The European Pulsar Timing Array (EPTA) is a European collaboration to combine five 100-m class radio-telescopes to observe an array of pulsars with the specific goal of detecting gravitational waves. It competes with the Australian and American pulsar timing arrays, the Parkes Pulsar Timing Array and the North American Nanohertz Observatory for Gravitational Waves.
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Pulsars are rapidly rotating, highly magnetized neutron stars that emit radio waves from their magnetic poles that are observed as pulses. Due to the extremely high density of neutron stars, their rotation periods are very stable, hence the observed arrival time of the pulses are highly regular. These arrival times are called TOA's (time of arrival) and can be used to perform high-precision timing experiments.
The stability of the TOA's from most pulsars is limited due to the presence of red noise, also called "timing noise"[1]. However, there is a special class of pulsars, called millisecond pulsars (MSP), that are shown to suffer from little or no timing noise. By keeping track of the TOA's of different MSP's over the sky allows for a high-precision timing experiment to detect one of the holy grails of physics: gravitational waves.
Gravitational waves (GW) are small disturbances in space-time, caused by the motion of masses. These waves are so weak that only the strongest waves, caused by rapid motion of dense stars or black-holes, have a chance of being detected. A pulsar timing array (PTA) uses an array of MSP's as the endpoints of a galaxy-scale GW detector. It is sensitive to GW's with a frequency in the nano-Hertz regime, which corresponds to the regime of the stochastic GW background caused by the coalescence of super-massive black holes in the early universe. This makes PTA's complementary to other GW detectors such as LIGO, VIRGO and LISA.
The EPTA uses five European telescopes. These are the Westerbork Synthesis Radio Telescope, the Effelsberg Radio Telescope, the Lovell Telescope, the Nançay Radio Telescope and the Sardinia Radio Telescope.
Recently the EPTA has made a giant "leap" forward thanks to European funding of the Large European Array for Pulsars (LEAP). This project involves coherently combining the five EPTA-telescopes to make the equivalent of a fully steerable 194-m dish. This will improve the accuracy with which the TOA's can be measured by an order of magnitude, possibly leading to a first detection of GW's around 2015 (citation needed).