Pathways to Discovery in Astronomy and Astrophysics for the 2020s (2023) / Chapter Skim
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2 A New Cosmic Perspective
2 A New Cosmic Perspective
Pages 33-82
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From page 33... ...
Newly discovered fossil structures from the formation of the Milky Way Galaxy open a window on the Milky Way's distant past, and observations take several steps closer to diagnosing the conditions present shortly after the Big Bang. The investments of previous decades bore fruit in this decade in the awarding of Nobel Prizes in Physics for six discoveries derived from astronomical measurements: dark energy, neutrino oscillations, gravitational waves, exoplanets, physical cosmology, and black holes (Figure 2.1)
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From page 34... ...
In 2011, Saul Perlmutter, Adam Riess, and Brian Schmidt received the prize "for the discovery of the accelerating expansion of the universe through observations of distant supernovae." The citation to Takaaki Kajita and Arthur McDonald in 2015 was "for the discovery of neutrino oscillations, which shows that neutrinos have mass." In 2017, Kip Thorne, Rainer Weiss, and Barry Barish were awarded the prize "for decisive contributions to the LIGO detector and the observations of gravitational waves." The year 2019 saw the awarding of the Nobel Prize in Physics to James Peebles "for theoretical discoveries in physical cosmology" and to Dider Queloz and Michael Mayor "for the discovery of an exoplanet orbiting a solar-type star." Most recently, in 2020, the topic of black holes received Nobel attention, with recognition to Roger Penrose "for the discovery that black hole formation is a robust theory of general relativity" and to Andrea Ghez and Reinhard Genzel "for the discovery of a supermassive compact object at the center of our galaxy." SOURCES: 2011: NASA/STScI/Ann Field; 2015: Kamioka Observatory, ICRR (Institute for Cosmic Ray Research) , The University of Tokyo; 2017: R
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From page 35... ...
It soon became clear that most of the science questions and discovery areas could be organized into three broad thematic areas: Worlds and Suns in Context highlights the extraordinary advances over the past decade in the study of exoplanets, stars, and their associated planetary systems, and the opportunities for transformational advances in these areas, including the ultimate search for and characterization of habitable planets, in the decades ahead. Cosmic Ecosystems represents an integration and culmination of understanding the origins of galaxies, stars, planets, and massive black holes, and the realization that the life cycles of the universe over this billionfold range of scales are intimately connected, through feedback processes propagating through the gas within, surrounding, and between galaxies.
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From page 36... ...
The coming decade will continue this trend: where Gaia enabled precision stellar parameters of roughly a billion stars in the Milky Way, the Rubin Observatory promises to expand by a factor of 10 the number of main sequence stars for which distances can be determined. 2.1.1 Stellar Demographics We are in the midst of a stellar renaissance, as astronomers come to know the individual journey of each star, separating these stars from anonymous points of light in the heavens to having the equivalent of detailed dossiers of physical characteristics and histories.
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From page 37... ...
The mass distribution of red giant stars probed by asteroseismology does not match predictions from stellar population models. Asteroseismology detects strong magnetic fields in the cores of red giant stars.
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From page 38... ...
The detection of gravitational waves from astrophysical objects this decade leads to invigorated research into the endpoints of stellar evolution for the next decade. Stars in multiple systems can have very different evolutionary pathways compared to their single counterparts; detailed photometric and spectroscopic electromagnetic observations spanning infrared through X-ray wavelengths, coupled with gravitational wave measurements and attention from theory, will elucidate these multiple routes and their consequences.
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From page 39... ...
Complementary radio observations could generate three-dimensional mapping of the magnetic field in sunspots, and magnetic field measurements in the global corona provide context for the more detailed, restricted field of view measurements.
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From page 40... ...
Work in the next decade will broaden the range to include a larger sample and provide the perspective of stars of different ages and spectral types. While hints exist now of a disconnect in the relationship between fundamental stellar parameters and magnetic properties -- magnetically active stars can have larger radii than predicted based on their temperatures; stellar twins identical in all other properties can have very differing levels and amounts of magnetic field distribution and magnetic activity signatures -- observations in the next decade will further this discovery space and fill important gaps in the foundation of stellar structure theory.
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From page 41... ...
The importance of magnetic fields in a stellar context was expected but is now unequivocal. Magnetism in massive stars has been confirmed (but not entirely explained)
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From page 42... ...
42 PATHWAYS TO DISCOVERY IN ASTRONOMY AND ASTROPHYSICS FOR THE 2020s 2010 (Minimum) Mass (Earth Masses)
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From page 43... ...
