Pathways to Discovery in Astronomy and Astrophysics for the 2020s (2023) / Chapter Skim
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Appendix C: Report of the Panel on Cosmology
Appendix C: Report of the Panel on Cosmology
Pages 255-270
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From page 255... ...
Realistic physical theories predict a wide range of observable signatures, and the opportunity to discover these signatures is the driving motivation for the coming decade of cosmological research. The foundation of modern cosmology theory is the Hot Big Bang, in which an initially hot, dense, and nearly smooth universe rapidly expands and cools.
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From page 256... ...
There is no doubt that the standard cosmological model is a triumph. By adopting simple versions of inflation, dark matter, and dark energy, the model can match observational results despite orders of magnitude of improvement in cosmological measurements over the past 20 years.
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From page 257... ...
project reports an 80 MHz spectral distortion consistent with 21 cm absorption from redshift 17, but with an amplitude several times larger than predicted. And there is a haze of gamma-ray emission that peaks at around 1 GeV from the inner Milky Way, consistent with a dark matter annihilation signal but also possibly explained as high-energy emissions from undetected pulsars.
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From page 258... ...
One of the fundamental discoveries of modern cosmology is that the primordial density fluctuations, the seeds of the structure of the universe observed throughout cosmic history, were created before the hot phase of the Big Bang. As a result, studying these primordial fluctuations provides a unique window into physics at extremely early times and at energy scales many orders of magnitude above what researchers can access in the laboratory.
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From page 259... ...
Minimal models of inflation involve a single field that evolves during inflation, serving as a clock that determines when inflation ends and the Hot Big Bang begins. Interactions between fluctuations of this clock field, or between such fluctuations and those of other fields during inflation, generically cause noticeable departures from Gaussian correlations in the distribution of the primordial structural seeds.
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From page 260... ...
Deviations at much smaller scales could be detected owing to their impact on CMB spectral distortions or on small-scale structure in early galaxy formation. Summary of Capabilities Needed for C-Q1 Capabilities needed include a next generation of CMB polarization experiments (both large and small angular scales)
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From page 261... ...
This area is being revolutionized by Gaia, and augmented by both LSST and the funded upcoming wide-field high-multiplex optical/infrared spectrographs. At yet lower masses, possibly as low as 10–14 solar mass, pulsar timing arrays offer a novel opportunity by searching for timing anomalies owing to gravitational lensing by dark matter lumps passing between the observer and the target pulsar, even if the lumps are not compact enough to generate microlensing of the flux.
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From page 262... ...
Understanding the physics behind cosmic expansion requires testing both the expansion of space and the growth of structure across cosmic time. Together, these observations will provide an end-to-end test of our standard cosmological model and measure the properties and masses of neutrinos -- the last known un-weighed constituent of our universe.
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From page 263... ...
It is critical to probe this acceleration with diverse and independent probes that provide constraints on both distance scales and the growth of structure.Key methods to explore this question in the coming decade include weak gravitational lensing of distant galaxies and the CMB, BAO and redshift-space distortion measurements from redshift surveys, supernovae Ia distance measurements, Hubble constant measurements, and galaxy cluster abundances. The funded upcoming wide-field survey facilities and CMB experiments will produce a major leap forward in both statistical reach and systematic control of these methods.
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From page 264... ...
C-Q3c. End-to-End Tests of Cosmology Diverse and precise cosmological measurements that probe multiple epochs of cosmic history will allow for stringent cross-epoch end-to-end tests of cosmology, enabling explorations of the consistency of cosmological models across cosmic time and providing ways to challenge the standard cosmological model.
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From page 265... ...
As the universe cools, phase transitions and their associated topological defects could produce gravitational waves. Gravitational waves far more intense than those expected from simple inflation models could also arise from the start of the Hot Big Bang, and comparison to the ultra-low-frequency waves being sought in CMB large-angle polarization measurements would probe a large lever arm in the spectrum of gravitational waves.
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From page 266... ...
DISCOVERY AREA: THE DARK AGES AS A COSMOLOGICAL PROBE Our understanding of how the universe began is measured through the fingerprints that the Hot Big Bang left in matter density fluctuations. Unfortunately, these fingerprints are often smudged.
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From page 267... ...
C-DA2. The Future of Primordial Density Mapping As described earlier, understanding the initial conditions of our universe requires observing the primordial density fluctuations.
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From page 268... ...
CROSSCUTTING CAPABILITIES This appendix identifies new observational capabilities needed to address the science questions and discovery area. In addition, the panel identified the following crosscutting capabilities needed to support the overall cosmological research enterprise.
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From page 269... ...
C-Q3b: The properties of neutrinos C-Q3c: End-to-end tests of cosmology C-Q4: How will measurements of gravitational waves reshape our C-Q4a: The stochastic gravitational wave background cosmological view? C-Q4b: Standard sirens as a new probe of the cosmic distance scale C-Q4c: Light fields and other novel phenomena C-DA1: Discovery Area: The Dark Ages as a cosmological probe C-DA1: The end of the Dark Ages C-DA2: The future of primordial density mapping
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From page 270... ...
novel terrestrial detectors not in Astro2020 cosmological gravitational wave phenomena scope. Large-scale Ubiquitous contributions These crosscutting capabilities will computation; theory require consistent attention and research; technology funding.
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