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Report of the Panel on Particle Astrophysics and Gravitation

L.1 EXECUTIVE SUMMARY

Our universe is almost certainly populated with sources more wondrous and consequential than anything we have seen or even imagined. This belief is well supported by astronomical history, where surprises have been common when new capabilities are developed. It drives the core work of astronomy—that is, observation, from which all else follows—to consistently reach for greater sensitivity.

Astronomy has been revolutionized by observations in increasingly broad swaths of the electromagnetic spectrum, for example, through imaging black holes with radio interferometry, seeing the dust-enshrouded hearts of galaxies with infrared light, and revealing constellations of high-energy sources with X-rays. Multiwavelength observations have also revolutionized the understanding of physics—for example, through establishing the foundations of the Hot Big Bang cosmology, testing general relativity with binary pulsars, and revealing the cauldrons where the elements are made. The hallmark of this work, for which projects with sensitivity up to hard X-ray energies have largely been developed, funded, and carried out as part of astronomy programs, is its precision for localizing and measuring sources.

Astronomy is now also being revolutionized by observations with new messengers—gravitational waves, neutrinos, gamma rays, and cosmic rays—that greatly complement and are leveraged by observations in conventional astronomy. The hallmark of this work, for which the projects have largely been developed, funded, and carried out as part of physics programs, is its ability to probe extremes of energy, fields, and density. The panel characterizes these four probes as new messengers owing to huge advances since the previous decadal survey. Gravitational waves and very-high-energy neutrinos were detected for the first time. For gamma rays, there have been dramatic advances in the energy range, angular resolution, and number of sources. For cosmic rays, there have been dramatic advances in the precision and composition of the spectra, plus hints of sources. The breakthrough discoveries enabled by these new messengers will be broadened and deepened by ongoing and planned experiments.

Astronomy can also be revolutionized by greater efforts on diversity, equity, and inclusion, which will lead to new perspectives and discoveries, as well as societal benefits.

From observations with these new messengers, we now know that our universe contains objects with surprising properties that were hidden from conventional astronomical studies. These objects include the following:

• Extreme gravitators, with incredibly strong fields that distort space-time (including electromagnetically dark mergers of black holes);
• Extreme accelerators, with total power and per-particle energy far beyond laboratory experiments (including neutrino sources that are currently unknown and that may be hidden from electromagnetic observations); and
• Multi-messenger sources, where some of these processes are also revealed by electromagnetic radiation, especially gamma rays (including mergers of neutron stars, gamma-ray bursts, flares of active galactic nuclei, and more).

The techniques of particle astrophysics and gravitation, in addition to probing such sources, are also essential for fundamental studies of cosmology, including inflation, dark matter, and tests of new physics.

The Particle Astrophysics and Gravitation (PAG) program panel (hereafter “the panel”) was charged to “identify and suggest to the Decadal Survey committee a program of federal investment in research activities” within its topical scope. The panel reviewed observatories using these new astronomical messengers and considered technology-development and other needs to support cutting-edge programs that probe both the sources noted above and the properties of the new messengers. While the panel reviewed white papers for many worthy potential projects, it suggests that only a fraction of them are compelling for significant investments in the 2020s. This report is focused on those projects.

The panel sees a compelling opportunity to dramatically open the discovery space of astronomy through a bold, broad multi-messenger program, with three components:

• Neutrino program: A large-scale (Major Research Equipment and Facilities Construction [MREFC]) investment by the National Science Foundation (NSF) in IceCube Generation 2 (IceCube-Gen2), to resolve the bright, hard-spectrum, TeV–PeV diffuse background discovered by IceCube into discrete sources and to make first detections at higher energies.
• Gravitational-wave program: Medium-scale investments in three bands (kHz, nHz, and mHz) to develop a rich observational program: Cosmic Explorer, with NSF support for technology development to set the stage for large-scale investments and huge detection rates in the 2030s; the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), with NSF support for expanded operations in the 2020s; and the Laser Interferometer Space Antenna (LISA), with National Aeronautics and Space Administration (NASA) support for a broad scope of activities to build a vibrant U.S. community for significant science contributions in the 2030s.
• Gamma-ray program: Medium-scale investments that support observations over a wide energy range, with two components. (For simplicity, this report uses “gamma ray” to mean photons at or above hard X-ray energies.) First, a NASA probe-scale mission, targeted to multi-messenger astronomy, with sensitivity in the keV–MeV–GeV range and with capabilities for the identification, localization, and characterization of transients. This would be selected by competitive review; potential projects include the All-Sky Medium Energy Gamma-Ray Observatory (AMEGO), the Advanced Particle-Astrophysics Telescope (APT), or the Transient Astrophysics Probe (TAP). Second, U.S. participation in TeV-range ground-based experiments for precision studies—for example, the Cherenkov Telescope Array (CTA) and the Southern Wide-Field Gamma-Ray Observatory (SWGO)—as NSF medium-scale projects. All

• of these projects will be valuable themselves—gamma rays reveal processes that longer-wavelength photons cannot—and will greatly enhance the returns of neutrino and gravitational-wave observatories.

In cosmic rays, the scientific opportunities are also outstanding, including the possibility of eventual directional astronomy with charged particles, but continued science and technology development is needed to drive sufficient advances over current and planned experiments.

For the whole multi-messenger program above, the costs would be modest (details below in Box L.2 and supporting text), while the scientific returns would be outstanding. Even greater returns will follow if these new observatories are operated simultaneously with each other and with the growing transient program in conventional electromagnetic astronomy, especially the Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST). Theory and computing support will be critical to all aspects of this, as those connect disparate observations to each other and to fundamental physics, predict new phenomena, and guide efficient experimental designs to observe them.

Until new discovery-class observatories for gravitational waves, neutrinos, and gamma rays begin operations, it is critical to maintain support for key existing experiments: the Laser Interferometer Gravitational-Wave Observatory (LIGO) and its A+ upgrade, NANOGrav, IceCube, the High-Altitude Water Cherenkov (HAWC) Observatory, the Very Energetic Imaging Telescope Array System (VERITAS), the Fermi Gamma-Ray Space Telescope (Fermi), the Neil Gehrels Swift Observatory (Swift), space- and ground-based cosmic-ray observatories, as well as a range of smaller experiments, especially those that are developing new technologies. The combined timeline for current and future facilities is shown later in Figure L.3, and individual capabilities are discussed below.

Developing this new program will require unprecedented coordination of these new projects with each other and with conventional astronomy programs, technology development to create significantly greater capabilities, and cultural work to develop the necessary connections between astronomy and physics and in how collaborations operate.

If this new program is realized, by 2030 the broad field of astronomy would look very different, with robust new astronomies in gravitational waves, neutrinos, and gamma rays built on incredible discoveries individually and especially through their synthesis, allowing detailed studies in the 2030s. As part of this, the United States would maintain and grow leading roles in forefront fields that it had huge roles

in developing and which are now rapidly expanding worldwide. Without this new program, the science opportunities—and especially the U.S. roles in them—would be greatly impoverished.

L.2 DREAMS OF NEW ASTRONOMIES

The fundamental goal of astronomy is to observe and understand the universe and its constituents. In 2015, the detection of gravitational waves from the collision of two black holes (GW150914) started a new era in gravitational wave astronomy. Two years later, the age of multi-messenger astronomy was ushered in by two breakthrough discoveries: the detection of gravitational waves by LIGO from a binary neutron star inspiral (GW170817) and the detection of an astrophysical neutrino by IceCube during a blazar flare (TXS 0506+056). These source detections, plus IceCube observations since 2013 of a bright, hard-spectrum TeV–PeV diffuse background, have revealed much richer prospects than can be seen with conventional astronomical observations. We now know that our universe contains:

• Extreme gravitators, where the dynamics of strong-field gravity produce deformations of the fabric of space-time that are detectable across cosmic distances. LIGO has observed dozens of electromagnetically dark mergers of binary stellar-mass black holes, showing that gravitational-wave observations are essential to astronomy.
• Extreme accelerators, with huge luminosities of charged particles and accompanying gamma rays and neutrinos, and with per-particle energies ranging up to the TeV–PeV range and sometimes much higher. IceCube observations of the diffuse neutrino flux suggest a dominant population of sources that are gamma-ray obscured, showing that neutrino observations are essential to astronomy.
• Many and varied multi-messenger sources, where simultaneous observations are critical. The 2017 observations of GW170817 and TXS 0506+056 achieved their discovery potential thanks to detections by Fermi and other gamma-ray observatories, plus optical and other telescopes, which facilitated quick, deep follow-up in astronomical electromagnetic bands. As the sensitivity of gravitational-wave and neutrino astronomy increases, facilitating the pathways between different types of observatories will become even more essential.

