Gauging the cosmic microwave background

Understand

We provide a new derivation of the anisotropies of the cosmic microwave background (CMB), and find an exact expression that can be readily expanded perturbatively.

• Close attention is paid to gauge issues, with the motivation to examine the effect of super-Hubble modes on the CMB.
• We calculate a transfer function that encodes the behaviour of the dipole, and examine its long-wavelength behaviour.
• We show that contributions to the dipole from adiabatic super-Hubble modes are strongly suppressed, even in the presence of a cosmological constant, contrary to claims in the literature.