Almost a hundred years since Albert Einstein proposed gravitational waves as part of his general theory of relatively, we are still no closer to finding them. Was Einstein wrong?
Astronomers have spent 11 years searching for evidence of the waves, which are ripples in the fabric of space-time, using high-precision telescopes. But rather than detecting the background 'rumbles' created as galaxies merge, they heard nothing. Scientists view the universe as being made up of a 'fabric of space-time' and this corresponds to Einstein's General Theory of Relativity, published in 1916. Objects in the universe bend this fabric, and more massive objects bend it more, with gravitational waves being considered as 'ripples' in this fabric.
They can be produced, for instance, when black holes orbit each other or by the merging of galaxies. Galaxies grow by merging and every large one is thought to have a supermassive black hole at its heart. When two galaxies merge, the black holes are drawn together and form an orbiting pair. At this point, Einstein's theory is expected to take hold, with the pair predicted to succumb to a death spiral sending ripples known as gravitational waves through space-time, the very fabric of the universe.
Although Einstein's general theory of relativity has been confirmed in every other respect, gravitational waves remain its only unconfirmed prediction. For scientists, gravitational waves are significant because it is believed they carry information allowing them to look back into the very beginnings of the universe. Gravitational waves, in particular, are thought to have been produced during the Big Bang. Although there is strong circumstantial evidence for their existence, they have not yet been directly detected.
The current research was led by Dr Ryan Shannon from CSIRO and the International Centre for Radio Astronomy Research, and is published in the journal Science. To look for the waves, Dr Shannon's team used the high-precision Parkes telescope to monitor a set of 'millisecond pulsars'. These small stars produce highly regular trains of radio pulses and act like clocks in space. The scientists recorded the arrival times of the pulsar signals to an accuracy of ten billionths of a second.
Einstein's theory states that a gravitational wave passing between Earth and a millisecond pulsar would squeeze and stretch space, changing the distance between them by about 33ft (10 metres) - a tiny fraction of the pulsar's distance from Earth. This would change the time that the pulsar's signals arrive on Earth, albeit very slightly. The scientists studied such pulsars for 11 years, which should have been long enough to reveal gravitational waves, but there were no signs of them.
'We heard nothing. Not even a whimper,' Dr Shannon said. 'It seems to be all quiet on the cosmic front - at least for the kind of waves we are looking for.' This doesn't mean that Einstein's theory about the waves has been proved wrong, instead the scientists suspect it is because black holes merge very fast. As a result they spend little time spiralling together, and therefore little time generating gravitational waves.
'There could be gas surrounding the black holes that creates friction and carries away their energy, letting them come to the clinch quite quickly,' added team member Dr Paul Lasky, a postdoctoral research fellow at Monash University. Whatever the explanation, it means that if astronomers want to detect gravitational waves by timing pulsars they'll have to record them for many more years.
'There might also be an advantage in going to a higher frequency,' said Dr Lindley Lentati of the University of Cambridge. For example, higher-frequency gravitational waves are said to be generated when neutron stars merge. Astronomers will also gain an advantage with the highly sensitive Square Kilometre Array telescope, set to start construction in 2018. Not finding gravitational waves through pulsar timing has no implications for ground-based gravitational wave detectors such as Laser Interferometer Gravitational-Wave Observatory (LIGO), which began its own observations of the Universe last week.