Only observations with new messengers can reveal these new sources and solve many long-standing questions. In astronomy, these questions include the details of stellar endpoints, the jets in active galactic nuclei, and the universe’s dark processes. In physics, these include fundamental tests of gravity, the nuclear equation of state, and the particle properties of neutrinos and dark matter. Figure L.1 highlights some of these potential discovery areas.

New-messenger observations are key to answering some of the high-priority questions identified by the Astro2020 science panels. For the Panel on Compact Objects and Energetic Phenomena (COEP), these include their overall discovery area plus several questions:

• COEPD: Transforming our view of the universe by combining new information from light, particles, and gravitational waves.
• COEP1: What are the mass and spin distributions of neutron stars and stellar black holes?
• COEP2: What powers the diversity of explosive phenomena across the electromagnetic spectrum?
• COEP3: Why do some compact objects eject material in nearly light-speed jets, and what is that material made of?
• COEP4: What seeds supermassive black holes, and how do they grow?

New-messenger observations are also key to questions identified by the Panel on Cosmology (COS) and the Panel on Stars, the Sun, and Stellar Populations (STARS), especially:

• COS3: What physics drives the cosmic expansion and large-scale evolution of the universe?
• COS4: How will measurements of gravitational waves reshape our cosmological view?
• STARS4: How do the Sun and other stars create space weather?

To enable discoveries that answer these questions, the first step is to build powerful observatories for new messengers. For sources with multi-messenger signals, such as binary neutron star mergers, flares of active galactic nuclei, and a Milky Way supernova, broad-based, simultaneous detections are critical. Gamma-ray monitors like Fermi and Swift, but with improved capabilities, are needed to find and localize events for follow-up in conventional electromagnetic observations, which provide precise details and localization in sky coordinates and redshift. Progress depends not only on new experiments but also on new investments in theory and computation, as well as on addressing broader issues, as discussed in Section L.6.

The increasing tilt of physics research toward astrophysics and cosmology arises from the recognition that these are powerful tools to address fundamental questions beyond the reach of laboratory experiments. Arguably, this tilt has been one of the most significant developments for astronomy in the past few decades, and it can be encouraged. When do observations with new messengers become astronomy per se? One answer is when we detect multiple localized sources. For gamma rays and gravitational waves, this has been attained; for high-energy neutrinos, it is within close reach; and for cosmic rays, it is a hope to be nurtured. Sources that appear only in gravitational waves and/or neutrinos are especially interesting, as they reveal the universe’s dark processes. By the end of this decade, astounding discoveries are near certain, provided that the field and funding agencies make the right choices. For neutrinos and gravitational

waves, observing even small numbers of multi-messenger point sources can be extremely significant, as this could reveal the origin of IceCube’s diffuse background and critical details about binary neutron-star mergers and their connection to gamma-ray bursts. Detections in the 2020s can pave the way for much more powerful observatories and higher statistics in the 2030s. Figure L.2 illustrates some of the prospects. This report highlights the most critical elements of a bold, broad program that is within reach.

L.3 DESCRIPTION OF ACTIVITIES CONSIDERED

This panel differs from other program panels in three ways. First, although its topics have long been pursued, its relevance for astronomy has increased greatly in the past several years. Second, many of the proposed investments within its scope would be funded outside the usual mechanisms in astronomy. Third, it is the natural home of multi-messenger astronomy, which requires a new level of coordination among the fields examined by the Astro2020 survey.

The panel inputs included the forefront questions identified by the science panels, 23 project white papers, responses to requests for information sent to many projects, and independent Technical, Risk, and Cost Evaluations (TRACEs; see Appendix O) of some projects. All of these materials were carefully read and considered by the panel over its two in-person meetings and its weekly online meetings. The panel took into account the international context and physics projects that did not submit white papers. It analyzed the capabilities needed to address science panel questions, through observing and understanding extreme gravitators, extreme accelerators, and multi-messenger sources, in comparison to opportunities. Importantly, the panel considered how to maximize the scientific return by developing diverse fields as a coherent whole on a viable, coordinated timeline.

The fields in the scope of the panel—gravitational waves, neutrinos, gamma rays, and cosmic rays—need to be considered coherently with each other and with the conventional astronomy program to maximize the value of research in each field and for astronomy as a whole. The most compelling programs for new investment thus strongly depend on the landscape of existing and planned experiments, in the United States and abroad, and its gaps. Figure L.3 illustrates the landscape of capabilities. In further detail:

• Gravitational waves: To probe extreme gravitators on a wide range of mass scales, sensitivity in multiple frequency bands is needed. In the audio (kHz) band, LIGO has made several discoveries and has clear upgrade plans in the 2020s but needs technology development for a successor in the 2030s. In the nHz band, NANOGrav has been operating successfully and has set important limits; the continuous long-term timing needed is threatened by decreased funding for the Arecibo¹ and Green Bank Observatories. In the mHz band, LISA will be a powerful new capability in the 2030s; if the United States is to have an important role in this project, it needs to increase its participation in the 2020s. In the future, it will be possible to expand coverage to the 0.1–1 Hz band, which would be important for observing the merger of intermediate-mass black holes with 10³–10⁴ M_☉ masses.
• Neutrinos: To probe extreme accelerators, neutrino observatories need greater sensitivity across a range of energies. In the TeV–PeV (very-high-energy [VHE]) range, IceCube has detected a bright, hard-spectrum diffuse background and one likely source. In the EeV–ZeV (ultra-high-energy [UHE]) range, we know from cosmic-ray data that powerful sources must exist, but experiments have set limits only on the neutrino flux. In both energy ranges, dramatic leaps in sensitivity are needed and are feasible if relevant research is adequately supported. To go further, there are a variety of proposed experiments with promising new ideas for UHE neutrino detection.

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¹ The catastrophic loss of the Arecibo facility occurred after the PAG panel completed its deliberations and presented its findings to the steering committee. Following the lead of the Panel on Radio, Millimeter, and Submillimeter Observations from the Ground (RMS), which evaluated such facilities, the PAG panel did not attempt to revise its report. Please see additional notes in the RMS panel report (Appendix M).

• Gamma rays: To probe multi-messenger sources, gamma-ray observations are critical both as sky monitors and as precision tools. In the keV–MeV–GeV (high-energy [HE]) range, Fermi and Swift have been indispensable. However, even as increasing investments are made in gravitational-wave and neutrino observatories, these satellites are reaching the end of their lives, with no successors planned. In the TeV–PeV range, there are exciting developments worldwide, but without U.S. support for and involvement in these activities, the United States will lose its leadership role.
• Cosmic rays: Although cosmic rays do not point back to their sources, owing to magnetic deflections, observations with the Pierre Auger Observatory, Telescope Array, and other facilities show the existence and high power of extreme accelerators. Further, because cosmic rays are samples of matter from distant sources, their composition information is valuable. Directional astronomy with UHE cosmic rays may be possible if a sufficient component of the flux is protons as opposed to nuclei; the flux of UHE neutrinos is sensitive to the nature of the cosmic rays, being higher for a lighter composition. Further measurements and technology development are needed to dramatically improve sensitivity for breakthroughs in the 2030s.

These new messengers will be especially powerful when combined with each other and with conventional astronomical observations. A key example is binary neutron star mergers, which will be plentifully observed when the LIGO A+ upgrade achieves its design sensitivity in the mid-2020s. Without sky monitoring capabilities and sensitive gamma-ray detectors in the keV–MeV range, it will be very challenging to localize these events promptly and study them in detail. Without GeV–TeV–PeV gamma-ray and neutrino data, it will be challenging to tell if these short gamma-ray bursts (GRBs) accelerate cosmic rays. Another example is active galactic nuclei (AGN) flares, for which gamma-ray data are critical to determining which sources have high-energy activity. Other examples include a Milky Way supernova, a nearby long GRB, and the tidal disruption of stars by black holes.

L.4 MISSIONS AND PROJECTS ENDORSED FOR THE SURVEY

A successful new-messenger program requires investments in this decade in new observatories for gravitational waves, neutrinos, and gamma rays. The greatest discovery potential comes from having a rich, connected observational program. Multiple discoveries of seminal importance to astronomy are likely. This program is realistic owing to the modest costs of these projects. Success also depends on continued support for present active experiments. In the following, the panel’s view of the most compelling large-, medium-, and small-scale investments are described in turn, selected from the many submitted white papers. Difficult choices had to be made.

