Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics (2011) / Chapter Skim
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1 Report of the Panel on Cosmology and Fundamental Physics
1 Report of the Panel on Cosmology and Fundamental Physics
Pages 3-52
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From page 3... ...
Fifteen years ago, cosmologists considered a wide range of possible models; their best estimates of the Hubble constant differed by nearly a factor of two, and estimates of the mass density of the universe differed by as much as a factor of five. Today, the Lambda cold dark matter model is remarkably successful in explaining current observations, and the key cosmological parameters in this model have been measured by multiple techniques to better than 10 percent.
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From page 4... ...
The scope of this panel report encompasses cosmology and fundamental physics, including the early universe; the cosmic microwave background; linear probes of large-scale structure using galaxies, intergalactic gas, and gravitational lensing; the determination of cosmological parameters; dark matter; dark energy; tests of gravity; astronomical measurements of physical constants; and fundamen tal physics derived from astronomical messengers such as neutrinos, gamma rays, and ultrahigh-energy cosmic rays. In response to its charge, the panel identified four central questions that are ripe for answering and one general area in which there is unusual discovery potential: • How did the universe begin?
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From page 5... ...
Over the past decade, cosmological observations have confirmed these predictions. Over the coming decade, it may be possible to detect the gravitational waves produced by inflation, and thereby infer the inflationary energy scale, through measurements of the polarization of the microwave background.
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Gravitational waves, on the verge of being detected, can be used both to study astrophysical objects of central importance to current astronomy and to perform precision tests of general relativity. The strongest known sources of gravitational waves involve extreme conditions -- black holes and neutron stars (and especially the tight binary systems containing them)
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This portion of the gravitational wave spectrum can be accessed only from space. Space-based detections can achieve much higher precision measurements of black hole mergers and thus much stronger tests of general relativity.
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Detection of these gravitational waves would determine the energy scale of inflation. • Search for isocurvature modes, non-Gaussian initial conditions, and other deviations from the fluctuations predicted by the simplest inflationary models.
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• Improve measurements of light-element abundances in combination with big bang nucleosynthesis theory to test neutrino properties and dark matter models. Gravitational Waves • Detect gravitational waves from mergers of neutron stars and stellar mass black holes.
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From page 10... ...
As part of this symbiotic relationship, astronomical observations have stimulated new advances in fundamental physics. Kepler, Galileo, and Newton devised new theories of motion, force, and universal gravity to explain the wandering of the planets across the sky.
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From page 11... ...
Most notably, observations of solar neutrinos and of atmospheric neutrinos produced by cosmic rays have demonstrated that the three neutrino species in the standard model of particle physics have non-zero mass and that they oscillate from one form to an
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From page 12... ...
Scientists expect the next decade to see the first direct detection of gravitational waves, the propagating ripples of space-time predicted by Einstein nearly a century ago. The strongest expected sources of gravitational waves are violent events such as mergers of black holes and neutron stars; gravitational wave measurements will provide unique insights into the physics of these events and allow powerful tests of general relativity in a completely new regime.
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From page 13... ...
Astrophysical measurements provide the most powerful and varied constraints on neutrino properties. Gravitational waves probe general relativity in the strong-field regime, a test that can be done only in the extreme environment near black holes.
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From page 14... ...
a stochastic back ground of inflationary gravitational waves (IGWs) with a nearly scale-invariant spectrum.
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From page 15... ...
If consistency with the predictions of SFSR inflation persists, then the range of allowable values of the scalar-field potential will need to be narrowed. If departures from the simplest predictions are found, this will provide insights into fundamental physics and into the first moments of the early universe.
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From page 16... ...
trum of fluctuations in the early universe through a three-dimensional, rather than two-dimensional, picture of the matter distribution extending to smaller scales inaccessible to the CMB. At present, only 10–6 of the linear spatial modes in the observable universe have been measured.3 Galaxy surveys over larger volumes could survey 108 to 109 modes out to redshift z < 2, characterizing the linear matter power spectrum with statistical uncertainties that approach the cosmic-variance limit.
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From page 17... ...
Gravitational Waves Gravitational waves produce a distinctive B-mode signal in the CMB that could provide the most promising method for detecting the "smoking gun" of inflation. Different inflationary models predict different amplitudes for r, the ratio of gravitational wave fluctuations to scalar fluctuations.
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2. If dark energy is causing the acceleration, is its energy density constant in space and time?
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. Vacuum energy, the simplest and arguably best-motivated model for dark energy, is constant with time, and so for this model w = -1 at all z.
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In a spatially flat universe, H2(z) is proportional to the total energy density -- the sum of matter, radiation, and dark energy -- and D(z)
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scale. Lower: The power spectrum measured from the final SDSS galaxy redshift survey, with the inset emphasizing the BAO signature of wiggles relative to the smooth power spectrum that would arise in a baryon-free universe.
