Driven by the priorities of the scientific community, the research program of the GBT covers a broad and dynamic spectrum that cuts across traditional disciplinary boundaries. Over decade, the unique capabilities of the GBT will allow for major advances in the study of the Solar System, interstellar chemistry, fundamental physics, the environment of black holes, star formation, the structure and evolution of galaxies and galaxy groups, and cosmology. Of the approximately 570 unique submission to the Astro2020 Call for Science White Papers, 38 (7%) explicitly requested the GBT to accomplish their desired scientific goals, while an additional 25- 50 papers would benefit greatly from using of the GBT to achieve their scientific goals. A breakdown of the scientific categories of these whitepapers is given below.
The Low Frequency Gravitational Wave Universe and Multi-messenger Astronomy
Detecting Gravitational Waves via Pulsar Timing Arrays: The rapid, clock-like rotation of pulsars makes them unique tools for studying fundamental physics and stellar evolution. Precision pulsar timing at the level of tens to hundreds of nanoseconds allows detection of nanohertz gravitational waves from supermassive binary black holes and, potentially, from exotic sources such as cosmic strings. With modest projected increases in current sensitivity in the near term as additional milli second pulsars are monitored, the stochastic background is expected to be detected within the next several years and GWs from individual SMBH binaries before the end of the next decade, enabling multi-messenger study of these systems. The GBT will provide sensitivity and sky coverage that is key to the success of this program (Taylor, et al. 2019;Cordes, et al. 2019). Pulsar timing also provides the best prospects of detecting gravitational waves from more exotic sources such as cosmic strings (Siemens, et al. 2007), phase transitions in the early Universe, and as relics from the inflationary era, all of which would provide unique windows into high-energy and early-Universe physics.
Imaging Gravitational Wave Events: The direct detection of gravitational waves by LIGO from merging objects has opened a new window on the Universe. Radio follow-up of Gravitational Wave events is essential to localizing the event and studying different facets of the merger; e.g., resolved imaging using VLBI techniques represents the only direct way to map the kinematic distribution of merger ejecta (Corsi, et al. 2019). The GBT used as an element of a long-baseline array will continue to provide unique data for radio studies of Gravitational Wave events, e.g., the weak radio emission associated with the binary neutron star merger GW170817 (Mooley, et al. 2018;Ghirlanda, et al. 2019). There is no instrument either current or proposed that can replace the GBT’s role in these long-baseline measurements.
Time Domain Astronomy: Rapid response is an essential part of multi-messenger science and this need will only grow as Gravitational Wave detectors become more sensitive and the LSST opens a new era of discovery space in time-domain astronomy. Uses of the GBT will include searches for pulsars in newly-discovered compact objects, bi-static radio studies of near-Earth objects, and VLBI observations to characterize the evolution of energetic events at an angular resolution of tens of micro-arcsec (Ray, et al. 2013;Michilli, et al. 2018;Casadio, et al. 2019).
Tests of gravity and ultra-dense matter: It is remarkable that although pulsars were first discovered 60 years ago (Hewish, et al. 1968), many of the best systems for studying extreme physics were only discovered, and their potential fully exploited, within the last decade. Pulsars can be used to test theories of gravity and compact matter at otherwise inaccessible energy scales. Binary pulsars have been used to measure high-order relativistic effects (Kramer, et al. 2009;Kramer, et al. 2006), place limits on theories of gravity (Antoniadis; et al. 2013), and to test the strong equivalence principle (Archibald, et al. 2018). Neutron star masses measured through pulsar timing have substantially constrained quark-matter models for the equation of state of dense matter (Demorest, et al. 2010;Fonseca, et al. 2019). Searches for new pulsars remain limited by telescope sensitivity, sky coverage, and the computational power needed to find systems in close binaries (which are often the most scientifically interesting). Single dish telescopes like the GBT are necessary for surveys to detect new pulsars, since searches with interferometers require orders of magnitude more computational power.