L.4.1 Large-Scale Investment: Neutrino Program

To develop discovery-class observatories for astrophysical neutrinos over a wide energy range, the panel endorses continued growth in this field under U.S. leadership. The centerpiece would be IceCubeGen2; compared to IceCube—one of the largest, most successful, and most visible NSF investments—it would have greatly increased sensitivity while having a comparable real-year (RY) cost. IceCube-Gen2 is designed to resolve IceCube’s observed TeV–PeV diffuse background into sources and to open new frontiers at higher energies, up to the EeV–ZeV range. In addition, as discussed in Section L.4.4, the panel also endorses technology development that may facilitate even larger future experiments at those higher energies.

L.4.1.1 NSF: IceCube-Gen2

The IceCube Neutrino Observatory was constructed at the South Pole during 2004–2010; data-taking and analysis with newly deployed hardware followed soon after each year’s commissioning. The capabilities of the original IceCube facility are being improved upon by the IceCube upgrade, a relatively small project that will be completed in 2023 and that adds a dense infill of optical sensors to improve sensitivity at low energies and to better calibrate IceCube. IceCube-Gen2 will create a third-generation observatory for HE neutrinos sited at the South Pole, greatly improving upon the capabilities of IceCube and its predecessor, the Antarctic Muon and Neutrino Detector Array (AMANDA). Construction could start in 2024 and would take about 10 years to complete, during which the experiment would be taking data with an increasingly larger detector. The primary component of IceCube-Gen2, intended for TeV–PeV neutrinos, is an array of optical sensors, much like IceCube but with significantly enhanced sensitivity and directionality. The secondary component (≅10 percent in cost), intended for higher-energy neutrinos, is an array of radio sensors, building on the heritage of several experiments, but with dramatically increased sensitivity. At present, there are no funded detectors worldwide that could compete with IceCube-Gen2, which is led by the United States, in either energy range.

Scientific Context: More than a century since the discovery of charged cosmic rays, their origins are still unknown, owing to magnetic deflections that obfuscate their sources. The dominant cosmic-ray component is nuclei, principally protons. From observations of HE gamma-ray sources, there are many proposed sites for where cosmic rays (including electrons, which are subdominant) are accelerated. However, there is an essential question that has not been resolved. Where nuclei are accelerated, both gamma rays and neutrinos are produced through hadronic processes, especially pion production and decay. Where electrons are accelerated, only gamma rays are produced, through leptonic processes, especially inverse-Compton scattering. Detecting neutrinos from some source class would be definitive evidence of efficient hadronic acceleration. Together with sufficiently sensitive non-detections of neutrinos from other source classes, which would favor efficient acceleration of electrons only, this would revolutionize our understanding of cosmic-ray origins and the nature of gamma-ray sources, each a long-standing mystery.

Starting in 2013, IceCube has detected a diffuse, hard-spectrum, TeV–PeV flux of astrophysical neutrinos, now with ≅60 events of energies ≳TeV that are distinct from the steeply falling foreground of atmospheric neutrinos that dominates at lower energies. The diffuse background is extragalactic, with a Milky Way component surprisingly absent. Comparison to the diffuse gamma-ray background suggests that the neutrino sources may be dominantly gamma-ray dark, which would make the results even more exciting. In 2017, IceCube detected one likely source, the blazar TXS 0506+056, with one HE neutrino detected in association with a gamma-ray flare (GeV by Fermi, TeV by Major Atmospheric Gamma Imaging Cherenkov Telescope [MAGIC]), with a significance of 3σ. IceCube then also detected, at a significance of 3.5σ, a 2014–2015 neutrino flare without accompanying gamma-ray emission, from the same source.

IceCube-Gen2 is expected to make several discoveries that go beyond the capabilities of the existing IceCube (including the Upgrade). First, IceCube-Gen2 would dramatically improve on the existing spectrum, skymap, and flavor information of the diffuse data, improving statistics by an order of magnitude, leading to important clues about the origin of these neutrinos. Second, IceCube-Gen2 has the power to resolve the diffuse extragalactic background into discrete sources, for which the case is based on the luminosity and number density of sources, and does not depend on the TXS 0506+056 source, and to make first detections of Milky Way sources, for which the case is based on high-energy gamma-ray observations. Third, by improving present sensitivity at higher energies by about two orders of magnitude, IceCube-Gen2 would enable first detections at such energies, including of the cosmogenic flux.

The cosmogenic neutrino flux, reaching the EeV–ZeV range, is owing to interactions of UHE protons with the cosmic microwave and infrared backgrounds. There is strong circumstantial evidence for these interactions, with a 20σ pileup feature below ~10^19.5 eV in the cosmic-ray spectra observed by the Pierre Auger Observatory and the Telescope Array. However, this feature could also indicate the breakup of nuclei. The neutrino flux is sensitive to the cosmic-ray composition, being larger for protons compared to nuclei, encouraging the development of new neutrino observatories to test the cosmic-ray composition. A light composition could open a window for future directional astronomy with charged cosmic rays. Encouraging the development of new neutrino observatories, with much larger statistics than IceCube-Gen2, to probe the most extreme accelerators and the cosmic-ray composition through measuring the neutrino spectrum over a broad energy range.

IceCube-Gen2 will provide critical input to the Astro2020 science questions COEPD, plus some of COEP1, COEP2, COEP3, COEP4, and STARS4, depending on what the sources of the observed neutrinos are. More generally, the first definitive source detections in high-energy neutrinos would have a huge, broad impact, much like that for the first detection of a binary neutron star merger in gravitational waves. Contemporaneous observations of IceCube-Gen2 and other facilities—including those for gamma rays (CTA, based in Europe, and the Large High Altitude Air Shower Observatory [LHAASO], based in China), gravitational waves (worldwide, but especially LIGO and its upgrades), and transient astronomy (worldwide, but especially the LSST project)—would have synergistic benefits that could lead to breakthroughs.

Implementation—The optical array of IceCube-Gen2, intended for the detection of TeV–PeV neutrinos, is principally a larger version of IceCube, for which the technology and science have been demonstrated. The instrumented volume would be 7.9 km³, with 120 vertical strings of 100 optical sensors buried in deep (2.6 km) holes with a horizontal spacing of 240 m. For IceCube, the comparable figures are 1 km³, 86 strings each with 60 sensors, and with a typical spacing of 125 m. IceCube-Gen2 would use improved optical sensors: instead of a single 8” photomultiplier tube per module, there would be 24 grouped 3” tubes. These differences provide benefits in flux sensitivity, angular resolution, and cost. The panel’s assessment is that the proposed size of the IceCube-Gen2 optical array is the minimum needed to ensure detection of point sources, the principal goal.

The radio array of IceCube-Gen2, intended for the detection of higher-energy neutrinos, builds on decades of technology development to make a huge leap in sensitivity. The proposed array design comprises 200 stations deployed over an area of 500 km². Each station would be instrumented with both horizontally and vertically polarized receiver antennas, deployed on the surface and/or to a depth of 100 m. This would build on the heritage of previous experiments, including RICE, ARA, ARIANNA, and ANITA.² There is a compelling case to include a radio array as part of IceCube-Gen2, as discussed below. The panel’s assessment is that the proposed size of the IceCube-Gen2 radio array is the minimum needed to make a first detection of cosmogenic neutrinos in nominal scenarios, the principal goal.

It is critical to have both the optical and the radio arrays to cover a wide range of energies and to cross-calibrate. Different spectral components are expected in the TeV–PeV range and at higher energies, and how they connect will be a powerful test of source properties and cosmic-ray composition. The combined detector and its calibrations will also allow unprecedented measurements of ice properties, at wavelengths from 100 nm to 1 m, which will have broader impacts for glaciology.

Delaying the proposed timeline would endanger achieving the scientific goals. Much of the project, installation, technical, and scientific expertise needed for IceCube-Gen2 dates back to IceCube and is cur-

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² RICE: Radio Ice Cherenkov Experiment; ARA: Askaryan Radio Array; ARIANNA: Antarctic Ross Ice-Shelf Antenna Neutrino Array; ANITA: Antarctic Impulsive Transient Antenna.

rently being used and expanded for the IceCube Upgrade; attrition of this personnel base would require the development of a new cadre of specialists. Particularly irreplaceable is the drilling team, which operated the 5 MegaWatt Enhanced Hot Water Drill for IceCube. This expertise has been sustained by the Deep Core and Upgrade efforts; a large gap after the end of the Upgrade would likely result in the loss of this team and a significant delay in commissioning the hardware. A limiting factor in how fast the detector can be built is the short duration (10 weeks) of the drilling season, which makes it critical to have an experienced, efficient team to complete the detector within the projected budget.

