Small-angle CMB temperature anisotropies induced by cosmic strings

Abstract

We use Nambu-Goto numerical simulations to compute the cosmic microwave background (CMB) temperature anisotropies induced at arcminute angular scales by a network of cosmic strings in a Friedmann-Lema\^{\i}tre-Robertson-Walker (FLRW) expanding universe. We generate 84 statistically independent maps on a $7.2\ifmmode^\circ\else\textdegree\fi{}$ field of view, which we use to derive basic statistical estimators such as the one-point distribution and two-point correlation functions. At high multipoles, the mean angular power spectrum of string-induced CMB temperature anisotropies can be described by a power law slowly decaying as ${\ensuremath{\ell}}^{\ensuremath{-}p}$, with $p=0.889$ $(+0.001,\ensuremath{-}0.090)$ (including only systematic errors). Such a behavior suggests that a nonvanishing string contribution to the overall CMB anisotropies may become the dominant source of fluctuations at small angular scales. We therefore discuss how well the temperature gradient magnitude operator can trace strings in the context of a typical arcminute diffraction-limited experiment. Including both the thermal and nonlinear kinetic Sunyaev-Zel'dovich effects, the Ostriker-Vishniac effect, and the currently favored adiabatic primary anisotropies, we find that, on such a map, strings should be ``eye visible,'' with at least of order ten distinctive string features observable on a $7.2\ifmmode^\circ\else\textdegree\fi{}$ gradient map, for tensions $U$ down to $GU\ensuremath{\simeq}2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$ (in Planck units). This suggests that, with upcoming experiments such as the Atacama Cosmology Telescope (ACT), optimal non-Gaussian, string-devoted statistical estimators applied to small-angle CMB temperature or gradient maps may put stringent constraints on a possible cosmic string contribution to the CMB anisotropies.