Abstract
Fluctuations in the intensity and polarization of the cosmic microwave background (CMB) and the large-scale distribution of matter in the universe each contain clues about the nature of the earliest moments of time. The next generation of CMB and large-scale structure (LSS) experiments are poised to test the leading paradigm for these earliest moments—the theory of cosmic inflation—and to detect the imprints of the inflationary epoch, thereby dramatically increasing our understanding of fundamental physics and the early universe. A future CMB experiment with sufficient angular resolution and frequency coverage that surveys at least 1% of the sky to a depth of 1 uK-arcmin can deliver a constraint on the tensor-to-scalar ratio that will either result in a 5σ measurement of the energy scale of inflation or rule out all large-field inflation models, even in the presence of foregrounds and the gravitational lensing B-mode signal. LSS experiments, particularly spectroscopic surveys such as the Dark Energy Spectroscopic Instrument, will complement the CMB effort by improving current constraints on running of the spectral index by up to a factor of four, improving constraints on curvature by a factor of ten, and providing non-Gaussianity constraints that are competitive with the current CMB bounds.
Highlights
Cosmic inflation, the theory that the universe underwent a violent, exponential expansion during the first moments of time, is the leading theoretical paradigm for the earliest history of the universe and for the origin of the structure in the universe
The cosmic microwave background (CMB) offers a unique window between the late-time universe dominated by dark matter and dark energy, and the early universe when the energy density was dominated by the potential that drove cosmic inflation
Advances but we are predicting error of 0.002 for Dark Energy Spectroscopic Instrument (DESI) Lyman-α forest measurements. These errors are larger than naturally predicted by inflation, so new physics will have to be at work if we detect running at these levels
Summary
Precision cosmological measurements push the boundaries of our understanding of the fundamental physics that governs our universe. A detection of tensor modes would constitute a stunning measurement of the quantum mechanical fluctuations of the gravitational field This motivates a next-generation CMB experiment with the sensitivity and systematics control to detect such a polarized signal at ≥ 5 σ significance, ensuring either a detection of inflationary gravitational waves or the ability to rule out large classes of inflationary models. A program to meet these goals by developing a Stage IV CMB experiment, CMB-S4, with O(500,000) detectors by 2020 is described in the companion cosmic frontier planning document Neutrino Physics from the Cosmic Microwave Background and Large Scale Structure [5] Such an experiment would contribute to inflationary science by strongly constraining the spectrum of primordial density fluctuations, allowing us to distinguish different families of inflationary models. The generation of large scale structure measurements will produce non-Gaussianity constraints that are an important cross-check of the CMB bound and will pave the way for more stringent bounds from future large scale structure measurements
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