Searching for Gravitational Waves from the Coalescence of High-mass Black Hole Binaries

Laser interferometer gravitational wave observatory (LIGO) consists of two complex large-scale laser interferometers designed for direct detection of gravitational waves from distant astrophysical sources in the frequency range 10Hz - 5kHz. Direct detection of space-time ripples will support Einstein's general theory of relativity and provide invaluable information and new insight into physics of the Universe. Initial phase of LIGO started in 2002, and since then data was collected during six science runs. Instrument sensitivity was improving from run to run due to the effort of commissioning team. Initial LIGO has reached designed sensitivity during the last science run, which ended in October 2010. In parallel with commissioning and data analysis with the initial detector, LIGO group worked on research and development of the next generation detectors. Major instrument upgrade from initial to advanced LIGO started in 2010 and lasted till 2014. This thesis describes results of commissioning work done at LIGO Livingston site from 2013 until 2015 in parallel with and after the installation of the instrument. This thesis also discusses new techniques and tools developed at the 40m prototype including adaptive filtering, estimation of quantization noise in digital filters and design of isolation kits for ground seismometers. The first part of this thesis is devoted to the description of methods for bringing interferometer to the linear regime when collection of data becomes possible. States of longitudinal and angular controls of interferometer degrees of freedom during lock acquisition process and in low noise configuration are discussed in details. Once interferometer is locked and transitioned to low noise regime, instrument produces astrophysics data that should be calibrated to units of meters or strain. The second part of this thesis describes online calibration technique set up in both observatories to monitor the quality of the collected data in real time. Sensitivity analysis was done to understand and eliminate noise sources of the instrument. Coupling of noise sources to gravitational wave channel can be reduced if robust feedforward and optimal feedback control loops are implemented. The last part of this thesis describes static and adaptive feedforward noise cancellation techniques applied to Advanced LIGO interferometers and tested at the 40m prototype. Applications of optimal time domain feedback control techniques and estimators to aLIGO control loops are also discussed. Commissioning work is still ongoing at the sites. First science run of advanced LIGO is planned for September 2015 and will last for 3-4 months. This run will be followed by a set of small instrument upgrades that will be installed on a time scale of few months. Second science run will start in spring 2016 and last for about 6 months. Since current sensitivity of advanced LIGO is already more than factor of 3 higher compared to initial detectors and keeps improving on a monthly basis, upcoming science runs have a good chance for the first direct detection of gravitational waves.

One hundred years after Albert Einstein predicted the existence of gravitational waves as a result of his theory of general relativity, the Laser Interferometer Gravitational-Wave Observatory (LIGO), made the first direct detection of a gravitational-wave signal from a binary black hole merger, GW150914. GW150914 was found not only by search methods specifically developed to find the distinctive waveform produced by coalescing binaries, but also by generic searches designed to find any arbitrary short-duration signal in the LIGO data. The impact of noise on the searches must be carefully investigated in order to reduce the search background and enable confident gravitational-wave detections. In this dissertation, I will present my work on characterizing transient noise sources in the detectors and implementing data quality vetoes to reduce their effects on the generic transient gravitational-wave searches. Chapters 3 and 4 describe my work on the data quality of the searches for generic transient gravitational waves. I worked on the development of data quality vetoes during the first observing run and the decisions about which vetoes to implement in the transient searches. I also analyzed the transient noise sources that the vetoes were unable to eliminate, using statistical methods to search for potential instrumental causes. Since the development of data quality vetoes requires a thorough understanding of every component of the detectors, I have also conducted a detailed investigation into the transients in the suspension systems used to isolate the LIGO optics from seismic motion. Chapter 5 presents the details of this work. The first gravitational wave detection was only the beginning an exciting era of gravitational-wave astronomy that will give us a new way of understanding the universe. Even in the first observing run, a second binary black hole merger was observed. The methods used in this dissertation to investigate and reduce background noise will continue to play an important role in making these detections possible. As the detectors improve in the future and continue to take data, more signals will be detected, bringing us a wealth of new information about black holes and other types of sources.

