Across Europe, some 10 000 antennas stand courtly, like squat flag poles. They may not look like much, but they are in a sense an incredibly powerful time machine.
Known collectively as the Low-Frequency Array (LOFAR), the antennas are receiving radio signals that have travelled billions of years to get here, from the depths of the cosmos. That means they are looking billions of years into the past, when the universe was almost featureless – and when planets, stars or galaxies didn’t exist.
Because light travels at a finite speed, all telescopes look into the past to some extent. But astrophysicist Professor Dominik Schwarz of Bielefeld University in Germany, who helped to plan the telescope, said the ‘cosmic dawn cannot be seen with any other instrument’.
The LOFAR antennas are concentrated in the Netherlands – indeed they are run by the Netherlands Institute for Radio Astronomy, and are mostly funded by the Dutch government. But the EU and several other European countries have also backed the project to host some of the instrumentation – Prof. Schwarz’s group operates a LOFAR station in Norderstedt, Germany, which is the size of a football pitch and contains nearly 200 LOFAR antennas.
LOFAR is not a telescope in the familiar sense. It is not a big tube with glass lenses and mirrors, nor is it one of those latticed cones that look like huge satellite dishes, pointed up to the heavens. Instead, it is an array of what are, essentially, metal sticks. These are omnidirectional antennas, which do not point in any one direction but receive radio signals from anywhere and everywhere.
By combining the signals coming from individual antennas in different ways, however, it is possible to point the antennas computationally. That requires some clever mathematics and a hefty supercomputer – the latter is situated at the University of Groningen in the Netherlands.
The radio signals of interest are emitted by hydrogen, the most abundant element in the universe. Given its predominance, an observation of hydrogen is an observation of matter more generally.
Hydrogen emits radio waves with a wavelength of 21 centimetres. But the universe is expanding, meaning that the radio waves of distant hydrogen atoms are stretched en route to us.
By focusing on the most stretched radio waves – up to two metres in wavelength – LOFAR can focus on the most distant hydrogen and, therefore, the oldest regions of the cosmos.
Of course, it is not quite that simple. The sparseness of the LOFAR antennas means that they cannot give a complete map of the heavens by themselves.
In their calculations, therefore, the astronomers must make certain assumptions about what the sky looks like. For instance, they know that it is mostly black, that the patches of light are not totally random, and that objects such as galaxies have familiar shapes like spirals and ellipses. ‘This cosmological signal will be a really good probe of our universe’s evolution.’
Dr Jérôme Bobin, CEA Saclay, France
‘This cosmological signal will be a really good probe of our universe’s evolution.’
To an extent, plugging these assumptions into the maths allows a complete image to be formed. But it is not perfect, as the actual appearances of certain astronomical phenomena do not have a simple relationship with the data recordings.
Astrophysicist Dr Jérôme Bobin and his colleagues are hoping to solve this problem, and help give a more accurate picture of the cosmic dawn, by developing new algorithms to interpret radio data. Their LENA project, backed by the EU’s European Research Council, will draw on advances in various aspects of applied mathematics, such as machine learning.
Dr Bobin believes the work will be particularly helpful for the study of the oldest, most distant phenomena. ‘This cosmological signal will be a really good probe of our universe’s evolution, precisely at a barely known period called the dark ages,’ he said.
LOFAR became operational in 2012, and the hope is that it will help scientists understand how the universe’s first structures began to form.
One of the first observations was of a giant elliptical galaxy, Messier 87, located in the centre of a cluster of galaxies in the Virgo constellation. The observations revealed that the super-massive black hole at the galactic centre is continuously sucking in nearby matter and firing it out to create a galaxy-sized bubble of plasma.
The study of Messier 87 was made possible thanks to RadioNet3, an EU project bringing together research infrastructures that gives astronomers from non-partner states free access to LOFAR and other radio telescopes.
‘It allows us to pull together all the radio telescopes we have, and offer access under one umbrella,’ said Dr Izabela Rottmann, an astrophysicist at the Max Planck Institute for Radio Astronomy in Bonn, Germany, and RadioNet3’s project manager.
Providing EU-wide access to world-class radio telescopes is key to maintaining the quality of European astronomy, says Dr Rottmann, as European funds are not enough to build one on their own. And there is nothing else quite like LOFAR.
Scientists hope that the telescope could also boost the inventory of known galaxies by several million, and probe the universe’s magnetism, which is thought to affect how galaxies have evolved over time.
The observations could also tackle very specific issues. For example, by timing the regular radio-bursts of pulsars – which are thought to be dense, rapidly rotating stars – LOFAR could accurately test that pillar of modern physics, Einstein’s general theory of relativity.
A brief history of the universe
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