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Scientific Discoveries and Technical Advances

The 2010 astronomy and astrophysics decadal survey, New Worlds, New Horizons in Astronomy and Astrophysics¹ (NWNH), organized its discussion of the decadal science program around the themes of Cosmic Dawn, New Worlds, Physics of the Universe, and The Larger Science Program. This summary of scientific discoveries since NWNH is organized in the same categories. The committee emphasizes, as did NWNH, that these themes capture only some of the highlights of an extraordinarily rich palette of astronomical discovery that ranges from our own solar system to the edge of the observable universe, and from the first instants of cosmic time to the present day.

COSMIC DAWN: SEARCHING FOR THE FIRST STARS, GALAXIES, AND BLACK HOLES

On the theme of cosmic dawn, the most important advances in the first half of the decade have come from large surveys of the high-redshift universe with the Hubble Space Telescope (HST), often exploiting the near-infrared and grism capabilities of the Wide Field Camera 3 (WFC3) installed as part of the 2009 Hubble servicing mission. These surveys, augmented by space- and ground-based observations at other wavelengths, have dramatically improved understanding of galaxy populations in the first billion years of cosmic history. At redshifts z = 6-8,

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¹ National Research Council (NRC), 2010, New Worlds, New Horizons in Astronomy and Astrophysics, The National Academies Press, Washington, D.C.

the field has advanced with astonishing speed from the first tentative detections to reliable measurements of luminosity functions that span a factor of 100 or more in galactic star formation rate. The new frontier of discovery is at redshifts z = 9-12, within 500 million years of the Big Bang. Galaxies in this era are being found both in large area surveys and in projects like the CLASH Treasury Program and the Hubble Frontier Fields, which use clusters of galaxies as gravitational telescopes to bring small patches of the distant universe into magnified view (Figure 1.1). Star-forming galaxies grow steadily in number from z = 10 to z = 6, roughly tracking

the theoretically predicted growth of their parent halos of dark matter. Hydrogen Lyman-α emission from these early galaxies appears to decline rapidly at z > 6, which suggests that the Lyman-α photons at early epochs are being absorbed by a blanket of intervening neutral hydrogen. Hydrogen absorption spectra of quasars and gamma-ray bursts at z > 6 also suggest that the transparency of the intergalactic medium is changing rapidly at this epoch. Extragalactic observations with Atacama Large Millimeter Array (ALMA) are beginning to indicate the potential of ALMA spectroscopy at high redshift.

Cosmic microwave background (CMB) observations from the Planck satellite now suggest that intergalactic hydrogen was mostly reionized by z ≈ 7, somewhat later than previous estimates from the Wilkinson Microwave Anisotropy Probe (WMAP). With plausible extrapolations in luminosity and redshift, ultraviolet photons from the observed population of high-redshift galaxies appear sufficient to achieve reionization by this epoch. However, the relative contributions of bright and faint galaxies, and the potential contribution of X-rays and ultraviolet photons from accreting black holes, remain matters of debate. A powerful way to address these questions is to directly map redshifted 21 cm emission and absorption by neutral hydrogen throughout the epoch of reionization. Specially designed radio experiments are now approaching the sensitivity thought needed to detect the predicted signals, and they have demonstrated the ability to remove contamination from astrophysical and terrestrial foregrounds, which present the most challenging technical obstacle to mapping reionization. Cosmological simulations and semi-analytic models, incorporating gravitational clustering, hydrodynamics, star formation, and radiative transfer, play a crucial role in connecting observed galaxy counts to the physical mechanisms of reionization and in making predictions for the complex structures expected in 21 cm maps.

The next 5 years should see major further advances in the study of the cosmic dawn. Observing the first stars and galaxies is a defining objective of the James Webb Space Telescope (JWST), and its superb near-infrared sensitivity and angular resolution will allow detailed characterization of z = 8-12 galaxies that are barely detectable with HST, as well as detection of galaxies down to the limiting mass imposed by the physics of atomic gas cooling in dark matter halos. JWST may also detect individual supernovae arising in galaxies at these redshifts, and even earlier.