Gravitational waves from coalescing compact binaries are searched for using the matched filtering technique. As the model waveform depends on a number of parameters, it is necessary to filter the data through a template bank covering the astrophysically interesting region of the parameter space. The choice of templates is defined by the maximum allowed drop in signal-to-noise ratio due to the discreteness of the template bank. In this paper we describe the template-bank algorithm that was used in the analysis of data from the Laser Interferometer Gravitational Wave Observatory (LIGO) and GEO 600 detectors to search for signals from binaries consisting of non-spinning compact objects. Using Monte Carlo simulations, we study the efficiency of the bank and show that its performance is satisfactory for the design sensitivity curves of ground-based interferometric gravitational wave detectors GEO 600, initial LIGO, advanced LIGO and Virgo. The bank is efficient in searching for various compact binaries such as binary primordial black holes, binary neutron stars, binary black holes, as well as a mixed binary consisting of a non-spinning black hole and a neutron star.

In this thesis, I present a number of studies intended to improve our understanding of black holes using gravitational waves. Although black holes are relatively well understood from a theory perspective, many questions remain about the nature of the black holes in our Universe. According to general relativity, astrophysical black holes are fully described by just their mass and spin. Yet, relying on electromagnetic-based observatories alone, we still know very little about the distribution of black hole masses or spins. Moreover, as merging black holes are invisible to these electromagnetic observatories, we cannot rely on them to provide us with information about the binary black hole merger rate or binary black hole formation channels. However, by observing gravitational wave signals from these inherently dark binaries, we will soon have some answers to these questions. Indeed, the Laser Interferometer Gravitational-Wave Observatory (LIGO) has already revealed a great deal of new information about binary black holes; giving us an early glimpse into their mass and spin distributions and placing the first constraints on the binary black hole merger rate. This thesis contributes to the goal of probing the nature of black holes with gravitational waves. Binary black holes can form as an isolated binary in the galactic field or through dynamical encounters in high-density environments. Dynamical formation can significantly alter the binary parameters, which then become imprinted on the gravitational waveform. By simulating varying black hole populations in high-density globular clusters, we identify a population of highly eccentric binary black hole mergers that are characteristic of dynamical formation. Although these systems would circularize by the time they are visible in LIGO's frequency band, the future Laser Interferometer Space Antenna (LISA) is capable of distinguishing this population of eccentric mergers from the circular mergers expected of isolated field-formed binaries. As these dynamically formed binaries depend on the size of the underlying black hole population in globular clusters, we can utilize the dynamically formed merger rate to infer globular cluster black hole populations -- allowing us to reveal information about binary black hole birth environments. In order to properly estimate the parameters of binary black holes from detected gravitational wave signals, such as their masses and spins, high-accuracy waveforms are a needed. The highest accuracy waveforms are those produced by numerical relativity simulations, which solve the full Einstein equations. Using the Spectral Einstein Code (SpEC), we expand the reach of numerical relativity to simulate binary black holes with nearly extremal spins, i.e., black holes with spins near the maximal value χ = 1. These waveforms are used to calibrate existing waveform approximants used in LIGO data analyses. This ensures that the systematic errors in these approximants are small enough that if highly-spinning systems are observed, the spins are recovered without bias. Although rapidly spinning binaries have remained elusive thus far, these waveforms ensure that the highest-spin systems can be detected in the quest to uncover the spin distribution of black holes. The end state of a binary black hole merger is a newly born, single black hole that rings down like a struck bell, sending its last few ripples of gravitational waves out into the spacetime. Embedded in this 'ringdown' signal are a multitude of specific frequencies. Einstein's theory of general relativity precisely predicts the ringdown frequencies of a black hole with a given mass and spin. The statement that a black hole is entirely described by just these two parameters is known as the no-hair theorem. For black holes that obey the laws of general relativity (and consequently, the no-hair theorem), these frequencies serve as a fingerprint for the black hole. However, if the objects we observe are not Einstein's black holes, but instead something more exotic, the frequencies will not have this property and this would be a spectacular surprise. A minimum of two tones are required for this test, each with an associated frequency and damping time that depend only on the mass and spin. The conventional no-hair test relies on the so-called 'fundamental' tones of a black hole. A test relying on the fundamental modes is not expected to be feasible for another ~10-15 years, after detector sensitivity has improved significantly. However, by analyzing the ringdown of high-accuracy numerical relativity waveforms, we show that modes beyond the fundamental, known as 'overtones', are detectable in current detectors. The overtones are short-lived, but this is countered by the fact that they can initially be much stronger than the fundamental mode. By measuring two tones in the ringdown of GW150914 we perform a first test of the no-hair theorem. While the current constraints are rather loose, this first test serves as a proof of principle. This is just one example of the powerful tests that can be employed with overtones using present day detectors and the even more precise tests that can be accomplished with LISA in the future.

