Dark energy camera snaps first images ahead of survey
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The camera comprises 62 separate CCDs, the same
kind of detector familiar from consumer cameras
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| Dark energy camera’s first image Fornax galaxy cluster |
The
most powerful sky-scanning camera yet built has begun its quest to pin down the
mysterious stuff that makes up nearly three-quarters of our Universe.
The
Dark Energy Survey's 570-million-pixel camera will scan some 300 million
galaxies in the coming five years.
The
goal is to discover the nature of dark energy, which is theorised to be
responsible for the ever-faster expansion of the Universe.
Its
first image, taken 12 September, focussed on the Fornax galaxy cluster.
In
time, along with its massive haul of individual galaxies, it
will study 100,000
galaxy clusters - the largest stable structures we know of - and 4,000
supernovae, the bright dying throes of stars.
This
enormous survey is a collaboration between US, UK, Brazilian, Spanish and
German astronomers.
The
phone box-sized Dark Energy Camera or DECam is mounted on the 4m Victor M
Blanco telescope at the Cerro Tololo Inter-American Observatory in central
Chile.
While
it is not the biggest astronomical camera - that honour goes to the Pan-Starss instrument in Hawaii - its extraordinary
sensitivity arguably makes it the world's most powerful device of its type.
DECam
is particularly sensitive to red and infrared light, to better study cosmic
objects as distant as eight billion light-years away.
What is redshift?
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The
term "redshift" arises from the fact that light from more distant
objects shows up on Earth more red than when it left its source
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The
colour shift comes about because of the Doppler effect, which acts to
"stretch" or "compress" waves from moving objects
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It
is at work in the sound of a moving siren: an approaching siren sounds
higher-pitched and a receding one sounds lower-pitched
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In
the case of light, approaching objects appear more blue and receding objects
appear more red
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The
expansion of the Universe is accelerating, so in general, more distant objects
are moving away from us (and each other, and everything else) more quickly than
nearer ones
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At
cosmic distances, this "cosmological redshift" can greatly affect the
colour - the factor by which the wavelength is "stretched" is called
redshift
More
distant objects are moving away from us - and each other - faster than nearer
objects, which causes a shift of their apparent colour toward the red end of
the spectrum - a "redshift". But the very stretching of space can
cause the same effect.
Careful
studies of the shifted light from distant supernovae were what first
demonstrated an acceleration in this expansion of the Universe, leading
to the 2011 Nobel prize in physics.
What
is believed to be causing this increase in the speed of expansion is called
dark energy, making up more than 70% of the mass-energy - all of the
"stuff" - of the Universe and the focus of the DECam's mission.
Other
efforts hope to get to the bottom of the mystery, including the Boss survey and a future space
telescope dedicated to the effort called Euclid.
But
for now, Will Percival from the University of Portsmouth, a Dark Energy Survey
collaborator, said DECam is an exciting prospect.
"This
will be the largest galaxy survey of its kind, and the galaxy shapes and
positions will tell us a great deal about the nature of the physical process
that we call dark energy, but do not currently understand," he said.
The
survey will tackle the problem in four ways.
It
will study the same kind of supernovae that led to the Nobel prize, in a bid to
unravel the "expansion history" of the Universe - when its expansion
increased and decreased over billions of years.
It
will also map out in 3D the distribution of galaxy clusters, measuring what are
known as baryon acoustic oscillations - literally relics of the sound echoes of
the Big Bang.
Dark energy and dark
matter mysteries
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Gravity
acting across vast distances does not seem to explain what astronomers see
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Galaxies,
for example, should fly apart; some other mass must be there holding them
together
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Astrophysicists
have thus postulated "dark matter" - invisible to us but clearly
acting on galactic scales
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At
the greatest distances, the Universe's expansion is accelerating
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Thus
we have also "dark energy" which acts to drive the expansion, in
opposition to gravity
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The
current theory holds that 73% of the Universe is dark energy, 23% is dark
matter, and just 4% the kind of matter we know well
By
counting the clusters and plotting out when they evidently formed, the survey
can feed back to computer models that map out how we think the Universe
organised itself in its earliest years.
And
studies of the way galaxies and galaxy clusters bend passing light - in a
process called weak gravitational lensing - will help to pin down the equally
mysterious "dark matter" that is believed to make up more than 80% of
the Universe's mass - most of the Universe's stuff that is not energy.
DECam
will now be run through a series of tests and will begin the official survey in
December.
With
each snapshot it acquires, it will see an apparent area of the sky 20 times
larger than the full moon.
In
its full five-year run, it should capture an eighth of the full sky.
"The
achievement of first light through the Dark Energy Camera begins a significant
new era in our exploration of the cosmic frontier," said James Siegrist,
associate director of science for high-energy physics at the US Department of
Energy, which oversaw the instrument's construction.
"The
results of this survey will bring us closer to understanding the mystery of
dark energy and what it means for the Universe."
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