Composition of the
Universe...
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In the past decade, systematic
and thorough observations of the sky have
lead cosmologists to measure the composition of our Universe. The most
recent observations show that it is made of about 75% of a mysterious
substance called "dark energy" which causes the Universe to expand
faster and faster; 21% comprises dark matter and only 4% is made of
ordinary, well-known baryonic matter in the form of gas, stars and
planets. Although cosmologists
have successfully dissected the Universe, they are still far from a
fundamental understanding of the nature of dark energy and dark matter.
Only high precision cosmological measurements are potentially capable
to capture the detailed signatures of these dark components leading to
deep insights of their true nature. A major leap forward in that
direction has recently been achieved by an international team of French
and Canadian astronomers lead by scientists based at the Institut
d'Astrophysique de Paris (IAP) and the Université Paris 6 (UPMC)
in
France, the University of British Columbia (UBC) and the University of
Victoria (UVic) in Canada.
By observing how dark matter is distributed in cosmic structures, the
scientists detected its presence out to unprecedented large scales. The
picture emerging from these observations confirms the "cold dark
matter" paradigm which predicts that galaxies and clusters of galaxies
are embedded in giant filamentary structures of dark matter, forming
what is called the "cosmic web". The
team of astronomers detected dark matter out to 4 degrees angular scale
on the sky, that is eight times the apparent size of the moon. This
corresponds to a typical structure size of about 270 million
light-years. The absence of such a detection would have led to a
profound revision of the cold dark matter paradigm.
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The CFHT Legacy Survey
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This remarkable result was made
possible by observing the so called
weak gravitational lensing effect at the 3.6 meter
Canada-France-Hawaii
Telescope (CFHT) taking advantage of the CCD camera MegaCam,
the
largest optical imager available to astronomers. These observations are
part of an extensive program, the CFHT Legacy Survey (CFHTLS).
This survey will be completed after 5 years by the end of 2008,
covering 170 square degrees. The French and Canadian team's analysis
was carried out on a preliminary set of the data on 57 square degrees -
300 times the area of the full Moon, and 30 times the area of the HST COSMOS survey
(see figure below) . They used the individual calibrated
images and the co-added images and catalogues produced by the Elixir
data pipeline at CFHT and by the Terapix
data center at IAP. |
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The observed patches
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One of three
observed patches (indicated in green) is situated
right next of the big
dipper. The zoom-in containing a few stars and
over a hundred faint
galaxies is 3 arc minutes in size, not much
larger than the
resolution limit of the naked eye. In all three
patches, two
million galaxies were observed in total.
High resolution image
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Gravitational lensing
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Gravitational
lensing is a consequence of General Relativity, which states that mass
curves the space-time structure in a way that light rays do not travel
on straight line as they would in an empty Universe. The dark
cosmic web bends space-time enough so that light coming from very
distant galaxies undergoes several deviations from a straight-line path,
leading to a distorted image of the galaxy when it reaches the
observer. The distortion is subtle and difficult to measure and it
requires to estimate the shape of millions of galaxies very
accurately. Similar to X-ray
radiography which illuminates the inner structure of
a human body, these distortions can be used to map the cosmic
structure. Gravitational
lensing theory turns this measurement into an
unambiguous constraint on how dark matter is distributed in the
Universe. However, cosmologists predict that gravitational
distortions should be weaker and weaker as the angular scale increases,
making the observations of very weak gravitational lensing by large
filaments impossible with previous surveys.
The CFHT MegaPrime/MegaCam
is a unique instrument for this purpose, thanks to its unrivalled
large field-of-view and its excellent image quality. The 3.6m mirror
telescope with its large light-collecting area uses the exceptional
observational site of Manua Kea on Hawaii. Already in 2000, the weak
gravitational distortions were detected for the first time
by the
same team using CFH12K, a precurser of MegaPrime/MegaCam.
