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A few centuries ago, maps of the world showed blank spaces – places
no voyager had visited, unknown territories at the centres of
continents. Gaps in our knowledge of things. Here today at the
beginning of the 21st century, every corner of the globe has been
mapped to millimetre precision by orbiting satellites and our
undiscovered countries are now all beyond the Earth. But most of the
matter in the Universe is dark unknown substance at the heart of
galaxies and clusters. Recently an international team of astronomers
has succeeded for the first time in tracing out the contours and
outlines of this land, this mysterious material which permeates
space throughout millenia of cosmic time.
A key component of astronomy is
cartography, finding out how much stuff there is in the Universe and
where it is located. Unfortunately astronomers now are in the
unenviable position of realising that most of the mass in the
Universe is comprised of material which emits no light. This stuff
is invisible, and it presence can only be divined indirectly by the
tug of gravity. In the past this meant careful measurements of the
positions and motions of galaxies which could give us an idea where
the dark matter might be hiding. But what about the dark matter
where there is are no galaxies? And its distribution on the largest
scales? Luckily for astronomers there is another way to find where
the dark matter in the Universe lies -- using light itself.
Rays of light themselves are deflected by mass -- one of the key
predictions of Einstein's general theory of relativity, and one
which was spectacularly verified by the measurement of the position
of a star during the eclipse expedition of 1919. Extragalactic
observations of this effect had to wait until the 1980s, when
charge-coupled devices provided images of sufficient quality that
galaxy shapes could be precisely measured. It was realised that arcs
and filaments seen at the cores of galaxy clusters were actually
heavily distorted images of distant background galaxies. This
provided a new way to measure the amount of dark matter inside
galaxy clusters.
Now Imagine a ray of light leaving a distant galaxy and traversing
the Universe. On its long journey to Earth it passes nearby
concentrations of dark matter. The shape of the galaxy is subtly
altered; it is distorted, deformed. On one single galaxy the effect
is undetectable, but on millions it can be measured. Determining the
amplitude of this 'cosmic shear' tells us about all the dark matter
along the line of sight between us and that distant galaxy. The
group at the Institut d'Astrophysique de Paris (IAP, CNRS,
Université Pierre & Marie Curie) lead by
Yannick Mellier, was amongst the first groups
in the world to convincingly measure this effect.
Such measurements tells us about all the dark matter between us and
this distant galaxy. What we would really like to know is how the
dark matter is distributed throughout cosmic epochs -- from the
distant past to the present day. And we would also like to know how
the distribution of the dark matter depends on the luminous matter.
The observations presented in Nature by Massey and collaborators
represent the very first time that such a map has been drawn. This
has only been possible thanks to a remarkably ambitious set of
observations called 'COSMOS'. Several astronomers at the IAP are
actively involved in this collaboration. The centrepiece of this
program is a very large allocation of Hubble Space Telescope time --
640 orbits, a bit less than 1000 hours of observation -- covering almost two square degrees of sky, more than
ninth times the size of the full moon. It is the largest contiguous area
of sky ever observed with the Hubble Space Telescope. These images
provide exquisitely precise morphological information for galaxies
in the COSMOS field. By measuring the extremely small distortion of
these 'cosmic wallpaper' galaxies by intervening dark matter, one
can compute the amount of dark matter between us and these distant
galaxies in each different part of the COSMOS field.
Such work has already been carried out
out; but the COSMOS field revolutionises the field in two important
ways. Firstly, the astounding resolution of space-based images means
that in each part of sky, one can greatly increase the number of
objects used to measure the distortion signal, meaning one is
sensitive to much smaller concentrations of matter along the line of
sight. Secondly, our knowledge of the 'wallpaper' galaxies is
extensive: all major space-based observatories have taken a long
look here, amongst them Spitzer, Chandra and the European XMM
sattelite and observatories on earth like Subaru
telescope, Japan, CFHT in Hawaii, Cerro-Tololo (CTIO) or the VLT in
Chile.. Future observatories like the Herschel satellite and the
ALMA radio array already have the COSMOS field in their mission
plans. Taken together this means that we can accurately measure the
distances to each galaxy using a technique known as 'photometric
redshifts', which profits from the happy fact that the more distant
an object is, the redder it becomes, a consequence of the expansion
of the Universe. By calibrating this method with a set of objects
whose distances are precisely known, it becomes possibly to derive
distance information for most objects out to distance where the
Universe was half it's current age of 13.6 billion years.
Armed with this information, we can repeat the exercise, measuring
once more the dark matter distribution but using each time as a
point of reference galaxies at progressively greater and greater
distances, giving us a set of 'slices' through the Universe. Each
slice shows it's own snapshot of the dark matter distribution in a
particular range of cosmic time. Some mathematical sleight-of-hand
allows us derive a composite image of where all this dark matter is
over the half the age of the Universe. Equivalently, one can also
trace the distribution of luminous matter over the same cosmic time.
What does one see when one does all this? Dark matter, like visible
matter, is not distributed uniformly throughout the Universe.
Instead, there are great voids, empty spaces extending over vast
distances; there are long, elongated filamentary structures; and
there are dense clumps and knots of matter. One sees approximately
the same structures in visible light. Other tracers tell a different
story; for example, x-rays are only emitted from the most dense
regions of the Universe, in the maps from the COSMOS collaboration,
they shine from the centres of galaxy clusters.
Where the dark matter resides in the COSMOS volume, and its
evolution with cosmic time, agrees quite well with the predictions
of our current 'best guess' for the formation of structure in the
Universe, the Cold Dark Matter model.
This model had posited some properties of dark
matter; by turning a numerical handle, scientists have been able to
create entire simulated universes filled with this material and
follow its evolution and distribution over cosmic time. The dark
matter in these simulations seems to be very similar stuff to the
dark matter observed in the COSMOS volume.
The real frontier, however, will be understanding
the precise role and relationship between the dark and luminous
matter -- essentially, how galaxies form. It seems that galaxies can
only form where there are 'haloes' of dark matter. It also seems
that the amount of dark matter in these haloes determines what kind
of galaxies form there. The more massive the hosting dark
matter haloes, the more massive are the galaxies which form there.
In the COSMOS maps there are also regions where
there are traces of luminous matter but no corresponding dark matter
component, and vice-versa. Although intriguing, this is probably an
instrumental effect; better maps are still needed. An interesting
new continent has been revealed by this work, and doubtless there
are many years of explorations still ahead of us.
This composite shows three different components of the COSMOS survey:
The normal matter (in red) determined mainly by the European Space
Agency’s XMM/Newton telescope, the dark matter (in blue) and the
stars and galaxies (in grey) observed in visible light with Hubble.
© NASA,
ESA and R. Massey (California
Institute of Technology)
Dark matter (blue) and baryons (red)
in Hubble Space Telescope COSMOS Survey.
© NASA,
ESA and R. Massey (California
Institute of Technology)
When the slices across the Universe and back into time are combined,
they make a three-dimensional map of dark matter in the Universe.
The three axes of the box correspond to sky position (in right
ascension and declination), and distance from the Earth increasing
from left to right (as measured by cosmological redshift). Note how
the clumping of the dark matter becomes more pronounced, moving
right to left across the volume map, from the early Universe to the
more recent Universe.
© NASA,
ESA and R. Massey (California
Institute of Technology)
These two false-colour images compare the distribution of normal
matter (red, left) with dark matter (blue, right) in the Universe.
© NASA,
ESA and R. Massey (California
Institute of Technology)
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