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State-Of-The-Art of LIGO Zeming Zhuang

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State-Of-The-Art of LIGO Zeming Zhuang 1 State-of-the-art of LIGO Zeming Zhuang Abstract—Scientists have done a lot on the research of grav- itational wave with the help of LIGO. LIGO itself also has several changes during since it was made. This article narrates the history and events of LIGO and the future of researchers. I. INDEX TERMS Astrophysics, Observatories, Interferometers II. INTRODUCTION In 2016, LIGO and VIRGO team together claimed detecting the gravitational wave emitted from two black holes’ merge. After that, LIGO has found many other gravitational waves and gravitational wave has been one of the most popular research area in cosmology and astrophysics. This is about to introduce the basic knowledge and state-of-the-art of LIGO. III. GRAVITATIONAL WAVES Fig. 1. The gravitational wave spectrum with sources and detectors. Credit: Gravitational waves are the disturbance in the curvature of NASA Goddard Space Flight Center spacetime generated by accelerated masses and propagate as waves outward from their source at the speed of light [1] (like the movement of waves away from a stone thrown into a C. Detector pond). Its existence is only provided within the general theory Divided by frequency, there are different kinds of detectors of relativity because of its speed limit of light speed. But in based on different features of gravitational wave: Newton’s classical mechanics, physical interactions propagate • Resonant mass antennas: Weber bar instantaneously, so the gravitational wave cannot exist. • Terrestrial Interferometers: LIGO, VIRGO • Space-based Interferometers: LISA A. History • Pulsar timing arrays: EPTA, NANOGrav It is first predicted in 1916 by Albert Einstein on the basis of his general theory of relativity. [2], [3] But it wasn’t actually IV. LIGO proved until 1974 by two astronomers working at the Arecibo Radio Observatory in Puerto Rico. They discovered a binary The Laser Interferometer Gravitational-Wave Observatory pulsar which was exactly the type of system that, according (LIGO) is a large-scale physics experiment and observatory to to general relativity, should radiate gravitational waves. Then detect cosmic gravitational waves and to develop gravitational- astronomers have begun to monitor it and found it precisely wave observations as an astronomical tool. [4] It was con- predicted by Einstein’s general relativity so far about 40 years. ceived, built, and are operated by Caltech and MIT. B. Source A. History Gravitational waves (changes in curvature) can be generated The first prototype of laser interferometer was conceived by emitted 5 systems or celestial bodies: in the 1960s by American and Soviet scientists. But until • Binaries: Two stars of similar or dissimilar mass with an 1994 has the LIGO team received of USD 395 million which in-spiral or decrease in circular orbits made the project a substantial progress. The initial operations • Black hole Binaries: Two black holes in their in-spiral, are started from 2002 and found nothing till 2010, when merger, and ring-down phase LIGO was shut down to upgrade. This major upgrade will • Supernova: A significant proportion of the matter in the increase the sensitivity of the LIGO instruments by a factor star is blown away at extremely high velocities asymmet- of 10, giving a one thousand-fold increase in the number of rically astrophysical candidates for gravitational wave signals. [5] In • Spinning neutron stars: Slight deformities on the surface 2015, the new aLIGO started its observatory. Only after several causing spherically asymmetric spinning days, LIGO detected its first gravitational wave of two ~30 • Inflation: Asymmetric inflation causing a gravitational solar mass black holes merging about 1.3 billion light-years wave background from Earth. [6] 2 detected (purple circle). Figure 3.2: A gravitational wave passing over the left arm (yellow) changes its length and thus the interference pattern. Fig. 2. Advanced LIGO: Gravitational Wave Detectors Upgraded Image Credit: LIGO, Caltech, MIT, NSF B. Observations So far, from 2015, LIGO has detected 6 signals of gravita- tional waves: TABLE I LIST OF GRAVITATIONAL WAVE EVENTS GW Detection Published Type Ref event Time Date GW 2015-09-14 2016-02- Binary Black [7], [8], 150914 09:50:45 11 hole [9] GW 2015-12-26 2016-06- Binary Black [10], [11] 151226 03:38:53 15 hole GW 2017-01-04 2017-06- Binary Black [12], [13] 170104 10:11:58 01 hole GW 2017-06-08 2017-11- Binary Black [14] 170608 02:01:16 16 hole GW 2017-08-14 2017-09- Binary Black [15], [16] 170814 10:30:43 27 hole GW 2017-08-17 2017-10- Binary [17], [18], 170817 12:41:04 16 Neutron Star [19] C. Operation LIGO consists of two 4-km long beam lines in Gires–Tournois etalon arms which make up a Michelson Fig. 3. Simplified operation of a gravitational wave observatory Image Credit: interferometer. A power recycle mirror is used to increase Wikipedia — LIGO the power