In the longer run, the Wide-Field Infrared Survey Telescope (WFIRST) will execute imaging surveys that equal the deepest HST observations over areas hundreds or thousands of times larger; both WFIRST and Athena will help to assess the relative contribution of galaxies and active galactic nuclei to reionization. CMB polarization experiments will provide sharper constraints on the timing and duration of the epoch of reionization. Building on the current generation of radio experiments, the Hydrogen Epoch of Reionization Array (HERA) project is proceeding with initial funding from the National Science Foundation’s (NSF’s)

Mid-Scale Innovations Program (MSIP). With further development, HERA and its successors will map the large-scale structure of the young universe and trace the process by which the first galaxies filled it with ultraviolet light.

NEW WORLDS: SEEKING NEARBY, HABITABLE PLANETS

Since 2010, the number of detected exoplanets has grown from fewer than 1,000 to more than 5,000, with the majority of new systems coming from NASA’s Kepler mission. But numbers alone understate Kepler’s impact: by opening new windows of parameter space, it has discovered a diversity of planets and planetary systems far beyond that previously known. Transit measurements of planet diameters, especially when combined with mass measurements from radial velocity follow-up or from transit timing variations, provide critical diagnostics of exoplanet compositions. Kepler discoveries range from dense, iron-rich planets like Mercury to planets of solid rock or molten lava, to water worlds and ice giants and gas giants puffier than Jupiter and Saturn. “Super-Earths,” intermediate in size between Earth and Neptune, have been found in great abundance, commonly in compact and flat systems of multiple planets in nearly circular orbits. The most tightly packed Kepler systems have four or five planets, all with orbital radii smaller than Mercury’s. Other systems have planets orbiting binary stars, natural realizations of the science-fiction vision of a world with two suns. Small planets are an order of magnitude more abundant than giant planets, confirming a generic prediction of the core accretion theory of planet formation. Sun-like stars, on average, are host to about one planet larger than Earth with an orbital period shorter than a year, and Kepler’s census implies billions of Earth-like worlds in the Milky Way.

Ground-based transit, radial velocity, microlensing, and direct imaging searches have also made great strides over the decade. Transit observations show that some hot Jupiters are inflated because of proximity to their parent stars, and roughly half of them have orbits misaligned with their parent star’s spin, which suggests emplacement by dynamical scattering rather than slow migration or in situ formation. High-precision radial velocity surveys can now measure stars moving at the speed of a slow walk, and some searches are now targeting cooler stars for which rocky planets in the habitable zone would produce a detectable signal. Several strong candidates for such potentially habitable worlds are now known. Microlensing is most sensitive to planets with orbital radii of several astronomical units, and the technique has so far discovered roughly 50 planets in 44 systems, showing that there is, on average, roughly one planet per star beyond the snow line (where ices can form in a protoplanetary disk) with mass between 5 M_Earth and 10 M_Jupiter. The ongoing Spitzer Space Telescope mission has been highly successful for the characterization of transiting extrasolar planets, particularly as the primary source of measurements of thermal emission from secondary eclipses. Coronagraphic instru-

ments that exploit adaptive optics on 8-meter telescopes are yielding direct images of massive planets around younger stars, probing the boundaries between planets and brown dwarfs, and between star formation and planet formation.

The disks of gas and dust around forming stars are the nurseries where planets and planetary systems are born. Images of protoplanetary and transitional disks at a range of wavelengths reveal detailed structures that trace the process of star and planet formation. In scattered light observations at infrared wavelengths, protostars with infalling dusty envelopes and obscured mid-planes are visible within large molecular clouds. The expected transformational power of ALMA at millimeter wavelengths is apparent in early observations of disks that probe rotational dynamics, fine spatial structure, and chemical composition. Most spectacularly, the disk of proto-star TW Hydrae is deeply sculpted with rings and gaps, possibly caused by orbiting planets (Figure 1.2). Other ALMA observations confirm the predicted disappearance of molecular gas species at the large separations where they freeze into solid particles. Disk observations with extreme adaptive optics systems reveal streams and spiral arms, whose structure may be related to the masses of embedded planets. High-resolution images of the young system LkCa15 show an annulus of cold dust and gas with an object that may be an accreting planet orbiting within its inner gap.

