Cosmic string, hydrodynamics and microanisotropies in the cosmic background radiation

Abstract

‘Cosmic string’ formed at the mass scale of GUT (grand unified theory) symmetry breaking (M_x ≊ 2×l0¹⁵GeV) is typified by a mass per unit length μG/c² - ≊2×10⁻⁶ in dimensionless units (where G is the universal gravitational constant and c is the speed of light), corresponding to μ ≊ 2.6×l0²¹ kgm⁻¹≊4×l0⁷M⊙ pc⁻¹ (where M⊙ is the mass of the Sun). It has been conjectured^1–3 that loops of such cosmic string, presumably with radii in the range 1–10 kpc, might provide the seed perturbations which could initiate galaxy formation. Here I demonstrate the possibility of detecting such loops by observing the distortions in the cosmic background radiation (CBR) induced by the hydrodynamic interaction of isolated loops of cosmic string with the cosmic fluid, during the epoch immediately preceding recombination. These distortions occur on angular scales of a few arc minutes, and may be as large as ΔT/T ≊10⁻³, but the areal density of kiloparsec-sized loops is anticipated to be only ∼10 per square degree in most cosmologies, so one would not expect limited-area searches for CBR microanisotropies^4,5 to have found them yet.