Costs/Risks: The project-estimated cost of IceCube-Gen2 is $345 million in fiscal year (FY) 2020 dollars ($420 million in RY dollars). This is approximately the same RY cost as IceCube. As with IceCube, funding for construction of IceCube-Gen2 would be sought through the NSF MREFC program, with support for operations through NSF Division of Physics and the Office of Polar Programs, plus substantial international contributions (much of which is already committed). The MREFC project investment would be stretched out over 10 years owing to the short South Pole construction season; the peak annual MREFC funding required would be $50 million in 2024.

IceCube-Gen2 is designed around mature technology. Since the initiation of neutrino telescopes at the South Pole (AMANDA construction began in 1993–1994, after 3 years of prototyping), the techniques for drilling and deploying a distributed optical array to 2500 m depths have been honed to the point that >99 percent of the optical sensors initially deployed for IceCube show no loss in performance after a decade of data-taking. Radio-based neutrino detection has developed similarly, with the first deployment of hardware at the South Pole in 1995. In addition to their dedicated physics programs and goals, the IceCube Upgrade at the South Pole and the Radio Neutrino Observatory in Greenland (RNO-G) will prototype a significant fraction of the hardware planned for IceCube-Gen2.

The costs of IceCube-Gen2 are significantly reduced by the ability to leverage the infrastructure built by IceCube, including the deployment and operations experience, the data-processing and analysis tools, and the established collaboration. In addition, there are important technological developments, plus the field experience to know that a larger instrument spacing is feasible for the observed hard-spectrum flux. The leverage to reduce costs is especially true for the radio array (about 10 percent of the total costs of IceCubeGen2), which would benefit greatly from the above plus by sharing facilities for electronics and personnel at the South Pole. Although the radio technology is newer than the optical technology, it builds on substantial heritage and its development would be accelerated by having a larger collaboration. Achieving comparable sensitivity in a dedicated radio array at another site and/or with a different collaboration would be substantially more expensive (but see the discussion of continued technology development in Section L.4.4).

An independent, external TRACE review was done to assess whether IceCube-Gen2, accounting for all risks, had been properly costed to achieve the desired scientific goals on the stated timeline. The TRACE review found that the programmatic risk (science and costs) and schedule risk of IceCube-Gen2 are both medium-to-low. The TRACE cost estimate is 20 percent higher than the project estimated cost, which the panel considers only a minor concern. The MREFC review process will lead to more accurate accounting. The MREFC-funded IceCube project was constructed with a fixed budget; the project was able to save costs and deploy an increased number of strings, for which there was a powerful science motivation. It is likely that IceCube-Gen2 could do the same. The TRACE projected a schedule that is 7 months longer than that estimated by the project, which, again, the panel considers only a minor concern.

L.4.2 Medium-Scale Investments: Gravitational-Wave Program

To develop discovery-class, multi-band experiments in gravitational waves, the panel endorses both the continued growth in sensitivity of current gravitational-wave observatories and the development of new

ones, in multiple gravitational-wave bands. On the ground, this includes planned upgrades to the LIGO facilities and technology development for its successor, Cosmic Explorer, and continuity and growth of NANOGrav observations. In space, this includes an increased U.S. presence in the science of the LISA mission. The United States has played a key role in the conception of all of these efforts and is currently either leading them or contributing critical input. LIGO is another one of NSF’s largest, most successful, and most visible investments. NANOGrav, whose data set is currently dominated by the Arecibo and Green Bank observatories, provides the bulk of the sensitivity to the International Pulsar Timing Array (IPTA). LISA was initially conceived as a partnership between NASA and the European Space Agency (ESA); ESA now leads the project, but key technology and analysis are under development in the United States with NASA support.

L.4.2.1 NSF: Cosmic Explorer

Cosmic Explorer is the U.S. component of a future network of third-generation, ground-based gravitational-wave detectors. In the current plan, Cosmic Explorer will be built on the same principles as LIGO, but with 10 times longer arms (40 km) and additional technological upgrades that will provide the ability to measure and characterize every stellar-mass black hole merger in the universe. The corresponding European-based project is the Einstein Telescope, with different design and implementation but with comparable sensitivity and time scale. Cosmic Explorer and the Einstein Telescope will be part of a detector network to provide source localization and coverage—critical ingredients for multi-messenger science. The details of how such a network will operate are still to be defined.

Scientific Context: At its current sensitivity, LIGO is able to detect gravitational-wave signals on a roughly weekly cadence. Once LIGO detectors achieve the sensitivity planned for their A+ upgrade, which is expected in this decade, their detection rate will increase by a factor of about 10. The third generation of observatories is intended to increase this rate by a factor of 1,000 or more. By lowering the low-frequency sensitivity limit of LIGO from 10 Hz to 5 Hz and reducing the noise by a factor of 10, Cosmic Explorer will reach gravitational-wave signals all the way back in cosmic time, for a powerful and diversified science program. The science goals for Cosmic Explorer include determining the nature of the densest matter in the universe, enabling multi-messenger observations of binary neutron star systems, and measuring the geometry and expansion rate of the universe independent of electromagnetic observations. Cosmic Explorer will also provide insights into the evolution of massive stars, the physics of supernovae, and the origin of pulsar glitches, and maybe find exotic sources. Cosmic Explorer will provide critical input to the Astro2020 science questions COEPD, COEP1, COEP2, COEP3, COEP4, COS3, and COS4.

Implementation: Cosmic Explorer will be an L-shaped laser interferometer built on the surface of geologically appropriate and seismically quiet land in a U.S. location still to be identified. Cosmic Explorer Stage 1 (CE1) will adopt technology tested in the 4 km LIGO facilities to a 40 km detector; the longer arms will increase the amplitude of the observed signal but not the noise, thus providing better sensitivity. Cosmic Explorer Stage 2 (CE2) will provide further sensitivity improvements with new technology that will mitigate quantum and thermal noise. This plan spans multiple decades and will ultimately be a large-scale program, but the requested investment in this decade is a medium-scale ground-based investment from NSF to implement technology developments that will proceed in parallel with the approved LIGO A+ upgrade. These developments will be needed to initiate observations with CE1 in the 2030s and CE2 in the 2040s. The strategy of performing technology development for future detectors while maintaining the operations of current detectors worked very well for the transition from the initial proof-of-concept LIGO to the currently operational LIGO; the same model is expected to work for the transition from the current facilities to Cosmic Explorer, with the bonus that technology being planned for the future may end up being affordable enough to be first installed in the current 4 km facilities.

Costs/Risks: The Cosmic Explorer request in this decadal survey is for medium-scale funding for a design study, at the level of $65.7 million in FYs 2020 to 2025: $33 million for two engineering studies, $20 million for a prototype CE chamber, and the remainder for other upgrades and for governance. Based on 2011 estimates for the Einstein Telescope and the historical cost of LIGO, the panel expects that this project will become a large-scale investment in the 2030s. The requested investments in the 2020s will yield results from engineering studies and experiments that will then help produce solid estimates for the costs and risks of the full Cosmic Explorer project. In addition, Cosmic Explorer is a 10-fold expansion of an experiment that has been demonstrated to work, and it is proposed by a team with world-class expertise and leadership in its field.

L.4.2.2 NSF: North American Nanohertz Observatory of Gravitational Waves (NANOGrav)

NANOGrav regularly observes 75–200 millisecond pulsars to detect and characterize gravitational-wave emission in the nHz band via the “pulsar timing” technique. Correlated changes in the arrival times of the pulses are analogous to correlated changes in laser phase in LIGO or LISA; NANOGrav is, therefore, a galactic-scale gravitational-wave interferometer with arm lengths of hundreds of parsecs. NANOGrav is currently funded as an NSF Physics Frontier Center. The IPTA is a consortium of pulsar timing arrays (PTAs), including NANOGrav, that shares data to increase the sensitivity to nHz gravitational waves. NANOGrav, and specifically the Green Bank and Arecibo³ observatories, provide the bulk of the sensitivity to the IPTA. NANOGrav has a strong tradition of conducting training programs for young scientists and significant outreach programs.

