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June 2009 swinburne
The Milky Way’s parade across Earth’s
night sky offers a glittering view of our
home galaxy and a glimpse of the billions
of galaxies in the cosmos beyond. However,
despite this night sky splendour, an image
taken by NASA’s Wilkinson Microwave
Anisotropy Probe (WMAP) satellite shows
that viewing platforms on Earth can see
2009 Year of Astronomy
This year is the International Year of Astronomy:
a worldwide celebration of astronomy, held to
mark the 400th anniversary of Galileo turning
a telescope to the sky. Australia is one of
63 countries participating.
More information
• www.astronomy2009.org.au
Emily Wisnioski (left) and
Dr Sarah Brough.
photo: paul Jones
only a tiny part, perhaps just 3 per cent, of
the known universe.
Most of the remainder appears to be
something quite new to human discovery –
dark energy. This is the mysterious property
that is causing the universe’s expansion to
accelerate in ways that confound fundamental
physics. It may eventually challenge
Einstein’s general theory of relativity and his
theory that gravity is the force that should
counter-balance cosmic expansion.
This dark energy has only been known
to science for about a decade and is not to
be confused with dark matter, which was
discovered more than half a century ago.
Dark matter has a gravitational ‘pull’ because
of its mass, but something is overcoming
this to allow the universe to ‘push’ outwards
and keep expanding. It is this ‘push’ that
scientists have called dark energy.
Dark energy’s discovery in the late-
1990s was a shock for astrophysicists, who
promptly initiated a suite of investigations to
understand the relevance of dark energy to
the origin, evolution, composition and fate of
the universe.
One of these initiatives was the WiggleZ
Dark Energy Survey, which is being
undertaken at the Centre for Astrophysics
and Supercomputing at Swinburne University
of Technology. The WiggleZ team is a
collaboration between Swinburne and several
other Australian institutions and is attempting
to measure the distances separating 200,000
galaxies as the universe expands.
These observations have been made
possible by a powerful new spectrograph
at the Anglo-Australian Telescope in
Coonabarabran, NSW. Called AAOmega,
the instrument can image 392 galaxies an
hour, despite the galaxies being located half
the distance of the universe away. Data from
NASA’s orbiting Galaxy Evolution Explorer
(GALEX) satellite is helping the Australian
team select where to probe in the night sky.
Swinburne astrophysicist Dr Sarah
Brough explains that AAOmega measures
the ‘redshift’ in light emitted by the target
galaxies. Redshift is the increase that
occurs in light’s wavelength if the emitting
light-source is moving away from us. The
existence of redshift in starlight is the
evidence used to support the theory that the
universe is expanding.
Dr Brough says that because redshift
increases the further a galaxy moves away
from us, the AAOmega observations provide
a measure of the physical distance between
Earth and the galaxy. By observing galaxies
located at a range of distances from Earth,
separation between galaxies can be measured
at various ages of the universe. It is this that
provides a history of cosmic expansion.
By observing 200,000 galaxies, the
WiggleZ team is creating a huge database of
separation distances. When enough of these
measurements are plotted, a characteristic
‘wiggle’ appears in the distribution.
“More accurately termed ‘baryon acoustic
oscillations’, wiggles indicate that galaxies
have a small but detectable preference for
a particular separation distance that was
imprinted into the universe shortly after
the Big Bang,” Dr Brough says. “Wiggles
formed as a result of acoustic waves
travelling through the baryon-photon plasma
before these particles cooled and separated
into matter and radiation.”
Since the imprinted separation
distance remains constant at a fixed scale,
astrophysicists are attempting to use them
as ‘rulers’ against which to measure cosmic
expansion.
Swinburne PhD student Emily Wisnioski,
from the US, explains that wiggles amount to
a small preference for pairs of galaxies to be
separated by a distance of 150 megaparsecs
(Mpc) (a measure of distance equivalent to
3.26 million light years).
“The WiggleZ survey will provide an
independent measure, over vast cosmic
distances of this 150Mpc ‘standard ruler’
that was first determined by the WMAP
satellite,” she says.
Taken together, the separation
measurements can be fed into equations that
describe the universe’s underlying contents,
allowing the team to deduce properties of
dark energy. The more accurately they can
measure distances that separate galaxies, the
more can be learnt about dark energy.
In 2009 the team is well past the half-way
point in its observations, with the survey
due for completion in 2010. With enough
data to start preliminary analysis, the team
is on track to confirm and measure dark
energy with the greatest level of accuracy
yet achieved.
Since fitting dark energy into existing
theoretical frameworks is impossible, this
means the universe has, paradoxically,
become more mysterious as observations
became more powerful. As the
astrophysicists see it, dark energy provides
an extraordinary opportunity to challenge
fundamental theories and they foresee
profound impacts in our understanding of
physics, string theory or quantum gravity.
Stay tuned … ••
Contact. .
Swinburne University of Technology
1300 MY SWIN (1300 697 946)
magazine@swinburne.edu.au
www.swinburne.edu.au/magazine
Astrophysics
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