Cosmic strings are hypothetical entities that might have been created in the Big Bang. They are thought to distort space in the following way: the ratio between the circumference and the radius of a circle drawn around such a string in the plane perpendicular to it will be less than $ 2 \pi \, $. The difference in angle is often referred to as angle deficit, denoted $ \Delta $. This angle can be related to the mass density per unit length $ \mu $ of the string using gravitational lensing arguments

$ \Delta \approx \frac{G \mu}{c^2} $

where $ G $ is the universal constant of gravity, and $ c $ is the speed of light.