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From page 22... ...
At present, weak lensing is the only observable whose fundamental physics is un derstood well enough to achieve the required, sub-percent-level mass calibration. While weak-lensing measurements are noisy for any individual cluster, they can precisely measure the mean mass profile of clusters binned by observable quantities.
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From page 23... ...
is more sensitive to dark energy than is D(z)
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From page 24... ...
Intensity mapping of redshifted 21-cm emission is an emerging method that may allow efficient BAO measurements over a wide redshift range. Weak Lensing and Cluster Surveys Weak lensing and cluster surveys require high-resolution imaging with a well characterized point-spread function (PSF)
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rePort Panel cosMoloGy fundaMental PHysIcs 25 of tHe on and FIGURE 1.4 Upper: Gravitational lensing by the massive galaxy cluster Abell 2218, shown here in a Hubble Space Telescope image, stretches the images of background galaxies into arcs, elongated tangentially with respect to the center of the mass distribution. Far away from massive clusters, the image distortions are much weaker, but they can still be detected statistically because galaxies lensed by the same foreground mass will be stretched in the same direction.
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From page 26... ...
Precision Measurements and Cosmic Acceleration Although there are many ideas about possible causes of cosmic acceleration, none of them is compelling. The current cosmological data are consistent with general relativity and a cosmological constant, which is a viable model for accel eration even though the magnitude of acceleration appears surprising.
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From page 27... ...
It was first noticed in the 1930s that there must be more matter in galaxy clusters than the luminous matter could provide, and the evidence that accrued over the intervening decades showed that this dark matter must be nonbaryonic. The existence of dark matter is now the strongest empirical evidence for physics beyond the standard model, and, as discussed below, the dark matter may well be tied to the physics of electroweak symmetry breaking, one of the central problems in particle physics today.
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From page 28... ...
and models with extra spatial dimensions. Such particles must have been in thermal equilibrium in the early universe until the cc → ff interactions (where c is the WIMP and f a standard model particle)
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FIGURE 1.6 Three complementary approaches to searching for dark matter. Read from top to bottom, the diagram shows two dark matter particles c annihilating to produce standard model particles f, which can be detected by indirect searches (e.g., for gamma rays or cosmic rays)
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From page 30... ...
Additional promising directions are low threshold experiments extending sensitivities to lower WIMP masses, experiments that significantly improve limits on spin-dependent cross sections, and detectors that are sensitive to WIMP direction. 5 National Research Council, Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics, The National Academies Press, Washington, D.C., 2006.
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From page 31... ...
The resulting neutrinos are distinctive because they come from the Sun but are far more energetic than the solar neutrinos produced by nuclear reactions. It is emphasized that unlike all other indirect signals, the flux of neutrinos from the Sun is determined by scattering cross sections, and so these searches may be directly compared to direct-detection searches.
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As in the case of gamma rays, these rates depend sensitively on the distribution of dark matter and its annihilation cross sections. Charged-particle targets include the following: • Positrons from nearby halo annihilations.
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From page 33... ...
Examples include sterile neutrinos; super-WIMPs (dark matter candidate particles with extremely small cross sections) ; dark matter candidates produced in decays during or after big bang nucleosynthesis; hidden-sector dark matter -- that is, dark matter without standard model gauge interactions; and metastable dark matter with lifetime greater than the age of the universe.
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From page 34... ...
Then in 2001 and 2002, the Sudbury Neutrino Observatory showed that oscillations were also responsible for the missing solar neutrinos, directly measuring the flux of muon and tau neutrinos into which the solar electron neutrinos had oscillated. The demonstration of neutrino oscillations is a marvelous example of the use of astrophysics as a laboratory for fundamental physics.
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From page 35... ...
Neutrinos are also unique probes of the cosmos. Rapid advances in large and sophisticated neutrino detectors have created new opportunities for measuring the metallicity of the solar core, determining properties of neutron stars, and probing the most distant regions of the universe for nature's most energetic particle accelerators.
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From page 36... ...
For each method, there are challenges similar to those discussed in the subsection above titled "Precision Measurements and Cosmic Acceleration." But the critical role of these measurements in both cosmology and neutrino astrophysics makes overcoming these challenges a very high priority. A determination of the absolute scale of neutrino mass would be of fundamen tal importance to particle physics, potentially probing new physics phenomena far beyond the reach of accelerators.
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From page 37... ...
Rawlings, Determining neutrino properties using future galaxy redshift surveys, Monthly Notices of the Royal Astronomical Society 381(4) :1313-1328, copyright Royal Astronomical Society, 2007.
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From page 38... ...
Snow, and the Pierre Auger Collaboration, Observation of the suppression of the flux of cosmic rays above 4 × 1019 eV, Physical Review Letters 101:061101, 2008, copyright 2008 by the American Physical Society, available at http://link.aps.org/doi/10.1103/PhysRevLett.101.061101. Right: Courtesy of the Pierre Auger Observatory.
