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Cosmic Hide and Seek: the
Search
for the Missing Mass
by Chris Miller
Copyright © 1995 by Chris Miller, all rights
reserved. This text may be freely
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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 overabundance 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, Princeton
University, 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).
Dark Matter and the Universe
The search for dark matter is about more than explaining
discrepancies in galactic mass calculations. The missing matter
problem has people questioning the validity of current theories
about how the universe formed, and how it will ultimately end.
The Big Bang. In the mid 1950's a new theory of how the
universe formed emerged. The Big Bang theory says that the
universe began with a great explosion. The theory evolved from
Doppler shift observations of galaxies (17). It seems that, no
matter which direction astronomers point their telescopes, the
light from the center of the galaxies is red shifted. (Doppler shift
caused by rotational velocity can only be detected at the sides of a
galaxy.) Observing red-shifted galaxies in every direction implies
expansion in all directions an expanding universe.
The Big Bang theory is a current model for the origin of our
universe which says all the matter that exists was, at one time,
compressed into a single point. The Big Bang distributed all the
matter evenly in all directions. Then the matter started to clump
together, attracted by gravity, to form the stars and galaxies that
we see today. The expansion generated by the Big Bang was great
enough to overcome gravity. We still see the effects of that force
when we see red-shifted galaxies.
Clumping. One of the problems with the Big Bang theory is its
failure to explain how stars and galaxies could form in a young
universe that was evenly distributed in all directions. What started
the clumping? In a smooth universe, every particle would have
the same gravitational effect on every other particle; the universe
would remain the same (6). But something supplied the initial
gravity to allow galaxies to form. Physicists suggest dark matter
WIMPs as the solution. Since WIMPs only affect baryon matter
gravitationally, physicists say this dark matter could be the "seed"
of galactic formation (6). "We don't have a completely successful
model of galaxy formation," explains Walter Stockwell, "but the
most successful models to date seem to need plenty of nonbaryonic dark matter" (10).
Closed, Open and Flat. There are three current scenarios that
predict the future of the universe (17). If the universe is closed,
gravity will catch up with the expansion and the universe will
eventually be pulled back into a single point. This model suggests
an endless series on Big Bangs and "Big Crunches." An open
universe has more bang than gravity--it will keep expanding
forever. And the flat universe has exactly enough mass to
gravitationally stop the universe from expanding, but not enough
to pull itself back in. A flat universe is said to have a critical
density of 1.
What does the expansion of the universe have to do with the
missing mass? The more mass, the more gravity. Whether the
universe is closed, open, or flat depends on how much mass there
is. This is where dark matter comes into the picture. Without dark
matter, critical density lies somewhere between 0.1 and 0.01, and
we live in an open universe. If there is a whole lot of dark matter,
we could live in a closed universe. Just the right amount of dark
matter, and we live in a flat universe. The amount of dark matter
that exists determines the fate of the universe.
Many Theories. Scientists are tossing theories back and forth.
Some are skeptical of WIMPs; particle physicists say MACHOs
will never account for 90% of the universe. Some, like H.C. Arp,
G. Burbage, F. Hoyle, and J.V. Narlikan claim that discrepancies
like the dark matter problem discredits the Big Bang theory. In
Nature they proclaim, "We do not believe that it is possible to
advance science profitably when the gap between theoretical
speculation becomes too wide, as we feel it has . . . over the past
two decades. The time has surely come to open doors, not to seek
to close them by attaching words like 'standard' and 'mature' to
theories that, judged from their continuing non-performance, are
inadequate" (18). Others say there is no missing mass. In his
book, What Matters: No Expanding Universe No Big Bang, J.L.
Riley claims that galactic red shift is just the effect of light
turning into matter as it ages, and not the universe expanding
(19).
But most scientists like Walter Stockwell have faith in the Big
Bang. "The theorists will come up with all sorts of reasons why
this or that can or cannot be and change their minds every other
year," he says. "We experimentalists will trudge ahead with our
experiments. The Big Bang theory will outlive any of this stuff. It
works very well as the overall framework to explain how the
universe is today" (10).
