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 Chile's
Atacama desert.
While it is not the highest-resolution camera - that honour goes to the
Pan-Starss instrument in Hawaii - its high resolution and extraordinary
sensitivity make it arguably the world's most powerful camera.
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?
Diagram of Doppler shift
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
The colour shift comes about because of the Doppler effect, which acts to
"stretch" or "compress" waves from moving objects
It is at work in the sound of a moving siren: an approaching siren sounds
higher-pitched and a receding one sounds lower-pitched
In the case of light, approaching objects appear more blue and receding objects
appear more red
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
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 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
Dark matter distribution simulation
Gravity acting across vast distances does not seem to explain what astronomers
see
Galaxies, for example, should fly apart; some other mass must be there holding
them together
Astrophysicists have thus postulated "dark matter" - invisible to us but
clearly acting on galactic scales
At the greatest distances, the Universe's expansion is accelerating
Thus we have also "dark energy" which acts to drive the expansion, in
opposition to gravity
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."