Progress should come on many fronts in the next 5 years. Compared to the Kepler prime mission, the continuing “K2” campaigns will target a greater number of stars with a greater range of properties and galactic environments, albeit with less sensitivity to small planets or long orbital periods. Because it is targeting a larger area, K2 is also finding planets around brighter stars than those in the primary mission. The Transiting Exoplanet Survey Satellite (TESS), scheduled for launch in December 2017, will use similar techniques to Kepler but will observe bright, relatively nearby stars over the whole sky, thus identifying targets that are ideal for radial velocity mass determinations and transit spectroscopy. A new generation of microlensing surveys will greatly expand the census of low-mass planets at distances near and beyond the snow line, testing basic predictions of the core accretion scenario. Large observing campaigns with the new generation of coronagraphic instruments will probe the populations and properties of gas giants at large orbital separations. These instruments and ALMA will provide a systematic census of structure in protoplanetary disks. Radial velocity instruments are now targeting the 0.1 m/s sensitivity needed to detect the reflex motion induced by Earth-like planets around Sun-like stars, a challenge that requires understanding and mitigating astrophysical sources of Doppler noise. JWST will provide a huge leap in sensitivity for transit spectroscopy of relatively cool planets, including many targets identified by TESS, allowing radically new insights into the composition and structure of exoplanet atmospheres. Phase curves may also be possible if relaxation of operational constraints currently under consideration is possible. In the longer

term, WFIRST’s microlensing census of planets beyond 1 AU will perfectly complement Kepler’s census of compact systems, and WFIRST will also be able to detect free-floating planets unbound from their parent star. Coronagraphic instruments on 20- to 30-meter telescopes will sharpen the sensitivity and angular resolution of direct imaging searches and spectroscopic characterization of gas giants, while the WFIRST coronagraph is expected to push spectroscopy to the regime of Neptunes and super-Earths, and to demonstrate the technology that would eventually allow images and spectra of habitable worlds around nearby stars.

PHYSICS OF THE UNIVERSE: UNDERSTANDING SCIENTIFIC PRINCIPLES

Building on discoveries of the 1990s, the first decade of the 21st century saw the establishment of a “standard cosmological model,” ΛCDM, incorporating cold dark matter, a cosmological constant, a flat universe, and Gaussian primordial fluctuations from inflation. Observations since 2010 have tested this model far more stringently than before, with new physical phenomena and redshift domains and greatly improved measurement precision. CMB data from WMAP, the Atacama Cosmology Telescope (ACT), the South Pole Telescope (SPT), and, especially, the Planck satellite have confirmed ΛCDM predictions in exquisite detail, including the long series of acoustic oscillation peaks imprinted by primordial sound waves, the polarization power spectrum expected for adiabatic initial conditions that arise from quantum fluctuations during inflation, and a 40 σ standard deviation detection of the lensing of CMB fluctuations by clustered foreground dark matter (Figure 1.3). One fundamental result is a clear demonstration of “tilt” in the large-scale power spectrum, confirming the inflationary prediction of a small departure from “generic” scale-invariant fluctuations.

At lower redshifts, baryon acoustic oscillation (BAO) measurements from the Sloan Digital Sky Survey (SDSS) have allowed 1 percent determinations of the absolute cosmic distance scale at z ≈ 0.6 and the first precise (2-3 percent) determinations of the expansion rate at high redshifts, z ≈ 2.5. Homogeneous analyses of large supernova data sets have achieved 1-2 percent measurements of the relative distance scale over the range 0 < z < 0.8 and lower precision measurements out to z ≈ 1.4. The ΛCDM model reproduces these percent- or subpercent-level measurements of the cosmic expansion history from the recombination epoch to the present day. The model also predicts the history of dark matter clustering, which can be measured with gravitational lensing and galaxy clustering. Here, the agreement with observations is less clear, and the level of systematic uncertainties in the measurements is higher. There is also some tension between the values of H0 inferred from CMB and BAO data and those inferred from local distance ladder measurements.