Scientific Context: The nHz gravitational-wave band is the only way to measure the cosmic merger rate of 10⁸–10⁹ M_☉ black-hole binaries, a critical test of the evolution of both black holes and galaxies. NANOGrav has placed limits on the nHz stochastic gravitational-wave background, already constraining current models of supermassive black-hole binary formation. Under current projections, NANOGrav expects to detect the stochastic background owing to supermassive black-hole binaries in the first half of this decade and to detect individual sources in the second half of this decade. The amplitude of the background will measure the scaling of black-hole mass with host-galaxy mass as well as dynamical-friction time scales. Further measurements—the spectrum of this background, in combination with the detection of individual 10⁸–10⁹ M_☉ binary mergers—will yield a definitive picture of how these supermassive black holes evolve in their galactic environments. With this, NANOGrav will provide crucial input on the Astro2020 science questions COEPD, COEP4, and COS4.

Implementation: To fulfill not just first detections but also to make detailed measurements of the nHz gravitational-wave band (both stochastic background and single sources), NANOGrav requires continued access to the Green Bank and Arecibo observatories, in addition to an expansion of their capabilities with future radio facilities with larger collecting area. This expansion is crucial to achieving the daily cadence of observations that NANOGrav requires to detect and characterize single continuous-wave sources of gravitational waves. With the Arecibo and Green Bank observatories alone, NANOGrav has sufficient collecting area but insufficient overall observing time for detecting single sources.

Costs/Risks: The budget for NANOGrav science is $118 million over the decade of the proposed plan. While the cost exceeds the nominal ground-based medium-scale range (see Box L.1), the panel considered it in this range based on its present scale and the possibilities of either changes in scope or attracting investments from other partners. These costs are dominated by telescope usage and personnel. The observational capabilities of NANOGrav would be greatly increased by the realization of new and expanded radio facilities; an analysis of the feasibility of such facilities was not in the scope of the panel but is included in the report of the Astro2020 Panel on Radio, Millimeter, and Submillimeter Observations from the Ground

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³ See note 1, above.

(RMS). In the absence or delay of such facilities, the defunding of Green Bank and Arecibo would place at risk NANOGrav science. The sensitivity increases as the time baseline of observations increases, so the loss of a telescope is not just a loss of sensitivity at the time of its loss, but also the deprecation of the data set for its long-wavelength capabilities. The Green Bank Observatory is operated as a partnership between federal, state, and private sources. The $118 million NANOGrav budget for the decade includes $30 million for telescope time on these two telescopes, and allows NANOGrav to use 750 hours per year, or about 10 percent of the on-sky time, on each telescope. That time is still a fraction of the total telescope time required by the project. In the ideal scenario specified in the NANOGrav white paper, the required time would come from potential future facilities (about one-half), Green Bank and Arecibo (about one-third), and the remainder (about one-sixth) from a combination of IPTA telescopes.

L.4.2.3 NASA: Laser Interferometer Space Antenna (LISA)

LISA is a project led by the ESA, with significant contributions anticipated from several ESA member states and NASA. LISA will be the first space-based gravitational-wave detector, sensitive to the mHz range. The 2020s will be a crucial decade for LISA, as activities ramp-up in preparation for a launch in 2034.

Scientific Context: Multi-messenger astronomy cannot reach maturity without an instrument capable of observing gravitational waves in the mHz band, which are emitted by some of the most interesting sources. Because of seismic noise, which dominates below 1 Hz, ground-based gravitational-wave detectors cannot be sensitive to miliHertz signals; however, LISA will have this capability by virtue of being space-based. LISA will be able to observe all merging supermassive black holes in the universe (10⁵–10⁷ M_☉ masses), the inspiral of small compact objects into supermassive black holes to redshifts of order one, white dwarf and neutron star binaries in the Milky Way, and stochastic backgrounds from the early universe. LISA will also study the dynamics of dense nuclear star clusters and explore the fundamental nature of gravity and black holes, as well as shed light on the existence of ultralight boson fields, a dark-matter candidate that would grow around black holes through super-radiance. Last but not least, LISA will probe the cosmic expansion rate, use stochastic gravitational-wave backgrounds to understand the early universe and TeV-scale particle physics, and listen for gravitational-wave bursts from serendipitous sources. LISA will provide critical input on the Astro2020 science questions COEPD, COEP2, COEP3, COEP4, COS1, COS2, COS3, and COS4.

Implementation: The panel was not asked to directly evaluate the LISA mission, as this is an already-approved ESA mission with NASA partnership, with a planned launch in 2034. Instead, the panel was asked to evaluate the possibility of changing the scope of LISA funding. The panel endorses investments in increasing the scope of U.S. participation in LISA, with two categories of contribution:

• Contribution 1: An increase in U.S. LISA science funding, with a decade-long, dedicated NASA program that researchers at universities and other centers could reliably count on to develop LISA-specific gravitational-wave data analysis tools, gravitational-waveform models, and science-extraction techniques.
• Contribution 2: Support for a U.S. LISA Science Facility to (1) implement and coordinate the U.S. role in the ESA-led project-level data analysis, (2) provide outside users at universities and research institutes access to mission data at a variety of levels, and (3) provide tools to facilitate working with LISA data and for combining LISA data with other facilities in multi-messenger investigations.

These two items are complementary, as the science facility would integrate the research sponsored by the dedicated U.S. funding program in the ESA-led program, which only supports science development within Europe. This increase in the scope of U.S. efforts would not duplicate other ESA efforts, but rather

would add a critical contribution to the development of tools and of the analysis framework that will be needed to extract the most science from the data by 2034. It would also empower U.S. scientists to work with LISA data, as they do with other observatory facilities, and overall strengthen the role and input of U.S. scientists in the LISA mission. A lack of support for U.S. scientists to work on LISA science (modeling, data, and computation) would severely diminish the U.S. capabilities in the future of gravitational-wave science.

The analysis of LISA data will be unlike that of LIGO and NANOGrav: the three instruments are based on distinct technologies, observe the gravitational-wave spectrum in nonoverlapping frequency ranges, and have unrelated noise sources, therefore observing different sources with different challenges. The unique character of LISA data requires new gravitational-wave models (both analytical and computational) and new analysis techniques. Just as an example, space-based detectors will routinely observe gravitational waves emitted by binary systems with intermediate and sometimes extreme mass ratios that inspiral in generic (eccentric and double spin-precessing) orbits, and the numerical, analytical, or semi-analytical models currently in existence either cannot describe such systems at all, or are, at best, not sufficiently accurate yet. Different groups of scientists in the United States are currently (and have been for more than 30 years) been working on these nonoverlapping techniques.

Costs/Risks: NASA is currently supporting a range of potential contributions to LISA, including instruments, spacecraft elements, and science analysis, in the medium-scale range of $400 million to $600 million. This would cover the cost of the hardware deliverables, as well as contributions to the science ground activities, the U.S. Guest Investigator programs, and NASA project overhead, including management, systems engineering, project science, and mission assurance. The suggested increase in LISA support from NASA would be $100 million for the decade, with $30 million to $40 million for sponsored science funding and $50 million to $60 million for the U.S. LISA Science Facility.

The scope of NASA’s LISA Preparatory Science program encompasses the science program described above, but at a lower level of effort, and it is not clear if the Preparatory Science program will be continued throughout the decade. Regarding the increase in U.S. LISA Science funding (Contribution 1), dedicated, reliable, and decade-long funding for a LISA science program at the level of about $3 million to $4 million per year ($30 million to $40 million per decade) would be in line with the dedicated funding channel established by the NSF Gravity program for LIGO research support, which was instrumental for LIGO’s success; a similar need can be envisioned for LISA. This new dedicated, decade-long funding program would enable support for roughly seven research groups on 3-year, $500,000 per year grants that researchers could count on over the decade to expand the field of gravitational-wave astronomy and the U.S. role in it.

Decade-long funding of $50 million to $60 million for a U.S. LISA Science Facility (Contribution 2) would enable U.S. community participation in a new and unexplored regime that requires new tools, techniques, and simulations, and is comparable to previous support for other science center activities in the United States. Based on past experience of NASA deploying missions using new messengers, significant investments have been needed to adequately prepare the astronomical community to take full advantage, even in cases where there is substantial heritage from prior missions.

L.4.3 Medium-Scale Investments: Gamma-Ray Program

To develop discovery-class capabilities in multi-messenger astronomy, the panel endorses a dedicated probe-scale space mission with a suite of capabilities for these sources, plus U.S. participation in international ground-based gamma-ray observatories. The timely deployment of a space-based high-energy gamma-ray mission is critical to take advantage of anticipated discoveries from LIGO and IceCube and their successors. The VHE gamma-ray band, which probes the most energetic particles in the universe, can

only be accessed from the ground. The panel endorses U.S. participation in the Cherenkov Telescope Array (CTA) and SWGO as VHE observatories that extend source sensitivities to fainter sources, higher redshifts, and faster emission time scales, providing complementary catalogs of sources that span distance scales from the Milky Way to the cosmos.