Now the missing mass problem is threatening humankind's
place in the universe again. If non-baryonic dark matter does
exist, then our world and the people in it will be removed even
farther from the center. Dr. Sadoulet tells the New York Times, "It
will be the ultimate Copernican revolution. Not only are we not at
the center of the universe as we know it, but we aren't even made
up of the same stuff as most of the universe. We are just this
small excess, an insignificant phenomenon, and the universe is
something completely different" (20).
A dark matter discovery could possibly affect our view of our
place in the universe. If scientists prove that non-baryonic matter
does exist, it would mean that our world and the people in it are
made of something which comprises an insignificant portion of
the physical universe. A discovery of this nature, however,
probably will not affect our day-to-day process of living. "It's
hard for me to imagine people getting bothered by the fact that
most of the universe is something other than baryonic. How many
people even know what baryonic means?" comments Walter
Stockwell, "Most of the universe is something other than human.
If their philosophy already accepts that humans are not the center
of the universe, then saying protons and neutrons aren't the center
of the universe doesn't seem like much of a stretch to me" (10).
Perhaps the only thing a dark matter discovery will give us is
some perspective.
Works Cited
1. West, Michael J. "Clusters of Galaxies." The Astronomy and
Astrophysics Encyclopedia. New York: Van Nostrand Reinhold,
1992.
2. Griest, Kim. "The Search for the Dark Matter: WIMPs and
MACHOs." Annals of the New York Academy of Sciences. vol
688. 15 June 1993: 390-407.
3. Gribben, John. The Omega Point: The Search for the Missing
Mass and the Ultimate Fate of the Universe. New York: Bantam,
1988.
4. Chase, Scott I. "What is Dark Matter?" physics-faq/part2.
sci.physics Newsgroup. 5 Dec. 1994.
5. McDonald, Kim A. "New Findings Deepen the Mystery of the
Universe's 'Missing Mass'." Chronicle of Higher Education. 23
Nov.1994: A8-A13.
6. Wilford, John Noble. "Astronomy Crisis Deepens As the
Hubble Telescope Finds No Missing Mass." New York Times. 29
Nov. 1994: C1-C13.
7. Zeilik, Michael., and John Gaustad. Astronomy: The Cosmic
Perspective. New York: John Wiley & Sons, Inc, 1990.
8. Trefil, James. "Dark Matter." Smithsonian. June 1993: 27- 35.
9. Mateo, Mario. "Searching for Dark Matter." Sky and
Telescope. Jan. 1994: 20-24.
10. Stockwell, Walter K. E-mail interview. 1 Feb. 1995.
11. McIrvin, Matt. "Some Frequently Asked Questions About
Black Holes." physics-faq/part2. sci.physics Newsgroup. 5 Dec.
1994.
12. Asker, James R. "'Missing Mass' Enigma Deepens." Aviation
Week & Space Technology. 21 Nov. 1994: 31.
13. Falco, Emilio and Nathaniel Cohen. "Gravity Lenses: A Focus
on the Cosmic Twins." Astronomy. July 1981: 18-22.
14. Wilford, John Noble. "New Galactic Evidence of Black
Holes." New York Times. 12 Jan. 1995: B9.
15. Miyoshi, Makoto., et al. "Evidence for a Black Hole from
High rotation Velocities in a Sub-parsec Region of NGC458."
Nature. 12 Jan. 1995: 127-129.
16. National Science Foundation. 1995 Center for Astrophysical
Research in Antarctica: Amundsen-Scott South Pole Station. In
Library of Congress LC Marvel [Online].
17. Abell, George O., and Marc Davis. "Cosmology." McGrawHill Encyclopedia of Science and Technology. 7th ed. New York:
McGraw-Hill, 1992.
18. Arp, H.C., et al. "Big Bang contd . . ." Nature. vol 357. 28
May 1992: 287-288.
19. Riley, J.L. What Matters: No Expanding Universe No Big
Bang. Plano TX: No Big Bang Publishing Co., 1993.
20. Wilford, John Noble. "Physicists Step Up Exotic Search for
the Universe's Missing Mass." New York Times. 26 May 1992:
C1-C11.
21. Miller, Christopher M. "Cosmic Hide and Seek: the Search
for the Missing Mass" online in Chris Miller's Home Page, 1995.
http://www.eclipse.net/~cmmiller/
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