As anticipated by NWNH, direct and indirect searches for dark matter have now achieved sufficient sensitivity to probe the core parameter space of broad classes of weakly interacting massive particle (WIMP) theories, such as those based on minimal supersymmetric extensions of the standard model of particle physics. The Fermi γ-ray satellite has been especially important because it detects photons in the energy range expected for typical WIMP annihilation channels, and it has deep, full-sky coverage. While there have been tantalizing claims of possible dark matter annihilation signals from the galactic center or from other galaxies or clusters, none of these signals is convincingly distinct from astrophysical sources, and the absence of signals from nearby dwarf galaxies sets interesting limits on WIMP annihilation cross sections. Underground direct detection experiments have

yielded some claimed signals, but none of these has yet convinced the community as a whole, and other experiments are now ruling out significant regions of the supersymmetric particle parameter space. The Large Hadron Collider (LHC) has confirmed, with the dramatic discovery of the Higgs boson, a central pillar of the standard model of particle physics, but it has not yet shown evidence for super-symmetry or other standard model extensions that could explain dark matter.

Neutrino astrophysics has seen major advances (and the 2015 Nobel Prize in physics), including precise measurements of many of the parameters that describe the neutrino sector. Upper limits on neutrino mass from cosmological data are now approaching the lower limits set by neutrino oscillation data. The most dramatic recent development in neutrino astronomy is the IceCube experiment’s detection of several dozen neutrinos in the peta-electronvolt (PeV) energy range, with arrival directions spread over much of the accessible sky. These are the first known astrophysical neutrinos from sources other than the Sun and Supernova 1987A, and their discovery opens the view to new messengers from the high-energy universe.

The most dramatic astronomical development of the century thus far is the detection of gravitational waves from merging black holes at a distance of 400 Mpc, during the first science run of the advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) (Figure 1.3). This discovery follows decades of work to build instruments that can measure displacements 10,000 times smaller than an atomic nucleus, pushing the extremes of quantum optics, mechanical engineering, and signal processing. NWNH anticipated detection of gravitational waves this decade based on improving sensitivity of LIGO and pulsar timing experiments. Nonetheless, the detection of such a strong signal so early in advanced LIGO operations is startling. This discovery confirmed some of the most exotic predictions of Einstein’s theory of general relativity, and it demonstrated that 30 solar mass black holes exist and form close binary systems, that black hole mergers produce gravitational wave bursts that match the predictions of numerical relativity simulations and analytic calculations of the merged remnant’s ringdown, and that the interferometric methods pioneered by LIGO are up to the challenge of detecting astrophysical sources of gravitational waves. Most importantly for the future, this discovery strongly suggests that sources within LIGO’s sensitivity range are fairly common and that gravitational wave observations will rapidly open a new window on some of the most energetic phenomena in the cosmos. Space-based gravitational wave observatories can probe different phenomena in frequency ranges inaccessible from the ground, and they can test general relativity predictions of black hole spacetimes at extremely high precision.

Many improvements are expected over the next 5 years, which may consolidate or challenge the current understanding of the physics of the cosmos. CMB experiments have set their sights on detecting the distinctively twisted polarization pattern induced by primordial gravitational waves, which would directly probe physics during an era when today’s observable universe occupied a volume smaller than a grapefruit. The ongoing Dark Energy Survey (DES) and Subaru Hyper-Suprime Camera (HSC) survey will sharpen weak lensing measurements of matter clustering to the 1 percent level, comparable to current measurements of the expansion history. The Dark Energy Spectroscopic Instrument (DESI), slated to begin observations in 2019, will map the three-dimensional distribution of tens of millions of galaxies, yielding finer and more detailed measurements of expansion and structure growth over the past 10 billion years. New underground dark matter experiments (Super-CDMS, LUX-ZEPLIN, ADMX-Gen2) are expected to achieve order-of-magnitude gains in sensitivity to the most widely investigated candidates for particle dark matter. The higher operating energy and increased luminosity of the LHC make it sensitive to previously undetectable particle species. A convincing discovery of dark matter could come from these experiments any day, or not at all. IceCube will continue to build its sample of PeV neutrinos, while more densely sampled or larger area detectors will expand the reach of neutrino experiments to

lower and higher energies. Analyses of several LIGO events are already in the works, and coordinated programs of follow-up for associated electromagnetic events are under way. By decade’s end, a far more comprehensive view of the gravitational wave universe will be complete.

