The construction of the Square Kilometre Array – the largest radio telescope ever conceived – has begun. With a vast number of antennas located across two continents, Australia and South Africa, it will provide unprecedented capability to scrutinise the faintest radio signals from the deepest regions of the cosmos.
In astronomy, a bigger instrument usually, though not always, means a better instrument. The Square Kilometre Array is as big as it gets for radio astronomy. However, some astronomers will turn it into an even bigger telescope – effectively the size of thousands of light years – tuned to observe a radically different kind of radiation: ultra-low frequency gravitational waves.
Gravitational waves are ripples of space-time that propagate at the speed of light throughout the universe. Black holes that are accelerated to speeds close to the speed of light, for example when two of these objects pair up and orbit around each other in a tight orbit, are ideal sources. Predicted by Einstein in 1916 as a fundamental consequence of his theory of general relativity, gravitational radiation was essentially relegated to oblivion for a century, but it made the front page of pretty much every newspaper in the world in February 2016.
If we could observe gravitational waves from these cosmic dances we could build a radically new map of how some of the fundamental building blocks of the Universe evolve, which is surrounded by mystery. We need a new kind of instrument to do this, because the radiation from massive black holes is fundamentally unobservable from a detector on Earth as its frequency is too low.Professor Alberto Vecchio, Director, Institute of Gravitational Wave Astronomy - University of Birmingham
LIGO (the Laser Interferometer Gravitational-wave Observatory) had just reported the very first observation of gravitational waves, a discovery to which researchers at the University of Birmingham made many contributions. The source of the faint signal detected by LIGO was the merger of two black holes, each of which about 30 times the mass of the Sun. This marked the beginning of what is now known as gravitational-wave astronomy, mapping the cosmos using not light, but gravitational radiation. Since this first detection, there have been many more, almost a hundred by now, which are revolutionising our understanding of many phenomena in astrophysics and fundamental physics, and we’re just at the beginning.
We now want to use gravitational waves to map the populations of the million-to-billion solar mass black holes that live at the centre of most galaxies, like our own. There is evidence that massive black holes are already at the core of galaxies early on in cosmic history. We also know that galaxies interact and merge frequently during the Universe’s lifetime, so binaries of massive black holes should also form, dissipate energy through gravitational waves and eventually merge.
If we could observe gravitational waves from these cosmic dances we could build a radically new map of how some of the fundamental building blocks of the Universe evolve, which is surrounded by mystery. We need a new kind of instrument to do this, because the radiation from massive black holes is fundamentally unobservable from a detector on Earth as its frequency is too low. We need to go into space, but even there it’s not quite so simple. The recipe for such an instrument has been known for over forty years, and goes as follows: build a large number of very stable clocks; put them into space and far from Earth, where there are all sorts of gravitational disturbances; engineer the clocks to continuously send to Earth a very sharp pulse of light every time they tick; measure the time at which each pulse arrives at the Earth. As light travels through space and time, the presence of gravitational waves affects its path in a characteristic way: some pulses will arrive a little (by a billionth of a second or thereabout) later, some a little earlier. The difference between the expected - with no gravitational waves - and recorded arrival time of these pulses carries the fingerprints of gravitational waves. Voila’: you’ve just built a telescope for ultra-low frequency gravitational radiation, which no one has ever observed.
A rather obvious objection to this plan is that it does not seem feasible at all. However, it just so happens that every galaxy, including our own, assembles many very stable ‘astronomical clocks’. They are called radio pulsars. They are stars with a mass comparable to the mass of the Sun and a radius of just about 10 km - the size of a city. They are constituted almost entirely of neutrons, and rotate up to a thousand times per second. Their enormous magnetic fields produce a beam of radio waves that hit the Earth, if it happens to be along the beam path, once every rotation. They are like cosmic lighthouses: every rotation is a tick of the clock, and it’s very stable, although not perfectly so.
With existing radio telescopes, we have discovered over two thousand of these radio pulsars, of which about a hundred we can ‘time’ precisely. This is called a ‘pulsar timing array’: an ultra-low frequency gravitational-wave telescope. The University of Birmingham is part of the European Pulsar Timing Array (EPTA), which together with the North American (NANOGrav) and Australian (PPTA) counterparts, forms the International Pulsar Timing Array. Some intriguing features in the data have recently emerged. Maybe we just don’t understand the clocks well enough. Maybe there is some gravitational wave signal starting to creep up from the noise. If the latter is true, what are secrets about the Universe is it holding? To find an answer, we need more pulsars, better timing precision and better sensitivity: we’re gonna need a bigger telescope, the Square Kilometre Array.