In 2015, the terrestrial LIGO gravitational-wave detectors observed the first gravitational-wave signal generated in a merger of two black holes about 1.3 billion light-years away. This event has marked the beginning of the new field of gravitational-wave astronomy. Gravitational-wave detectors have since started to routinely observe objects and events in the universe that are not accessible to the traditional electromagnetic observations. Furthermore, in 2016-2017 the successful LISA Pathfinder mission demonstrated some of the key technologies needed for developing a space-borne gravitational-wave detector. Coming on the heels of these remarkable breakthroughs, the European Space Agency (ESA) recently selected the Laser Interferometer Space Antenna (LISA) as the third large-class mission in ESA's Science Programme. LISA is expected to be launched in 2034, and with a significant contribution from NASA, it will be the first space-borne gravitational-wave observatory.
This proposal focuses on the stochastic gravitational-wave background (SGWB), which is one of the science targets of the LISA mission. The SGWB is expected to arise as a superposition (sum) of many incoherent sources of gravitational waves. It could be of astrophysical origin, for example due to contributions of numerous binary systems in the Milky Way or throughout the universe. It could also be of cosmological origin, generated by very energetic processes in the early universe. Detection of a cosmological SGWB would provide unique information about the fundamental physical laws that apply at very high energy scales, inaccessible to standard laboratory experiments. Detection of an astrophysical SGWB would provide unique information about properties and evolution of structure we observe today in the universe, including objects such as neutron stars and black holes.
This proposal will develop a new method for measuring the properties of the SGWB using LISA data, drawing from similar methods developed in the context of terrestrial gravitational-wave detectors and of pulsar timing experiments. The new method, known as the phase-coherent mapping approach, will enable direct estimates of the frequency, directionality, and polarization content of the SGWB. It is also compatible with the global-solution approach to LISA data analysis, which advocates doing a global fit to all of the gravitational-wave signal and noise components in the data.
Furthermore, a statistical framework will be developed to perform the SGWB model selection and parameter estimation for LISA. By leveraging differences in directional, polarization, and frequency structure in different SGWB models, this framework will enable identification of contributions from different SGWB sources, hence separating the cosmological SGWB from the astrophysical foregrounds. The framework will therefore combine the development of SGWB models and SGWB search techniques to fully exploit the science potential of SGWB searches with LISA.
The LISA observatory will allow us to study the milli-Hertz band of the gravitational wave spectrum, which is thought to contain many millions of sources. The challenge of extracting thousands of individual signals in the presence of fluctuating instrument noise, glitches and data drop-outs remains an unsolved problem. The success of the mission depends on solving this problem, which is very different, and far more challenging, than anything that has been encountered with the ground-based LIGO-Virgo gravitational wave detectors. We propose to develop a flexible, trans-dimensional analysis algorithm that dynamically determines the number of resolvable sources and their physical parameters, while simultaneously modeling the instrument noise and accounting for gaps in the data. We plan to build on our experience using stochastic analysis techniques to extract tens of thousands of galactic binary signals and hundreds of binary black hole signals from simulated LISA data sets. We also propose to develop techniques for detecting and characterize un-anticipated and un-modeled sources of gravitational waves. The signal extraction will be done in concert with advanced noise modeling techniques to develop a prototype global solution to the LISA science analysis problem.
The ultimate expression of strong gravity in the Universe is a black hole, a region of spacetime from which nothing, not even light, can escape. Hidden from our direct view by their very nature, black holes are only "seen" by their interaction with a light- producing environment. Two black holes merging together light up the cosmos in gravitational waves, while remaining completely dark in electromagnetic light. On September 14, 2015, such a cataclysmic collision was observed for the first time when LIGO detected the gravitational waves emitted by two black holes merging, called GW150914. Gravitational waves, or ripples in spacetime, are the only means to detect black holes directly. LISA will offer unprecedented views of black holes, detecting gravitational waves from black hole binaries across multiple mass scales and deep into the history of the Universe inaccessible by ground based detectors like LIGO. This proposal prepares for such detections by providing theoretical predictions of black holes at multiple mass scales. The central thesis of the proposal is the determination of the signature of black hole binaries of unequal mass ratio in which one of the black holes is a hundred or more times the mass of its companion. These sources are expected to be relatively common and current methods fail to capture this likely LISA source.
