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The Nature of Inflation As described previously, the inflation hypothesis proposes that the universe began to expand exponentially some 10 -36 seconds after the big bang. This hypothesis explains why the present universe has almost the same temperature everywhere we look, as measured by the microwave background radiation, over the entire sky. Despite the power of the hypothesis, the mechanism by which inflation happened⎯its origin⎯remains a great mystery. Directly confirming inflation and understanding its fundamental underlying mechanism lie at the frontier of particle physics, because inflation probes scales of energy far beyond anything that can be achieved in accelerators on Earth. Inflation is central to astrophysics: the quantum fluctuations present during inflation formed the seeds that grew into the CMB fluctuations and the large-scale structure of the universe we see around us today. Perhaps the most profound reason to understand inflation is that its nature and duration might have spelled the difference between a universe of sufficient vastness to house galaxies, planets, and life, and a “microverse” so small that matter as we know it could not be contained therein. To understand the origin of our macroscopic universe—why we exist—requires understanding inflation. The last decade was one of stunning progress in our understanding of the first moments of the universe. NSF-supported South Pole and Chilean ground-based work, and NASA’s balloon-based and the Wilkinson Microwave Anisotropy Probe Explorer mission, mapped the spatial pattern of temperature fluctuations that occur in the relic cosmic microwave background from the big bang. The state of the young universe during the epoch of inflation, prior to the existence of stars or galaxies, is imprinted as minute fluctuations in the CMB, and the character of these fluctuations is broadly consistent with the theory of inflation. Armed with theoretical advances and complementary balloon-borne and groundbased measurements, we are now ready to move beyond foundational knowledge of the very early universe and apply increasingly more precise measurements of the CMB to new questions. One important test of inflation involves making highly detailed measurements of the structure of the universe by mapping the distribution of hundreds of millions of galaxies. Inflation makes very specific predictions about the spatial distribution of the dark matter halos that host these galaxies. However, the most exciting quest of all is to hunt for evidence of gravitational waves that are the product of inflation itself. Just as the light we see with our own eyes can be polarized, the CMB radiation may also carry a pattern of polarization⎯the so-called B-modes⎯imprinted by inflationary gravitational waves. Different models of inflation predict distinguishable patterns and levels of polarization, so the next great quest of CMB research is to detect this polarization, thereby probing the behavior of the particles or fields driving inflation. Today we stand at a crossroads. If we discover the signature of inflation in the CMB in the next few years, future studies would focus on follow-up precision measurements of that signal. If, on the other hand, the signal is not seen, then we will need to develop increasingly sensitive experiments that may ultimately lead us to revise our theoretical models. More detailed measurements of the CMB are a path to exciting future discoveries⎯fed by both technology development and theoretical inquiry. The Accelerating Universe About twelve years ago, the simple picture of a universe decelerating because of gravity began to fall apart. Due in large part to supernova distance measurements, we have since come to realize that instead of decelerating, the expansion of the cosmos is accelerating. Why this is so is an outstanding puzzle in our modern picture of the universe. The observation that the universe is accelerating is presently consistent with Einstein's postulate of a cosmological constant or equivalently with the idea that empty space carries energy. It is also consistent with the more general idea that spacetime is permeated with gravitationally repulsive dark energy, a mysterious substance that accounts for over 70 percent of the energy content of the universe. Alternatively, cosmic acceleration could be an indication that Einstein's theory of gravity⎯general PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION 2-26
elativity⎯must be modified on large scales. In Einstein's theory, the growth of structure and the expansion of the universe are linked by gravity; in modifications of gravity, that link is altered. 5 Understanding the underlying cause of acceleration therefore requires precision measurements of the expansion of the universe with time and of the rate at which cosmic structure grows. Comparing the expansion history of the universe with the history of the growth of structure will in principle enable us to test whether dark energy or modifications of general relativity are responsible for cosmic acceleration. Fortunately, the supernova distance measurement techniques are advancing dramatically, and a few other independent techniques are being developed that also promise advances in precision measurement of the expansion history, as well as adding measurements of the growth of structure. Knowing how the size of the universe changes with time means that we can now chart the rate at which the universe grew over its long history. By combining all these data we can test whether the theory of relativity is correct and also determine whether the Einstein’s cosmological constant gives an accurate description of the way dark energy determines the fate of our universe. The Nature of Dark Matter “Normal” matter — the stuff of which we, the Earth, and the stars are made, as well as the more exotic particles created in Earth-bound accelerators or in natural accelerators such as supernova remnants — appears to be only a minority of the matter in the cosmos (Figure 2-11). This discovery through measurements of the rotation rate of galaxies was presaged by work as early as the 1930s in which astronomers noticed that the speeds at which galaxies orbit around the centers of the clusters to which they belong are far higher than needed to counteract the gravitational pull of the stars in those clusters. To keep these clusters from rapidly flying apart, astronomers argued, they must contain far more material than that visible to telescopes. A lot of astronomical detective work ruled out the hypothesis that the invisible mass might simply be unobservable planets and dead stars, and so it became known as a mysterious dark matter. By now, the evidence for such dark matter in almost all galaxy-sized and larger astronomical systems is overwhelming and comes from a wide variety of techniques⎯among others, gravitational lensing measured by the Hubble Space Telescope and ground-based telescopes, the distribution of hot X- ray emitting gas measured by the Chandra X-Ray Observatory, and the rotation speed of hydrogen gas disks surrounding galaxies measured by ground-based radio telescopes (Figure 2-12). With improved observations, astronomers have determined precisely how much dark matter there is, and learned that it interacts only with itself and very feebly with familiar matter only through gravity. These normal matter constituents are small islands in a vast sea of dark matter of some unknown form. 5 The extremely small value of a cosmological constant that would be consistent with the observed acceleration is not a natural fit for current physics theories. PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION 2-27
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THE NATIONAL ACADEMIES PRESS 500 Fi
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COMMITTEE FOR A DECADAL SURVEY OF A
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Panel on Stars and Stellar Evolutio
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DOUGLAS FINKBEINER, Harvard Univers
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SPACE STUDIES BOARD CHARLES F. KENN
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activities 2 that have emerged from
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In addition to the 27 panel meeting
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Acknowledgment of Reviewers This re
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The Accelerating Universe 2-26 The
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Page 28 and 29: RECOMMENDED PROGRAM Maintaining a b
Page 30 and 31: TABLE ES.4 Space: Recommended Activ
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Page 54 and 55: FIGURE 2‐2 The source 3C 75, show
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Page 60 and 61: FIGURE 2‐6 Top: Schematic of the
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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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y optical and radio astronomers are
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FIGURE 5‐8 Number of PhDs per yea
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ated by the Program Prioritization
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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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3. Investment in new and upgraded i
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etween several competing elements i
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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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penalty based on an assessment of s
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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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