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The Max Planck Institute for Gravitational Physics (Albert Einstein Institute (AEI)) is an institute of the Max Planck Society with sub-institutes in Potsdam-Golm (outside Berlin) and Hannover, where it is closely related to the Leibniz Universität. Since its foundation in 1995, the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) has established itself as a leading international research center. The research program is pursued in five divisions and several independent research groups cover the entire spectrum of gravitational physics: from the giant dimensions of the Universe to the tiny scales of strings. The AEI is the only institute in the world that brings together all of these key research fields. Three of its five divisions are part of the LIGO Scientific Collaboration and played a major role in realizing the first direct detection of gravitational waves.

The Institute for Gravitational Physics of Leibniz Universität Hannover is co-located with the AEI Hannover. Under one roof, scientists from both institutions collaborate closely on all aspects of gravitational wave research. More than 50 PhD students are working towards their doctoral degree at Leibniz Universität Hannover in the joint International Max Planck Research School (IMPRS) on Gravitational Wave Astronomy.

Gravitational wave research in the Max Planck Society has a long history and goes back to the very beginning of the field in the 1960s.  It was the Max Planck group that conducted coincidence experiments between resonant mass detectors to disprove the early claims of gravitational wave detection in the 60s. The group then turned to laser interferometry and built the first serious prototypes of laser interferometric gravitational wave detectors, developing and/or demonstrating most of the key concepts that are now an integral part of the large observatories, among them optical mode cleaners, stray light suppression, power recycling, and later in collaboration with Leibniz Universität Hannover, dual recycling, resonant sideband extraction, thermally adaptive optics, multi-stage monolithic suspensions, and high-power stable lasers.

Gravitational waves are an important prediction of Einstein's theory of general relativity. Accelerated motions of large masses create ripples in space-time, which lead to tiny relative distance changes between far-away objects. Even gravitational waves emitted by astrophysical sources, like stellar explosions or merging black holes, change the length of a one-kilometer measurement distance on Earth by only one thousandth of the diameter of a proton (10-18 meters). Only now have the detectors reached a level of sensitivity at which they can measure gravitational waves. The observation of the until now dark “Gravitational Universe” ushers in a new era in astronomy. The interferometric gravitational-wave detectors such as LIGO (in the USA), GEO600 (in Germany), and Virgo (in Italy), as well as planned detectors in Japan and India collaborate closely. A low-frequency gravitational wave detector in space (LISA) is under preparation by ESA and NASA and scientists from Leibniz Universität Hannover and AEI play a leading role.

The signal reported now is referred to as GW150914 since it arrived at Earth on September 14, 2015 at 09:40:45 UTC. It was detected by both LIGO instruments in Hanford and Livingston. It was observed for about 0.2 seconds during which the signal increased both in frequency and amplitude. Over 0.2 seconds its frequency grew from 35 Hz to 250 Hz and it had a peak amplitude (gravitational-wave strain) of 10-21.

By looking at the time of arrival of the signals—the detector in Livingston recorded the event 7 milliseconds before the detector in Hanford—scientists can say that the source was located in the Southern Hemisphere.

The signal matches the predictions of general relativity for those of an inspiral and merger of two black holes with masses of 36 and 29 solar masses, respectively. The black hole resulting from the merger has a mass about 62 times that of our Sun. About 3 times the mass of the sun was converted into gravitational waves in a fraction of a second—with a peak power output about 50 times that of the whole visible Universe. From the observations a distance of about 410 Megaparsecs (1.3 billion light years) to the black hole system was inferred.

By characterising the random noise fluctuations of the Advanced LIGO detectors the researchers estimate the statistical significance of the signal to be 5.1 standard deviations. This means that such signals in 16 days of observation happen less then once in 200,000 years through coincidence of random detector fluctuations.

Advanced LIGO consists of interferometric gravitational-wave detectors at two sites, one in Hanford (Washington State, USA) and one in Livingston (Louisiana, USA).  At each site, lasers beams are bounced down four kilometer long L-shaped vacuum tubes to very precisely monitor the distance between mirrors at each end. According to Einstein’s theory, the distance between the mirrors will change by an infinitesimal amount when gravitational waves pass by the detector. A change in the lengths of the arms smaller than one-ten-thousandth the diameter of a proton (10-19 meter) can be detected.

Independent and widely separated observatories are necessary to verify that the signals come from space and also to determine the direction to the gravitational wave source.

Advanced LIGO concluded its first coordinated three-month data-taking run on January 12, 2016. During that run the sensitivity was 3 to 5 times higher than that of initial LIGO. At design sensitivity, a ten-fold increase in sensitivity over initial LIGO is expected.

GEO600 is an interferometric gravitational-wave detector with 600 meter long laser beam tubes, located near Hannover, Germany. It is designed and operated by scientists from the Max Planck Institute for Gravitational Physics and the Institute for Gravitational Physics at Leibniz Universität Hannover, along with partners in the United Kingdom. GEO600 is part of a worldwide network of gravitational wave detectors and at the moment the only detector taking data almost continuously. GEO600 also is a test bed for advanced detector technologies, such as non-classical (squeezed) light, signal and power recycling, and monolithic suspensions.

Atlas is a large computer cluster at the Albert Einstein Institute in Hannover with enormous computing capacities. Atlas consists of more than 14,000 CPU cores and 250,000 GPU cores, making it the largest computer cluster worldwide dedicated to gravitational-wave data analysis. Atlas is primarily supported (investment and operations) by the Max Planck Society but also receives operational support from Leibniz Universität Hannover.

Funding information
LIGO operations are funded by the US National Science Foundation (NSF), and the facility is operated by Caltech and MIT. The LIGO upgrade was funded by the NSF with substantial financial and technical contributions from the German Max Planck Society, the UK’s Science and Technology Facilities Council (STFC), and the Australian Research Council (ARC).

GEO600 is funded by the Federal Ministry of Education and Research, the State of Lower Saxony, the Max Planck Society, the Science and Technology Facilities Council (STFC), and the VolkswagenStiftung.