The theoretical foundation of black hole mergers and their generation of gravitational waves is Einstein's theory of general relativity. There are several methods to produce accurate gravitational waveforms for the coalescence of black hole binaries. This work will require knowledge of these approaches and new ones. Numerical relativity provides a means to solve Einstein's equations during the nonlinear regime when the two black holes merge. While advances in numerical relativity over the last 13 years have been monumental, as of yet numerical relativity cannot solve systems of black holes with mass ratios more disparate than about one to one hundred. For systems with vastly different black-hole masses, called extreme mass ratio inspirals, perturbation theory provides solutions. Unfortunately, perturbation theory is best suited for mass ratios in the thousands or more rather than hundreds. When the black holes are effectively point masses moving at a fraction of the speed of light, an approximation to Einstein's theory known as post-Newtonian provides the answer. All of these approaches will be important in the binary black hole landscape of LISA. Black-hole binaries of moderate mass ratios in the hundreds will require pushing the boundaries of numerical relativity and perturbation theory in order to provide reasonable solutions for which neither approach can do alone.
Several of the driving scientific objectives for LISA will depend on successful studies like that proposed. LISA is to "trace the origin, growth and merger history of massive black holes across cosmic ages" and "to explore the fundamental nature of gravity and black holes" to name just two objectives from the LISA Mission Proposal. Our work will contribute to this goal and that of the overall mission of NASA to probe the Cosmos by providing the theoretical foundations necessary to interpret gravitational wave detections in terms of their astrophysical origins, providing a map of the gravitational waves of binary black holes of unequal-mass.
There are several products this proposal will produce for LISA science if funded. The main product will be waveforms of black-hole mergers with mass ratios in the hundreds. These waveforms will be provided to the LISA Mock Data Challenges to stress test the community's ability to detect and interpret gravitational waves from these systems. The proposed work will also produce papers and numerical algorithms that provide guidance and insight into gravitational waves from moderate mass ratio black hole binaries and their interpretation.
The analysis of LISA data will be significantly different than data from ground-based detectors, owing to the fact that there will be thousands of overlapping signals from multiple source classes constantly in the data. This poses a variety of interesting problems to develop strategies to identify, isolate, and extract signals and parameters from individual sources. Astrophysical inference and interpretation of signals from single and whole populations of sources will require targeted, quantitative models for observational selection biases. Additionally, multi-messenger observations of LISA sources, before, during, and after the LISA mission will play critical roles in interpreting and understanding the LISA source catalogs.
Observational bias has many origins, including selection effects derived from the fundamental astrophysical parameters of a source, uncharacterized physical effects that systematically alter data relative to naive expectations, the response of the instrument to different sources at different frequencies and sky locations, and on the methods used to identify and extract source parameters from the data.
In our proposal we will consider the problem of identifying and quantifying astrophysical information contained in LISA data for the population of ultra-compact binaries. We will produce a set of public tools that the astrophysical community can use to simulate data catalogs for inference and interpretation studies of compact binaries in LISA analysis, and foundational data for simulating populations of ultra-compact binaries for studies in LISA data analysis, stellar evolution, and gravitational wave source simulation. We are principally interested in how astrophysical information imprints in LISA observations compact stellar mass binaries, and what biases are introduced in catalogs derived from different approaches to LISA analysis of the compact binaries. We will include non- standard sources, e.g., eccentric binary black holes and interacting double white dwarfs with inverse gravitational-wave chirps, which hold special promise for multi-wavelength and multi-messenger studies and can be used to leverage maximal information.