Changes of fundamental constants: Comparisons between the redshifts of multiple spectral transitions from distant galaxies provide a sensitive probe of secular evolution in fundamental constants such as the fine structure constant and the proton-electron mass ratio over cosmological epochs. The GBT will provide a substantial increase in the number of redshifted radio absorbers in 4 mm-wavelength lines of CO and HCO+ toward distant sources, including new and improved samples of SDSS Type-2 quasars (Ghosh, et al. 2019).
Sunyaev-Zeldovich Effect: This well established tool is used for performing high-resolution studies of the warm and hot ionized gas in and between galaxies, groups, and clusters. Galaxy groups and clusters are powerful probes of cosmology, and serve as hosts for roughly half of the galaxies in the Universe. The GBT, with its high resolution and sensitivity at 3-mm, particularly when outfitted with broadband, wide field bolometric arrays such as MUSTANG-2 and potential upgrades will probe scales from 10’s of kpc to those nearly as large as the cluster virial radius (> 1 Mpc) across the epoch of cluster formation (z < 2) (Mroczkowski, et al. 2019). Sunyaev-Zeldovich studies would also be fundamental for the study of filamentary structures between galaxy clusters and shed light on the open problem of the missing baryons in our Universe as well as on the hierarchical structure formation scenario (Battistelli, et al. 2019). Finally, Sunyaev-Zeldovich studies will be used to understand the co-evolution of massive galaxies at high-z with their circumgalactic medium (Emonts, et al. 2019).
Formation & Evolution of Compact Objects
Fast Radio Bursts & other Transients: The last decade has seen the rapid growth in the study of new classes of energetic astrophysical transients – Fast Radio Bursts; extremely luminous transients, such as the super luminous supernova; and “ultra long” GRBs with durations exceeding thousands of seconds. It is possible these transients are all manifestations of magnetar birth (Law, et al. 2019). The GBT, with its wide frequency range, 85% sky coverage, and high instantaneous sensitivity, is one of the critical instruments in this field, used as a single dish to detect and characterize the transients (e.g., Michilli, et al. 2018) and for high angular resolution high sensitivity interferometry (e.g., Mooley, et al. 2018).
Neutron Stars: Our understanding of the neutron star population is informed to a great degree by large surveys that have been carried out by radio facilities during the past fifty years. The GBT has been, and will continue to be, a key instrument for these surveys. By the end of the next decade, a significantly more complete census of the Galactic pulsar population will be done. Among the anticipated discoveries are pulsar–black hole binary systems that will provide further of our understanding of gravity in strong-fields, as well as large numbers of millisecond pulsars that are crucial to enhancing the sensitivity of timing arrays for low-frequency gravitational waves (Lorimer, et al. 2019).
Black Holes and Active Galactic Nuclei: The GBT will continue to contribute its enormous sensitivity to VLBI studies of AGN and black holes. It will produce high-resolution maps of H2O masers in accretion disks and mass constraints for supermassive black holes with uncertainties of only a few percent (e.g. Zhao, et al. 2018). The appearance of a prominent, jet-like structure observed through GBT VLBI in a merging galaxy is interpreted as tidal disruption of a star by a black hole (Mattila, et al. 2018). In just the last few years the GBT has developed the ability to participate in VLBI observations at 3mm, but these are already producing spectacular results on the jet in M87 (Hada, et al. 2016; Kim, et al. 2018) and the Milky Way’s central black hole, Sgr A* (Issaoun, et al. 2019).
Galaxy Formation & Evolution
Milky Way: The GBT will make several spectral surveys of the inner Milky Way in the next decade. The Fermi Bubbles, two giant plasma lobes emanating from the Galactic center, are a local example of energetic nuclear feedback that can shape a galaxy’s evolution. Sensitive 21-cm HI and CO observations with the GBT, combined with optical and UV spectroscopy, will track gas entrained within the outflow, revealing its morphology, extent, kinematics, and mass loss (Fox, et al. 2019b). The GBT will map molecules in the dust lane within the Galactic Bar to determine its physical properties, connection to the Central Molecular Zone, and potential for star formation (Butterfield, et al. 2019).