2.1.2.1 Planet Formation The detailed process of planet formation is one of the great unknown frontiers in astrophysics. The core concept of gas and tiny dust particles in a circumstellar disk assembling into planets is well known.
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From page 44... ...
VII. Precise planet radii leveraging Gaia DR2 reveal the stellar mass dependence of the planet radius gap, The Astronomical Journal 156:264, © AAS.
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From page 45... ...
New radio and far-IR surveys of planetforming disks would enable leaps in understanding that would surpass this era's ALMA-driven revolution. There have been tremendous advances in the theory that underlies planet formation, as new ideas such as the "streaming instability" and "pebble accretion" use gas and particle interplay and physical changes in the disk to trap planet-forming material and rapidly form giant planets.
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From page 46... ...
, to examine the atmospheres of potentially Earth-like planets around Sun-like stars will require further development of a specialized space telescope for high-contrast imaging to measure a reflection spectrum that could show oxygen, methane, water vapor, and carbon dioxide. SOURCE: Courtesy of N
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From page 47... ...
Do they show signs of life? 2.1.2.3 Connections to the Solar System Studies of exoplanets and of the solar system are tightly intertwined and have enjoyed a profitable give and take in contributing to the understanding of planet formation and atmospheres.
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From page 48... ...
Directly imaging non-transiting, potentially habitable planets of the nearest low-mass stars requires extreme angular resolution but is only moderately demanding in terms of relative brightness, and hence is feasible from the ground with high-performance adaptive optics. Achieving this is a key science goal of the proposed ground-based extremely large telescopes.
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From page 49... ...
Time-domain surveys such as the Vera Rubin Observatory's Legacy Survey of Space and Time will greatly expand the number of known small bodies in the outer solar system, and provide information about its early evolution. Results from recent studies analyzing dynamics of small bodies in the Kuiper Belt provide tantalizing hints, to be confirmed, about the possibility of additional planets.
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From page 50... ...
The star's properties and evolution influence the evolution and habitability of the planets, particularly of terrestrial planets. The end stages of star formation provide the initial conditions for planet formation.
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From page 51... ...
Magnetic fields of low-mass stars may prevent some eruptive events from ejecting mass, lending complexity to a blind application of the solar analog, and there are currently few observational constraints on stellar
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From page 52... ...
, and with an aperture of at least 6 m and coronagraphic imaging capability, should be capable of observing approximately 100 nearby stars and of successfully detecting potentially habitable planets around at least a quarter of the systems. Such an observatory would also provide valuable information on other extrasolar planets, and be versatile enough to carry out groundbreaking observations of stars, galaxies, black holes, and the gases and baryons within and between galaxies, with a scientific impact rivaling that of previous "great observatories" such as HST.
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From page 53... ...
In doing so, X-ray observations revealed the dynamic coronae of the Sun and other stars, accreting neutron stars and black holes, and the hot plasma that pervades interstellar and circumgalactic environments. Radio observations revealed the existence of neutron stars, whose remarkably stable rotation rates have since been used to discover planets and confirm the theory of general relativity's prediction of orbital decay via the emission of gravitational waves.
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From page 54... ...
In 2013, the south pole IceCube observatory detected a diffuse high-energy neutrino flux of astrophysical, but unknown, origin. Starting in 2015, LIGO opened up the gravitational wave view of the universe by detecting merging binary black holes.
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From page 55... ...
Finding innovative ways to probe cosmology in the "dark ages" prior to any significant star formation is one of the discovery areas identified here. The development of "LambdaCDM," our current standard cosmological model, is one of the major intellectual triumphs of the past few years; the nature and origin of its key ingredients -- dark matter, dark energy, and a nearly scale-invariant spectrum of primeval mass fluctuations -- remain, however, some of the biggest mysteries in science.
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From page 56... ...
In parallel with lensing studies on large scales, more detailed, focused studies of individual lensed galaxies, supernovae, and quasars (accreting black holes) , and even lensed stars at cosmological distances, will test the predictions of the cold dark matter paradigm on the smallest scales, where we expect deviations to be most evident.
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From page 57... ...
The fluctuations seen in the CMB are believed to have been imprinted in the earliest phases of the Big Bang during a period of cosmological inflation in which extraordinarily rapid expansion established the large-scale homogeneity and flatness of the universe while also causing quantum fluctuations that subsequently grew into the fluctuations we observe. One of the most exciting opportunities in the coming decade is that CMB measurements may reveal remnant gravitational waves from this early epoch, as depicted in Figure 2.16, later in this chapter.