The Laser Interferometer Space Antenna (LISA) aims to observe gravitational waves from a cornucopia of low-frequency astrophysical sources including extragalactic black holes with masses ranging from tens to tens of millions of solar masses, thousands of binary star systems in our own Milky Way galaxy and cosmological stochastic background sources and exotic physics in the early universe. To achieve this aim, LISA will employ a pm-precision interferometric measurement of six test masses in three spacecraft, disturbed by accelerations of order fm/s2 and separated by 2.5 million km. Unlike almost any astrophysics mission that has preceded it, the LISA instrument and spacecraft are inextricably linked in determining the quality of the final scientific product. The scientific performance of the mission depends on the interplay between almost every element of the payload and platform. For example, the optical metrology system (OMS) is used not only as the primary distance measurement between test masses but also for spacecraft attitude control. The need to maintain the interferometric reference test masses in pure free fall drives the spacecraft dynamics through the on-board micro-propulsion system and the quality of free-fall depends critically on the stability of the platform.
Studying the performance of the instrument and generating realistic synthetic data with which to train data processing algorithms will require detailed simulations capable of reproducing the full complexity of the LISA instrument. The work described in this proposal will contribute to this effort in three areas: simulating the drag-free and attitude control system (DFACS) of LISA that governs the dynamics of the test masses and spacecraft; simulating the relevant light path for the entire optical chain of each laser link, including modeling the interferometric length and alignment sensing; and developing tools to predict laser heterodyne frequencies based on orbit data and plan necessary switches or tunings of the offset-frequencies in the phase-locked loops.
We propose to develop simulation tools describing the LISA DFACS, incorporating non-trivial test mass acceleration and read-out noise, the motion of the moving optical sub-assemblies (MOSAs) and the interaction of spacecraft and test mass dynamics through the control laws and actuators.
We will develop a simulation of the LISA optical metrology system based on a Hermite-Gauss mode expansion and scatter matrices capable of modeling tilt-to-length coupling, as well as the effects of other imperfections in the optical path such as telescope wavefront errors, mode mismatches between interfered beams, and beam spot mis-centering on quadrant photodetectors.
We propose to develop a tool that will rapidly devise suitable frequency plans to keep the heterodyne beat signals within a workable range based on inputs of orbital data. The integration of this tool with the OMS and DFACS simulation tools will allow us to transition from a solely frequency-limit-based figure of merit to one that is driven by the actual impact of the heterodyne frequency on the end-to-end noise.
To provide maximum benefit to the LISA mission, the products from each of these tasks will be built for compatibility with an end-to-end performance simulator architecture. The LISANODE infrastructure is being developed within the LISA Consortium to facilitate the implementation of, and communication between, distinct but not isolated simulations of the LISA subsystems. The distinct simulations will be developed as modules called Nodes within the simulation environment.
Ultra-compact binaries are one of the major classes of sources detectable by LISA. The number of such sources is estimated to be very large – more than 10,000 individually detectable sources, and an even larger number of sources contributing to (and dominating) the LISA background below about 2 mHz.
Much remains to be learned about the nature of ultra-compact binaries. To date, only a hand-full of LISA Verification Sources have been identified and the total number of known binaries with periods less than 60 minutes is only a few dozen.
The Zwicky Transient Facility (ZTF) is a major new large-area ground-based optical survey instrument, having just begun survey observations in May-June 2019. ZTF will likely be the most powerful near-term instrument for detection and characterization of ultra-compact binaries at least until LSST becomes available. Combined with Gaia source distant estimates, ZTF will significantly increase the number of known LISA verification binaries as well as detect hundreds of other compact binaries that will provide a deeper understanding of the astrophysics of the various binary channels that will contribute to the LISA GW signal. The proposed work involves observational detection and characterization of ZTF identified compact binaries. Our proposal team includes observers experienced in time-domain analysis as well as consultant theorists.
What are the properties of accretion flows in the vicinity of coalescing massive black hole binaries (MBHBs)? The answer to this question has direct implications for the feasibility of coincident detections of electromagnetic (EM) and gravitational wave (GW) signals from coalescences. Such detections are considered to be the next observational grand challenge that will provide a more complete understanding of evolution and growth of these massive objects.
