I | INTRODUCTION |
Dark
Matter, in astronomy, designation for matter that does not give off or
reflect detectable electromagnetic radiation, the radiant energy that includes
visible light, radio waves, infrared radiation, X rays, and gamma rays. Although
dark matter is practically invisible, astrophysicists have determined its
existence by detecting its gravitational interaction with matter that does give
off detectable electromagnetic radiation, such as stars, galaxies, and clusters
of galaxies. Dark matter has become a vital component of modern theories of
cosmology and elementary particle physics. Along with the phenomenon of dark
energy, the puzzle of what dark matter is represents one of the most important
questions in physics today.
II | THE DISCOVERY OF DARK MATTER |
The existence of dark matter was first
suggested in the early 20th century by the Swiss American astronomer Fritz
Zwicky, but convincing and overwhelming evidence of its existence was gathered
by the American astronomer Vera Rubin in the 1970s. In the early 1930s, Zwicky
studied the rotational motions of thousands of galaxies clustered together in a
large group of galaxies known as the Coma Cluster. He found that the orbital
motion of the galaxies around their common center of mass could only be
explained by the presence of unseen matter, which astronomers now call dark
matter. Zwicky’s suggestion was not taken very seriously at first because there
was not a great amount of evidence to support such a radical suggestion.
In the early 1970s, however, Rubin studied the
orbital motions of stars in a large number of galaxies. As these stars orbited
their galactic centers, Rubin noticed that the outlying stars in the galaxies
were moving so fast that they should have been flung out of the galaxies. But
since they were still part of the galaxies, Rubin proposed that unseen matter
was keeping them gravitationally bound to the galaxies. A similar observation
was reported in the early 1930s for the stars in our own Milky Way Galaxy by
Dutch astronomer Jan Oort, but the dark matter interpretation was not considered
at that time.
Following up on the observational data
gathered by Oort, Zwicky, Rubin, and other astronomers, two American theoretical
astrophysicists, Jeremiah P. Ostriker and P. J. E. Peebles, contributed
important theoretical analyses. The data and the analyses helped scientists
determine that dark matter probably constitutes as much as 90 percent of all the
matter in the universe. Scientists verified that the orbital motions of stars in
galaxies cannot be explained by the mutual gravitational influence of all the
other visible stars. To explain this orbital motion, dark matter must be
present. Rubin and other astronomers and astrophysicists showed that this dark
matter seems to be distributed in a large envelope or “halo” around the visible
matter of the galaxy. As a result the galaxies are much larger than what can
actually be observed through a telescope.
Some scientists have theorized that there may
be other explanations for the orbital motions of stars besides dark matter, such
as a new type of force in nature that exerts itself over vast distances or a
modification of the law of gravity. But so far, neither a new force nor a new
understanding of gravity has been found. The hypothetical existence of dark
matter, however, does explain the observed interactions with ordinary matter
extremely well without resorting to a new long-range force.
III | WHAT IS DARK MATTER? |
To gain a fuller understanding of our
universe, it is vital to determine exactly what dark matter is made of.
Scientists think that dark matter occurs in several different forms. Moreover,
observations and experiments place limits on the quantity and distribution of
each type. There are two broad categories of dark matter: “hot” dark matter,
which moves at speeds comparable to the speed of light (about 299,000 km per
second or 186,000 mi per second), and “cold” dark matter, which moves at speeds
well below that of light.
A | Hot Dark Matter |
The elementary particle called the
neutrino, discovered in 1956, is an example of a hot dark matter candidate.
Various experiments and observations, such as those reported in 1998 by the
Super-Kamiokande experiment in Japan, have shown that the neutrino has mass.
Mass is the quality that causes gravitational attraction. The mass of the
neutrino is extremely small, which is why the particle travels at speeds
comparable to that of light. Neutrinos are extremely abundant in the universe
because they are produced in enormous numbers in nuclear interactions that take
place at the core of every star. For example, several trillion neutrinos pass
through each person on Earth each second as a result of the nuclear reactions
that cause the Sun to shine. Because neutrinos are electrically neutral they can
pass easily through ordinary matter, such as through people, and so are able to
spread throughout a region near ordinary matter. Their large numbers could
enable them to be a significant component of dark matter despite the tiny amount
of mass in an individual neutrino.
However, there is evidence that dark matter
cannot be made up mostly of neutrinos. This is due to two reasons. First, their
likely mass is still too small to provide enough matter to account for the
gravitational effects seen in the orbital motions of stars. In addition, some
form of dark matter was necessary in the early universe to create the early
structures that eventually led to the formation of stars and galaxies. Neutrinos
could not have played this role, in part because they could not have been
created in the required quantities until stars actually formed.
Secondly, neutrinos are too energetic to
have helped seed the process of star and galaxy formation. Some other form of
dark matter must have contributed to star and galaxy formation, which developed
from localized structures, or lumps, known as anisotropies. These lumps have
been detected in the cosmic microwave background radiation, the radiation left
over from the formation of the universe in the big bang about 13.7 billion years
ago.
