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Posted by PB on February 11, 2016
On September 14, 2015 at 09:50:45 UTC, nearly 100 years after Albert Einstein finalized his theory of general relativity, an extraordinary discovery was made by the two detectors of the Laser Interferometer Gravitational-wave Observatory (LIGO). Over one billion years ago, two orbiting black holes violently merged and radiated three solar masses of gravitational-wave energy. This energy propagated outward as ripples in spacetime moving at the speed of light. These ripples stretch and squeeze spacetime in an alternating pattern as they pass by. Only three minutes after the signal passed by Earth, we had an alert of the first ever direct detection of gravitational waves. We tagged this signal as GW150914. It was also the first observation of a binary black hole merger.
The LIGO detectors had been undergoing upgrades for several years leading up to the discovery of GW150914. The initial LIGO detectors ran from 2002-2010 before being upgraded into “Advanced LIGO.” Advanced LIGO had only been collecting data for a few days when this first signal reached us. The detectors are designed as modified Michelson interferometers with two orthogonal arms 4 km long. Passing gravitational waves alter the arm lengths so that we can measure a gravitational-wave strain from the differences in length. The peak strain measured from GW150914 was 10-21, corresponding to a displacement of the interferometer arms of +/- 0.002 fermi, less than the radius of a proton! The detectors are isolated from seismic noise above 10 Hz and minimize the impact of photon shot noise at high frequencies. GW150914 swept upwards in frequency from 35 Hz to 150 Hz, precisely in the most sensitive frequency band of the detectors.
GW150914 arrived first at the LIGO Livingston detector and then 6.9 ms later at the LIGO Hanford detector. The position of the source was determined to be in a 600 square degree region, located mostly in the Southern Hemisphere. We quickly sent alerts to astronomers, who searched the area for potential electromagnetic signals associated with GW150914, though these are unlikely in the case of binary black hole mergers.
Several independent gravitational-wave search algorithms confirmed the discovery. Within three minutes, the signal had been detected by a generic transient search, which makes minimal assumptions about the structure of gravitational waveforms. Two weeks later, several deeper searches confirmed the detection. Two of these deeper searches specifically targeted gravitational-wave emission from binary systems with individual masses 1-99 solar masses, total mass less than 100 solar masses, and dimensionless spins up to 0.99. These searches found the significance of GW150914 to be greater than 5.1 sigma. Put another way, we expect an event like GW150914 to happen simply by chance---without a real gravitational wave present---only once every 200,000 years.
Follow-up investigations using fits to numerical simulations of binary black hole mergers found a primary black hole mass of 36 solar masses, a secondary black hole mass of 29 solar masses, a final black hole mass of 62 solar masses, a final black hole spin of 0.67, and a luminosity distance of 410 Mpc. Additionally, the signal is found to be consistent with the predictions of general relativity in the strong-field gravity regime. Finally, this observation allows us to constrain the rate of stellar-mass binary black hole mergers in the local universe to be in the range 2-400 Gpc-3 yr-1.
University of Wisconsin-Milwaukee scientists played an important part in this discovery. At the time of collection, the data were calibrated and made ready for analysis by computer code that was largely written by UWM faculty, postdocs, and students. UWM scientists also played a vital role in writing and running search pipelines. Finally, UWM hosts and provides critical computing infrastructure for the LIGO Scientific Collaboration.
A number of companion papers detailing GW150914 are available at https://dcc.ligo.org/P150914/public. Additional detailed information on GW150914 can be found at http://dx.doi.org/10.7935/K5MW2F23. GW150914 was found in a subset of the data from the first observational period of Advanced LIGO (O1). Stay tuned for the rest of the results from the analyses of O1!