The Quest for B Modes from Inflationary Gravitational Waves (1510.06042v1)

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.

• The paper demonstrates that B modes in CMB polarization directly probe inflationary gravitational waves and early-universe physics.
• The methodology uses geometric decomposition to distinguish inflationary signals from lensing and Galactic foregrounds.
• Numerical results show that upcoming experiments could constrain the tensor-to-scalar ratio to around 0.001, testing key inflationary models.

Overview of "The Quest for B Modes from Inflationary Gravitational Waves"

The paper authored by Marc Kamionkowski and Ely D. Kovetz presents a comprehensive analysis of the search for B modes—a crucial aspect of the cosmic microwave background (CMB) polarization induced by inflationary gravitational waves. The paper systematically elucidates the underlying physical processes, observational strategies, and scientific implications associated with detecting these elusive cosmic signatures.

Focused on the canonical single-field slow-roll (SFSR) inflationary model, the authors delve into the quantum mechanical production mechanisms of primordial density perturbations and gravitational waves during inflation. The resulting gravitational waves impart a distinct curl component (B mode) in the CMB polarization, which differs qualitatively from the curl-free (E mode) component initiated by density perturbations. This distinction underpins the concerted effort to detect B modes, as their discovery would serve as a direct probe of the inflationary epoch and the gravitational wave background it predicts.

The manuscript explores the geometric decomposition of CMB polarization, emphasizing the unique B mode contribution from inflationary gravitational waves. It further distinguishes B modes produced by gravitational lensing and Galactic foregrounds, offering insights into experimental methodologies for disentangling these effects to isolate the inflationary signal.

In their discourse on observational strategies, the authors identify the technological and analytical challenges in measuring B modes, discussing various experimental setups, including suborbital missions and potential satellite experiments. These endeavors aim to achieve the sensitivity required to detect a tensor-to-scalar ratio $r$ as low as 0.001—a crucial threshold in testing inflationary predictions.

Numerical results presented in the paper demonstrate the power spectra's amplitude and scale dependence, providing evidence that contemporary and near-future experiments can reach significant constraints on $r$, potentially confirming or ruling out many inflationary models.

The authors conclude by speculating on the profound implications of detecting inflationary B modes, which extend beyond confirming cosmic inflation. Such a detection would lend support to high-energy physics theories near the grand unified theory (GUT) scale, provide empirical data on quantum fluctuations of spacetime, and offer insights into the tensor perturbation spectrum, including aspects like chirality and statistical properties.

Overall, this paper positions the quest for cosmic B modes as a pivotal exploration in modern cosmology, bridging fundamental physics and observational cosmology with the potential to reshape our understanding of the universe's earliest moments.