Looking for cosmic strings

Abstract

Cosmic strings predicted in some grand unified theories can produce double images of distant galaxies and quasars^1–3, and it has been argued that some of the known double quasars may be due to strings^2–4. To test this hypothesis, one can look for other characteristic effects of strings in the vicinity of double quasars, such as a discontinuous change of the microwave background temperature across the string^3,5, lines of double galaxies², and images of galaxies with a sharp edge⁴. In most of the previous work on the gravitational effects of strings it was assumed either that the string is static or that both the string and its velocity are perpendicular to the line of sight. Here I show that in more general configurations the expressions for the angular separation of double images and for the temperature discontinuity δT/T across the string are modified by trigonometric and relativistic factors. More importantly, the line connecting the double images is not, in general, perpendicular to the apparent direction of the string (contrary to what one would naively expect). Although the analysis presented here is rather elementary, the results may nevertheless be useful for interpreting the observations.