Pulsars are rapidly rotating neutron stars. By timing the arrival time of signals from pulsars, astronomers using the Parkes radio telescope hope to make the first direct detection of gravitational waves.
The Parkes Pulsar Timing Array team includes astronomers from Australia and USA. The project will observe about twenty pulsars regularly for five years.
What are gravity waves?
Einstein’s theory of general relativity predicts the existence of gravity waves. Whenever a massive body such as a star accelerates the theory predicts that gravitational waves, disturbances in the fabric of spacetime, are produced.
These waves travel at the speed of light but are different from electromagnetic radiation such as visible light and radio waves. We cannot see them but may be able to detect them by their effect on other objects.
Gravity waves are extremely weak and the only sources likely to be detectable are collapsing or orbiting stars, orbiting black holes or events in the early Universe.
Recent observations have convinced most astronomers that galaxies such us our own Milky Way have supermassive black holes at their centre. These bizarre objects have a mass of a million to a billion times that of our Sun.
When two large galaxies collide and merge, their central supermassive black holes may get trapped in close orbit around each other. The gravitational waves produced by such an event are of the right frequency to be detectable with the Pulsar Timing Array project.
How do we find them?
Several teams around the world are looking for gravitational waves. Most use a method called laser interferometry involving laser beams travelling through vacuum tubes over several kilometres.
The Parkes Pulsar Timing Array project adopts a different but complementary approach based on observations of millisecond pulsars. These neutron stars are the collapsed cores of massive stars spinning hundreds of times per second.
As they rotate these pulsars emit an intense beam or radiation, much like the beam from a lighthouse. Radio telescopes such as Parkes detect these radio signals as regular pulses.
Millisecond pulsars are incredibly precise clocks. Astronomers can predict the arrival time of an individual pulse to less than a millionth of a second over many years.
If a gravity wave passes over the Earth the interval between the observed pulses will be shortened then lengthened. With enough pulsars being observed, local effects can be compensated for and a distinctive pattern due to the gravitational wave observed.
The Parkes Pulsar Timing Array project currently observes about 20 millisecond pulsars at least once each week or so for five years. Each pulsar is observed at three frequencies: 630, 1400 and 3100 MHz, providing a wealth of data.
Astronomers using the Parkes radio telescope have already discovered over two-thirds of the 1,7 00 pulsars now known.
Problems to be solved
The team are building new hardware that can measure the arrival times of pulses to 100 nanoseconds and writing new software to search for the gravitational wave signature in the data. Theoretical calculations help them determine what they expect to detect and how they can prove that it is a gravitational wave.
The precision required for timing the arrival of pulsar signals poses problems for the astronomers. The millisecond pulsars themselves are probably more stable than the best atomic clocks on Earth.
Digital television broadcasts and other radio signals in the same frequency range as the timing array observations pose a real problem for the astronomers. This so-called radio frequency interference requires sophisticated software and hardware tools to remove it before the final data analysis.
What do we learn?
Observational evidence for gravitational waves would test general relativity and provide a new way of studying the Universe. Pulsar timing observations have already ruled out some cosmic string models for the early Universe.
By-products of these observations are stringent checks of our current time standards and a more accurate model for the Solar System. Astronomers are also using the data to find out more about the interstellar medium, the tenuous gas found between the stars in our Galaxy.
Direct detection of gravitational waves would open up in a whole new field of gravitational wave astronomy.
About our scientists
The Pulsar Timing Array project is led by Dr Dick Manchester, a world expert on pulsars and a Federation Fellow at CSIRO’s ATNF. His team includes other CSIRO scientists:
Dr George Hobbs
Dr Russell Edwards
Mr John Sarkissian
Other collaborators include:
Professor Matthew Bailes from the Centre for Astrophysics and Supercomputing at the Swinburne University of Technology in Victoria
Professor Fredrick Jenet from the Center for Gravitational Wave Astronomy, University of Texas at Brownsville in the USA
More information
Australia Telescope National Facility
