Linear polarized fluctuations in the cosmic microwave background

Abstract

It has been recognized since the early work of Silk¹ and Sachs and Wolfe² that inhomogeneities in density, velocity fields or gravitational potential can produce small amplitude fluctuations in the intensity of the cosmic microwave background on small angular scales. Searches³ for such intensity fluctuations on scales of arc seconds to degrees have now reached sensitivities of 10⁻⁴ in ΔT/T or better (where T is the temperature of the background). It is less often noted that density inhomogeneities can introduce small-scale fluctuations in linear polarization into the microwave background. Work by Bond and Efstathiou⁴ in the context of cold dark matter models for the origin of galaxies, for instance, suggests that fluctuations in polarization will be present on characteristically smaller angular scales than total intensity fluctuations, and will be ∼1/10 their amplitude. Lubin et al.⁵ have shown that the large-scale linear polarization is ≲0.2 mK, or ≲7 x 10−⁵ in ΔT/T. We report here limits on the linear (and circular) polarization of the cosmic microwave background on small angular scales,18″≤ Θ≤160″.