of laser. With the effect of two orthogonal arms of Fabry-Perot cavities, the equivalent distance can be much bigger, which makes the power of laser increase thousands of times. When a gravitational wave passes through the interfer- D. Difficulties ometer, the spacetime in the local area is altered. This changes 1) Seismic Isolation: LIGO‘s greatest strength also be- the length of one or both cavities, making the light currently come its greatest weakness. It is so sensitive that both vibration in the cavity very slightly out of phase with the incoming near (a bypass truck) or far (an earthquake at the other side of light and the beams, which were interfering, will have a slight the world) would cause the noise of detector. To that end, detuning, which can be measured by the detector. LIGO uses multiple means of eliminating vibration falling The image below indicates the operation of a simplified into two broad categories: “active” and “passive” damping LIGO. Figure 3.1: A beamsplitter (green line) splits coherent systems. The active damping is made by the ISI system. light (from the white box) into two beams which reflect off The Internal Seismic Isolation (ISI) system senses ground the mirrors (cyan oblongs); only one outgoing and reflected movement and deliberately perform counter movements to beam in each arm is shown, and separated for clarity. The eliminate them. LIGO’s passive damping system holds the all- reflected beams recombine and an interference pattern is important mirrors perfectly still through a 4-stage pendulum 3 called a "quad". In the quad, LIGO’s test masses (its mirrors) [11] R. Nemiroff and J. Bonnell, “Gw151226: A second confirmed are suspended at the end of four pendulums by 0.4 mm thick source of gravitational radiation,” 2016. [Online]. Available: https: //apod.nasa.gov/apod/ap160615.html fused-silica (glass) fibers. The sheer weight of the suspension [12] B. P. Abbott, L. S. Collaboration, and V. Collaboration, “Gw170104: components (each mirror weighs 40 kg) also helps to prevent Observation of a 50-solar-mass binary black hole coalescence at motion of the mirrors. [20] redshift 0.2,” Physical Review Letters, vol. 118, no. 22, p. 1101, 2017. [Online]. Available: 10.1103/PhysRevLett.118.221101 2) Vacuum: LIGO has the purest sustained vacuum on [13] D. Overbye, “Gravitational waves felt from black-hole merger 3 billion Earth which is one-trillionth that of air pressure at sea level. light-years away,” New York Times, 2017. It is because even just a few molecules of air can create noise [14] B. P. Abbott, S. Collaboration, and V. Collaboration, “Gw170608: Observation of a 19-solar-mass binary black hole coalescence,” The which disturb the detector and dust may fall onto the mirrors Astrophysical Journal Letters, vol. 851, no. 2, 2017. [Online]. Available: and cause the light to scatter. At last, it took 40 days (1100 10.3847/2041-8213/aa9f0c hours) to remove all 10,000 m3(353,000 ft3) of air and other [15] ——, “Gw170814: A three-detector observation of gravitational waves from a binary black hole coalescence,” Physical Review residual gases from each of LIGO’s vacuum tubes to reach an Letters, vol. 119, no. 14, p. 1101, 2017. [Online]. Available: air pressure one-trillionth that at sea level. 10.1103/PhysRevLett.119.141101 [16] D. Overbye, “New gravitational wave detection from colliding black holes,” The New York Times. Retrieved, 9 2017. E. Future [17] B. P. Abbott, L. S. Collaboration, and V. Collaboration, “Multi- 1) INDIGO: INDIGO, or LIGO-India, is a world-class messenger observations of a binary neutron star merger,” The Astrophysical Journal, vol. 848, no. 2, p. L12, 2017. [Online]. gravitational-wave detector created by the LIGO Laboratory Available: 10.3847/2041-8213/aa91c9 located in India. [18] A. Cho, “Merging neutron stars generate gravitational waves and a 2) A+: Like Enhanced LIGO, scientists are planning to celestial light show,” Science, 2017. [19] B. P. Abbott, L. S. Collaboration, and V. Collaboration, “Gw170817: improve the instruments of aLIGO over the decade from 2017 Observation of gravitational waves from a binary neutron star inspiral,” to 2026, which would almost double the sensitivity of aLIGO. Physical Review Letters, vol. 119, no. 16, p. 1101, 2017. [Online]. 3) Other: LIGO Voyager & Einstein Telescope are third- Available: 10.1103/PhysRevLett.119.161101 [20] L. Caltech, “Ligo technology.” [Online]. Available: https://www.ligo. generation detector which is far more sensitive than existing caltech.edu/page/ligo-technology LIGO. V. CONCLUSION 100 years after Einstein published the paper first mentioned the gravitational wave, people had great progress on astro- physics. Together with LIGO, many countries and regions also have started their research on gravitational wave. In the future, people may be able to observe the universe from an unprecedented aspect. REFERENCES [1] “Gravitational wave - wikipedia,” 2018, wikipedia Gravitational Wave; https://en.wikipedia.org/wiki/Gravitational\_wave.
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