According to General Relativity a perturbed black hole will return to a stable configuration by the emission of gravitational radiation in a superposition of quasi-normal modes. Such a perturbation will occur due to the coalescence of a black hole binary, following their inspiral and subsequent merger. At late times the waveform, which we refer to as a ringdown, is expected to be dominated by a single mode. As the waveform is well-known the method of matched filtering can be implemented to search for this signal using LIGO data. LIGO is sensitive to the dominant mode of perturbed black holes with masses between 10 and 500 $M_odot$, the regime of intermediate-mass black holes, to a distance of up to 300 Mpc. We present a search for gravitational waves from black hole ringdowns using data from the fourth LIGO science run. We implement a blind analysis of the data. We use Monte Carlo simulations of the expected waveform, and an estimation of the background from timeslides to tune the search. We present an analysis of the waveform parameter estimation and estimate the efficiency of the search. As there were no gravitational wave candidates found, we place an upper limit on the rate of black hole ringdowns in the local universe.

Both the Laser Interferometer Gravitational Wave Observatory (LIGO) and the Laser Interferometer Space Antenna (LISA) will over the next decade detect gravitational waves emitted by the motion of compact objects (e.g. black hole and neutron star binaries). This thesis presents methods to improve (i) LIGO detector quality, (ii) our knowledge of waveforms for certain LIGO and LISA sources, and (iii) models for the rate of detectability of a particular LISA source. 1) Plunge of compact object into a supermassive black hole: LISA should detect many inspirals of compact objects into supermassive black holes ($sim 10^5-10^7 M_odot$). Since the inspiral of each compact object terminates shortly after the inspiralling object reaches its last stable orbit, the late-stage inspiral waveform provides insight into the location of the last stable orbit and strong-field relativity. I discovered that while LISA will easily see the overall inspiral (consisting of many cycles before plunge), the present LISA design will just miss detecting the waves emitted from the transition from inspiral to plunge. 2) Scheme to reduce thermoelastic noise in advanced LIGO: After its first upgrade, LIGO will have its sensitivity limited by thermoelastic noise. [Thermoelastic noise occurs because milimeter-scale thermal fluctuations in the mirror bulk expand and contract, causing the mirror surface to shimmer.] The interferometer's sensitivity could be enhanced substantially by reducing thermoelastic noise. In collaboration with Kip Thorne, Erika d'Ambrosio, Sergey Vyatchanin, and Sergey Strigin, I developed a proposal to reduce thermoelastic noise in advanced-LIGO by switching the LIGO cavity optics from simple spherical mirrors to a new, Mexican-hat shape. 3) Geometric-optics-based analysis of stability of symmetric-hyperbolic formulations of Einstein's equations: Einstein's equations must be evolved numerically to predict accurate waveforms for the late stages of binary black hole inspiral and merger. But no matter which representation of Einstein's equations is used, numerical simulations rarely run long. For examle, for first-order symmetric-hyperbolic (FOSH) formulations of Einstein's evolution equations, sometimes exact but unphysical solutions grow so large that the evolution fails. For FOSH formulations, I found easily-understood solutions (wave packets) and used them to predict which formulations will be particularly ill-behaved.

