CMB Polarization

The cosmic microwave background is faintly polarized, and the pattern of that polarization probes the early universe, the epoch of reionization, and possibly primordial gravitational waves.

Definition

CMB polarization is the small linear polarization imprinted on the cosmic microwave background when its photons last scattered off electrons in the presence of local temperature quadrupoles, conventionally separated into E-mode and B-mode patterns with distinct physical origins.

Scope

This topic covers the origin of cosmic microwave background polarization in Thomson scattering of an anisotropic radiation field, the decomposition of the polarization pattern into curl-free E modes and curl-like B modes, the cosmological information each carries, and the search for primordial B modes as a signature of inflationary gravitational waves.

Core questions

• How does the cosmic microwave background become polarized?
• What is the difference between E-mode and B-mode polarization?
• Why are primordial B modes considered a signature of inflation?

Key concepts

• Linear polarization
• E modes
• B modes
• Thomson scattering
• Primordial gravitational waves
• Reionization bump
• Lensing B modes

Key theories

Thomson-scattering polarization

Polarization arises because Thomson scattering of radiation with a local quadrupole anisotropy produces a net linear polarization, so the cosmic microwave background carries a polarized component correlated with its temperature pattern.

E and B mode decomposition

The polarization field splits into a curl-free E-mode pattern, produced by density perturbations, and a curl-like B-mode pattern, which on large scales can only come from primordial gravitational waves or gravitational lensing, providing a clean test for inflation.

Mechanisms

When the cosmic microwave background photons last scattered, a quadrupole anisotropy in the radiation seen by each electron produced a small linear polarization; density perturbations generate only E modes, whereas tensor perturbations from inflation can generate B modes, and gravitational lensing converts some E modes into B modes at small scales.

Clinical relevance

Polarization sharpens and extends the cosmological information in the cosmic microwave background: E modes confirm the acoustic physics and constrain reionization, lensing B modes probe the growth of structure and neutrino masses, and a detection of primordial B modes would provide direct evidence for inflation and measure its energy scale.

History

The E/B decomposition was introduced in 1997 as a way to isolate the signature of primordial gravitational waves; E-mode polarization was first detected by DASI in 2002, Planck measured it precisely, and experiments such as BICEP and the Simons Observatory continue searching for primordial B modes.

Debates

Primordial B-mode detection

A robust detection of primordial B modes would confirm inflation, but foreground contamination from galactic dust has caused false alarms, so disentangling a genuine cosmological signal from foregrounds remains a central challenge.

Key figures

• Marc Kamionkowski
• Arthur Kosowsky
• Albert Stebbins
• Uros Seljak
• Matias Zaldarriaga

Seminal works

• kamionkowski1997

Frequently asked questions

Why is the cosmic microwave background polarized at all?

Polarization is generated when photons scatter off electrons that see a directional, quadrupole variation in the radiation around them; this happened at last scattering, leaving a faint polarized signal correlated with the temperature anisotropies.

Would detecting B modes prove inflation happened?

A confirmed primordial B-mode signal would be very strong evidence for inflation, because inflationary gravitational waves are the most natural source on large scales; however, galactic dust can mimic the signal, so any claim requires careful separation of foregrounds.