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Cosmic Dawn: Searching for the First Stars, Galaxies, and Black Holes We have learned much in recent years about the history of the universe, from the big bang to the present day. A great mystery now confronts us: When and how did the first galaxies form out of cold clumps of hydrogen gas and start to shine—when was our “cosmic dawn” Observations and calculations suggest that this phenomenon occurred when the universe was roughly half a billion years old, when light from the first stars was able to ionize the hydrogen gas in the universe from atoms into electrons and protons—known as the epoch of reionization. Scientists think that the first stars were massive and shortlived. They quickly exploded as supernovas, creating and dispersing the first elements with nuclei heavier than those of hydrogen, helium, and lithium, and leaving behind the first black holes. Astronomers must now search the sky for these infant galaxies and find out how they behaved and interacted with their surroundings. After the cosmic dawn, more and more galaxies formed, merged, and evolved as their gas turned into stars and those stars aged. Many of the faintest images from current telescopes are of these growing infant galaxies. Their properties are just starting to be revealed. In particular, it is now known that such galaxies quickly grow black holes in their nuclei with masses that can exceed a billion times the mass of the Sun and become extraordinarily luminous quasars. How this happens is a mystery. We also know that the giant galaxies we see around us today were built up from the mergers of smaller galaxies and the accretion of cold gas. Not only do the stars and gas commingle, but the central black holes also merge. Amazingly, it should be possible to detect waves in the fabric of space-time— gravitational waves—that result from the dramatic unions when galaxies and black holes are young and relatively small. Another approach to understanding our cosmic dawn is to carry out “cosmic paleontology” by finding the rare stars that have the lowest concentrations of heavy elements and were formed at the earliest times. Today, we can scrutinize only stars in our galaxy; in the future, we will be able to explore other nearby galaxies to uncover stellar fossils and use them to reconstruct the assembly of young galaxies. Exploring the first stars, galaxies, and quasars is a tremendous challenge, but one astronomers and astrophysicists are ready to tackle and overcome, thereby continuing the story of how our universe came to be. New Worlds: Seeking Nearby, Habitable Planets On Christmas Eve, 1968, Apollo 8 astronaut William Anders took an iconic photograph of the rising Earth from his vantage point orbiting the Moon. It highlighted, to more people than ever before, that we humans share a common home that is both small and fragile. It also brought into focus the question, What does Earth look like from much farther away Remarkable discoveries over the past 15 years have led us to the point that we can ask and hope to answer the question, Can we find another planet like Earth orbiting a nearby star To find such a planet would complete the revolution, started by Copernicus nearly 500 years ago, that displaced Earth as the center of the universe. Almost two decades ago, astronomers found evidence for planets around neutron stars, and then, in 1995, a star just like the Sun in the constellation Pegasus was shown to vary regularly in its radial velocity—resulting from motion toward or away from us here on Earth—in response to the gravitational pull of an orbiting planet. This planet was roughly as massive as Jupiter but orbited its star every 4 days, far more quickly than any of our Sun’s planets. So, in a single set of observations we solved an age-old puzzle: yes, there are other planetary systems around stars like our Sun. However, they do not necessarily look like our solar system. Today, in mid 2010, we know of almost 500 extrasolar planets with masses ranging from a few to a few thousand times the mass of Earth. PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION 1-2
We have greatly expanded our discovery techniques since 1995. Radial velocity detection of planets is much more sensitive, reaching down below 10 Earth masses. We can detect tiny changes in the combined light of a star and planet as they transit in front of one other, a technique currently being exploited very successfully by the Kepler space telescope. We can also probe planetary systems by measuring microlensing as their gravitational fields bend rays of light from a more distant star. Telescopes on the ground and in space have even directly imaged as distinct point sources a few large planets. In other cases, we can learn about planetary systems by measuring infrared and radio emission from giant disks of gas out of which planets can form. Finally, in a most important development, the Hubble Space Telescope and the Spitzer Space Telescope have found the spectral lines of carbon dioxide, water, and the first organic molecule, methane, in the atmospheres of orbiting planets. This is extraordinarily rapid progress. Astronomers are now ready to embark on the next stage in the quest for life beyond the solar system—to search for nearby, habitable, rocky or terrestrial planets with liquid water and oxygen. The host star of such a planet may be one like our Sun, or it could be one of the more plentiful but less hospitable cooler red stars. Cooler red stars are attractive targets for planet searches because light from a planet will be more easily detected above the stellar background. Making the search harder, terrestrial planets are relatively small and dim, and are easily lost in the exozodiacal light that is scattered by the dusty disks that typically orbit stars. The observational challenge is great, but armed with new technologies and advances in understanding of the architectures of nearby planetary systems, astronomers are poised to rise to it. Physics of the Universe: Understanding Scientific Principles Astronomy and physics have always been closely related. Observations of orbiting planets furnished verifications of Newton’s law of gravitation and Einstein’s theory of gravity—general relativity. In more recent years, observations of solar system objects and radio pulsars have provided exquisitely sensitive proof that general relativity is, indeed, correct when gravity is weak. The universe continues to be a laboratory that offers access to regimes not available on Earth, helping us to both understand and discover new elements of the basic laws of nature. Scientists can study the universe on the largest observable scales—more than 10 trillion, trillion times larger than the size of a person. The past decade has seen the confirmation from measurements of the truly remarkable discovery that the expansion of the universe is accelerating. In modern language, this acceleration is attributed to the effect of a mysterious substance called dark energy that accounts for 75 percent of the mass-energy of the universe today causing galaxies to separate at ever faster speeds. The remainder of the mass-energy comprises 4.6 percent regular matter and 20 percent a new type of matter, dubbed dark matter, that is believed to comprise new types of elementary particles not yet found in terrestrial laboratories. The effects of dark energy are undetectable on the scale of an experiment on Earth. The only way forward is to use the universe at large to infer the properties of dark energy by measuring its effects on the expansion rate and the growth of structure. Amazingly, we can ask and hope to answer questions about the universe as it was very soon after the big bang. Recent observations of the microwave background are consistent with the theory that the universe underwent a burst of inflation when the expansion also accelerated and the scale of the universe grew from its infinitesimally small beginnings to about the size of a person. Gravitational waves created at this time can propagate all the way to us and carry information about the behavior of gravity and other forces during the first moments after the big bang. These waves can be detected through the distinctive polarization pattern 1 that they impose on the relic cosmic microwave background radiation. Detection of 1 The cosmic microwave radiation signal can be decomposed into two components: an E-mode and a B-mode. Patterns in these polarization modes allow determination of conditions when the radiation was emitted. PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION 1-3
