A perplexing picture of cosmology has emerged over the last decade. Several different observations concur that the Universe has entered a startling phase of accelerating expansion propelled by a mysterious repulsive component of gravity known as "dark energy". This component apparently constitutes about 70% of the current energy density of the Universe.  Intriguingly, the cosmic acceleration began at roughly the same time as the formation of life on our planet.


There is currently no theoretical explanation for the existence or magnitude of dark energy.  The accelerating cosmos therefore provides an outstanding opportunity for cosmologists to challenge fundamental physics, with two possible outcomes.  Firstly, the Universe could be dominated by a new and unsuspected form of matter which exerts a negative pressure.  The leading candidate here is the "cosmological constant", the energy of the quantum vacuum.  However, it is extraordinarily difficult if not impossible for fundamental quantum theory to reproduce the observed amplitude of dark energy; typical predictions are over-estimates by more than one hundred orders of magnitude.  These problems have motivated a set of alternative suggestions in which dark energy is codified as a "scalar field" which fills all space.  Examples of other hypothesized scalar fields in physics include the mechanism which propelled cosmic inflation just after the Big Bang , and the Higgs field which may endow particles with mass.


The galaxy distribution contains the imprint of a standard ruler.  Sound waves propagate in the baryon-radiation plasma of the early Universe. Dark matter haloes launch spherical wavefronts which travel outwards until recombination, when the Cosmic Microwave Background is generated.  These wavefronts then correspond to "bulls-eye" patterns of matter on the surface of last-scattering, at the top of the Figure, with an accurately-known radius equal to the "sound horizon".  As structure grows with time, proceeding down the figure, galaxies form preferentially in the denser wavefronts.  The result is a small excess of pairs of galaxies separated by the sound horizon, illustrated by the horizontal bar.  Cosmologists can exploit this property as a standard ruler for measuring cosmic distances. [Image credit: Sam Moorfield]



The second possible outcome is that Einstein's vision of gravity, general relativity, is incorrect on large cosmological scales.  In this case dark energy would describe the discrepancy between relativity and an unknown modified theory of gravity, for which a number of suggestions already exist.  Alternatively, perhaps our derivation of the equations of cosmology, in which we approximate the Universe as homogeneous despite its extreme clumpiness, can induce the illusion of apparent accelerating expansion


In summary, dark energy is a precious clue in the quest to understand fundamental theory.  Unravelling its origin is likely to entail a revolution in our understanding of physics, string theory or quantum gravity.  A rich landscape of cosmological surveys has grown in recent years to meet this goal, measuring the influence of dark energy via a variety of observational techniques.  A combination of observations is in fact required to identify unequivocally the nature of dark energy. In particular, we must seek out discrepancies between the expansion history of the Universe and the rate of growth of cosmic structure within that overall expansion.  This comparison can distinguish between the different propositions of dark energy such as modified gravity and scalar fields.


The WiggleZ Dark Energy Survey at the Anglo-Australian Telescope is one of the earliest of the new generation of dark energy-focussed surveys, commencing in August 2006 and scheduled to finish in July 2010.  The goal of the project is to perform a new large-scale galaxy redshift survey spanning a deep and wide slab of the cosmos from moderate-redshift (z ~ 0.25) to high-redshift (z ~ 1) over a sky area of 1000 sq deg.


This unprecedented combination of redshift depth and survey area will allow the WiggleZ Survey to probe the nature of dark energy with multiple methods, covering the redshift range where the transformation from decelerating to accelerating expansion is thought to occur. WiggleZ is much deeper in redshift than previous wide-area spectroscopic surveys such as the 2-degree Field Galaxy Redshift Survey (2dFGRS) and the Sloan Digital Sky Survey (SDSS).  It is orders of magnitude wider in areal coverage than previous high-redshift surveys such as the Deep Extragalactic Evolutionary Probe (DEEP2).


The redshift range and cosmic volume mapped by the WiggleZ Survey (red line) compared to various current (black) and future (pink) galaxy redshift surveys.  Key to acronyms -- 2dFGRS: 2-degree Field Galaxy Redshift Survey; SDSS-DR6: Sloan Digital Sky Survey; BOSS: Baryon Oscillation Spectroscopic Survey to be undertaken by the Sloan Consortium; WFMOS: survey performed by the proposed Wide-Field Multi-Object Spectrograph.



The large-scale structure of the Universe is one of the most fertile sources of cosmological information and a long-standing pillar of cosmology's "standard model".  The patterns of galaxy clustering, shaped by the competing forces of inward gravity and outward expansion, contain information about the constituents of the Universe and the physical processes by which cosmic structure condenses and amplifies from initial small fluctuations.  In the early 1990s, combined analyses of galaxy clustering and the Cosmic Microwave Background (CMB) fluctuations provided the first strong evidence for the currently-accepted dark energy-dominated cosmological model. This model received spectacular confirmation through later observations of faint supernovae as standard candles.


The spectrum of temperature fluctuations in the CMB, as determined by the Wilkinson Microwave Anisotropy Probe.  The "wiggles" are produced by acoustic waves in the baryon-photon plasma before recombination.  The peak in the power spectrum at an angular scale of about 1 deg corresponds to the angular size of the 150-Mpc sound horizon projected at the distance of the CMB.