Magellanic clouds: The Magellanic Stream is the most spectacular example of a gaseous stream in the local Universe. Its interwoven filaments trailing the Magellanic Clouds as they orbit the Milky Way are thought to be created by tidal forces, ram pressure, and halo interactions, making it an excellent benchmark for dynamical models of galaxy evolution. A combination of 21-cm HI observations from the GBT and UV spectroscopy will resolve a number of key issues, including the mass inflow rate of gas and the Stream’s total spatial extent. (Fox, et al. 2019a).
High redshift galaxies: There is a growing focus on measuring the total molecular/atomic gas that surrounds gas-rich galaxies in proto-clusters. The GBT will complement interferometric studies of these redshifted systems by supplying information on low surface-brightness molecular emission over angular scales missing from the interferometer data. The GBT will map the redshifted CI, CO(1-0; 2-1; 3-2) line emission surrounding the most over-dense regions at 3.3 < z < 5.6 searching for previously undetected, low-excitation, gas-rich systems. This will reveal how the gas depletion time changes with distance to the proto-cluster core, and how the brightness temperature of dense gas tracers depends on the circumgalactic medium (Harrington, et al. 2019; Emonts, et al. 2019; Casey, et al. 2019).
Local Universe: Lyman-α and metal line absorption observations have established the ubiquity of a gas-rich circumgalactic medium around star-forming galaxies at z<0.2, potentially containing half of the missing baryonic mass within galaxy halos. However, such studies leave open the question as to how this gas flows from the circumgalactic medium onto the disks of galaxies to fuel ongoing star formation. Planned observations with the GBT will complete the census of HI in the local circumgalactic medium at NHI<1017cm−2, a sensitivity level unattainable by interferometers (Pisano, et al. 2019). Through deep HI mapping the GBT will also shed light on the origin of high-velocity clouds, which may mediate between the circumgalactic medium and the disk of galaxies, thus constraining gas accretion processes in the local universe (Lockman, et al. 2019).
Star and Planet Formation
A fundamental question in astronomy is how stars form, and how this impacts the formation of planetary systems and the evolution of galaxies over time.
Star Formation Mechanisms: The mechanisms that regulate star formation span a huge range of spatial scales, from the sub-parsec scales on which individual stars form to the the kiloparsec scales of galactic encounters. Molecular transitions of NH3, CO, HCN, N2H+, and HCO+ are important tracers of gas at different temperatures and densities. While telescopes like ALMA provide excellent sensitivity to dense, compact structures, single-dish telescopes like the GBT are essential to map mid-sized to large spatial scales (Friesen, et al. 2019;Kauffmann, et al. 2019).
Chemical Inventory: The GBT will be key in determining the chemical inventory of the Galaxy and the relation between chemistry and phases of star and planet formation through spectral line surveys and mapping of molecular clouds. More than 200 distinct molecular species have been detected in various astronomical environments, but we still lack a chemical theory for their formation. This is a critical gap as chemistry is an integral part of star-formation and the processes that eventually results in the organic molecules that have been detected in meteors and, likely, life on Earth (Remijan, et al. 2019;McGuire, et al. 2019a). In the last several years it has been shown that there may be a significant amount of previously undetected carbon, which is influencing the star and planet formation process. GBT observations will be crucial in understanding the role of these PAHs in stellar (and planetary) life cycles (McGuire, et al. 2019b;2019c).
Low Density Star Formation: The current knowledge of star formation in low-density, HI-dominated gas is significantly sparser than our understanding of star formation in the metal-rich disks of spiral galaxies. Understanding these low density environments is vital to understanding dwarf galaxies, the most common galaxies in the Universe. A multi-wavelength approach is required over the next decade, including deep, sensitive maps of molecular gas in a wide variety of low-density environments made with multi-pixel cameras on GBT (Thilker, et al. 2019).
