In September 2015 the Advanced LIGO detectors inaugurated the era of gravitational-wave astrophysics with the observation of GW150914. In this proceeding, I will describe the upgrade that made the detection possible, review the binary black hole observations of the ﬁrst aLIGO observing run, and discuss plans for the future

Hoak, D.

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DOI 10.1393/ncc/i2017-17119-1Colloquia: SciNeGHE 2016IL NUOVO CIMENTO 40 C (2017) 119Advanced LIGO: Status and prospectsD. Hoak on behalf of the LIGO Scientific CollaborationEuropean Gravitational Observatory - Cascina (PI), Italyreceived 14 June 2017Summary. — In September 2015 the Advanced LIGO detectors inaugurated theera of gravitational-wave astrophysics with the observation of GW150914. In thisproceeding, I will describe the upgrade that made the detection possible, review thebinary black hole observations of the first aLIGO observing run, and discuss plansfor the future.1. – IntroductionOn September 14, 2015, the Advanced LIGO detectors observed the coalescence of apair of black holes [1]. This observation represents the first direct detection of gravita-tional waves and the first observation of a binary black hole merger, and demonstratesthe existence of binary stellar-mass black hole systems. The event was detected follow-ing an ambitious upgrade of the initial LIGO detectors, which will ultimately provideten times the sensitivity of the initial detectors and three times the sensitivity of theinstruments at the time of the first detection.2. – The instrumentsThe Advanced LIGO interferometers [2] are twin dual-recycled Fabry-Perot Michel-son interferometers, designed to observe gravitational waves from astrophysical sources.These second-generation instruments are designed to realize a factor-of-ten improvementin sensitivity beyond the initial LIGO interferometers. Ultimately, the global networkof terrestrial gravitational-wave detectors — comprising the two current LIGO sites, theVirgo detector in Italy, KAGRA in Japan, and possibly a third LIGO detector in India —is expected to revolutionize the fields of gravitation and high-energy astrophysics throughthe direct detection of gravitational waves.Creative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0) 12 D. HOAK on behalf of the LIGO SCIENTIFIC COLLABORATIONFig. 1. – Optical configuration of the Advanced LIGO interferometers. Figure from [2].In Advanced LIGO (fig. 1) the basic design from initial LIGO of a Michelson inter-ferometer with Fabry-Perot arm cavities and power-recycling at the input port has beenretained, with the following additions and upgrades:• A signal recycling mirror has been added to improve the frequency response of thedetector.• Larger test masses (40 kg instead of 10 kg) are used to reduce the effect of suspensionthermal noise.• The isolation systems have undergone a complete redesign and provide a factor of1010 suppression at 10 Hz.• The reflectivity of the mirrors has been increased, which increases the number ofround-trips made by the laser field in the Fabry-Perot arms and amplifies the effectof a gravitational wave signal.• The laser has been upgraded to a 200 W design.• The thermal compensation system, to counteract thermal lenses in the optics dueto absorption of the main laser power, has been upgraded to include ring heatersand wave-front sensors to actively control the thermal effects.ADVANCED LIGO: STATUS AND PROSPECTS 3Fig. 2. – Sensitivity of the Advanced LIGO detectors during the first observing run. A compar-ison is made to the previous best sensitivity (from the last observing run of the first-generationdetectors), and the ultimate sensitivity achievable by the advanced detectors.• All sensing of length and alignment degrees of freedom is performed with in-vacuumphotodiodes to reduce the coupling of acoustic noise into control signals.The introduction of signal recycling has increased the complexity of the detector con-trol topology, and required a redesign of the procedure to bring the instruments froman uncontrolled, nonresonant state, to a condition in which they are sensitive to gravi-tational signals. This lock-acquisition sequence is described in [3], and utilizes auxiliarylasers to maintain independent control of the different optical cavities before they havereached a fully resonant condition.The benefit of signal recycling can be seen in fig. 2. Compared to the first-generationdetectors, the Advanced LIGO detectors have a broader response to gravitational-waveperturbations. The improvement at high frequency