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From page 58... ...
This led to the identification of time-domain astronomy as a key discovery area in Astro2010. These seemingly separate advances in observational techniques are in fact intimately related: most of the known and anticipated sources of gravitational waves, neutrinos, and cosmic rays are also time variable or transient electromagnetic sources (e.g., neutron star mergers, gamma-ray bursts, black hole jets, and stellar explosions)
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From page 59... ...
Source parameters and basic inferences measurement with GW170817 is not as precise as other techniques, multi-messenger cosmology will increase The physical properties of GW sources are inferred by matching the observed data with in importance in the coming decade as we detect ever more binary neutron star and black hole mergers. waveforms generated following the prescriptions of general relativity, which makes detailed Observations of SN predictions for 1987A andand the inspiral the Sun in light coalescence signaland neutrinos of merging andBHs.
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From page 60... ...
This is an indication of just how difficult neutrinos are to detect relative to how common they are, but in this difficulty lies their promise. Precisely because neutrinos interact so little with matter, neutrinos carry information about the inner workings of some of nature's most important energy sources: the nuclear furnaces inside stars, the formation of neutron stars in stellar explosions, and the conditions in the jets of relativistic particles that originate near the event horizons of black holes in galaxy nuclei.
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From page 61... ...
Are the TeV–PeV neutrinos and UHECRs produced in the same sources? They are widely surmised to be accelerated in the relativistic jets of accreting supermassive black holes or GRBs, but this has yet to be tested observationally.
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From page 62... ...
. The importance of gravitational waves lies in part with the central role that black holes and neutron stars play in many areas of astronomy, from stellar evolution to galaxy formation.
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From page 63... ...
The population of neutron star and black hole mergers with masses of ~2–5 Msun4 in the "mass gap" between neutron stars and black holes will provide key constraints on our understanding of massive stellar evolution, the maximum mass of neutron stars, and core-collapse explosion physics. Combined gravitational wave and electromagnetic observations have the potential to finally crack the long-standing puzzle of the origin and growth of massive black holes, one that lies at the intersection of the understanding of stars, galaxies, accretion disks, and cosmology.
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From page 64... ...
Over the few years since then, gravitational wave observations have become an indispensable astronomical tool. The coming decade, with the potential of detections in other parts of the gravitational wave spectrum, signals from new sources, and large numbers of black hole and neutron star detections, will be the start of a new era of precision and multi-wavelength gravitational wave astronomy.
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From page 65... ...
This work will ultimately provide a mapping between transients, the energy sources and central objects (e.g., black holes, neutron stars, stars) that power them, and their broader astronomical consequences (e.g., feedback and nucleosynthesis)
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From page 66... ...
Priority Science Area: New Windows on the Dynamic Universe The combination of new multi-messenger probes of astronomical phenomena with the maturation of time-domain observations opens up tremendous discovery spaces across nearly all areas of astrophysics. Within this discovery landscape, driven by improvements in gravitational wave and neutrino detection, and upcoming facilities such as the Rubin Observatory, one priority area stands out: the application of these new tools to the formation, evolution, and nature of compact stellar remnants such as white dwarfs, neutron stars, and black holes, as probed by the gravitational wave signatures of their mergers, together with rare explosive events that can be explored by the unique cadence and multi-color sensitivity of the Rubin Observatory.
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From page 67... ...
2.3 COSMIC ECOSYSTEMS Processes on a wide range of time and length scales together drive the formation, evolution, and interaction of the remarkable diversity of objects we observe, from exoplanets and stars to black holes and galaxies. A confluence of advances in theory, computational modeling, and observational capabilities expected in the next decade will transform our understanding by identifying the key mechanisms shaping this web of interconnected systems.
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From page 68... ...
An example is ionizing radiation, which is produced by massive stars and black hole accretion disks and can regulate star formation and black hole accretion on sub-parsec scales. Some of this radiation can escape from the dense gaseous environments in which it was produced and propagate out of galaxies into the intergalactic medium.
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From page 69... ...
Even in the local universe, for example, there are significant gaps in the understanding of stellar winds. This leads to large uncertainties about which massive stars become neutron stars and which become black holes.
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From page 70... ...
and local analogs in the coming decade may finally resolve this long-standing puzzle. Regions of high-mass star formation are often buried behind huge layers of dusty gas, so improved long-wavelength observations (far-infrared, sub-mm, radio)
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From page 71... ...