THE LARGER SCIENCE PROGRAM

The NWNH science themes represent three slices from the broad panorama of astronomical research. Exciting advances have occurred in many other fields of astronomy and astrophysics, and just a small selection is summarized here.

The fields of stellar astrophysics and stellar populations have been transformed by asteroseismology measurements from Kepler and by highly multiplexed spectroscopic surveys of stellar radial velocities and chemical compositions. Kepler’s superb photometric precision and time sampling have yielded asteroseismic signals for many thousands of stars, allowing measurements of their internal structure and rotation profiles, and even the strength of their core magnetic fields. These measurements provide an unprecedented look at the inner life of stars. Combined with composition measurements, Kepler asteroseismology allows determination of stellar ages with an accuracy of 10 to 20 percent, adding a new dimension to studies of galactic structure. The SDSS APOGEE survey has mapped the multielement abundances of 100,000 red giant stars across the full span of the Milky Way, finding patterns that reveal the inward and outward migration of stars by many kiloparsecs over the life of the galactic disk. ALMA has opened new windows on the study of the mass loss in evolved stars, and its sensitivity, resolution, and millimeter-wave spectroscopic capabilities are already having an impact on stellar astrophysics with detailed chemical, as well as dynamical, studies of evolving stars and planetary nebulae.

Other observations have mapped extremes of stellar properties and stellar fates. Programs that sift through stellar samples to identify the most chemically primitive stars have found some with heavy element abundances 1,000 to 10,000 times lower than the Sun’s. The pattern of abundances in these metal-poor stars, including enhanced levels of carbon relative to other elements, provides clues to the physics of the first stars and the earliest stellar explosions. The Wide-Field Infrared Survey Explorer has revealed a large population of “Y dwarfs,” objects with masses below the hydrogen-burning main sequence and surface temperatures below about 500 K. One such Y dwarf has an effective temperature of only 250 K, cooler than Earth; recently, two brown dwarfs in a binary have been found only 2 parsecs away, making these brown dwarfs some of the closest “stars” to the Sun.

At another extreme, measurements of relativistic time delay have identified two neutron stars with masses of 2.0 solar masses, and uncertainties of a few percent, eliminating many theories of the equation-of-state of nuclear matter. Supernova

surveys with robotic telescopes, including the Palomar Transient Factory (PTF) and the All Sky Automated Survey for Supernovae (ASAS-SN), have uncovered many new classes of stellar explosions, including events that are 10 times fainter or several hundred times brighter than canonical supernovae. These discoveries challenge the standard picture of supernova physics. Other studies of early light curves and pre-explosion environments suggest that even the conventional Type Ia supernovae may well be a mix of two populations, powered by single- and double-degenerate explosions, respectively. X-ray observations of element distributions in supernova remnants, from the Chandra X-ray Observatory and the Nuclear Spectroscopic Telescope Array (NuSTAR) Explorer satellite, suggest clumpy and asymmetric explosion morphologies, providing a new testing ground for three-dimensional supernova calculations. PTF, ASAS-SN, and the Swift gamma-ray satellite have also begun to discover substantial numbers of tidal disruption events, in which a star is effectively turned inside out by passing too close to the supermassive black hole at its galaxy’s center.

