2-1
b
History
Early
history
The hypothesis of dark matter has an elaborate history. In a talk
given in
1884, Lord Kelvin estimated the number of dark bodies in the Milky
Way
from the observed velocity dispersion of the stars orbiting around
the center
of the galaxy. By using these measurements, he estimated the mass
of the
galaxy, which he determined is different from the mass of visible
stars.
Lord Kelvin thus concluded “many of our stars, perhaps a
great majority
of them, may be dark bodies”. In 1906 Henri Poincaré
in “The Milky Way
and Theory of Gases” used “dark matter”, or “matière
obscure” in French,
in discussing Kelvin’s work.
The first
to suggest the existence of dark matter using stellar velocities
was
Dutch astronomer Jacobus Kapteyn in 1922. Fellow Dutchman and
radio
astronomy pioneer Jan Oort also hypothesized the existence of
dark matter
in 1932. Oort was studying stellar motions in the local galactic
neighborhood
and found the mass in the galactic plane must be greater than
what
was observed, but this measurement was later determined to be
erroneous.
In 1933, Swiss astrophysicist Fritz Zwicky, who studied galaxy
clusters
while working at the California Institute of Technology, made
a similar
inference. Zwicky applied the virial theorem to the Coma Cluster
and obtained
evidence of unseen mass he called dunkle Materie (‘dark matter’).
Zwicky estimated its mass based on the motions of galaxies near
its edge
and compared that to an estimate based on its brightness and number
of
galaxies. He estimated the cluster had about 400 times more mass
than was
visually observable. The gravity effect of the visible galaxies
was far too
small for such fast orbits, thus mass must be hidden from view.
Based on
these conclusions, Zwicky inferred some unseen matter provided
the mass
and associated gravitation attraction to hold the cluster together.
Zwicky’s
estimates were off by more than an order of magnitude, mainly
due to an
obsolete value of the Hubble constant; the same calculation today
shows
a smaller fraction, using greater values for luminous mass. Nonetheless,
Zwicky did correctly conclude from his calculation that the bulk
of the
matter was dark.
Further
indications the mass-to-light ratio was not unity came from measurements
of galaxy rotation curves. In 1939, Horace W. Babcock reported
the
rotation curve for the Andromeda nebula (known now as the Andromeda
Galaxy), which suggested the mass-to-luminosity ratio increases
radially.
He attributed it to either light absorption within the galaxy
or modified
dynamics in the outer portions of the spiral and not to the missing
matter
he had uncovered. Following Babcock’s 1939 report of unexpectedly
rapid
rotation in the outskirts of the Andromeda galaxy and a mass-to-light
ratio
of 50; in 1940 Jan Oort discovered and wrote about the large non-visible
halo of NGC 3115.
1970s
Vera Rubin, Kent Ford, and Ken Freeman’s work in the 1960s
and 1970s
provided further strong evidence, also using galaxy rotation curves.
Rubin
and Ford worked with a new spectrograph to measure the velocity
curve of
edge-on spiral galaxies with greater accuracy. This result was
confirmed
in 1978. An influential paper presented Rubin and Ford’s
results in 1980.
They showed most galaxies must contain about six times as much
dark as
visible mass; thus, by around 1980 the apparent need for dark
matter was
widely recognized as a major unsolved problem in astronomy.
At the
same time Rubin and Ford were exploring optical rotation curves,
radio astronomers were making use of new radio telescopes to map
the 21
cm line of atomic hydrogen in nearby galaxies. The radial distribution
of
interstellar atomic hydrogen (H-I) often extends to much larger
galactic
radii than those accessible by optical studies, extending the
sampling of
rotation curves – and thus of the total mass distribution
– to a new dynamical
regime. Early mapping of Andromeda with the 300 foot telescope
at
Green Bank and the 250 foot dish at Jodrell Bank already showed
the H-I
rotation curve did not trace the expected Keplerian decline. As
more sensitive
receivers became available, Morton Roberts and Robert Whitehurst
were able to trace the rotational velocity of Andromeda to 30
kpc, much
beyond the optical measurements. Illustrating the advantage of
tracing
the gas disk at large radii, Figure 16 of that paper combines
the optical
data (the cluster of points at radii of less than 15 kpc with
a single point
further out) with the H-I data between 20–30 kpc, exhibiting
the flatness
of the outer galaxy rotation curve; the solid curve peaking at
the center
is the optical surface density, while the other curve shows the
cumulative
mass, still rising linearly at the outermost measurement. In parallel,
the
use of interferometric arrays for extragalactic H-I spectroscopy
was being
developed. In 1972, David Rogstad and Seth Shostak published H-I
rotation
curves of five spirals mapped with the Owens Valley interferometer;
the
rotation curves of all five were very flat, suggesting very large
values of
mass-to-light ratio in the outer parts of their extended H-I disks.
A stream of observations in the 1980s supported the presence of
dark matter,
including gravitational lensing of background objects by galaxy
clusters,
the temperature distribution of hot gas in galaxies and clusters,
and the
pattern of anisotropies in the cosmic microwave background. According
to consensus among cosmologists, dark matter is composed primarily
of
a not yet characterized type of subatomic particle. The search
for this particle,
by a variety of means, is one of the major efforts in particle
physics.