In September 2015, the Laser Interferometer Gravitational-wave Observatory (LIGO) initiated the era of gravitational wave astronomy, a new window on the universe. In its first 4 months of operation, the Advanced LIGO instrument made the first, two direct detections of gravitational waves (ripples in the fabric of space-time). Each of these events were the result of merger of a pair of black holes into a single larger black hole. The first detected system consisted of two black holes of about 30 solar masses each which merged at a distance of 400 mega-parsecs or 1.4 billion years ago, revealing a new population of black holes. As of October 2017 five black hole mergers have been announced. In August 2017, after some further improvements and commissioning, the LIGO and VIRGO collaborations announced the first direct detection of gravitational waves associated with a gamma ray burst and the electromagnetic emission (visible, infrared, radio) of the afterglow of a kilonova -- the spectacular collision of two neutron stars at a distance of 40 mega-parsecs. This marks the beginning of multi-messenger astronomy. The discovery was made using the U.S.-based LIGO; the Europe-based Virgo detector; and some 70 ground- and space-based observatories. The Advanced LIGO gravitational wave detectors are second generation instruments designed and built for the two LIGO observatories in Hanford, WA and Livingston, LA. These two identically designed instruments employ coupled optical cavities in a specialized version of a Michelson interferometer with 4 kilometer long arms. Fabry-Perot cavities are used in the arms to increase the interaction time with a gravitational wave, power recycling is used to increase the effective laser power and signal recycling is used to improve the frequency response. In the most sensitive frequency region around 100 Hz, the displacement sensitivity is 10-22 meters rms, or about 10 million times smaller than a proton. In order to achieve this unsurpassed measurement sensitivity Advanced LIGO employs a wide range of cutting-edge, high performance technologies, including a ultra-high vacuum system; an extremely stable laser source; multiple stages of active vibration isolation; super-polished and ion milled, ultra-low loss, fused silica optics with high performance multi-layer dielectric coatings; wavefront sensing; active thermal compensation; very low noise analog and digital electronics; complex, nonlinear multi-input, multi-output control systems; and a custom, scalable and easily re-configurable data acquisition and state control system.

In his 1916 article [1] predicting the existence of gravitational waves, Einstein wrote that they were too weak to be of any consequence. A century later, the world’s attention was captured by the February 11th announcement, demonstrating how wrong this was. In a landmark publication in Physical Review Letters [2], an international team reports the first direct detection of the gravitational waves emitted by a pair of black holes (29 and 36 solar masses) during their final few orbits before merging to form a single 62 solar-mass black hole. Physicists love extremes, because pushing the limits leads to insight and understanding. This discovery, a short chirp lasting about a quarter-second (see Fig. 1, adapted from [2]), pushes the limits in many different directions. Let’s start with the human side. The first unsuccessful attempts to detect gravitational waves were smallinvestigator experiments made about fifty years ago. In contrast, the discovery paper had about 1000 authors (20% are from the Max Planck Institute for Gravitational Physics, for which I am the Managing Director). Some of these authors have worked towards the discovery for more than forty years. The detector technology also pushes the limits. The advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) instruments are located 3000 km apart, in Hanford, Washington and Livingston, Louisiana. The effect of the gravitational waves is to distort the effective path length between pairs of mirrors hanging 4 km apart in a seismically-isolated high-vacuum system. At the peak of the signal the measured length change in each detector arm is about 0.002 femtometers, 1/1000 the diameter of a hydrogen nucleus. This is a fractional precision of a part in 1021; analogous to measuring the distance to the nearest star (Proxima Centuri) to an accuracy of 40 μm. I don’t know of any other measurement done with comparable precision. The binary system we observed was about one billion light years distant from earth; over some tens of milliseconds it converted about three solar masses of gravitational binding energy into gravitational waves. During that brief time, the system emitted more power than the optical luminosity of every star in every galaxy in the visible universe. My kids, jaded by generations of Star Wars movies, were unimpressed until I told them that in comparison with this, the “Death Star”, capable of vaporizing entire planets, is child’s toy, a harmless plaything. Three solarmasses is enough energy to vaporize every planet in many galaxies! Binary black hole systems like this one could not have been detected in any other way, because (being black!) they do not emit any light or electromagnetic energy. But there are intriguing reports [3] that a team analyzing data from the Gamma-ray Burst Monitor (GBM) detector on board NASA’s Fermi satellite observed a gammaray burst 0.4 seconds after the merger. So Nature might still surprise us. Could such systems be surrounded by clouds of gas or dust? We can’t be sure now, but within a few years, I am confident that we will know the answer to this and to many other questions. The rate at which our knowledge will now increase is breathtaking. In their first observing run the advanced LIGOdetectors were a factor of three below their final design sensitivity. That doesn’t seem like much, but keep in mind that the expected number of sources/detections is proportional to the observable volume of space, which scales like the cube of the sensitivity. So the second observing run, starting this September and lasting six months, should observe about a dozen black hole mergers, and the third observing run, starting in 2017, should see about one hundred. It’s going to be quite a ride – a golden age of gravitational wave astronomy. By the end of it, we’ll know themass distribution and spatial density of these stellarmass black hole binary systems. And perhaps our catalog of observations will include new extremes, such as systemswith high spin, or neutron-star/black-hole binaries, or other surprises.

