Feature Article
The Enigma of Dark
Matter
P.K. MUKHERJEE
The mystery of the dark matter continues to intrigue scientists the world over.
W
HAT is most of the universe made
of? This is one of the most
challenging and fundamental
questions in cosmology today. All visible
celestial objects such as planets, stars and
galaxies are believed to account for only
five per cent of the stuff of the universe. A
quarter is made of dark matter, an invisible
and mysterious substance that scientists
believe is there because of the
gravitational force it exerts. The remaining
70 per cent is a mysterious energy that has
been called dark energy. It is a sort of
repulsive force that acts against the
gravitational force.
The enigmatic dark matter eludes
detection as it does not emit or reflect light,
or the light is too feeble to be detected by
the best instruments of astronomy or particle
physics available today.
The first revelation that there might be
more to the universe than accounted for in
previous theory came in 1933 when Fritz
Zwicky, an astronomer at the California
Institute of Technology, observed that some
distant galaxy clusters and galaxies including
the one named NGC 3184 were behaving
oddly. The galaxies in the clusters instead of
moving away from one another, as
suggested by the Big Bang theory, were in
fact found closely held together as if bound
by a tremendous force of gravity.
Also, some galaxies were found to
travel at unexpectedly high speeds in a
cluster of other galaxies. Since the
observable mass of the cluster was not
sufficient to provide the gravity needed to
pull the galaxies at such speeds, Zwicky
reasoned that there must be strong source
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of gravity surrounding the clusters. However,
the source of gravity strangely remained
invisible.
Zwicky’s idea about the invisible mass
did not gain ground immediately. The
observations on distant galaxies made by
astronomers revealed that the orbital speeds
of planets in the solar system did not fall off
steadily with increase in the planetar y
distance from the sun as one would expect
going by the Newtonian laws. Anomalies like
this and more observations of the speeds,
rotations and shapes of galaxies made it
increasingly clear to the astronomers that
there must be some vast but invisible matter
surrounding the galaxies.
But, what did the vast mass consist of?
This question has baffled astronomers and
scientists so much that finding the stuff of
this hidden mass has to date remained an
enigma.
SCIENCE REPORTER, OCTOBER 2011
Feature Article
Favourite with some
physicists, the theory of
super-symmetry
predicts that every kind
of particle in the
universe is paired with
a heavier twin.
It is central to
theoretical efforts, like
string theory, which
unify all the forces of
nature into one
mathematical
expression.
Scientists have suggested a number of
candidates for the dark matter. Intense
vortices of gravity, known as black holes,
were initially supposed to fit into the list of
dark matter candidates. However, scientists
soon came out with the theory that dark
matter instead could consist of some
mysterious particles.
The first suspects were the neutrinos.
These exotic particles are mass-less,
electrically neutral elementary particles that
travel with velocities close to the velocity of
light. They experience only weak interaction
and so, under normal conditions, they do
not react with ordinar y substances.
Therefore, they are extremely difficult to
detect.
Three types of neutrinos are already
known to exist. These are the electron
neutrino, the muon neutrino and the tau
neutrino. Besides, a fourth kind of neutrino
has also been suggested by some American
and Russian scientists. In fact, it was the
Candian physicist John Simpson who in 1985
first proposed the existence of such
“ massive” neutrinos having non-zero
mass.
The theor y based on “hot ” or swift
neutrinos moving with velocities nearing the
velocity of light has been dubbed the ‘hot
matter theory.’ Although this theory is able
to explain the presence of dark matter in
SCIENCE REPORTER, OCTOBER 2011
20
Feature Article
The galaxies in the clusters
instead of moving away
from one another, as
suggested by the Big Bang
theory, were in fact found
closely held together as if
bound by a tremendous
force of gravity.
Astronomers Hunt for
Dark Matter
While the particle physicists are hunting for dark matter underground in the
defunct mines, astronomers are looking for it in the outer regions of galaxies.
In 2008, the Hubble Space Telescope photographed indirect evidence in the
form of a ghostly halo around a distant galaxy. According to astronomers, this
halo was possibly caused by lumps or dark matter bending light from stars as
it passed by. A year before that, scientists led by the British astronomer Richard
Massey, at the California Institute of Technology, published the first 3D map
of dark matter. This 3D map clearly revealed how the dark matter clung around
galaxies and held clusters of them together.