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Page 4 and 5: THE NATIONAL ACADEMIES PRESS 500 Fi
Page 7 and 8: COMMITTEE FOR A DECADAL SURVEY OF A
Page 9 and 10: Panel on Stars and Stellar Evolutio
Page 11 and 12: DOUGLAS FINKBEINER, Harvard Univers
Page 13: SPACE STUDIES BOARD CHARLES F. KENN
Page 16 and 17: activities 2 that have emerged from
Page 18 and 19: In addition to the 27 panel meeting
Page 20 and 21: Acknowledgment of Reviewers This re
Page 22 and 23: The Accelerating Universe 2-26 The
Page 24 and 25: APPENDIXES A Summary of Science Fro
Page 26 and 27: light up the universe, marking the
Page 28 and 29: RECOMMENDED PROGRAM Maintaining a b
Page 30 and 31: TABLE ES.4 Space: Recommended Activ
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Page 38 and 39: estimated to be on the order of $20
Page 40 and 41: observatories. For NASA an annual b
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FIGURE 2‐14 A possible pathway is
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As astronomy research has blossomed
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TABLE 3-1 Currently Operating OIR F
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29% 1990 44% US Priv US Fed Other E
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CONCLUSION: Complex and high-cost f
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Space Observatories The Laser Inter
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Physics Division (PHY); the Mathema
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4 Astronomy in Society Astronomy of
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FIGURE 4‐2 The dust sculptures of
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FIGURE 4‐5 President Obama takes
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Astronomy Inspires in the Classroom
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teaching the scientific method. The
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where the fraction of astronomers h
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Percentage of AAS Membership by Fie
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0.4 0.3 0.2 0.1 PL SO IM AG SF IN S
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FIGURE 4‐12 Number (top panel) an
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Professional training needs to acco
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assistant and associate professor p
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fluctuating level of funding for as
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THEORY Emerging Trends in Theoretic
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Central to the Galaxies across Cosm
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TABLE 5‐2 Support for astrophysic
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would normally be for a five-year p
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FIGURE 5‐7 Highly cited HST publi
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FIGURE 5‐8 Number of PhDs per yea
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nulling interferometry. The appropr
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FIGURE 5‐9 The richness of the su
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RECOMMENDATION: NASA and NSF suppor
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FIGURE 6‐1. All sky map as observ
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FIGURE 6‐3 NASA mission cost over
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FIGURE 6‐5 Gemini North with Sout
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TOWARD FUTURE PROJECTS, MISSIONS, A
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7 Realizing the Opportunities The p
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FIGURE 7.1 Survey time line showing
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SCIENCE OBJECTIVES FOR THE DECADE T
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Box 7.1 Implementing a Cosmic Dawn
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JWST, with its superb mid-infrared
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optical imaging of brighter galaxie
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Box 7.3 Implementing the Physics of
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FIGURE 7.2 Multiwavelength images o
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Recommendations for New Space Activ
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FIGURE 7.4 Science accomplishments
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significance of mergers in the buil
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independent advice committee review
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committee review. It could range be
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Laboratory Astrophysics As describe
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Recommendations for New Ground-Base
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Program for instrumentation and fac
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excel at high spectral and spatial
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The committee reviewed a technical
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FIGURE 7.10 ACTA would be, like the
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Advanced Technologies and Instrumen
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LISA as soon as possible subject to
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FIGURE 7.14 DOE recommended program
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A Summary of Science Frontiers Pane
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PLANETARY SYSTEMS AND STAR FORMATIO
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A preliminary recommended program w
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certified as independent work perfo
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The results show excellent correlat
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$12 MILLION TO $40 MILLION RANGE CM
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decadal research strategy with reco
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CFP CHIPSat CIP CGRO CMB CMU CoBRA
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NRC NSB NSF NSF-AGS NSF-ARC NSF-AST
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