Slow Transient events: Large, single dish telescopes working alone and as part of high sensitivity VLBI observations, are the major contributor to the discovery and study of slow, secular transient events. These phenomena include tidal disruption events caused by stars passing within the tidal radius of SMBH, core collapse of supernovae, and transient maser emission typically associated with OH/IR stars (Salter, 2019).
More than half of the dust and heavy elements in galaxies originates from the winds and out-flows of low-to-intermediate mass asymptotic giant branch stars, and many questions regarding this process remain. In the coming decade, more sensitive VLBI observations which include the GBT will provide high time- and spatial-resolution images of masers in a sample of nearby (d <1 kpc) AGB stars spanning a wide range of properties. This will enable a deeper understanding of the atmospheric physics and mass-loss processes in these objects (Matthews, et al. 2019).
Structure in the ISM: The organization of the interstellar medium (ISM) into filaments comprising molecular dust and gas has been observed for many years (see e.g. Schneider & Elmegreen, 1979). However, the exact nature and specification of these filaments, as well as the processes within that contribute to the overall Galactic star formation rate are still being explored.
Establishing a more definitive physical description of the star-forming process requires a better understanding of the level of thermal and turbulent support in small-scale clouds and the relationship of this support to environmental conditions. These questions may all be answered through the systematic observations of the dense gas present in cores and filaments (Friesen et al, 2019).
The GBT will continue to be used to measure properties of objects in solar systems, both our own and others, as a stand-alone facility, for long-baseline interferometry, and as the passive element of bistatic radar studies.
Bistatic Radar: Understanding the nature of planetary cores is vital to understanding the formation and evolution of the planets. For the Solar System’s terrestrial planets and ocean worlds this can be done through a bistatic radar speckle technique which will show not only the liquidity of the bodies’ core but also the nature of the coupling between the worlds’ exterior shell and interior core (Margot, et al. 2019a). GBT bistatic radar observations of the Solar System’s terrestrial planets and ocean worlds reveal details that otherwise cannot be seen without dedicated missions to visit the planetary bodies. Observations of Venus in the coming decade will test numerous hypotheses regarding the dynamics and super-rotation of the planet’s atmosphere, and its connection to surface features on the planet. Bistatic observations will also allow for detailed maps of the surface of the Jovian satellites, Mercury, and Mars in the coming decade (Margot, et al. 2019a, Campbell, et al. 2019).
Asteroids: Characterizing and tracking the asteroid population of the Solar System relates the properties and origins of these objects to the formation and evolution of the Solar System, and is a vital part of the National Science and Technology Council’s strong recommendation to address the hazards of near-Earth object (NEO) impacts over the next 10 years. In the next decade the number of known asteroids will skyrocket with LSST and NEOCam coming online. Characterization of these asteroids, though, requires mono- and bi-static radar observations that would include the GBT (Rivera-Valentín, et al. 2019).
Planetary Magnetic Fields: Measurement of the magnetic field of a planet is one of the few remote sensing means of constraining the properties of planetary interiors. Because Earth’s magnetic field may be partially responsible for its habitability, knowledge of an extra-solar planet’s magnetic field may be critical to understanding its possible habitability. Accurate measurements of the magnetic field of Solar System bodies will refine our understanding of when and how to apply this technique outside the Solar System. Young planets may sustain large magnetic fields, detectable by the GBT at cm-wavelengths (Lazio et al. 2019).
Technosignatures: The search for technosignatures from outside our Solar System will continue to advance in the 2020s. Studies of the Earth’s radio usage and transmissions will provide a baseline understanding of the types of signal expected through radio leakage from Earth-like civilizations (DeMarines, et al. 2019; Haqq-Misra, et. al. 2019). Significant improvements in the data science field, such as improved detection and deep learning techniques, will also greatly enhance the use of data from the GBT and other telescopes (Berea, et al. 2019). New surveys will greatly increase the fraction of the available volume and frequency space sampled (Margot, et al. 2019b).