○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○
the universe on a large scale, it fails to
account for the existence of dark matter on
the smaller scale. Therefore, scientists
proposed a theory, based on the existence
of “cold” or slow-moving particles, called
‘cold matter theory.’
In the ‘cold matter theory’, scientists
have hypothesised the existence of some
strange particles that have been given
exotic names like axions (ver y light
elementary particles) and weakly interacting
massive particles (WIMPs).
To solve the riddle of the mysterious
dark matter, scientists have also
hypothesised the existence of small and
dark or dim stars called Massive Compact
Halos Objects (Machos). Antithesis of
Wimps, Machos are sometimes also called
faint red dwarf stars. However, based on
observations taken by the Hubble Space
Telescope, a team of scientists led by John
Bahcall of the Institute of Advanced Study
in Princeton had raised in 1995 serious
doubts regarding the existence of Machos.
According to these astronomers, such dim
and small stars simply do not exist, for below
a certain mass limit “nature does not like to
make stars.”
Scientists the world over are desperately
hunting for the elusive dark matter. All these
experiments are being carried out
underground in deep mines. Scientists are
now zeroing in on weakly interacting massive
particles, or Wimps as potential candidates
for dark matter. Vast amounts of these
particles are thought to be constantly
moving through the Earth and everything on
it, us included, as the solar system spins
around our galaxy. The experiments looking
for dark matter particles have to be
conducted underground to shield the
detectors from other kinds of particles,
especially cosmic rays that bombard Earth
from space.
An international team of physicists has
recently succeeded in getting some hints
about the existence of dark matter. In the
bottom of a defunct iron mine in Minnesota,
the team christened Cryogenic Dark Matter
Search (CDMS) was able to detect two
particles that had all the expected
characteristics of dark matter. Announcing
the discover y at the SIAC National
Accelerator Laboratory in California, the
leader of the team Dan Bauer said that there
was more than a 20 per cent chance that
the tiny pulses of heat received by
these detectors were created by fluctuations
in the background radiation of their cavern.
Bauer also announced the results of his team
before a group of scientists in a seminar at
the Fermi National Accelerator Laboratory
in Illinois near Chicago.
The CDMS group used a set of 30
detectors buried in the Soudan mine,
about half a mile or a quarter kilometre
deep, in Minnesota. These detectors
consisted of stacks of very thin crystals of
germanium and silicon cooled to 0.01 K,
a temperature near absolute zero. This was
necessary in order to reduce the atomic
vibrations so that vibrations induced by
Wimps could be detected without any
interference.
The principle of the experiment carried
out by the CDMS group is as follows: When
a particle hits one of the detectors, it
produces an electrical charge and deposits
a bit of energy in the form of heat. It is
possible to independently measure both the
charge and the slight rise in temperature
due to the heat deposition. Through
measurements of amounts of charge and
the temperature rise the physicists are able
to distinguish between Wimps and neutrons,
which are expected to flood the
underground site from radioactivity in the
rocks around it.
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The CDMS group was able to detect
lattice vibrations, called phonons, due to two
Wimps impinging on their detectors. In the
search for the elusive dark matter, this is
being regarded as a big success by a
number of US laboratories. The claim, if
confirmed next year, will rank as one of the
most spectacular discoveries in physics in
the past century, echo these laboratories.
But why, after all, this discovery is being
considered so great? It is because it will not
only solve the riddle of dark matter, which is
thought to make up 25 per cent of the
universe, but it will also shed light on some
of the most profound mysteries of physics.
For instance, some of the particles
constituting dark matter could possibly
explain why ordinar y matter is not
radioactive while others may help physicists
understand why time—so far as we know—
always runs forward.
Many physicists believe that
confirmation of dark matter particles would
also be the first evidence for a new feature
of nature, called super-symmetr y, that
physicists have been seeking as avidly as
astronomers have been seeking dark matter.
Favourite with some physicists, the theory of
super-symmetry predicts that every kind of
particle in the universe is paired with a
heavier twin. It is central to theoretical efforts,
like string theory, which unify all the forces of
nature into one mathematical expression.
It may be remarked that finding
evidence for super-symmetry is one of the
major goals of the Large Hadron Collider
(LHC), the world’s largest particle smasher,
at CERN in Switzerland.
Dr P.K. Mukherjee is Associate Professor of
Physics at the Deshbandhu College, University of
Delhi, New Delhi. Address: 43, Deshbandhu
Society, 15, Patparganj, Delhi-110092
SCIENCE REPORTER, OCTOBER 2011