X-ray telescopes and next-generation gravitational wave experiments can constrain the black hole spins, and their masses will be better constrained by measurements with next-generation optical and radio telescopes. Observations of galaxy clusters have revealed the critical role of black hole feedback by jets in the intracluster medium (see Figure 2.19)
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From page 72... ...
Directly observing these gas flows is challenging because of the diffuse nature of the gas in galaxy halos and the high spatial resolution required to peer into regions of ongoing star, planet, and massive black hole formation. New theoretical and observational capabilities are, however, allowing this critical aspect of the tug-of-war between gravity and feedback to be tackled.
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From page 73... ...
Spectral-line observations indicate velocity structures in the gas and reveal flows similar to what has been predicted for the early stages of planet formation. In each image, the lower-left icon indicates the beam size and the lower-right icon is a 10-AU scalebar.
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From page 74... ...
Or does an entirely different set of processes seed galactic nuclei with massive black holes at high redshift? A combination of gravitational wave measurements of black hole mergers across cosmic time (see Section 2.2)
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From page 75... ...
It is unclear how much of the colder gas is launched from the galaxy and how much forms in situ in the outflow. Recent far-infrared dust polarization maps showing smooth vertical magnetic fields over large portions of galaxy disks argue that the cold gas in which this field is embedded is influenced by the wind; future infrared data will test these ideas and provide direct information on feedback processes in and around the cold ISM.
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From page 76... ...
Giant rivers of pristine gas move inward, from multiple directions, delivering the fuel for future generations of stars. These inflows are impeded as they encounter equally impressive galaxy-scale outflows, driven by star formation and black hole accretion deep inside the galaxies.
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From page 77... ...
Box These are all major frontiers highlighted by the science panels. 2.3 continued FIGURE 2.3.1 Illustration of the circumgalactic medium, where intense outflows from galaxies -- driven by FIGURE 2.3.1 Illustration ofsupernovae the circumgalactic medium, where intense outflows from galaxies -- driven by supernovae and black holes -- interact with inflowing pristine gas from intergalactic space.
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From page 78... ...
These are the sites where baryons accrete on to galaxies, star formation is triggered, central black holes accrete and grow, and the feedback processes regulating galaxy growth are manifested. The key observational probes of all of these processes are emission and absorption-line spectroscopy of the diffuse gas, which contains a wealth of diagnostic information on the physical conditions, compositions, and dynamics of the gas.
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From page 79... ...
Planet formation and evolution is influenced by the local stellar environment; star formation is affected by galactic environments and strongly influences them through supernovae and other feedback processes; and galaxy formation and evolution depends on the circumgalactic and intergalactic environment and interactions therein as well as on the energy output produced by massive black hole growth in galactic nuclei. Measurements of the positions and properties of individual stars in nearby galaxies enables an extraction of their evolution and life history from the larger forces at play in the galaxy's structure, formation, and evolution.
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From page 80... ...
In the gravitational wave arena, space-based detection of inspiraling intermediate mass black hole mergers can signal the need for ground-based gravitational wave observation of the final merger, with searches for possible electromagnetic counterparts. Transients in electromagnetic emission will come by the millions per night from the Rubin Observatory once it is operational, with follow-up in other ground- and space-based telescopes needed for further characterization of the most interesting transient events.
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From page 81... ...
What seeds supermassive black holes and how to they grow? New Messengers and New Physics, Cosmic Ecosystems Panel on Cosmology What set the Hot Big Bang in motion?
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From page 82... ...
82 PATHWAYS TO DISCOVERY IN ASTRONOMY AND ASTROPHYSICS FOR THE 2020s TABLE 2.2 Science Panel Discovery Areas Discovery Area Theme(s) Panel on Compact Objects and Energetic Phenomena New Messengers and New Physics Transforming Our View of the Universe by Combining New Information from Light, Particles, and Gravitational Waves Panel on Cosmology New Messengers and New Physics The Dark Ages as a Cosmological Probe Panel on Galaxies Cosmic Ecosystems Mapping the Circumgalactic Medium and Intergalactic Medium in Emission Panel on Exoplanets, Astrobiology, and the Solar System Worlds and Suns in Context The Search for Life on Exoplanets Panel on the Interstellar Medium and Star and Planet Formation Worlds and Suns in Context Detecting and Characterizing Forming Planets Panel on Stars, the Sun, and Stellar Populations Worlds and Suns in Context, "Industrial Scale" Spectroscopy Cosmic Ecosystems
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