Supermassive black holes remain a central theme of observations at many wavelengths, especially the high energies that probe physical conditions close to the event horizon. Spectroscopy, reverberation mapping, and microlensing variability studies using Chandra, XMM-Newton, and NuSTAR have confirmed key aspects of the long-standing theoretical picture of accretion disks at a few tens of gravitational radii, although remaining puzzles may provide clues to the inner structure of accretion flows. ALMA has also yielded direct kinematic measurements of the mass of supermassive blackholes in nearby galaxies with a precision better than HST. Results from the Fermi Gamma-ray Space Telescope show that the extragalactic gamma-ray background is dominated by blazars, in which supermassive black hole accretion powers highly beamed relativistic jets. Some of Fermi’s most dramatic results have come from our own galactic center, where a pair of giant gamma-ray bubbles spanning nearly 90 degrees of the sky (40,000 light years) suggest that the Sgr A* black hole was a far more luminous active galactic nucleus some 1-2 million years ago, pumping enormous energy into its surroundings. X-ray “echo mapping” of the galactic center, based on more than a decade of Chandra data, implies outbursts of activity within just the last few centuries, with the X-ray luminosity of Sgr A* flaring to a million times its present value. At millimeter wavelengths, coordinated observations with the global Event Horizon Telescope are approaching the extraordinary angular resolution needed to image the shadow of the Sgr A* event horizon, and they are already constraining the black hole spin and the physical structure of its accretion flow.

In addition to mapping the z > 6 galaxies of cosmic dawn, surveys from HST, Spitzer, Herschel Space Observatory, Chandra, ALMA, and ground-based optical telescopes have provided a much more detailed account of galaxy evolution through the epoch when the majority of stars in the universe formed. These obser-

vations reveal the often complex connections among stellar mass, star formation, gas content, morphology, size, metal abundance, and nuclear activity. At nearly all redshifts, stars form most effectively within dark matter halos that are similar in mass to the Milky Way’s (roughly a trillion solar masses), and galaxies in more massive halos have largely ceased forming stars. The mechanisms that quench star formation—especially the relative importance of central black holes, stellar bulge formation, gas stripping, and “strangulation” of fresh gas accretion—remain hotly debated. Circumgalactic gas is both the reservoir that feeds galaxy growth and the repository of material ejected by galactic winds. Large programs with HST at low redshift and the Keck Observatory telescope and European Very Large Telescope (VLT) at high redshift have mapped the distributions of hydrogen, carbon, oxygen, and silicon in the circumgalactic medium, using ultraviolet absorption lines in the spectra of background quasars. ALMA has been used to observe [C II] and [CO] in order to measure the dynamics of galaxies at high redshift (z~6). These observations show that most star-forming galaxies are surrounded by large reservoirs of cool gas, comparable in mass to their stellar disks. Even the million-degree halos of hot gas that surround quenched elliptical galaxies are found to harbor large amounts of cool gas within them. Chandra and XMM-Newton observations have mapped the massive hot gas halo of the Milky Way, which imprints X-ray absorption lines on the spectra of bright background sources.

In the local universe, surveys using integral field spectrographs are providing detailed and unified maps of the stellar populations, chemical enrichment, gravitational dynamics, and gas flows in large samples of nearby galaxies spanning a wide range of properties. These observations provide direct insights into the ecology of galaxies, the physics of star formation, and the origin of galactic winds, and they provide a new testing ground for cosmological simulations of galaxy formation. Building on discoveries from the SDSS, the first 2 years of data from DES have revealed at least 15 new satellite companions of the Milky Way that are too faint or too diffuse to have been detected in previous sky surveys. These systems provide clues to the assembly history of our galaxy, tests of the properties of dark matter and the physics of low mass galaxies, and potential sources for the detection of dark matter annihilation into gamma rays. The Dragonfly telescope, an innovative array of telephoto lenses designed to produce highly uniform images over large areas, has discovered a population of “ultra-diffuse” dwarfs in nearby clusters, a new class of galaxies whose origin is not yet understood. In previous observations, these galaxies were literally too big to see.

