The Quest for B Modes from Inflationary Gravitational Waves

Abstract

The search for the curl component (B mode) in the cosmic microwave background (CMB) polarization induced by inflationary gravitational waves is described. The canonical single-field slow-roll model of inflation is presented, and we explain the quantum production of primordial density perturbations and gravitational waves. It is shown how these gravitational waves then give rise to polarization in the CMB. We then describe the geometric decomposition of the CMB polarization pattern into a curl-free component (E mode) and curl component (B mode) and show explicitly that gravitational waves induce B modes. We discuss the B modes induced by gravitational lensing and by Galactic foregrounds and show how both are distinguished from those induced by inflationary gravitational waves. Issues involved in the experimental pursuit of these B modes are described, and we summarize some of the strategies being pursued. We close with a brief discussion of some other avenues toward detecting/characterizing the inflationary gravitational-wave background.

Statistical simulations of the dust foreground to cosmic microwave background polarization

“…The detection of this signal would have a deep impact on cosmology and fundamental physics, motivating a number of experiments designed to measure the sky polarization at microwave frequencies. Current projects have achieved the sensitivity required to detect the CMB B-mode signal predicted by the simplest models of inflation (Abazajian et al 2015; Kamionkowski & Kovetz 2016). …”

CMB Polarization B-mode Delensing with SPTpol and Herschel

“…Inflation generically predicts a stochastic background of gravitational waves (see e.g., Kamionkowski & Kovetz 2016 for a review). These primordial gravitational waves (PGWs) imprint a unique signature on the polarized anisotropies of the CMB.…”

“…Measuring r from CMB B modes provides the cleanest known observational window onto the PGW background. While the latter has not been detected, a variety of arguments predict  -r 10 3 , which should be observable in the near future (Kamionkowski & Kovetz 2016). Therefore, measuring r is a major objective of current and future CMB experiments.…”

Reassessing the link betweenB-modes and inflation

“…However, such a paradigm also makes predictions for the shape and amplitude of primordial gravity waves, which should be observable in the B-mode polarization of the CMB. The fact that, so far, such B-modes have not been observed, has been used to severely constrain the set of viable inflationary models [1][2][3]. Moreover, the recent detection of gravity waves by LIGO [4] removes all lingering doubts about the reality of such waves (if there were any remaining, given the dramatic indirect evidence provided by the Binary Pulsar studies [5]).…”

“…As ǫ is also connected with the tilt in the scalar spectrum [13], empirical constraints on that quantity can be used to limit the viability of various simple inflationary models. In particular, measurements of the amplitude of scalar perturbations, combined with the failure to detect tensor perturbations, have been used to discriminate among inflationary models and even to rule-out some of the simplest ones [1][2][3]. There are, however, a few problematic aspects within the standard approach that call into question some of its conclusions.…”