L.4.3.1 NASA: Probe-Scale Mission for Multi-Messenger Sources

Gamma-ray observations play a critical role in understanding extreme gravitators and extreme accelerators. A probe-scale mission dedicated to the study of multi-messenger sources would provide wide-field multiwavelength observations, at keV–MeV–GeV energies, at the sensitivities needed to achieve multimessenger discoveries and to directly answer questions about compact objects and stellar astrophysics. The ideal mission would provide rapid alerts and sky localization for transient sources, enabling timely follow-up observations by other telescopes with narrower fields of view.

Scientific Context: Space-based gamma-ray observations are needed to probe astrophysical sources at high energies, where non-thermal activity is easily distinguished and indicates extreme physical conditions and possibly cosmic-ray acceleration. Continuum gamma-ray observations provide unique information on the structure and composition of relativistic winds and jets in sources such as pulsar wind nebulae, active galactic nuclei, supernovae, and gamma-ray bursts. To match the growing sensitivity of gravitational-wave observatories like LIGO and neutrino facilities like IceCube, advances in gamma-ray sensitivity are urgently needed to increase detection rates and distance horizons. Increasing the populations of well-studied cosmic accelerators and multi-messenger events will require the wide-field, high-cadence, all-sky monitoring made possible by space-based missions.

A space-based mission is also needed to place multi-messenger observations into the broader astronomical context. The combination of gravitational-wave and gamma-ray observations for GRB 170817A bracketed the inspiral of the binary neutron stars and the first emergence of light from the resulting burst, enabling constraints on theories of gravity and initiating a massive campaign of groundbreaking follow-up observations that probed aspects such as heavy-element formation. The association of a neutrino, X rays, and gamma rays from the TXS 0506+056 blazar demonstrated the combined use of these observations to peer into the workings of relativistic jets. Sky localization and rapid-alert capabilities enable the detection of electromagnetic emission by telescopes with narrower fields of view (e.g., radio, optical, X-ray, or VHE gamma rays). The identification of host galaxies (e.g., as possible at radio or optical wavelengths) is crucial to distance determination, standard-siren cosmology to measure the expansion rate, and population studies to constrain source-formation channels. A probe-scale mission for multi-messenger sources will provide critical input to the Astro2020 science questions COEPD, COEP1, COEP2, COEP3, COEP4, COS3, COS4, and STARS4.

Implementation: A probe-scale mission opportunity in the next decade focused on multi-messenger astronomy would provide key capabilities for multi-messenger discovery. Competed instrumentation on a multi-messenger-themed probe would allow evaluation among options for capabilities. Launching this in this decade is critical for this science area owing to the necessity of coordination with other planned programs (see Section L.3). A single probe mission cannot meet all the needs for an ambitious program of discovery in multi-messenger astronomy. In addition to a probe dedicated to this topic, it is important to enable supporting capabilities for multi-messenger astronomy where possible in the implementation of other NASA astrophysics missions, including flagship missions.

Costs/Risks: The panel reviewed submissions presenting gamma-ray probe-scale mission concepts capable of providing key multi-messenger capabilities (e.g., AMEGO, APT, and TAP) primarily for their scientific importance to particle astrophysics and gravitation. The existence of several concepts with a high

level of technical readiness and mission plans designed to meet the probe schedule and budget demonstrate the feasibility of addressing this need with a probe-scale mission.

L.4.3.2 NSF: U.S. Participation in the Cherenkov Telescope Array (CTA) and the Southern Wide-Field Gamma-Ray Observatory (SWGO)

Astronomical observations in the VHE gamma-ray energy band probe the sites of cosmic-ray acceleration. These observations require a combination of high-resolution and high-sensitivity measurements on point sources as well as full-sky monitoring capabilities. A combined program of imaging atmospheric Cherenkov telescopes (IACTs) and particle detector observatories will create a powerful synergistic capability. The nexus of these facilities is critical to the understanding of the emission of the highest-energy nonthermal radiation extending from galactic to our nearby cosmological neighborhood, and to support simultaneous multi-messenger observations with upgraded gravitational-wave and high-energy neutrino astronomy capabilities.

Scientific Context: Ground-based observations of VHE gamma-ray sources provide a critical probe of nonthermal processes in extreme astrophysical environments, such as GRBs, pulsar wind nebulae/supernova remnants, jet emission in AGNs, and accretion disks near Be stars/binary systems. The CTA IACT observatory will provide seasonal pointed observations over modest fields of view distributed across the full astronomical sky. CTA’s large detection area (up to 1 km in each hemisphere) provides the large photon statistics necessary for observing VHE spectra from extragalactic sources with weak emission (e.g., the starburst galaxy M82), exploring fast emission time scales (VHE flares from gamma-ray bursts like 1901140C, which lasted for tens of seconds), and detecting morphological variations in extended objects (e.g., the supernova remnant IC443). CTA’s low energy threshold extends the cosmological horizon for VHE astronomy beyond a redshift of one, thereby enabling VHE observations into the peak epoch of activity for active galactic nuclei and gamma-ray bursts.

High-altitude arrays of particle detectors—such as HAWC, LHAASO (2021 completion), and SWGO—allow wide field-of-view, continuous all-sky monitoring of the visible sky. SWGO and LHAASO can provide an order of magnitude increase in gamma-ray sensitivity for similar observations, critical for triggering on transients outside the narrow fields of view of IACTs. Particle detector observatories also have superior ability to detect extended, diffuse sources such as supernova remnants and pulsar wind nebulae (e.g., Geminga), the Galactic Plane, and complex, extended-emission regions containing dense clusters of astrophysical sources and phenomena (e.g., the Cygnus region). Using this capability, HAWC discovered a population of previously unknown, angularly extended, hard-spectrum Milky Way sources in the multi-TeV energy range around pulsar-wind nebulae (called “TeV halos”).

The CTA observatory, consisting of both Northern and Southern Hemisphere sites, provides access to the highest sensitivity pointed observations across the full astronomical sky, with the highest energy and angular resolution. SWGO’s Southern Hemisphere sky coverage will complement the Northern Hemisphere coverage of the LHAASO observatory, providing daily coverage of the full sky at very high energies. The combination of CTA and LHAASO/SWGO provides an integrated observational capability that maximizes the scientific opportunities for all-sky multi-messenger astronomy. The success of the broad U.S. program in multi-messenger astrophysics would be greatly enhanced by access to these world-leading facilities. The development of these facilities depends critically on decades of U.S. investment that cannot be capitalized upon without continued U.S. involvement.

Implementation: The international CTA observatory has been under development for more than a decade. U.S. participation in CTA was recommended as a ranked priority in the New Worlds, New Horizons

in Astronomy and Astrophysics⁴ (Astro2010) decadal survey. Since then, the U.S. CTA group has developed and built a prototype medium size (9.7 m diameter) two-mirror Schwartzchild-Couder IACT Telescope (SCT) using funds from the NSF Major Research Instrumentation (MRI) program. The prototype SCT detected the Crab Nebula in Spring 2020 with a partially filled focal plane. The prototype SCT focal plane will be fully populated by 2022 through a second NSF MRI grant. The United States will contribute 10 SCT telescopes to the larger CTA array, which will roughly double the number of medium-scale telescopes. Detailed studies show that this U.S. contribution would dramatically enhance many of CTA’s Key Science Projects, ranging from studies of astrophysical sources to searches for dark matter annihilation signals.

The HAWC observatory has demonstrated the synergistic capabilities that high-altitude particle detectors provide to IACT arrays. HAWC also developed the use of a large, modular water-Cherenkov detector design that is scalable to larger arrays and higher altitudes. SWGO is based on this modular design. The increased size of SWGO requires a large international collaboration to manage the construction, operation, and data analysis.

Costs/Risks: The design of the CTA observatory is mature, including a detailed science case, completed site acquisition, optimized observatory design, and prototype testing of every telescope in the array. The project is refining a multi-level work breakdown structure for cost and schedule, a project execution plan, and a plan for assessing and mitigating project risks to cost and schedule. The cost of CTA is estimated to be $500 million (with a U.S. contribution of $40 million) for construction and $3 million per year for U.S. operations, which is well below the $100 million U.S. contribution envisioned in the Astro2010 decadal survey.

