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13. Searching for the unknown

Supernova explosions, gamma ray bursts and the merger of a binary system are all expected to be sources of gravitational waves. Due to the difficulty in predicting the mass distributions of these sources we are not able to produce accurate predictions for the gravitational wave emission of these sources.

Therefore, as well as developing techniques to search for known gravitational wave signals (such as the inspiral "chirp" emitted just prior to the merger of a binary system) in noisy detector data we also develop techniques to search for unmodelled signals from unexpected or complicated sources.

A real gravitational wave signal will cause similar a response in each gravitational wave detector. Conversely, each detector will be afflicted by its own local noise sources which will not, in general affect the other detectors. By examining the data gathered by different gravitational wave detectors for common features we can indicate the presence of a gravitational wave without us having to know the form of the gravitational wave in advance. We are able to detect gravitational waves from exotic sources we have not even thought of!

Due to the detector's different location on the surface of the Earth we know that gravitational wave signals from a given position on the sky will reach each detector at a slightly different time (Remember that gravitational waves travel at the speed of light meaning that a given wave front will pass through the earth in around 1/16th of a second - the typical time delay between detectors is of the order of milliseconds). By taking into account the direction-dependent sensitivity of each detector we can time-delay and sum the data from each detector in order to search for gravitational waves from different regions of the sky.

Members of the Cardiff Gravity group search LIGO data for the gravitational waves emitted by gamma ray bursts. The sky position of these events can be determined through the use of electromagnetic observations enabling us to search for gravitational wave signals from the direction of the gamma ray burst. Although no gravitational waves have been directly observed the lack of detection has been used to place constraints on the distance of gamma ray bursts from Earth. (Ref GRB070201 paper).