Key science projects

> Cradle of Life
> Probing the Dark Ages
> The origin and evolution of Cosmic Magnetism
> Strong field tests of gravity using pulsars and black holes
> Galaxy evolution, cosmology and dark energy

Recommendations for the Key science projects (PDF), Bryan Gaensler

The SKA will enable revolutionary progress in the fields of galaxy evolution and large-scale structure. In particular, it will map out the cosmic distribution of neutral hydrogen (by detecting the HI 21cm transition) at cosmological distances that are almost entirely inaccessible to current instrumentation. With the SKA, it will be possible to detect typical galaxies at redshift z=3 via 21cm emission, in a reasonable integration time. The cosmic history of neutral hydrogen is a critical ingredient of galaxy evolution about which almost nothing is presently known. An HI emission-line survey is able to map out galaxies independently of dust extinction, with one additional advantage: once the galaxy has been located on the sky, the observed wavelength of the emission line automatically provides an accurate redshift, locating the object's position in the three-dimensional cosmic web. The SKA will hence become the premier machine for delineating the large-scale structure of the universe. Our simulations indicate that with a wide enough field-of-view (> 10 sq deg) the SKA can survey the entire visible sky in a year of operation, locating a billion HI emission galaxies over a vast volume stretching to redshift z=1.5.

Simulation of bayonic oscillations in the power spectrum of the clustering of HI emission galaxies as a function of redshift. Credit: Chris Blake

The result would be the most accurate measurement of the clustering pattern of galaxies ever achieved, testing theoretical models for the growth of structure in the Universe and pinpointing the cosmological parameters (in conjunction with Cosmic Microwave Background data from the Planck satellite). For example, this survey would permit an accurate quantification of the properties of the mysterious "dark energy", which is believed to compose 70 per cent of the current energy density of the Universe, and which is driving the cosmic expansion to accelerate (as evidenced by observations of distant supernovae). One of the cleanest methods of measuring dark energy in the Universe is by accurately delineating the small-amplitude "acoustic oscillations" in the clustering power spectrum. This baryonic signature has an identical physical origin to the acoustic peaks already identified in the CMB, which act as an accurate standard ruler for the experiment. Their recovery in the SKA HI survey, as a function of redshift, permits an extremely accurate determination of the rate of evolution of the equation of state of dark energy with cosmic time, discriminating between theoretical dark energy models. Thus an SKA cosmic structure survey can potentially disprove Einstein's cosmological constant.

In addition to detecting vast numbers of HI emission line galaxies, the SKA can also perform the deepest-ever radio continuum survey, probing the star formation history of the Universe as a function of redshift in a manner independent of the dust extinction that confuses experiments in optical wavebands. It will be of great interest to link the star formation properties of galaxies to their HI contents, as a function of redshift and environment. Furthermore, a high-resolution radio continuum survey over wide areas allows a precise measurement of the coherent shape distortions of distant galaxies imparted by the foreground cosmic web. This "weak gravitational lensing" encodes a vast body of cosmological information, and its exploitation will become one of our key cosmological probes within the next decade. Radio wavebands are particularly advantageous for this experiment because the point-spread function is well-determined and stable (being simply the interferometer baseline distribution), solving the principle systematic difficulty inherent in the method.

More: Galaxy evolution, cosmology and dark energy with the Square Kilometre Array S.Rawlings, F.B. Abdalla, S.L Bridle, C.A.Blake, C.M. Baugh, L.J. Greenhill, J.M. van der Hulst - in " Science with the Square Kilometre Array", 2004

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