The field of observational gravitational wave astronomy has begun in earnest, starting with the detection of the strain signal from the binary black hole merger GW150914 by the Laser Interferometer Gravitational-wave Observatory (LIGO) in 2015. The current incarnation of the LIGO observatories, known as Advanced LIGO, has achieved strain sensitivities on the order of 10−23/√Hz in the hundreds of Hz region, which has enabled unambiguous detection of astrophysical gravitational wave signals. Nevertheless, the scientific output from the LIGO observatories is constrained by the instrumental performance and sensitivity, as there remain many more distant and exotic sources to be observed. This thesis describes a few topics in experimental gravitational physics, broadly unified by the desire to improve the performance and sensitivity of gravitational wave interferometers. First, it describes an experimental effort to search for a novel form of nonlinear mechanical noise that may be relevant for the ultimate performance of the mirror sus- pension systems used throughout the instrument. Next, it summarizes work done at the CalTech 40m LIGO controls prototype to realize its fully operational state, and a novel automated controls algorithm developed and tested there that may be useful in simplifying the control of current and future interferometers. Finally, it describes work done on a system to identify and subtract unwanted noise couplings out of recorded aLIGO strain data in an automated fashion. The noise subtraction system applied to GW150914 is demonstrated to reduce the uncertainties of the black hole mass parameters by about 10%.

Late in 2015, gravitational physics reached a watershed moment with the first direct detections of gravitational waves. Two events, each from the coalescence of a binary black hole system, were detected by the Laser Interferometer Gravitational-wave Observatory (LIGO). At present, LIGO comprises two 4 km laser interferometers, one in Washington and the other in Louisiana; a third detector is planned to be installed in India. These interferometers, known as Advanced LIGO, belong to the so-called “second generation” of gravitational-wave detectors. Compared to the first-generation LIGO detectors (Initial and Enhanced LIGO), these instruments use multi-stage active seismic isolation, heavier and higher-quality mirrors, and more laser power to achieve an unprecedented sensitivity to gravitational waves. In 2015, both Advanced LIGO detectors achieved a strain sensitivity better than 10-23/Hz1/2 at a few hundred hertz; ultimately, these detectors are designed to achieve a sensitivity of a few parts in 10-24/Hz1/2 at a few hundred hertz. This thesis covers several topics in gravitational physics and laser interferometry. First, it presents the design, control scheme, and noise performance of the Advanced LIGO detector in Washington during the first observing run (O1). Second, it discusses some issues relating to interferometer calibration, and the impact of calibration errors on astrophysical parameter estimation. Third, it discusses the prospects for using terrestrial and space-based laser interferometers as dark matter detectors. This thesis has the internal LIGO document number P1600295.