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Modeling the Universe
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The CFHTLS weak
gravitational lensing observations reported by
the team are only comptatible with
a small number of cosmological models. Astronomers derived what cosmological parameters are permitted from their
data by comparing the observed amplitude of
distortion as function
of angular scale with theoretical predictions
(right
panel). The best model (black line) is a very
good fit to the data. A
clumpy (green) and a smooth (blue) universe are
also shown for
comparison. Both do not reproduce the
measurements.
Systematic errors (open black circles), coming
for example from
artefacts induced by the telescope optics and
atmosphere, are
consistent with zero on most scales. The colored areas on the left panel represent
the allowed regions consistent with the right
panel for two cosmological parameters that are
the most
sensitive to gravitational lensing effects, the
matter density in the
Universe are OmegaM and the "clumpiness parameter" sigma8. The permitted
regions for CFHTLS lensing data only (blue),
WMAP3 cosmic microwave background anisotropy
data only
(green) and their combination (yellow) are
shown. The permitted
ranges are OmegaM = 0.248
± 0.038, sigma8
= 0.771 ± 0.058. Illustrations of the cosmic web of three universes
correpond to the blue, black and green curves of
the right panels are
also shown, each corresponding to a different
parameter combination (OmegaM , sigma8) as indicated by the cross.
High resolution image
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Paving the way for the
future...
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The detection of
very weak gravitational lensing on large scales opens a new regime for
cosmology. The theory describing the cosmic web on small scales is not
very well known to date. This uncertainty hampered precise
cosmological interpretation of earlier lensing surveys, and strongly limited
the conclusions that could be drawn from them. For the first time,
these difficulties have been overcome with the CFHTLS measurement, and
robust cosmological results have been obtained.
The
CFHTLS paves the way for future large-scale surveys covering the whole
sky which are expected to take place in the next decade. Observation
of the dark matter distribution over the entire sky will hopefully
reveal the nature of dark matter and dark energy. In
particular, a fundamental question is whether or not a modification of
General Relativity can account for the dark matter and dark energy
signatures measured by gravitational lensing, or if there really is a new
type of matter and source of energy that remains to be discovered. |
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CFHT - The telescope with MegaPrime/MegaCam
The Canada-France-Hawaii telescope structure is based on an equatorial
mount design, with one of the axis of rotation set parallel to the axis
of the Earth¹s rotation. The mirror cell (the white circular
structure
at the bottom of the telescope, seen just above the person giving the
scale
on this photograph) holds and protects the most precious element: the
3.6-meter
diameter mirror.
Light from distant objects enters the dome through
the slit, bounces back from the mirror such that it will focus, and
create
a crisp image at the prime focus, MegaPrime, where the image is
captured
by MegaCam, CFHT's
wide-field digital camera (340 MegaPixels!) .
By today's standards, the CFHT telescope is a heavy
structure compared to the diameter of its mirror: the total mass of the
telescope is 325 tons, with 250 tons for the mobile section alone. Yet
the telescope can point to any location in the sky with an accuracy of
3 thousandths of a degree - and can follow astronomical objects with an
even better accuracy, thanks to an automated guiding mechanism that
compensates
for the apparent motion of the sky due to the Earth's rotation.
High
Resolution image
© Canada-France-Hawaii
Telescope Corporation 2008
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The MegaCam
CCD mosaic
At the focal plane of MeagCam seen in this
photograph, there are 40 CCDs. Each of them, known as the 'e2v
'CCD42-90'', account for more than 9.5 megapixels. 36 of them are
used to image the sky, bringing the total number
of pixels for the MegaCam mosaic to a staggering 340 million!
Want to know more about MegaPrime/MegaCam? You can go here (first light of the camera) or here for more technical
information.
Various resolution downloads:
[Full scale -
954x805
- JPEG - 84Kb]
[Scaled
1/2th
- 476x402 - JPEG - 19Kb]
© Canada-France-Hawaii
Telescope Corporation 2008
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