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Appendix C: Report of the Panel on Cosmology
Pages 247-263

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From page 247...
... 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.
From page 248...
...  Measurements of the Hubble constant from the direct distance scale and from strong gravitational lensing have improved to a precision of about 2 percent, while supernovae measurements at cosmological distances have driven precision on the dark energy equation of state w below 5 percent.  The first gravitational wave events have been detected, including an initial application of the standard siren method of constraining the cosmic expansion rate.
From page 249...
... 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.
From page 250...
... In the past decade, the Planck satellite measured departures from scale invariance of the power spectrum of density fluctuations, in line with the expectations of the simplest inflationary models, and placed exquisite constraints on departures from Gaussianity and fluctuations in the composition of the universe. The BICEP/Keck CMB polarization experiments put stringent upper limits on the amplitude of inflationary gravitational waves.
From page 251...
... 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.
From page 252...
... New forces also generically appear in a broad range of dark matter models, giving rise to dark matter self-interactions and modifications of CDM predictions for the abundance and PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION C-6
From page 253...
... These models typically predict dark matter candidates at the few-TeV mass scale, with cross sections for annihilation to photons that make them observable with next-generation Cherenkov telescopes that have sufficient scope to detect an PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION C-7
From page 254...
... 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 unweighed constituent of our universe.
From page 255...
... a next generation of densely sampled galaxy redshift and lensing surveys to test the impact of dark energy on structure formation with greater sensitivity. In order to thoroughly test the predictions of the cosmological model, robust and modelindependent tests for deviations from GR's prediction for the growth of large-scale structure are an essential element of the research program of the field.
From page 256...
... These tests concern comparison of absolute distance scales in the early and late universe and require improved measurements across cosmic time: local expansion rate measurements at low redshift; galaxies, quasar, and Lyman-alpha forest BAO measurements, strong lensing cosmography, supernovae, and gravitational wave standard siren measurements at intermediate redshifts; and small-angle CMB measurements at high redshift. If tensions persist, efforts will be needed to probe scales that can distinguish between changes in the early expansion rate and the speed of sound in the primordial plasma.
From page 257...
... C-Q4b. Standard Sirens as a New Probe of the Cosmic Distance Scale With the discovery in 2017 of both gravitational waves and electromagnetic signatures from a binary neutron star merger, scientists were able to make the first "standard siren" measurement of the cosmic distance scale.
From page 258...
... During the Dark Ages, astrophysical structure formation responded dramatically to the dark matter power spectrum at the smallest scales. Models of dark matter that suppress small-scale power (e.g., warm dark matter or dark sector interactions)
From page 259...
... A 30- to 40year goal would be to map the density fluctuations in the pre-reionization universe with an unprecedented number of modes traceable to the primordial density fluctuations, using the power spectrum and nonGaussianity to measure the statistical initial conditions of the universe. Summary of Capabilities Needed for the Discovery Area Needed capabilities include next-generation 21 cm interferometers targeting both the reionization epoch and lower redshifts, along with planning toward very high redshift mapping.
From page 260...
... Table C.1 presents the highest profile yet-unfunded capabilities needed to address the cosmology science questions and discovery area. These questions build on the successful framework of the standard cosmological model to search for distinctive signatures from the dark sector, the early universe, the cosmic expansion history, the gravitational wave window, and the Dark Ages, all of which can reveal rich new phenomena in realistic physical theories.
From page 261...
... C-Q2b: Dark sector imprints on Big Bang nucleosynthesis and recombination C-Q2c: Annihilation by-products C-Q3: What physics drives the cosmic C-Q3a: The physics of cosmic acceleration expansion and large-scale evolution of the C-Q3b: The properties of neutrinos universe? C-Q3c: End-to-end tests of cosmology C-Q4: How will measurements of C-Q4a: The stochastic gravitational wave background gravitational waves reshape our C-Q4b: Standard sirens as a new probe of the cosmic cosmological view?
From page 262...
... BBN light element abundances Next-generation large-aperture OIR dispersion spectroscopy telescopes with high-dispersion spectrographs. TeV imaging (C-2c)
From page 263...
... standard sirens; (C-4c) novel supernovae follow-up, gravitational cosmological gravitational wave phenomena waves counterparts, and strong lensing cosmography; improved pan-chromatic sensitivity and access.


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