The cosmological power of galaxy redshift surveys was boosted further by the realization that the pattern of galaxy

clustering encoded a robust "standard ruler" which could be used to map out the cosmic expansion history in a manner analogous to supernova standard candles.  The nature of this standard ruler is a preference for pairs of galaxies to be separated by a co-moving distance of 150 Mpc. This favoured scale is an echo of sound waves which propagated 13.7 billion years ago through the matter-radiation plasma before CMB last-scattering, less than 380,000 years after the Big Bang.  These sound waves emanated from primordial dark matter haloes, which launched spherical wavefronts driven by radiation pressure from the compressed baryon-photon plasma.  These sound waves travelled rapidly in the early Universe, at 58% of the speed of light, covering a co-moving distance of 150 Mpc between the Big Bang and the last-scattering epoch.  Both the initial dark matter halo and the spherical baryonic shell preferentially seed the later formation of galaxies, imprinting the standard ruler into the large-scale clustering pattern.  Because the radiation is tightly coupled to the baryons, the signature of the acoustic waves is also imprinted in the microwave background photons.


The spectrum of density fluctuations in the distribution of Luminous Red Galaxies in the Sloan Digital Sky Survey.  The bump at separation 105 Mpc/h = 150 Mpc corresponds to a preferred clustering scale and is the corresponding imprint of the acoustic waves in the galaxy distribution.  The amplitude of this signal is much lower than that seen in the CMB radiation because the baryons, which carry the imprint of acoustic oscillations, are sub-dominant to cold dark matter.



A powerful aspect of this cosmological distance probe that differentiates it from supernova observations is that the standard ruler can be applied in both the tangential and radial directions, i.e. perpendicular and parallel to our line-of-sight. Tangentially we measure a preferred angular clustering scale of about 3 degrees, which determines the cosmic distance in a similar manner to a supernova flux. Radially we extract a preferred redshift-space clustering scale, which specifies the cosmic expansion rate or Hubble parameter at high redshift.  This expansion parameter depends directly on the underlying energy densities and is extremely difficult to measure with other methods.


The standard ruler scale of 150 Mpc can be predicted very accurately from the densities of dark matter and baryons inferred from observations of the CMB fluctuations.  The Wilkinson Microwave Anisotropy Probe (WMAP) has already measured the scale with an error of 1.3%, and this will be improved even further by the European Space Agency's Planck satellite. The difficulty in applying the ruler lies in measuring the signal over vast cosmic distances.  The accuracy with which a galaxy survey can measure the underlying spectrum of clustering fluctuations is limited ultimately by its volume, which determines the number of Fourier modes of a given scale that the survey can resolve.  In Fourier space, the 150 Mpc preferred scale corresponds to a series of harmonics ("wiggles" or "baryon oscillations") in the galaxy power spectrum, by analogy with the set of frequency harmonics excited by an organ pipe of fixed length.  It is interesting to note that current galaxy redshift surveys have mapped less than 0.01% of the volume information (or Fourier amplitudes) contained in the observable Universe.


Behaviour of the scale factor of the Universe, a, for a cosmological constant model with matter density Omega_m=0.3 and cosmological constant density Omega_Lambda = 0.7.  Top: Variation with redshift of the scale factor.  Middle: Variation with redshift of the time derivative of the scale factor, which represents the "velocity" of the cosmic expansion.  The y-axis is in units of H_0, the Hubble constant at z=0.  Bottom: Variation with redshift of the second time derivative of the scale factor, which represents the "acceleration" of the cosmic expansion.  The y-axis is in units of H_0^2.  Notice that the transition from decelerating to accelerating expansion occurs at redshift z~0.7, which lies near the centre of the target redshift range of the WiggleZ Survey, as illustrated in the middle plot.



The baryon oscillation standard ruler has already been detected at low redshifts (z=0.2 and z=0.35) in the distribution of Luminous Red Galaxies in the SDSS.  The aim of the WiggleZ Dark Energy Survey is to extend this delineation of the redshift-distance relation by adding further data points up to z = 1. After the addition of the WiggleZ results, which equate to a roughly 2% measure of the cosmic distance scale at z = 0.7, the accuracy with which baryon oscillation data can probe the properties of dark energy will be similar to supernova data. This will provide a detailed cross-check of the two techniques, in which disagreements could be indicative of systematic errors or new physics.


A simulation of the power spectrum of galaxy clustering which will be measured from the final WiggleZ Dark Energy Survey.  The power spectrum has been divided by a smooth "reference" spectrum fit in order to illustrate the detection of the baryon oscillations in the clustering pattern.



The WiggleZ Survey will also determine the growth rate of cosmic structure and test modified gravity theories.  This is possible through the measurement of "redshift-space distortions" -- the coherent flows of galaxies into their local clusters or superclusters, which imprint an additional "peculiar velocity" Doppler shift into each galaxy spectrum on top of the Hubble flow.  These distortions can be mapped statistically and their amplitude can be used to recover the growth rate of structure.  The self-consistency of the measured expansion history (from baryon oscillations) and growth history (from redshift-space distortions) is a robust test of the dark energy or modified gravity model.


Simulated cosmological parameter measurements using baryon oscillations in the final WiggleZ Dark Energy Survey.  This figure focuses on measurements of the matter density, Omega_m, and the equation of state of dark energy, w.  We assume a fiducial cosmological constant model with Omega_m = 0.27 and w = -1.  The yellow ellipse indicates the 68% confidence region for measurement of these parameters using the WiggleZ Survey baryon oscillations combined with the CMB "acoustic scale parameter", which calibrates the baryon oscillation preferred scale. The orange band indicates the confidence region obtained from WMAP measurements of the CMB "shift parameter".  The red region displays the confidence region for latest supernova measurements.  When the WiggleZ Survey is complete, the baryon oscillation data will measure the properties of dark energy with a similar precision to the supernova data, providing a detailed cross-check of the two techniques.  The central confidence circles illustrate the dark energy measurements obtained by combining all the datasets.



 

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