This summary has emphasized observational developments, but theoretical work underpins the analysis and interpretation of many of these observational programs, and it is ultimately the means by which to go from empirical measurements to knowledge of the workings of nature. The discovery of gravitational waves followed decades of work on expected event rates and signal forms and a decade

of extraordinarily rapid progress in numerical relativity calculations of black hole mergers, which show remarkable agreement with the first detection of this phenomenon. The design of cosmological surveys, from DES to WFIRST, relies heavily on theoretical optimization methods, and the inference of cosmological parameters from these experiments often involves large simulation programs that create mock data sets with realistic treatments of nonlinear gravitational clustering and bias between galaxies and dark matter. The powerful constraints inferred from CMB measurements also rely on sophisticated theoretical modeling and highly optimized computational and statistical methods. Advances in computational power and algorithms have allowed hydrodynamic simulations of galaxy formation to reach much greater levels of realism, resolving critical processes of stellar feedback on scales of a few parsecs while tracking the growth of primordial fluctuations into a Milky Way–like galaxy. Relative to previous generations, current simulations are much more successful at reproducing the observed properties and evolution of galaxies, and they are playing a key role in interpreting new observations of galactic outflows and circumgalactic gas.

On stellar scales, multiple groups around the world are now performing three-dimensional simulations of supernova explosions with increasing levels of sophistication, yielding new insights into the complex and long-standing puzzles of core collapse and thermonuclear supernova mechanisms. Kepler data have stimulated a burst of theoretical work on asteroseismology of red giants, which enables the Kepler measurements to probe the internal structure, rotation, and magnetic fields of evolved stars. The extraordinary diversity of Kepler planetary systems has stimulated theoretical investigations of the dynamical stability of tightly packed orbital configurations, the mechanisms that regulate orbital radii and eccentricity, and the physics that governs habitability. Transit spectroscopy and direct detection measurements draw on increasingly sophisticated models of planetary atmospheres, which include detailed molecular chemistry, cloud formation, and circulation flows driven by stellar irradiation.

The significance of theoretical developments often takes many years to emerge, as once speculative ideas are tested by observations enabled by new technology. The high-precision cosmological measurements of this decade, for example, provide spectacular confirmation of theoretical work on cosmological perturbations and non-baryonic dark matter from the 1970s and 1980s. The detection of gravitational waves confirms Einstein’s once-radical views of spacetime and gravity from the early 20th century. The interpretation of Kepler discoveries is heavily influenced by classic work on celestial mechanics and by theoretical descriptions of planet formation and migration first advanced 30 to 50 years ago. The interaction between theory and observations sometimes takes surprising forms, such as the recent suggestion that the anomalous clustering of the orbits of newly discovered solar system bodies may be explained by the existence of a ninth planet that is 10 times more

massive than Earth, which presently orbits hundreds of astronomical units from the Sun. If the existence of “Planet IX” is confirmed by direct detection, it will write a new chapter in the history of the solar system, echoing the discovery of Neptune through its gravitational effects nearly two centuries ago.

TECHNICAL ADVANCES

The time since the release of NWNH has seen great progress in the technological underpinnings of astronomy. Advances abound, so the committee only presents examples of this progress below.

In gravity wave astronomy, the impressive early results from the LISA Pathfinder Mission have demonstrated the key technologies needed for a future space mission to cover the source-rich millihertz portion of the gravitational wave spectrum, and of course the outstanding results from LIGO demonstrate the success of the underlying technological approach.

In X-ray astronomy, in both Europe (as part of the Athena technology program) and in the United States (as a continuation of Con-X technology), there has been the demonstration of sub-10-arcsec lightweight X-ray mirrors; the successful development of high-resolution, high-efficiency X-ray gratings that allow an order of magnitude improvement on the performance of the Chandra and XMM gratings; and the demonstration of electronvolt resolution imaging X-ray spectrometers, as shown by the successful operation of the Hitomi soft X-ray spectrometer. NuStar has utilized broadband (5-80 keV) multilayer focusing optics and low background imaging hard X-ray detectors. High-speed (100 ns) silicon drift detectors have been developed for the NICER (Neutron Star Interior Composition Explorer) mission, and there has been a major improvement in X-ray polarimeters. There have also been major developments in gamma-ray technology, including major advances in Si strip technology, as pioneered by Fermi and the LHC, and in CdZnTe technology.

