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Gravitational Waves: a New Era in Astrophysics Has Begun

March 14, 2017

20170314_170535Erik Katsavounidis gave a much awaited talk after the afternoon coffee break at Neutel today. He started by introducing the topic of cosmic explosions. Gravitational waves arise as distortions that are a consequence of Einstein’s general relativity, whenever matter moves or changes its configuration. They distort space itself, stretching one direction and squeezing the perpendicular in the first half period and vice-versa in the second half.

To make a big quadrupole derivative to give a measurable strain, you need large masses moving fast. For instance, two neutron stars orbiting one another in a tight orbit. The deformation is proportional to the size of the object. The strain is the fractional deformation in space, and is inversely proportional to the distance in space.


One can detect this with orthogonal arms that vary their lenght in different ways as they interact with the gravitational wave. The idea is to use length changes with laser and an interference pattern at the anti-simmetric port.

LIGO has one detector in Livingston, Louisiana, and one 3000 km to the north-west in Hanford, Washington. The data is noisy, and its amount is very large. The instrument is a transducer of gravitational waves that gives optical power and relative motion of the mirrors as a time series.

To reduce the noise, a painstaking work to have better seismic isolation, suspension of the masses, and heavy mirrors that are insensitive to photon pressure from high power. And then LIGO uses a 200W Nd:YAG laser to get a sensitive interference pattern.

LIGO defines the sensitivity as the maximum distance at which a neutron star merger can be observed, and this is now 50 megaparsecs.

Erik showed the famous plot of frequency versus time for GW150914 referring to it as a “glissando”. What he defined as a “money plot” was instead the measure of the significance of the signal, which he then went on to explain in detail. He then described the measurement of the masses coalescing in the GW150914 event, which were assessed at 36 and 29 solar masses, with 20% uncertainties.

A second observing run has started on Nov 30 2016. There were only 12 days of coincidence data for the two detectors. They want to build a network of detectors by 2020. One in Hannover, one in Pisa (advanced VIRGO), and one in India and one in Japan. With three active detectors observing a wave, there are significant uncertainties in the provenance; with a fourth detector these are strongly reduced.

To summarize, the direct detection of GWs has been one of the most challenging measurements ever undertaken. The detection has demonstrated the existence of stellar mass black holes. A new era in astrophysics has begun. Gravitational waves will provide a completely new way of observing the Universe.




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