Detailed observations of the cosmic
background radiation show that these localized lumps in the early universe were
too small to be seeded by fast-moving particles such as neutrinos. For the
anisotropies to have formed in the early universe, a large component of slower,
cold dark matter must have been present.
B | Cold Dark Matter |
Another candidate for the dark matter is
known as baryonic cold dark matter. Baryonic cold dark matter is made of protons
and neutrons, the subatomic particles known as baryons that make up ordinary
matter and combine with electrons to form atoms. Baryonic cold dark matter could
be found in celestial objects that were not massive enough to initiate the
fusion processes that make stars shine. It could also be made of matter that
collapsed to form dense objects such as neutron stars or even black holes
(objects so dense that not even light can escape their gravitational field).
Such objects are collectively referred to as “MACHOs,” which means “massive
compact halo objects.”
Astronomical observations limit the amount
of MACHOs that could exist. For example, if they were numerous, such objects
would inevitably pass close to the line of sight between an observer on Earth
and a distant visible object. Albert Einstein’s theory of gravity, known as the
theory of general relativity, describes how light is bent by the presence of
mass and energy. Mass and energy set up a gravitational field through which
light passes. The MACHO object’s gravitational field would therefore bend the
light of a more distant visible object and produce a distortion of its image.
Such a process is called a gravitational lens. Several such gravitational lenses
have been observed, and the measured frequency of such events has placed limits
on how much dark matter can take the form of MACHOs. From the observations that
have been made by astronomers, it is now known that MACHOs cannot be the
dominant constituent of dark matter. There are simply not enough such
gravitational lenses.
Physicists suspect that a more exotic form
of cold dark matter must exist. This form is not baryonic. Like neutrinos, this
form barely interacts with ordinary matter, but is some type of massive
particle. Such candidates are often called WIMPs (for “weakly interacting
massive particles”).
Various theories of the fundamental
elementary particles and their interactions in nature predict the existence of
new particles that have not yet been discovered. These hypothetical particles
are excellent candidates for WIMPs. For example, supersymmetric theories propose
that a new fundamental symmetry of nature was present when the universe was very
young and energetic. If this symmetry existed in the past, it requires the
hypothetical particles to exist. The symmetry would have disappeared as a result
of natural dynamical effects as the universe aged and became less energetic.
See also Theory of Everything; Unified Field Theory.
As a result we do not observe this
symmetry today. The loss of symmetry over time is very similar to what happens
as water freezes and turns to ice as it is cooled. Water is more symmetrical
than ice since its molecules point in all directions in space, whereas ice forms
a crystal lattice with the molecules pointing in the preferred directions of the
lattice. As the universe expands, it also cools, and this can give rise to the
disappearance of important symmetries in later epochs.
IV | FUTURE EXPERIMENTS |
The supersymmetric theories predict that
relics of the earlier, more symmetric phase of the universe exist and that they
exist in the form of a very specific family of massive particles. New
experiments, such as those planned for the Large Hadron Collider (LHC), a
particle accelerator at the European Organization for Nuclear Research (CERN) in
Switzerland, are expected to be able to test some of these theories. The
experiments will try to create the predicted particles directly, using the
energetic collisions produced in the LHC. If the particles are discovered, the
LHC and other proposed experiments will systematically study their properties to
determine if they are indeed the sought-after principal components of dark
matter. This type of dark matter would be the key agent of structure formation
that gave rise to the galaxies and clusters of galaxies that constitute our home
in the universe.
In 2006 astronomers announced what may be the
first direct evidence for dark matter in the cosmos, based on observations from
the Chandra X-Ray Observatory telescope, the Hubble Space Telescope (HST), and
advanced telescopes on Earth. A collision between two clusters of galaxies over
4 billion light-years away apparently caused visible matter and dark matter to
separate on a gigantic scale. Huge gas clouds between and around the galaxies in
each group smashed together as the two clusters raced past each other, heating
the gas to a plasma state and dragging it away from the dark matter halos
thought to surround the galaxies. The massive presence of dark matter was
detected by gravitational lensing that bent the light from more-distant
background galaxies in the telescopes’ line of sight. The findings appear to
support theories that dark matter exists and that gravity behaves the same in
giant clusters of galaxies as it does on Earth.
In early 2007 an international team of
astronomers published the results of the first search for dark matter in a wide
region of the sky using high-resolution images from the HST. Findings from the
Cosmic Evolution Survey (COSMOS) conducted with the HST were used to create a
three-dimensional map that shows the distribution of dark matter through periods
of time as far back as 6.5 billion years ago, almost halfway to the big bang.
The scientists studied the shapes of half a million galaxies to find distortions
in their apparent shapes caused by gravitational lensing around large
concentrations of mass lying between very distant galaxies and Earth. Such
concentrations of mass indicate the presence of dark matter. The new map reveals
how areas of dark matter provided the scaffolding for clusters of galaxies made
of visible matter. The galaxies accumulated in the densest regions of dark
matter as gravity gradually drew the dark matter into more compact filaments
that form a massive networklike structure.
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