The design and costs of SWGO are at the initial stages, with an SWGO construction cost estimate of $60 million (with a U.S. contribution of $20 million) based on extrapolations of cost and schedule of the HAWC observatory. The international SWGO collaboration has only recently been formed (2019) and is in the process of selecting the observatory site and developing a more detailed instrument cost and schedule.

The technologies for both projects are well understood and demonstrated, and so the risks to cost and schedule of both projects are modest. Significant delays in construction funding will result in missed opportunities for U.S. participation in multi-messenger science. Because these projects are being led by international partners, the costs of U.S. participation are greatly reduced.

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⁴ National Research Council, 2010, New Worlds, New Horizons in Astronomy and Astrophysics, The National Academies Press, Washington, DC.

L.4.4 Small-Scale Investments: Technology Development for Improved Capabilities

In addition to the large- and medium-scale investments described above, it will be critical to support new small efforts that foster new ideas and have the potential to become future larger implementations.

L.4.4.1 Future Observatories for Neutrinos and Cosmic Rays

Highly sensitive neutrino observatories will be needed at ultra-high energies to probe the origins and composition of ultra-high-energy cosmic rays, building on a possible first detection of the cosmogenic neutrino flux by IceCube-Gen2. These will allow precise measurement of the spectrum and other properties of the cosmogenic flux—probing the cosmic evolution of the accelerators of ultra-high-energy cosmic rays—if the flux is as large as hoped for, or, at a minimum, an overall flux measurement if the flux is smaller than expected. In detection, these neutrino events will probe center-of-momentum frame energies well above that of the Large Hadron Collider (e.g., for E_ν ~10¹⁹ eV, √s ~100 TeV). To develop detectors with exposures well beyond that of IceCube-Gen2 requires funding technology development now. Most of these projects have very modest costs and potentially huge payoffs. Because these projects are led by physics groups, not all submitted white papers to the Astro2020 survey.

At present, all proposals for detecting cosmogenic neutrinos are based on radio instrumentation of natural formations, as radio has tremendous advantages of long attenuation lengths and sensitive, low-cost detectors. The proposed projects involve sites worldwide, at least for the development phase. Ultimately, some may propose to locate in Antarctica or even the South Pole specifically, but not necessarily. Neutrino interactions lead to energetic particle showers. In a dense medium like ice, a shower emits coherent radio signals through the Askaryan effect. In a tenuous medium like air, neutrino interactions in a sufficiently thick nearby mass such as a mountain can lead to tau leptons that decay in air, producing extensive air showers that can be detected via radio signals emitted by geomagnetic synchrotron processes. In addition, active detection of showers in dense media through radar is possible owing to the reflectivity of radio waves on the ionization the showers leave behind, recently measured for the first time in a laboratory experiment.

The panel endorses continued development of technologies for neutrino observatories, which may lead to a high-statistics detection of cosmogenic neutrinos. A particularly challenging aspect is self-triggering using radio data alone at sites near human populations and thus radio backgrounds. As this field has many proposed techniques and the instrumental technology is developing rapidly, the choice of the observational approach for field implementations would preferably be determined through competitive peer review. Owing to the potentially enormous scientific return, it is critical that this direction must be aggressively pursued during the coming decade. Collaboration with astronomers using large arrays for radio astronomy is also encouraged.

Closely related to the above, technology development is needed to work toward dramatic improvements in UHE cosmic ray sensitivity, as an order-of-magnitude expansion of existing arrays would require deployment over greater than tens of thousands of square kilometers. Such detectors could also be sensitive to ultra-high-energy gamma rays from the nearest sources of ultra-high-energy cosmic rays. Modest funding for small development efforts may be available through NSF PI programs, or the NSF MRI program, but significantly increased development opportunities are needed. Below the ultra-high-energy scale, there are a wide variety of successful or planned cosmic-ray experiments, and these are critical for testing the origins of Milky Way cosmic rays, finding the PeVatrons, and probing dark matter.

L.4.4.2 Scientific Opportunities for Gamma-Ray Observatories

NASA’s Explorers program could provide high-impact opportunities to conduct multi-messenger astronomy, either through missions targeted for that purpose or through the additional capabilities of mis-

sions targeted to other specific purposes. Continuing the current frequency of opportunities for the openly competed Explorers program is highly desirable, and an increase in the maximum allowed cost for missions would be warranted. The Astrophysics Research and Analysis (APRA) program and the new Astrophysics Pioneers program are vital for opening new areas for gamma-ray astrophysics and enabling technical developments that will lead to future-generation observatories. The shorter implementation time for these programs is also important to support imminent multi-messenger and particle astrophysics advances and allow responsiveness to emerging discoveries.

The panel reviewed a rich array of concepts for new space-based gamma-ray observatories. These covered a range of scientific opportunities and made use of a range of techniques and mission scales, representing significant progress made since 2010, in particular for the energy band from 100 keV to 100 MeV, which is markedly underdeveloped compared to lower and higher energy bands. In many cases, the technical readiness for concepts is already high and would allow significant observations in targeted capability areas. Examples of exciting scientific opportunities at the small scale include studies of the history of nucleosynthesis in the Milky Way, measurements of dynamic tomography of Type Ia supernovae, and measurements of gamma-ray polarization.

Several concepts were considered that would make excellent pathfinder studies. Small-mission opportunities in the next decade can also lead to future observatories that will provide additional leaps in the number and type of gamma-ray sources that can be studied in the higher energy band from 100 MeV to 100 GeV. Continuing the current pace of opportunities for smaller missions will be critical for advancing observational progress in these new areas. Additionally, it will be necessary to support technology development to enable broadly capable next-generation high-energy space-based observatories. A vital piece of the development program for gamma-ray and cosmic-ray observatories is a well-supported balloon program. Development work in the next decade will be critical to attaining the capabilities that will be needed to support the breakthrough multi-messenger science possible in the 2030s.

The capabilities for wide-field detection of energetic phenomena and rapid multiwavelength follow-up provided by currently operating HE space missions, such as Fermi and Swift—which have finite lifetimes and no clear successors—have supported an extremely rich range of synergistic science discoveries involving gravitational waves, neutrinos, and VHE gamma-ray observations. Until missions that exceed the current sensitivity and output become available, it is critical to continue support of these high-impact facilities.

L.5 RATIONALE FOR THE PROGRAM

As regards the topics considered by the panel, Astro2020 is a special decadal survey. Compared to Astro2010, the scope of PAG’s activities that have direct relevance to astronomy has dramatically increased owing to breakthrough discoveries. LIGO has detected dozens of gravitational-wave sources; IceCube has detected a bright, hard-spectrum diffuse background and one likely source; and gamma-ray observatories have played critical roles in leveraging those observations plus in making their own discoveries. In contrast, at the time of Astro2010, there were no direct detections of astrophysical sources of gravitational waves or VHE neutrinos, and the impetus for multi-messenger astronomy was notional at best.

Astro2020 is also special compared to Astro2030: the 2020s are a critical time to act to maximize the returns of current and pending investments. Much of the science and technology in PAG’s scope was developed in the United States, which excited worldwide interest, leading to huge investments abroad that take advantage of the early investments here. Action is needed now to maintain U.S. leadership and to develop the next stages of the science and technology. In some cases, it would be advisable for the United States to lead bold efforts; in others, it would be acceptable if it participated as a junior partner. But the United States must not cede its leadership in particle astrophysics and gravitation, which is of central and growing importance to both astronomy and physics.

The projects considered by the panel span a wide range of topics—gravitational waves, neutrinos, gamma rays, and cosmic rays—where continued investment in each area nurtures unique power to address multiple science-panel questions. The first overall observation of the panel is that astronomy with new messengers is astronomy per se. There is a high priority for observations of extreme gravitators, which include electromagnetically dark mergers of black holes, as well as of extreme accelerators, which include gamma-ray–obscured sources of high-energy neutrinos. But this is not the whole story, as emphasized through the COEP panel’s discovery area of multi-messenger astronomy. The second overall observation of the PAG panel is that coordinating new-messenger capabilities is essential. Without this coordination, we will not be able to fully understand multi-messenger sources like binary neutron star mergers, cosmic-ray accelerators, a Milky Way supernova, and more. Multi-messenger observations are especially critical for rare, spectacular transients, where the opportunity for incredible insights depends on the completeness of the coverage. The panel has thus chosen to endorse key projects in multiple areas. Despite the number of projects, the overall costs are modest, and they would be partially funded from physics programs. Projects using individual new messengers could each lead to major discoveries in astronomy. Together, as multimessenger astronomy, the prospects are even greater.