Matched filtering is used to search for gravitational waves emitted by inspiralling compact binaries in data from the ground-based interferometers. One of the key aspects of the detection process is the design of a template bank that covers the astrophysically pertinent parameter space. In an earlier paper, we described a template bank that is based on a square lattice. Although robust, we showed that the square placement is overefficient, with the implication that it is computationally more demanding than required. In this paper, we present a template bank based on an hexagonal lattice, which size is reduced by 40% with respect to the proposed square placement. We describe the practical aspects of the hexagonal template bank implementation, its size, and computational cost. We have also performed exhaustive simulations to characterize its efficiency and safeness. We show that the bank is adequate to search for a wide variety of binary systems (primordial black holes, neutron stars, and stellar-mass black holes) and in data from both current detectors (initial LIGO, Virgo and GEO600) as well as future detectors (advanced LIGO and EGO). Remarkably, although our template bank placement uses a metric arising from a particular template family, namely, stationary phase approximation, we show that it can be used successfully with other template families (e.g., Pad\'e resummation and effective one-body approximation). This quality of being effective for different template families makes the proposed bank suitable for a search that would use several of them in parallel (e.g., in a binary black hole search). The hexagonal template bank described in this paper is currently used to search for nonspinning inspiralling compact binaries in data from the Laser Interferometer Gravitational-Wave Observatory (LIGO).

Twenty years ago, construction began on the Laser Interferometer Gravitational-wave Observatory (LIGO). Advanced LIGO, with a factor of ten better design sensitivity than Initial LIGO, will begin taking data this year, and should soon make detections a monthly occurrence. While Advanced LIGO promises to make first detections of gravitational waves from the nearby universe, an additional factor of ten increase in sensitivity would put exciting science targets within reach by providing observations of binary black hole inspirals throughout most of the history of star formation, and high signal to noise observations of nearby events. Design studies for future detectors to date rely on significant technological advances that are futuristic and risky. In this paper we propose a different direction. We resurrect the idea of a using longer arm lengths coupled with largely proven technologies. Since the major noise sources that limit gravitational wave detectors do not scale trivially with the length of the detector, we study their impact and find that 40~km arm lengths are nearly optimal, and can incorporate currently available technologies to detect gravitational wave sources at cosmological distances $(z \gtrsim 7)$.

One hundred years after Albert Einstein predicted the existence of gravitational waves in his general theory of relativity, the Laser Interferometer Gravitational-Wave Observatory (LIGO) made the first direct detection of gravitational waves. Since the first detection of gravitational waves from a binary black hole merger, LIGO has gone on to detect gravitational waves from multiple binary black hole mergers, and more recently from a binary neutron star merger in collaboration with telescopes around the world. The detection of gravitational waves has opened a new window to the universe and has launched the era of gravitational wave astronomy. With the first detection of gravitational waves now two years behind us, work has already begun on improving the sensitivity of Advanced LIGO and planning for future generations of gravitational wave interferometers. One of the main limiting noise sources for current and future gravitational wave detectors is quantum noise, which includes quantum radiation pressure noise that originates from the quantum nature of the photons that reflect off of the test masses. Chapter one provides an introduction to gravitational wave sources and detectors. It also describes the noise sources that limit the sensitivity of interferometeric gravitational wave detectors like Advanced LIGO and includes a detailed description of the origin of quantum noise and its effect in interferometers. Chapter two introduces the concept and properties of optical springs. Much of the experimental work presented in the rest of this thesis utilizes an optical spring. This thesis investigates quantum radiation pressure noise and techniques to reduce quantum noise in gravitational wave interferometers. The experimental research contained in this thesis uses an optomechanical Fabry-Perot cavity in which one of the cavity mirrors is a microresonator consisting of a micro-mirror suspended by a cantilever structure. Chapter three outlines the design and construction of the optomechanical cavity that is housed in a vacuum chamber and sits on a suspended optical breadboard to provide isolation from seismic motion. Chapter three also includes details on the design of the cantilever micro-mirror used in the optomechanical cavity. The experiments in this thesis can be divided into two main categories: the characterization of optical springs and the measurement of broadband quantum radiation pressure noise. Chapter four of this thesis focuses on the characterization of optical springs. I present results from an experiment that uses radiation pressure to control an optomechanical cavity and investigates the feedback control needed to keep the system stable. In

