The first terrestrial Gravitational Wave (GW) interferometers have dramatically underscored the scientific value of observing the Universe through an entirely different window and of folding this new channel of information with traditional astronomical data for a multimessenger view. The Laser Interferometer Space Antenna (LISA) will broaden the reach of GW astronomy by conducting the first survey of the millihertz GW sky, detecting tens of thousands of individual astrophysical sources ranging from white dwarf binaries in our own galaxy to mergers of massive black holes (MBHs) at redshifts extending beyond the epoch of reionization. These observations will inform and transform our understanding of the end state of stellar evolution, MBH birth, and the co-evolution of galaxies and black holes through cosmic time. LISA also has the potential to detect GW emission from elusive astrophysical sources such as intermediate-mass black holes as well as exotic cosmological sources such as inflationary fields and cosmic string cusps. LISA is now in Phase A as a European Space Agency (ESA) led mission with significant contributions anticipated from several ESA member states and NASA. The mission concept retains all essential features of the NASA/ESA LISA mission that was ranked as the 3rd priority for Large class missions in the 2010 Decadal Survey, including the full three-arm triangular configuration that measures GW polarization and improves robustness. Since that ranking, LISA's technical readiness has been greatly advanced through two flight demonstrations: the ESA-led LISA Pathfinder mission (2015-2017) and the Laser Ranging Instrument on board the US/German Gravity Recovery And Climate Experiment Follow-On mission (2018-). The Midterm Assessment of the 2010 Decadal Survey recommended that the US participate as a "strong technical and scientific partner" in an ESA-led LISA mission. NASA is currently supporting preproject activities to support a range of potential contributions to LISA including instruments, spacecraft elements, and science analysis. The currently envisioned scale of these contributions is at the lower end of the medium-scale cost range identified by Astro2020 ($500M - $1.5B). A recommendation for an upscope in US participation in LISA would provide opportunities to more fully exploit heritage from prior US investments, balance technical and programmatic risks across the partnership, and expand opportunities for future US leadership in this new field of astronomy.
Ultra-compact binaries (UCBs) are systems containing compact or degenerate stars with orbital periods less than one hour. Tens of millions of UCBs are predicted to exist within the Galaxy emitting gravitational waves (GWs) at mHz frequencies. Combining GW searches with electromagnetic (EM) surveys like Gaia and LSST will yield a comprehensive, multimessenger catalog of UCBs in the galaxy. Joint EM and GW observations enable measurements of masses, radii, and orbital dynamics far beyond what can be achieved by independent EM or GW studies. GW+EM surveys of UCBs in the galaxy will yield a trove of unique insights into the nature of white dwarfs, the formation of compact objects, dynamical interactions in binaries, and energetic, accretion-driven phenomena like Type Ia supernovae.
There are two big questions cosmologists would like to answer – How does the Universe work, and what are its origin and destiny? A long wavelength gravitational wave detector – with million km interferometer arms, achievable only from space – gives a unique opportunity to address both of these questions. A sensitive, mHz frequency observatory could use the inspiral and merger of massive black hole binaries as standard sirens, extending our ability to characterize the expansion history of the Universe from the onset of dark energy-domination out to a redshift z ∼ 10. A low-frequency detector, furthermore, offers the best chance for discovery of exotic gravitational wave sources, including a primordial stochastic background, that could reveal clues to the origin of our Universe.
Coalescing, massive black-hole (MBH) binaries are the most powerful sources of gravitational waves (GWs) in the Universe, which makes MBH science a prime focus for ongoing and upcoming GW observatories. The Laser Interferometer Space Antenna (LISA) – a gigameter scale spacebased GW observatory – will grant us access to an immense cosmological volume, revealing MBHs merging when the first cosmic structures assembled in the Dark Ages. LISA will unveil the yet unknown origin of the first quasars, and detect the teeming population of MBHs of 104-7 M⊙ forming within protogalactic halos. The Pulsar Timing Array, a galactic-scale GW survey, can access the largest MBHs the Universe, detecting the cosmic GW foreground from inspiraling MBH binaries of ∼ 109 M⊙. LISA can measure MBH spins and masses with precision far exceeding that from electromagnetic (EM) probes, and together, both GW observatories will provide the first full census of binary MBHs, and their orbital dynamics, across cosmic time. Detecting the loud gravitational signal of these MBH binaries will also trigger alerts for EM counterpart searches, from decades (PTAs) to hours (LISA) prior to the final merger. By witnessing both the GW and EM signals of MBH mergers, precious information will be gathered about the rich and complex environment in the aftermath of a galaxy collision. The unique GW characterization of MBHs will shed light on the deep link between MBHs of 104 – 1010 M⊙ and the grand design of galaxy assembly, as well as on the complex dynamics that drive MBHs to coalescence.
