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

The most striking feature of the distribution of galaxies that surrounds us is "clustering". Galaxies are not sprinkled uniformly through the Universe, but are concentrated into a complex network of dense knots, walls and filaments. This pattern - called "large-scale structure" - is an important source of data for cosmologists. It encodes a wealth of valuable information about the underlying contents of the Universe, and about the processes by which galaxies formed and grew under gravity.

The local galaxy distribution has been mapped in exquisite detail over the last few years by projects such as the 2dF Galaxy Redshift Survey and the Sloan Digital Sky Survey. Our challenge now is to provide equally good measurements in the distant Universe. Owing to the time taken for light from distant galaxies to reach us, such observations see the Universe at an earlier epoch. Comparing the properties of galaxies then and now enables us to test models which describe the formation of structure in the Universe, and to link different classes of galaxy in an evolutionary sequence.

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

The SKA can be used to observe radio waves originating from atomic processes in neutral hydrogen atoms ("21cm emission"). Hydrogen is the most common atomic element in the Universe, and a critical component in the formation of galaxies. However, observations of 21cm emission from distant galaxies have hitherto been almost impossible owing to the lack of sensitivity of current instruments. The vast collecting area of the SKA will make such observations possible for the first time.

The results will be astounding: in one year of operation, the SKA will be able to identify 21cm emission from a billion galaxies over the whole sky. Radio waves from the most distant of these objects will have taken 9.5 billion years to reach us, two-thirds of the age of the Universe (which is 13.7 billion years). Furthermore, mapping the 21cm emission line also determines an accurate distance to each galaxy: as these radio waves travel through the cosmos to our telescope, the overall expansion of the Universe stretches their wavelength. The more distant the galaxy, the greater the stretching factor, and the higher the observed wavelength. Hence as it collects data, the SKA forms a three-dimensional picture of the cosmic web of galaxies.

This unprecedented map will enable a host of interesting measurements. In particular, we will be able to probe the mysterious "dark energy" which is believed to constitute the majority of today's Universe. In the last five years, astronomers have made the astonishing discovery that the overall expansion of the Universe is not slowing down (due to the pull of gravity) but is actually accelerating! This remarkable fact was deduced from observations of distant supernovae, which appear significantly dimmer than one would expect in a decelerating Universe. This accelerating expansion must be driven by a new component of the Universe which is gravitationally repulsive, known as "dark energy". Using the exquisite galaxy clustering maps produced by the SKA, we will be able to investigate the nature and effects of dark energy.

More
- Galaxy evolution, cosmology and dark energy for scientists.
- 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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