Astronomers at the Hartebeesthoek Radio Astronomy Observatory (HartRAO) west of Johannesburg are working on a new radio telescope that will shed new light on the very earliest moments of the universe.
The C-Band All-Sky Survey (C-BASS) is a project to map the sky in microwave (short-wavelength radio) radiation. Like the European space Agency’s Planck satellite, it will survey the whole sky, mapping out brightness as well as the orientation of the waves (polarisation). While Planck observes very short wavelengths, C-BASS observes longer wavelengths that are easier to observe from the ground.
“Because we want to observe at these longer wavelengths, the C-BASS telescope has to be much bigger than the telescope on Planck,” explained South African C-BASS team member Charles Copley. “The C-BASS dish is over seven metres across – much too big to launch on a rocket.”
In order to observe the entire sky, C-BASS needs to use two different telescopes, one in the northern hemisphere and one in the southern hemisphere.
The northern telescope is already operating in California, while the southern system is undergoing final commissioning at HartRAO. After all the systems have been thoroughly checked out it will be moved to the Square Kilometre Array (SKA) site in the Northern Cape where the full survey will be done.
C-BASS is the latest in a long line of efforts to measure the properties of the oldest light in the Universe, the Cosmic Microwave Background (CMB). Tiny variations in brightness and polarisation of the CMB contain information about the conditions present in the early universe, only a few hundred thousand years after the Big Bang. The universe then was nothing like what we see today – there were no planets, stars or galaxies. However, the seeds of all the structures we see today can be seen in the CMB. Understanding these beginnings provides us with an insight into the universe today.
The recently released Planck results focus on CMB brightness, providing more accurate measurements than previous experiments.
Possibly the most exciting data is yet to come in a year or so when Planck releases its CMB polarisation measurements. These measurements may tell us about the physics of the universe when it was a tiny fraction of a second old.
In order to do this the Planck mission is faced with a large problem – the Milky Way. Over a large fraction of the sky any CMB polarisation signal is completely obscured by radiation from Earth’s own galaxy, the Milky Way. In order to successfully measure the CMB polarisation it is essential for astronomers to understand this foreground radiation.
This is where C-BASS will play a key role. Leader of the C-BASS team at Oxford University, Dr Angela Taylor, explained: “C-BASS acts like an extra frequency channel for Planck, hugely extending the range of radio wavelengths we have available. C-BASS will measure the polarisation signal from our galaxy with great accuracy and will hugely improve our ability to remove the galactic signal from Planck data, revealing the true CMB signal.”
Both the C-BASS telescopes, north and south, were originally built to communicate with satellites and have been adapted by the C-BASS team to look into deep space. This re-purposing of communications technology for science is also the idea behind the African VLBI Network (AVN) project, which plans to convert redundant satellite dishes across the continent in to a giant radio telescope network. A twin of the southern C-BASS dish is currently being refitted to ship to Mozambique as part of the AVN.
C-BASS also highlights the growing collaboration between South Africa and the rest of the world in radio astronomy.
South African astronomer Justin Jonas was a member of the group which conceived the project and two South African students have studied for doctorates at Oxford University where the radio receivers were designed and built. The southern receiver also uses digital hardware developed by the MeerKAT team in Cape Town.
C-BASS is a collaborative project between Hartebeesthoek Radio Astronomy Observatory, supported by the Square Kilometre Array (SKA) project in South Africa, the Universities of Oxford and Manchester in the UK, the California Institute of Technology (supported by the National Science Foundation) in the US and the King Abdul-Aziz City for Science and Technology (KACST) in Saudi Arabia.
C-BASS observes at a wavelength of 6 cm (a frequency of 5 gigahertz), compared to the longest wavelength of Planck of 1 cm (30 gigahertz). The southern telescope is a 7.6m dish donated to the project by Telkom. The northern telescope is a 6.1m dish donated by Nasa’s Jet Propulsion Laboratory.