In survey cosmology, which is central to studies of dark energy and cosmological parameters, the principal technical advances have been increasing power of large format detectors and highly multiplexed fiber systems. The DES uses a 520 megapixel charge-coupled device (CCD) camera on the 4-m Blanco Telescope at the Cerro Tololo Inter-American Observatory, and it will eventually be superseded by the 3.2 gigapixel camera being built for the Large Synoptic Survey Telescope (LSST).

The maturation of H4RG near-infrared detectors, with four times the pixel count of the previous generation H2RGs, has had a major impact on the design of WFIRST. For redshift surveys, the 1,000-fiber spectrographs used for the SDSS-III Baryon Oscillation Spectroscopic Survey (BOSS) will be superseded at decade’s end by the 5,000-fiber DESI for the 4-m Mayall Telescope at the Kitt Peak National Observatory. In CMB studies, individual detectors are already at the quantum sen-

sitivity limit, and higher sensitivity is being achieved by building ever larger and more integrated arrays of bolometers. This approach has led to substantial gains, especially for CMB polarization experiments where the signals being measured are one or more orders-of-magnitude down from the temperature anisotropy.

Previous generations of high-contrast imager designs were optimized for ideal unobscured apertures. These solutions would not work for the 2.4-meter obscured WFIRST pupil. Rapid development in response to this led to multiple concepts for coronagraphs that control the diffraction pattern from the secondary mirror and its supports, either with shaped-pupil masks, phase-induced amplitude apodization mirrors, or multiple deformable mirrors. Laboratory demonstrations of these technologies show contrast and stability levels sufficient to detect extrasolar planets with WFIRST.²

Dedicated high-contrast ground-based exoplanet imaging instruments, such as the Gemini Planet Imager, the VLT SPHERE (Spectro-Polarimetric High-contrast Exoplanet Research) facility, and the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) testbed, are now operational, producing significant exoplanet discoveries and proving techniques and technologies that will be needed for future high-contrast imagers on extremely large telescopes or in space.

Powerful infrared spectrographs, such as the Keck MOSFIRE multi-object near-infrared spectrograph, open up new scientific possibilities. By accessing the Doppler-shifted emission peak of galaxies, they can carry out large surveys of high-redshift galaxies on a scale previous restricted to visible-light programs. The successors to the SDSS, such as the Dark Energy Camera, are producing revolutionary science and showing the future potential of LSST. Massively multiplexed optical spectrographs (Hobby-Eberly Telescope Dark Energy Experiment, DESI, and Subaru Prime Focus Spectrograph) will bring similar scales to spectrographic surveys for BAO dark-energy measurements and other programs.

Adaptive optics (AO) systems continue to increase in capability, with fully operational laser guide star systems on both Keck telescopes, adaptive secondary mirror systems with high performance on both the Large Binocular Telescope and Magellan (including some visible-light capabilities, albeit limited to bright stars), and the wide-field multi-conjugate AO system on Gemini South.

At radio wavelengths, advances in digital signal processing technology combined with reductions in cost are making possible arrays composed of a large number of relatively small antenna elements, giving large fields of view with large collecting areas. These arrays enable comprehensive sky surveys with implications for cosmol-

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² NASA, 2015, Wide-Field InfraRed Survey Telescope-Astrophysics Focused Telescope Assets WFIRST-AFTA 2015 Report, Science Definition Team and WFIRST Study Office, https://arxiv.org/ftp/arxiv/papers/1503/1503.03757.pdf.

ogy, for the study of transients, and for charting the evolution of galaxies and clusters over cosmic timescales.

It is worth noting, however, that the above advances in ground-based astronomy technologies were set in motion in an era when NSF supported a variety of instrumentation programs, including the Telescope Systems Instrumentation Program to build instruments for large telescopes. In the remainder of this decade, with the exception of LSST, the Department of Energy-funded dark energy program building the DESI spectrograph, and some smaller-scale programs funded by the Major Research Instrumentation Program, ground-based optical and infrared instrumentation will likely slow significantly.