is due to a combination of signalrecycling and the increase of the finesse of the Fabry-Perot arm cavities. At low frequen-cies, the noise has been dramatically improved due to the upgraded seismic isolationsystems; the improvement is about a factor of six at 60 Hz.The sensitivity of the Advanced LIGO instruments is limited by a mixture of technicaland fundamental noise sources [4]. At high frequency (above 200 Hz), the detectors arelimited by quantum shot noise, due to the discrete nature of light. At low frequenciesthe sensitivity limit is defined by noise from auxiliary degrees-of-freedom that couple intothe gravitational-wave channel.3. – The first observing runThe two LIGO instruments performed round-the-clock observing operations betweenSeptember 12, 2015 and January 19, 2016. This observing period is called “O1”, forthe first observing run of the advanced detector era. During this period the detectorscollected about 50 days of coincident operation, with a sufficient sensitivity to detect theinspiral and merger of a binary neutron star (BNS) system out to 70 Mpc [5], a factor ofthree improvement over the best sensitivities achieved in initial LIGO.4 D. HOAK on behalf of the LIGO SCIENTIFIC COLLABORATIONDuring the observing run, the LIGO instruments made two confident detections ofgravitational-wave signals, from the coalescence of binary black hole systems. Theseevents, labeled GW150914 [1] and GW151226 [6], have already become the most stringenttest of general relativity in the dynamical strong-field regime [7]. The observations areconstraining the rate of BBH in the local universe and have raised questions about theprogenitor mechanism for BBH systems [8].To date, the LIGO-Virgo Collaboration has published several analysis results usingthe O1 dataset. In particular, the search for gravitational waves from two hundred knownpulsars has placed limits on gravitational-wave emission that beats the spin-down limit oneight pulsars [9], and the search for a stochastic background of gravitational wave energyhas improved the limit from the first-generation instruments by a factor of about thirty-three [10]. This search is particularly interesting, as rate of BBH coalescences implied byGW150914 and GW151226 indicates that a stochastic background of gravitational wavesfrom unresolved BBH sources may be detectable by the Advanced LIGO instruments attheir design sensitivity.4. – Prospects for the futureIn the near future, the LIGO observatories plan to carry out longer-duration observ-ing runs at progressively greater sensitivity, joined by the Advanced Virgo and KAGRAinstruments. An approximate timeline for these observing periods, the target sensitivity,and some estimates of the rate of detections is given in [11]. The addition of Virgo andKAGRA, and possibly a third LIGO detector in India, will improve position reconstruc-tion and parameter estimation for gravitational wave events.The LIGO Collaboration is exploring a variety of options for upgrading the detectorsbeyond the design sensitivity of Advanced LIGO [12]. In the near term, larger mirrorsand improved dielectric coatings will reduce the fundamental noise limit at low frequency,and the use of squeezed light injection will improve the sensitivity at high frequencies.More ambitious upgrades in the next decade could utilize cryogenic environments toreduce the thermal noise from the mirror coatings and suspensions. Completely newfacilities, either underground or above ground with significantly longer interferometricarms, could achieve ten times the design sensitivity of Advanced LIGO, and allow us toobserve binary black hole coalescences out to redshifts of z = 3.REFERENCES[1] Abbott B. P. et al., Phys. Rev. Lett., 116 (2016) 6.[2] Aasi J. et al., Class. Quantum Grav., 32 (2015) 074001.[3] Staley A. et al., Class. Quantum Grav., 31 (2014) 245010.[4] Martynov D. V., Hall E. D. et al., Phys. Rev. D, 93 (2016) 112004.[5] Abbott B. P. et al., Astrophys. J. Lett., 832 (2016) L21.[6] Abbott B. P. et al., Phys. Rev. Lett., 116 (2016) 241103.[7] Abbott B. P. et al., Phys. Rev. X, 6 (2016) 041015.[8] Abbott B. P. et al., Astrophys. J. Lett., 818 (2016) L22.[9] Abbott B. P. et al., arXiv:1701.07709 (2017).[10] Abbott B. P. et al., Phys. Rev. Lett., 118 (2017) 121101.[11] Abbott B. P. et al., Living Rev. Relat., 19 (2016) 1.[12] Abbott B. P. et al., Phys. Rev. Lett., 116 (2016) 131103.

Advanced LIGO: Status and prospects

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Advanced LIGO: Status and prospects

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