The LIGO (Laser Interferometer Gravitational-wave Observatory) project has begun its search for gravitational waves, and efforts are being made to improve its ability to detect these. The LIGO observatories are long, Fabry-Perot-Michelson interferometers, where the interferometer mirrors are also the gravitational wave test masses. LIGO is designed to detect the ripples in spacetime caused by cataclysmic astrophysical events, with a target gravitational wave minimum strain sensitivity of 4 x 10^-22 around 100 Hz. The Advanced LIGO concept calls for an order of magnitude improvement in strain sensitivity, with a better signal to noise ratio to increase the rate of detection of events. Some of Advanced LIGO's major requirements are improvements over the LIGO design for thermal noise in the test mass substrates and reflective coatings. Thermal noise in the interferometer mirrors is a significant challenge in LIGO's development. This thesis reviews the theory of test mass thermal noise and reports on several experiments conducted to understand this theory. Experiments to measure the thermal expansion of mirror substrates and coatings use the photothermal effect in a cross-polarized Fabry-Perot interferometer, with displacement sensitivity of 10^-15 m/rHz. Data are presented from 10 Hz to 4kHz on solid aluminum, and on sapphire, BK7, and fused silica, with and without commercial TiO2/SiO2 dielectric mirror coatings. The substrate contribution to thermal expansion is compared to theories by Cerdonio et al. and Braginsky, Vyatchanin, and Gorodetsky. New theoretical models are presented for estimating the coating contribution to the thermal expansion. These results can also provide insight into how heat flows between coatings and substrates relevant to predicting coating thermoelastic noise. The Thermal Noise Interferometer (TNI) project is a interferometer built specifically to study thermal noise, and this thesis describes its construction and commissioning. Using LIGO-like designs, components, and processes, the TNI has a minimum length noise in each of two arm cavities of 5 x 10^-18 m/rHz around 1 kHz.

In classical General Relativity (GR), an observer falling into an\nastrophysical black hole is not expected to experience anything dramatic as she\ncrosses the event horizon. However, tentative resolutions to problems in\nquantum gravity, such as the cosmological constant problem, or the black hole\ninformation paradox, invoke significant departures from classicality in the\nvicinity of the horizon. It was recently pointed out that such near-horizon\nstructures can lead to late-time echoes in the black hole merger gravitational\nwave signals that are otherwise indistinguishable from GR. We search for\nobservational signatures of these echoes in the gravitational wave data\nreleased by advanced Laser Interferometer Gravitational-Wave Observatory\n(LIGO), following the three black hole merger events GW150914, GW151226, and\nLVT151012. In particular, we look for repeating damped echoes with time-delays\nof $8 M \\log M$ (+spin corrections, in Planck units), corresponding to\nPlanck-scale departures from GR near their respective horizons. Accounting for\nthe "look elsewhere" effect due to uncertainty in the echo template, we find\ntentative evidence for Planck-scale structure near black hole horizons at false\ndetection probability of $1\\%$ (corresponding to $2.5\\sigma$ significance\nlevel). Future observations from interferometric detectors at higher\nsensitivity, along with more physical echo templates, will be able to confirm\n(or rule out) this finding, providing possible empirical evidence for\nalternatives to classical black holes, such as in ${\\it firewall}$ or ${\\it\nfuzzball}$ paradigms.\n