Brane World Cosmic String Interaction

It is shown that brane world gravity produces an attractive force between cosmic strings, in contrast to conventional Einstein gravity. This force is sufficient for stable bound states of Type II strings (which normally repel each other) to form. There is a small separation between the strings, giving them a rope-like structure. At astronomical scales these ‘cosmic ropes’ resemble higher winding number strings, and so all brane world cosmic strings are effectively Type I. There has been a renewed interest in the line-like topological defects known as cosmic strings [1], particularly since they have been found to occur in many superstring theory scenarios [2]. They may also be making a small contribution to the cosmic microwave background (CMB) radiation. Cosmic strings have been ruled out as the dominant source of CMB anisotropies, but their contribution could still be large enough to be detected by upcoming experiments. Most string network simulations, from which the above result is obtained, assume that at short distances two cosmic strings repel. This means that only strings with winding number n = 1 are stable, and that when two strings collide they will intercommute (exchange partners). However this is not true for all strings. Denoting the masses of the string’s Higgs and gauge bosons by ms and mv, the above statement only holds for Type II strings, for which the ratio β = m s /m v > 1. On the other hand Type I strings, for which β < 1, are attracted to each other. Higher winding number cosmic strings are stable in this case, and colliding strings do not always intercommute [1, 3]. When considering the force between two strings, the effects of gravity are generally ignored. For conventional Einstein gravity this is completely reasonable, as in this case the force is zero. However this need not be true for more exotic gravity theories. Consider the superstring motivated brane world scenario, in which our apparently 3+1 dimensional universe is a 3-brane embedded in a higher dimensional, bulk spacetime. The RandallSundrum scenario [4], which has a single warped extra dimension, provides the simplest ∗[email protected]