University of Minnesota
School of Physics & Astronomy

Spotlight

Searching for Gravitational Waves

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Tanner Prestegard is part of the search for gravitational waves with Vuk Mandic. He is a graduate student working on the Laser Infermeter Gravitational Observatory (LIGO) experiment. Prestegard’s role is primarily in data analysis-- looking through data and trying to pick out anything that is an actual gravitational wave.

Prestegard says the experiment has a lot of problems with detector noise. Part of his job has been to create a search algorithm that will filter out that noise. “We are looking for a type of signal that has not been looked for before. People have looked for short signals lasting one second or less, or things that are persistent. We are looking for signals between 10 to few hundred seconds long.” This type of signal is called a long transient signal. Prestegard says they are looking for this signal length because these longer signals are not as well understood. “There are a lot of good models for sub-second signals, whereas longer signals not as well understood.” Prestegard says that If you have a good idea what you are looking for with a model you can search in a more specific way. “We have to search more generally because it’s an unmodeled search.” Mandic’s group has tried to tune their search to what theorists say they should expect to find.
Gravitational waves were predicted by Einstein’s theory of relativity, but physicists have yet to actually detect them. These waves are produced when a massive object moves in a way that is not perfectly spherical. If you imagine a sphere with a big mountain on one side, spinning it would be moving asymetrically. Massive objects have a similar effect on gravitational waves: if you have a gravitational wave and it’s passing through this circle, it will be stretched, and oscillate between compressed and stretched ellipse. “That motivates the design for our detector- it’s just a big L shape. When a gravitational wave passes through the detector, it will shrink on one side depending on the shape of the wave.” Prestegard says that they think the longer signal might be created by a black hole with an accretion disc (a bunch of stuff orbiting the black hole). The accretion disc forms clumps that are tied to black hole through magnetic field. Because it’s clumpy it’s not symmetric and it’s causing the gravitational waves to be emitted.
Prestegard says that gravitational waves that would be detectable are created by a massive object, between two and ten times more massive than our sun. Objects such as massive stars, neutron stars and black holes can create detectable gravitational waves. If an object is too massive, say a galaxy, the gravitational wave will probably not be detectable because galaxies rotate so slowly that their frequency would be very low.
The next generation of detectors, Advanced LIGO, will come on line in 2015. The new detector will have increased power in the lasers, use new mirrors and mirror codings, and have better seismic isolation. The overall result will be a factor of ten improvement in sensitivity. The new LIGO can detect down to 10 hertz, while previously 40 hertz was the lower limit. While physicists have not yet detected gravitational waves, they continue to set new limits for their detection and have great hope that Advance LIGO will be able to detect the waves.