The tidal disruption of stars by (super-)massive black holes in galactic nuclei has been discussed in theoretical terms for about 30 years but only in the past decade have we been able to detect such events in substantial numbers. Thus, we are now starting to carry out observational tests of models for the disruption. We are also formulating expectations for the inspiral and disruption of white dwarfs by "intermediate-mass" black holes with masses ≲ 105 M⊙. Such events are very rich with information and open a new window to intermediate-mass black holes, thought to live in dwarf galaxies and star clusters. They can inform us of the demographics of intermediate-mass black holes, stellar populations and dynamics in their immediate vicinity, and the physics of accretion of hydrogen-deficient material. The combination of upcoming transient surveys using ground-based, electromagnetic observatories and low-frequency gravitational wave observations is ideal for exploiting tidal disruptions of white dwarfs. The detection rate of gravitational wave signals, optimistically, may reach a few dozen per year in a volume up to z ≈ 0.1. Gravitational wave observations are particularly useful because they yield the masses of the objects involved and allow determination of the spin of the black hole, affording tests of physical models for black hole formation and growth. They also give us advance warning of the electromagnetic flares by weeks or more. The right computing infrastructure for modern models for the disruption process and event rates will allow us to make the most of the upcoming observing facilities.
The LIGO/Virgo gravitational-wave (GW) interferometers have to-date detected ten merging black hole (BH) binaries, some with masses considerably larger than had been anticipated. Stellar-mass BH binaries at the high end of the observed mass range (with "chirp mass" ℳ ≳ 25M⊙) should be detectable by a space-based GW observatory years before those binaries become visible to ground-based GW detectors. This white paper discusses some of the synergies that result when the same binaries are observed by instruments in space and on the ground. We consider intermediate-mass black hole binaries (with total mass M ∼ 102 – 104M⊙) as well as stellar-mass black hole binaries. We illustrate how combining space-based and ground-based data sets can break degeneracies and thereby improve our understanding of the binary's physical parameters. While early work focused on how space-based observatories can forecast precisely when some mergers will be observed on the ground, the reverse is also important: ground-based detections will allow us to "dig deeper" into archived, space-based data to confidently identify black hole inspirals whose signal-to-noise ratios were originally sub-threshold, increasing the number of binaries observed in both bands by a factor of ∼ 4 – 7.
LISA will open the mHz band of gravitational waves (GWs) to the astronomy community. The strong gravity which powers the variety of GW sources in this band is also crucial in a number of important astrophysical processes at the current frontiers of astronomy. These range from the beginning of structure formation in the early universe, through the origin and cosmic evolution of massive black holes in concert with their galactic environments, to the evolution of stellar remnant binaries in the Milky Way and in nearby galaxies. These processes and their associated populations also drive current and future observations across the electromagnetic (EM) spectrum. We review opportunities for science breakthroughs, involving either direct coincident EM+GW observations, or indirect multimessenger studies. We argue that for the US community to fully capitalize on the opportunities from the LISA mission, the US efforts should be accompanied by a coordinated and sustained program of multi-disciplinary science investment, following the GW data through to its impact on broad areas of astrophysics. Support for LISA-related multimessenger observers and theorists should be sized appropriately for a flagship observatory and may be coordinated through a dedicated mHz GW research center.
