You can't hide gravity
Since Newton's time, Astronomers have delighted in the
powerful method of finding new objects in space through the effects of
their gravity, before the objects themselves are actually seen
directly.
The first great success of this idea was the discovery of Neptune,
by the reconstruction of where it must be in the Solar System in order
to gravitationally perturb the motions of the other planets. Similarly
the inference that some of the bright nearby stars must be circled by
faint but heavy companions lead to the discovery
of white dwarf stars.
The case of the missing matter
This venerable technique is still in use today, as it is
currently being used to infer the existance of large amounts of unseen
matter around galaxies, in clusters of
galaxies , and perhaps even dominating the Cosmos as a whole. The
mystery of the dark matter began
in the 1930's, when it was noticed that the motions of individual
galaxies in large clusters of galaxies was so rapid that the clusters
ought simply to fly apart.
The matter deepened in the 1970's and 1980's when it was
discovered that the motions of the stars
within individual galaxies were so rapid that they ought to fly apart,
too. If our understanding of Newtonian gravity is correct, then the
implication is that there is a lot of matter in
these structures, holding them together, but which we cannot yet
see.
Blame it on the
little guys
The most conventional idea for what these dark objects
could be is in
, stars
which are so intrinsically faint that they could not be detected
directly with our best telescopes until the launching of the Hubble Space
Telescope . Some such stars are known to exist, since a
few happen to be close to the Earth, but if the
dark matter in the Galaxy were composed of such stars, the vast
majority would be so far from us that ground based telescopes are
unable to distinguish between these faint stars and the much more
numerous faint galaxies. These stars are red because they are smaller
than the
sun and their surfaces are cooler. The superb resolving power of
Space Telescope allows us to get around this problem for the first
time: with the ten times sharper images we get from space, above the
distortion of Earth's atmosphere, the difference between faint stars
and faint
galaxies can be easily seen.
At Tuorla
Observatory, we have examined the images taken of the so-called Hubble Deep Field
, and saw far fewer faint stars than predicted
if the dark matter were made entirely of these objects. The Hubble Deep
Field is a one week long exposure taken with the Space Telescope of a
small patch of sky; it shows the faintest stars and galaxies ever
seen.
Case dismissed!
If the
dark matter in our own Galaxy were
composed of faint red stars, t
hen we would expect to see dozens or even hundreds of them on this
image, whereas
we found no red stars at all.
This puts a strict limit on the amount of
dark matter which could be in faint objects of this type.
This leaves the
possibility that the dark matter is made of something fainter still,
such as brown dwarfs, (basically big versions of Jupiter) which
are not hot enough internally to begin nuclear fusion and so do not
glow like true stars. Some of these
long predicted objects have been actually located in just the last
two years, but they would be too faint to see in this image. Then
again, the dark matter may be something completely different, such
as fundamental particles like the neutrino, or WIMPS, or axions or
black holes (quite a list!); or we may even need to modify the laws of
Gravity. All of these possibilities and more are in active research
around
the world.
Flynn,
Gould and Bahcall, 1996, Astrophysical Journal , 466
, 55
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A faint cluster of galaxies observed with Hubble Space
Telescope. Clusters contain anything from
a few to a few thousand galaxies bunched together and moving around
within their combined gravitational field. The Hubble image above shows
distorted images of background galaxies which have been "lensed" by the
dark matter which is holding this cluster of galaxies together.
" Rotation curve " of the Milky
Way Galaxy showing how fast stars move in circular orbits as a function
of their distance from the center of the galaxy. One expects the stars
to get slower the further they are from the center, whereas their speed
remains instead close to constant. This indicates that there is matter
in the Galaxy whose
gravity controls the stars almost totally. And yet we cannot find
this matter!
The NASA/ESA Hubble Space Telescope during the mission to
upgrade instruments (STS-61). Astronauts Story Musgrave and Jeffrey
Hoffman are seen during the last of the five EVAs. Australia's west
coast can be seen in the background.
The faintest stars and
galaxies ever seen. There are thousands of galaxies and just a handful
of stars (shown inside yellow circles) on this image. The number of
stars is so small that they can in no way account for the large amount
of "dark matter" thought to dominate the mass of the Milky Way Galaxy.
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