In anticipation of future detections by the Laser Interferometer Space Antenna (LISA), we propose to investigate the coincident EM and GW signatures of MBHBs immersed in accretion flows. The proposed program of research centers on a suite of high-resolution simulations with the radiation-magnetohydrodynamic code Athena++ , which will follow binaries as they evolve through the LISA frequency band. This study will set the stage for the first simulations of accreting MBHBs with radiative transfer and will use them to predict the resulting EM spectra, light curves, and gravitational waveforms.
Various astrophysical sources have benefited from multi-wavelength and multi- messenger analyses which provided valuable information on the behavior of these sources. The proposed study aims to explore the combination of LISA with other observatories.
In particular, we suggest investigating two possible counterparts to LISA detections, (1) an electromagnetic precursor in the form of a supernova, and (2) LIGO gravitational wave (GW) detections. Both counterparts have the same underlying physical system; they are based on the astrophysical evolution of binaries in galactic nuclei.
Within the vicinity of a supermassive black hole (SMBH), the members of a stable binary have a tighter orbital configuration than the orbit of their mutual center of mass around the SMBH. The SMBH can induce collisions and mergers, between stars, and between compact objects, such as black holes and neutron stars. During the supernova that led to the formation of the stellar-compact object, the stars may undergo a supernova kick which can change the velocity magnitude and its direction. We recently showed, in a proof-of-concept study (Lu & Naoz 2018), that this could lead to one of the compact objects plunging into the nearby SMBH. This plunge forms an extreme mass ratio inspiral which will be detected by LISA. Thus, this detection will have both a LISA GW emission detection with an optical counterpart precursor. On the other hand, if the binary survived and formed a stellar-mass compact object binary, the proximity of the binary to the SMBH, yields strong acceleration and thus produces a large phase shift in the GW signal measurable by LISA (e.g., Meiron et al. 2017; Inayoshi et al. 2017). This LISA measurement will inform LIGO of an incoming merger. Depending on the merger rates (Hoang, Naoz, et al. 2018) can provide a powerful tool for synergy between LISA and LIGO observations.
Our proposal will be guided by the following questions: What is the rate of these sources? What is the probability of detecting these sources? Will LISA be able to distinguish these sources from other sources?
The planned investigation will focus on the interpretation and analysis of LISA's data from these sources. Therefore, the proposed plan will assess the capability of LISA to differentiate these sources from others. Thus, this proposed research is in accordance with NASA's goal to "Conduct astrophysics investigation that prepares for the analysis and interpretation of the LISA data, and to Evaluate the capability of LISA data for enabling astrophysics investigations."
The recent direct detections of gravitational waves (GWs) by LIGO have opened a new window on the Universe. The LIGO detections, and the success of the LISA Pathfinder mission, together have led to a resurgent interest in an evolved LISA mission, currently positioned as the ESA L3 mission, with NASA providing a 20% contribution, and with launch in the early 2030s. LISA will offer unparalleled science returns, including a view of supermassive black-hole mergers (SMBHs) to high redshifts, extreme-mass-ratio inspirals (EMRIs) of stellar-mass black holes into supermassive black holes, a census of thousands of compact binaries in the Galaxy, precision tests of general relativity and black-hole structure, and the possibility of detecting stochastic signals from the early Universe.
This proposal supports the early development of data-analysis and interpretation techniques that will maximize the future science yield of the Laser Interferometer Space Antenna (LISA), the planned low-frequency gravitational-wave (GW) observatory in space. We aim to refine numerical methods, map likelihood surfaces, and demonstrate prototype software to optimally determine the system parameters of LISA's premier GW sources (massive black-hole binaries or MBHBs, and extreme mass-ratio inspirals or EMRIs). We also plan to develop a broadly applicable technique for estimating astrophysical population parameters from an ensemble of detected and characterized systems, with special emphasis on the population of MBHBs. These contributions fit within the work-package framework outlined by the LISA Consortium Science Group, which aims to deliver a data-and science-analysis system that will meet all of the mission's key science objectives. Furthermore, the proposed work will be carried out in close collaboration and coordination with the new LISA Data Challenges (LDCs). Working within the LDCs will enable us to compare our solutions with the contributions of other LISA Consortium members, and to make them immediately available to ongoing studies of mission parameters and science requirements.