The origin and properties of black hole seeds that grow to produce the detected population of supermassive black holes are unconstrained at present. Despite the existence of several potentially feasible channels for the production of initial seeds in the high redshift universe, since even actively growing seeds are not directly observable at these epochs, discriminating between models remains challenging. Several new observables that encapsulate information about seeding have been proposed in recent years, and these offer exciting prospects for truly unraveling the nature of black hole seeds in the coming years. One of the key challenges for this task lies in the complexity of the problem, the required disentangling of the confounding effects of accretion physics and mergers, as mergers and accretion events over cosmic time stand to erase these initial conditions. Nevertheless, some unique signatures of seeding do survive and still exist in: local scaling relations between black holes and their galaxy hosts at low-masses; in high-redshift luminosity functions of accreting black holes; and in the total number and mass functions of gravitational wave coalescence events from merging binary black holes. One of the clearest discriminants for seed models are these high redshift gravitational wave detections of mergers from space detectable in the milliHertz range. These predicted event rates offer the most direct constraints on the properties of initial black hole seeds. Improving our theoretical understanding of black hole dynamics and accretion will also be pivotal in constraining seeding models in combination with the wide range of multi-messenger data.
The inspiral of a stellar-mass compact object into a massive (∼ 104 – 107 M⊙) black hole produces an intricate gravitational-wave signal. Due to the extreme-mass ratios involved, these systems complete ∼ 104 – 105orbits, most of them in the strong-field region of the massive black hole, emitting in the frequency range ∼ 104 – 1 Hz. This makes them prime sources for the space-based observatory LISA (Laser Interferometer Space Antenna). LISA observations will enable high-precision measurements of the physical characteristics of these extreme-mass-ratio inspirals (EMRIs): redshifted masses, massive black hole spin and orbital eccentricity can be determined with fractional errors ∼ 104 – 106, the luminosity distance with better than ∼10% precision, and the sky localization to within a few square degrees. EMRIs will provide valuable information about stellar dynamics in galactic nuclei, as well as precise data about massive black hole populations, including the distribution of masses and spins. They will enable percent-level measurements of the multipolar structure of massive black holes, precisely testing the strong-gravity properties of their spacetimes. EMRIs may also provide cosmographical data regarding the expansion of the Universe if inferred source locations can be correlated with galaxy catalogs.
A space-based interferometer operating in the previously unexplored mHz gravitational band has tremendous discovery potential. If history is any guide, the opening of a new spectral band will lead to the discovery of entirely new sources and phenomena. The mHz band is ideally suited to exploring beyond standard model processes in the early universe, and with the sensitivities that can be reached with current technologies, the discovery space for exotic astrophysical systems is vast.
While still in its infancy, the budding field of gravitational-wave astronomy has so far exceeded most expectations, and the achievements that have already been made bode well for the decade to come. While the discoveries made possible by LIGO have captured the imagination of experts and nonexperts alike, it is important when looking ahead to consider those discoveries in the context of the field as a whole. Just as radio, optical, and X-ray radiation probe different physical phenomena occurring on a range of length and energy scales, the future of gravitational-wave astrophysics depends on our ability to open up the entire spectrum. We will describe the scientific prospects for the field of gravitational-wave astronomy as a whole as we enter the coming decade, and we will place the specific contributions from a future space-based gravitational-wave observatory within this context.
Low-frequency gravitational-wave astronomy can perform precision tests of general relativity and probe fundamental physics in a regime previously inaccessible. A space-based detector will be a formidable tool to explore gravity' s role in the cosmos, potentially telling us if and where Einstein' s theory fails and providing clues about some of the greatest mysteries in physics and astronomy, such as dark matter and the origin of the Universe.
Observational evidence has been mounting for the existence of intermediate mass black holes (IMBHs, 102 – 105 M⊙), but observing them at all, much less constraining their masses, is very challenging. In one theorized formation channel, IMBHs are the seeds for supermassive black holes in the early universe. As a result, IMBHs are predicted to exist in the local universe in dwarf galaxies, as well as wandering in more massive galaxy halos. However, these environments are not conducive to the accretion events or dynamical signatures that allow us to detect IMBHs. The Laser Interferometer Space Antenna (LISA) will demystify IMBHs by detecting the mergers of these objects out to extremely high redshifts, while measuring their masses with extremely high precision. These observations of merging IMBHs will allow us to constrain the formation mechanism and subsequent evolution of massive black holes, from the 'dark ages' to the present day, and reveal the role that IMBHs play in hierarchical galaxy evolution.