University of Birmingham Sensitivity of the Advanced LIGO detectors at the beginning of gravitational wave astronomy

The Laser Interferometer Gravitational Wave Observatory (LIGO) consists of two widely sepa-rated 4 km laser interferometers designed to detect gravitational waves from distant astrophysical sources in the frequency range from 10 Hz to 10 kHz. The first observation run of the Advanced LIGO detectors started in September 2015 and ended in January 2016. A strain sensitivity of better than 10 − 23 / √ Hz was achieved around 100 Hz. Understanding both the fundamental and the technical noise sources was critical for increasing the astrophsyical strain sensitivity. The average distance at which coalescing binary black hole systems with individual masses of 30 M (cid:12) could be detected above a signal-to-noise ratio (SNR) of 8 was 1.3 Gpc, and the range for binary neutron star inspirals was about 75 Mpc. With respect to the initial detectors, the observable volume of the Universe increased by a factor 69 and 43, respectively. These improvements helped Advanced LIGO to detect the gravitational wave signal from the binary black hole coalescence, known as GW150914. local sensors on the individual suspensions. Quantum noise in the signal recycling cavity length is signiticantly affected by the differential arm offset below 10 Hz. In addition to coupling to the gravitational wave channel, auxiliary degrees of freedom also couple to each other. For example, beam splitter motion above 10 Hz is caused by the Michelson control loop and dominates the power and signal recycling cavity length fluctuations in the frequency range 10-50 Hz.