidence" for the existence of a massive black hole at the American Astronomical Society meeting (14). Led by Dr. Makoto Miyosi of the Mizusawa Astrogeodynamics Observatory and Dr. James Moran of the Harvard-Smithsonian Center for Astrophysics, this group calculated the rotational velocity from the Doppler shifts of circling stars to determine the mass of the black hole. This black hole has a mass equivalent to 36 million of our suns (15). While this finding and others like it are encouraging, MACHO researchers have not turned up enough brown dwarfs and black holes to account for the missing mass. Thus, most scientists concede that dark matter is a combination of baryonic MACHOs and non-baryonic WIMPs.
WIMPs
In their efforts to find the missing 90% of the universe, particle physicists theorize the existence of tiny non-baryonic particles that are different from what we call "ordinary" matter. Smaller than atoms, Weakly Interactive Massive Particles are thought to have mass, but usually interact with baryonic matter gravitationally--they pass right through ordinary matter. Since each WIMP has only a small amount of mass, there needs to be a large number of them to make up the bulk of the missing matter. That means that millions of WIMPs are passing through ordinary matter--the Earth and you and me--every few seconds (8). Although some people claim that WIMPs were proposed only because they provide a "quick fix" to the missing matter problem, most physicists believe that WIMPs do exist (4). According to Walter Stockwell, astronomers also concede that at least some of the missing matter must be WIMPs. "I think the MACHO groups themselves would tell you that they can't say MACHOs make up the dark matter" (10). The problem with searching for WIMPs is that they rarely interact with ordinary matter, which makes them difficult to detect.
Detecting WIMPs. All hope of proving WIMPs exist rest on the theory that, on occasion, a WIMP will interact with ordinary matter. Because WIMPs can pass through ordinary matter, a rare WIMP interaction can take place inside a solid object. The trick to detecting a WIMP is to witness one of these interactions. Dr. Bernard Sadoulet and Walter Stockwell at the Center for Particle Astrophysics hope to do just that. Their project involves cooling a large crystal to almost absolute zero, which restricts the motions of its atoms. The energy created by a WIMP interaction with an atom in the crystal will then register on their instruments as heat (8). Because their research is still in progress, there are no results available.
A similar WIMP detection project is under way in Antarctica. The AMANDA project (Antarctica Muon and Neutrino Detector Array) is a collaboration of the University of Chicago, PrincetonUniversity, and AT&T, which is partially funded by the National Science Foundation. AMANDA scientists are placing detection instruments deep within the Antarctic ice. Instead of using a crystal, like the Berkeley team, the AMANDA group is using the Antarctic ice sheet itself as a WIMP detector (16).