Dark mysteries lure cosmic surveyors
Story by Gio Braidotti
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 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 ...
This article was originally published in Swinburne Magazine issue 6, June 2009.