Figure L.4 summarizes how science capabilities would be enhanced in the 2020s and 2030s by the endorsed program, as well as the essential need for time and capability coordination between experiments to maximize the potential for multi-messenger astronomy. In addition to new observatories in the 2020s, research and development investments are needed to enable even larger scientific payoffs in the 2030s.

L.6 PROGRAMMATIC ISSUES

There are programmatic issues that need to be addressed to nurture the fields in the panel’s scope. Some are crosscutting, applying to the whole Astro2020 decadal survey.

Funding: The program above would fit within existing funding profiles for the NSF MREFC, NASA probe-scale, and NSF mid-scale programs. However, these programs are intensely competed among many fields. Further, several of the PAG-endorsed programs would be competing with each other in the NSF mid-scale program, which makes it more difficult to enable a complementary suite of projects for multimessenger astronomy. Another issue is that there needs to be a well-thought-out plan to fund operations of projects built by the MREFC program without negatively impacting grant funding in the corresponding research areas. NSF has suggested that the scope of the MREFC program could be changed to include initial funding for operations. The panel endorses this. Longer term, the MREFC budget is not likely to be sufficient to support large projects such as Cosmic Explorer. For all the reasons above, it is vital to the field of astronomy to push for the budgets of all of these programs to be expanded while ensuring cost caps so that overruns for larger projects do not erode support for smaller projects and investigator-led programs.

For many projects within the scope of the panel, the Department of Energy (DOE) could be a good fit in terms of agency objectives, and DOE leadership or partnership would bring important topical expertise, plus critical experience in managing large facilities and collaborations. Astronomical observations with new messengers directly address fundamental questions in particle and nuclear physics, including the properties of neutrinos and particles beyond the standard model, the high-density equation of state and the production of the chemical elements, and many aspects of cosmology. Full or joint funding by DOE has been done successfully and could be repeated.

Given the wide range of participating observatories on the ground and in space, unprecedented coordination within and between agencies, and among international partners, is needed to establish a robust program to ensure that the right projects are operating at the same time to maximize the science opportunities. This requires increased coordination over multiple funding cycles and special care that potential

breakthrough projects do not fall between the cracks. The panel encourages enhanced communication between NASA, NSF, DOE, and international partners to guarantee that science opportunities are not missed. In particular, the panel encourages that agencies develop a coherent long-range strategic plan that ensures coordinated coverage over the multiple wavelengths of multiple messengers, paying special attention to the timing for new instruments to come online and old instruments to be decommissioned.

Despite the necessary trend toward “big” science, avenues for funding small groups acting independently of the dominant trends are critical to innovating and laying the seeds for the science of subsequent decades. Several of the white papers relevant to the panel describe large, bold projects that would open new areas of observation space, such as localized cosmic ray sources, for example, but these projects are only at the early stages of conception. NASA and NSF support for technology development and small precursor missions is essential so that by the 2030s new capabilities will be available and ready. For example, the NASA Strategic Astrophysics Technology (SAT) program could support multi-messenger technology development. As the precision of large observatories improves, it will be increasingly important to also support measurements of fundamental atomic, nuclear, and particle physics processes needed to interpret the observations.

Theoretical and computational investigations play a key role in the advancement of knowledge by developing new laws, techniques, and ideas for experiments. For multi-messenger astronomy in particular, there is also a critical need to connect the results of different projects. However, by “following the science,” investigators may find themselves unable to fit within the boxes defined by the agencies, especially across the physics-astronomy boundary. In addition, many programs—for example, the NSF Astronomy program and the NASA Astrophysics Theory Program—are extremely oversubscribed. The panel encourages new, larger lines of support for theory critical to multi-messenger astronomy, particularly at the intersection of agency-defined traditional boundaries, plus a robust theory component to Guest-Investigator programs of future supported facilities and missions.

Operations Ecosystem: The success of multi-messenger astronomy relies on the coordinated efforts of astronomers spanning a wide range of groups, nations, and observational facilities. A robust “ecosystem” of observatories, some outside the direct purview of this panel, is needed to localize and characterize the electromagnetic counterparts of gravitational-wave and neutrino sources. Given the wide fields of view of gravitational-wave and neutrino observatories, optical sky monitors that cover the whole sky are required. Although the Rubin Observatory’s LSST project will provide broad, deep coverage in the Southern Hemisphere, complementary facilities are needed in the Northern Hemisphere, as are full-sky monitors (e.g., the All-Sky Automated Survey for Supernovae [ASAS-SN]) for optical transients too bright for the LSST project. In addition, optical spectroscopic facilities with flexible scheduling are required to confirm and characterize the huge numbers of photometric candidates and identify the source redshifts. As multi-messenger, time-domain astrophysics grows to become a larger part of astronomy, capabilities for multi-messenger follow-up become a fundamental consideration of data management/operations plans at the early stage of all mission concepts, even if the principal science drivers are not multi-messenger focused. It would be ill advised for projects to eliminate critical capabilities—such as the ability to slew rapidly or to accept and issue real-time alerts—without explicit consideration of the cost and trade-offs associated with multimessenger capabilities.

The timely release of public alerts is critical to maximizing the scientific gains by multi-messenger discoveries. As the number of discoveries by LIGO, IceCube, and other projects grow, the competition for precious multi-wavelength follow-up observations on large facilities will increase accordingly. To make efficient use of sparse resources, the panel endorses an open data policy, particularly regarding information—timing, sky position, and other decision-critical information—needed to coordinate and prioritize multi-messenger follow-up. This could be achieved by explicitly considering the timing and detailed

information content of public data releases in evaluating mission and facility proposals. Requirements for timely public data releases will require dedicated funding for validating and documenting the releases.

A new level of coordination across scientific communities is needed. There are several possible elements. For transients, coordinated observations are critical, but this can be very difficult to arrange, owing to requiring separate proposals, often with time scales incompatible with each other and with a quick response. As recommended by the NASA Gravitational Wave–Electromagnetic Counterpart Task Force, there is a need for proposal calls that enable and prioritize joint observations. Once the data are in hand, the analysis of multi-messenger data is made more difficult if every data set comes in a different format and requires different tools. It would save a great deal of time for the community as a whole if there were adequate support to develop formats, tools, and alert standards that could be adopted by many projects. Likewise, there is a need for centralized, standardized ways of archiving and serving data. Last, it is important to build cross-project community ties to foster cooperation and innovation, including between astronomy and physics. The best solutions for these issues for particle astrophysics and gravitation are yet to be developed. Some aspects of the solutions may draw from the examples of the NSF-supported National Optical-Infrared Astronomy Research Laboratory (NOIRLab) and NASA’s High Energy Astrophysics Science Research Center (HEASARC), which provide integrated platforms for multi-observatory science proposals, common data formats, and integrated analysis tools within their respective sub-disciplines. Last, the panel endorses the LISA Science Support Center as a mechanism to connect the U.S. and international communities; this may also be an example to other projects.

Culture: The fields within the scope of the panel span a wide range of scientific communities with disparate backgrounds and cultures. There is significant representation of people from physics, often from backgrounds in particle and nuclear physics and funded through different mechanisms. Although such cultural differences are, on one hand, an obstacle that must be overcome to enable the vigorous research program laid out here, they also represent a great opportunity to bring fresh ideas and perspectives and to build a new field “from the ground up.” New cultural work is needed to better connect physics and astronomy, as well as the sub-fields of gravitational-wave, neutrino, gamma-ray, and cosmic-ray astronomy, and to train students to cross these boundaries.

New cultural work is also needed to manage the increasingly large, international collaborations in ways that encourage the growth of communities that emphasize diversity, equity, and inclusion through proactive policies, mentoring, and accountability. The high visibility of particle astrophysics and gravitation, particularly among junior researchers, provides an opportunity to grow a joint community of outstanding vibrancy and diversity, one where individuals can have a big impact. Success will lead to new perspectives and discoveries, as well as societal benefits.

The responsibility for addressing these issues lies in multiple places. The funding agencies have a responsibility to take positive action to support traditionally excluded groups (race, gender, sexual orientation, and other minorities) in large collaborations, to give small grants to scientists whose work is outside the boundaries or the traditions of the large collaborations, and to explicitly fund development at the intersections between physics and astronomy. Scientists themselves have the responsibility to engage in anti-racist, anti-sexist, and more general anti-discriminatory practices; to leverage the excitement of their fields to recruit underrepresented populations; and to nurture the next generation of scientists to develop interest and talent at the intersections between fields. The results will be transformational for both the community and the science.