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CMBPol Mission Concept Study: Probing Inflation with CMB Polarization

Daniel Baumann, Mark G. Jackson, Peter Adshead, Alexandre Amblard, Amjad Ashoorioon, Nicola Bartolo, Rachel Bean, Maria Beltran, Francesco de Bernardis, Simeon Bird, Xingang Chen, Daniel J. H. Chung, Loris Colombo, Asantha Cooray, Paolo Creminelli, Scott Dodelson, Joanna Dunkley, Cora Dvorkin, Richard Easther, Fabio Finelli, Raphael Flauger, Mark Hertzberg, Katherine Jones-Smith, Shamit Kachru, Kenji Kadota, Justin Khoury, William H. Kinney, Eiichiro Komatsu, Lawrence M. Krauss, Julien Lesgourgues, Andrew Liddle, Michele Liguori, Eugene Lim, Andrei Linde, Sabino Matarrese, Harsh Mathur, Liam McAllister, Alessandro Melchiorri, Alberto Nicolis, Luca Pagano, Hiranya V. Peiris, Marco Peloso, Levon Pogosian, Elena Pierpaoli, Antonio Riotto, Uros Seljak, Leonardo Senatore, Sarah Shandera, Eva Silverstein, Tristan Smith, Pascal Vaudrevange, Licia Verde, Ben Wandelt, David Wands, Scott Watson, Mark Wyman, Amit Yadav, Wessel Valkenburg, Matias Zaldarriaga

Abstract

We summarize the utility of precise cosmic microwave background (CMB) polarization measurements as probes of the physics of inflation. We focus on the prospects for using CMB measurements to differentiate various inflationary mechanisms. In particular, a detection of primordial B-mode polarization would demonstrate that inflation occurred at a very high energy scale, and that the inflaton traversed a super-Planckian distance in field space. We explain how such a detection or constraint would illuminate aspects of physics at the Planck scale. Moreover, CMB measurements can constrain the scale-dependence and non-Gaussianity of the primordial fluctuations and limit the possibility of a significant isocurvature contribution. Each such limit provides crucial information on the underlying inflationary dynamics. Finally, we quantify these considerations by presenting forecasts for the sensitivities of a future satellite experiment to the inflationary parameters.

CMBPol Mission Concept Study: Probing Inflation with CMB Polarization

Abstract

We summarize the utility of precise cosmic microwave background (CMB) polarization measurements as probes of the physics of inflation. We focus on the prospects for using CMB measurements to differentiate various inflationary mechanisms. In particular, a detection of primordial B-mode polarization would demonstrate that inflation occurred at a very high energy scale, and that the inflaton traversed a super-Planckian distance in field space. We explain how such a detection or constraint would illuminate aspects of physics at the Planck scale. Moreover, CMB measurements can constrain the scale-dependence and non-Gaussianity of the primordial fluctuations and limit the possibility of a significant isocurvature contribution. Each such limit provides crucial information on the underlying inflationary dynamics. Finally, we quantify these considerations by presenting forecasts for the sensitivities of a future satellite experiment to the inflationary parameters.

Paper Structure

This paper contains 62 sections, 109 equations, 14 figures, 17 tables.

Table of Contents

  1. Precision Cosmology: 'From What to Why'
  2. Introduction
  3. The Next Decade
  4. Outline
  5. Cosmological Observables: An Overview
  6. The Concordance Cosmology
  7. The Inflationary Sector
  8. Inflationary Cosmology
  9. Inflation as a Solution to the Big Bang Puzzles
  10. The Physics of Inflation
  11. Cosmological Observables
  12. SVT Decomposition in Fourier Space
  13. Scalar (Density) Perturbations
  14. Vector (Vorticity) Perturbations
  15. Tensor (Gravitational Wave) Perturbations
  16. ...and 47 more sections

Figures (14)

  • Figure 1: Examples of Inflaton Potentials. Acceleration occurs when the potential energy of the field $V$ dominates over its kinetic energy $\frac{1}{2} \dot \phi^2$. Inflation ends at $\phi_{\rm end}$ when the slow-roll conditions are violated, $\epsilon \to 1$. CMB fluctuations are created by quantum fluctuations $\delta \phi$ about 60 $e$-folds before the end of inflation. At reheating, the energy density of the inflaton is converted into radiation. Left: A typical small-field potential. Right: A typical large-field potential.
  • Figure 2: Creation and evolution of perturbations in the inflationary universe. Fluctuations are created quantum mechanically on sub-horizon scales. While comoving scales, $k^{-1}$, remain constant the comoving Hubble radius during inflation, $(aH)^{-1}$, shrinks and the perturbations exit the horizon. Causal physics cannot act on superhorizon perturbations and they freeze until horizon re-entry at late times.
  • Figure 3: Thomson scattering of radiation with a quadrupole anisotropy generates linear polarization HuWhite. Red colors (thick lines) represent hot radiation, and blue colors (thin lines) cold radiation.
  • Figure 4: Examples of $E$-mode and $B$-mode patterns of polarization. Note that if reflected across a line going through the center the $E$-patterns are unchanged, while the positive and negative $B$-patterns get interchanged.
  • Figure 5: Power spectrum of the cross-correlation between temperature and $E$-mode polarization anisotropies WMAP5. The anti-correlation for $\ell = 50-200$ (corresponding to angular separations $5^\circ > \theta > 1^\circ$) is a distinctive signature of adiabatic fluctuations on superhorizon scales at the epoch of decoupling spergel/zaldarriaga:1997Dodelson:2003ip, confirming a fundamental prediction of the inflationary paradigm.
  • ...and 9 more figures