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From page 39... ...
It is fortunate, in the quest to understand cosmic rays and to exploit them as a probe of new astrophysical sources (and potentially new fundamental physics) , that the field has multiple probes, including nucleons and nuclei, neutrinos, and gamma rays.
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From page 40... ...
The neutrino flux provides a very accurate measurement of the gravitational energy released in core collapse, and it also marks the onset of core collapse, provid ing a "clock" against which gravitational wave and optical signals can be compared. Changes in the late-time neutrino cooling curve could signal the onset of phase transitions at supernuclear densities; a sudden termination would accompany black hole formation.
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From page 41... ...
The measurements of the abundance of 6Li in halo stars require improved three-dimensional stellar atmosphere models, whereas interpretation of 7Li measurements would benefit from stellar models that incorporate more physical treatments of turbulence. Panel Conclusions Regarding Neutrinos The conclusions of the panel with respect to neutrinos are presented in Box 1.3.
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From page 42... ...
In the coming decade, some of the most exciting discoveries may come from opening a new ob servational window with the first direct detections of gravitational waves. In the same way that the sense of hearing complements the sense of sight, gravitational wave observations complement and enrich what can be learned elec
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From page 43... ...
In contrast, electromagnetic waves tell only about the thermal and magnetic environment of the gas that surrounds a source, and they can be bent or absorbed along their propagation paths to telescopes. However, the weak coupling to matter that allows gravitational waves to travel unimpeded also makes them very hard to detect: the merger of two stellar remnant black holes at 10 Mpc would bathe Earth with a peak energy flux exceeding 10 percent of the solar constant, but so little of this energy is captured that the mirrors of a kilometer-scale detector are displaced by less than 10 percent of the width of a proton.
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From page 44... ...
Koppitz, Binary black hole late inspiral: Simulations for gravitational wave observations, Physical Review (Section) D: Particles and Fields 75:124024, 2007, copyright 2007 by the American Physical Society.
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From page 45... ...
Above this noise it will still be possible to detect black hole mergers in the mass range 3 × 103 to 107 solar masses out to redshift 20 or greater. These observations will reveal the masses and spins of the black holes and will indicate the merger rate as a function of distance.
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From page 46... ...
The radiation from these and other exotic processes occurring in the early universe when the temperature was 0.1 to 1,000 TeV will have been redshifted to the frequency range explored by a space-based instrument. This nice coincidence means that gravitational waves have the potential to explore weak-scale physics.
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From page 47... ...
In addition to providing determinations of the black hole's mass and angular momentum to fractions of a percent, the observations can also be used to test whether the space-time that encodes the waves is the unique Kerr geometry that general relativity predicts for rotating black holes. Gravitational waves can also be used to test specific theories alternative to general relativity.
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From page 48... ...
If these ar rays achieve their proposed sensitivities of roughly 4 × 10–24/(Hz) 1/2 at 100 Hz, there will be a very good chance of discovery of gravitational waves from stellar remnant inspirals and mergers.
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From page 49... ...
THEORY AND SYNTHESIS Theory is and has been essential to what we choose to observe and how we arrange to do so. Many of the ideas that are central to the next decade's empirical investigations -- inflation, supersymmetric dark matter, neutrino oscillations, black holes, and gravitational waves -- began life decades ago as theoretical speculations.
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From page 50... ...
Examples that are central to the themes of this report include numerical simulations of structure formation needed to interpret maps of the galaxy distribution or to predict signals of dark matter annihilation; computational studies of core collapse and thermonuclear superno vae; calculations of gravitational wave emission from mergers of spinning black holes; statistical analyses of large and complex data sets from CMB observations, LSS surveys, neutrino observations, and gravitational wave searches; and massive searches through high-dimensional parameter spaces to evaluate the statistical
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From page 51... ...
The frontiers of cosmology today present grand theoretical challenges: rooting models of inflation in more fundamental descriptions of underlying physics; explaining the asymmetry between matter and antimatter and thus the origin of the particles that form Earth and the life on its surface; describing the interior structure of black holes and explaining their entropy in terms of quantum gravity; determining whether there are spatial dimensions beyond the three of everyday experience; explaining the surprising magnitude of cosmic acceleration and the seeming coincidence of the densities of baryons, dark matter, and dark energy; and determining whether our observable cosmos is a fully representative sample of the universe or one of many disparate bubbles in a much larger inflationary sea. Robust support for the full span of theoretical activities is essential in order to reap the return from large investments in observational facilities over the next decade, and also to ensure that the scientific opportunities in the 2020-2030 decade will be as exciting as those of today.
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From page 52... ...
2009. Searching for new physics with ultra-high energy cosmic rays.
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