Cosmic (super)strings and LIGO

Xavier Siemens, University of Wisconsin-Milwaukee
Through much of the last two and a half decades, cosmic strings were of great interest to the cosmology and high energy physics communities. Unlike other simple topological defects, such as monopoles and domain walls, strings do not cause cosmological disasters. Indeed, cosmic strings formed at the GUT scale would lead to cosmological density perturbations of the right amplitude to seed the formation of galaxies and clusters. Thus, cosmic strings became a leading candidate for structure formation. For a review see [1].

Cosmic strings were also appealing because their cosmological evolution (at least the gross features) turned out to be quite simple. Regardless of the details of the initial conditions, a string network in an expanding universe will quickly evolve toward an attractor solution called the ``scaling regime''. In this regime, the energy density of the string network becomes a small constant fraction of the radiation or matter density, and the statistical properties of the system, such as the correlation lengths of long strings and average sizes of loops, scale with the cosmic time.

The attractor solution is possible due to reconnections, which for field theoretic strings, essentially always occur when two string segments meet. Reconnections produce cosmic string loops, which in turn decay by radiating gravitationally. This takes energy out of the string network, converting it to gravitational waves. If the density of string in the network becomes large, then strings will meet more often, producing extra loops. The loops then decay gravitationally, removing the surplus energy from the network. If, on the other hand, the density of strings becomes too low, strings will not meet to often enough to produce loops, and their density will start to grow. Thus, the network is driven toward an equilibrium.

During the 1990s, cosmic microwave background data showed that strings could not give rise to the density fluctuations that seed structure formation. These observations placed upper limits on the string tension below the GUT scale, relegating strings to (at most) a sub-dominant role in the seeding of structure formation. As a result of these discoveries, the cosmology community's interest in cosmic strings dwindled through the late 1990s, and into the millennium. Recently, however, a few developments have contributed to a resurgence of interest in cosmic strings.

In 2000, Damour and Vilenkin found that cosmic strings could lead to the production of sizable gravitational wave bursts [2]. These bursts may detectable with first generation ground-based interferometric gravitational wave detectors, such as LIGO and VIRGO, at design sensitivity. Remarkably, they found values of the string tension that would result in a measurable signal, that are below the upper limits placed by cosmic microwave background observations.

The bursts we are most likely to be able to detect are produced at cosmic string cusps. These are regions of string which acquire phenomenal Lorentz boosts, and emit a powerful burst of gravitational waves in the direction of motion of the string. The formation of cusps on cosmic string loops and long strings is generic, and their gravitational waveforms simple and robust [3].

More recently, Jones, Stoica and Tye [4], and Sarangi and Tye [5] realized that string theory inspired inflation scenarios lead to the production of cosmic strings. Thus, the very exciting possibility arises [6] that a certain class of string theories may have consequences observable in the near future: Just like ordinary field theoretic strings, the cosmic superstrings formed could lead to the production of a detectable gravitational wave signal.

Fortunately, much of what was learned about the evolution of field theoretic cosmic string networks can be applied to the evolution of cosmic superstrings. Aside from the possibility of forming more than one type of string, the most significant difference is that cosmic superstring interactions are probabilistic. Pairs of strings do not always reconnect when they meet. Furthermore, strings in higher dimensional spaces may more readily avoid intersections [7]. The net effect is to lower the reconnection probability. If there is only one type of string, the network still enters a scaling regime [8], albeit at a density higher by a factor inversely proportional to the reconnection probability [9]. It turns out that the smaller reconnection probability of superstrings actually increases the chances of detection through the production of gravitational wave bursts [9]

Finally, there are direct observations that suggest a gravitational lens produced by a long cosmic string [10], as well as an oscillating cosmic string loop [11].

The LIGO Scientific Collaboration is currently involved in the development of a templated search for bursts from strings. At the end of February 2005, the collaboration plans to start its fourth science run (S4). The interferometers are within factors of a few from design sensitivity. Although with current sensitivities a detection seems unlikely, it may become possible to place constraints on the types of fundamental particle theories that describe our world.


[1] A. Vilenkin and E.P.S Shellard, Cosmic strings and other Topological Defects. Cambridge University Press, 2000; M. Hindmarsh and T.W.B. Kibble, Rept. Prog. Phys. 58 (1995) 477.
[2] T. Damour, A. Vilenkin, Phys.Rev.Lett. 85 (2000) 3761; T. Damour, A. Vilenkin, Phys.Rev.D64 (2001) 064008.
[3] X. Siemens, K.D. Olum, Phys.Rev. D68 (2003) 085017.
[4] N. Jones, H. Stoica, S.H.Henry Tye, JHEP 0207 (2002) 051, hep-th/0203163.
[5] S. Sarangi, S.H.Henry Tye, Phys.Lett.B 536 (2002) 185, hep-th/0204074.
[6] J. Polchinski, hep-th/0410082; J. Polchinski, hep-th/0412244.
[7] G. Dvali, A. Vilenkin, JCAP 0403 (2004) 010. [8] N. Jones, H. Stoica, S.H.Henry Tye, Phys.Lett. B563 (2003) 6.
[9] T. Damour, A. Vilenkin, hep-th/0410222.
[10] M. Sazhin et al., Mon. Not. Roy. Astron. Soc. 343 (2003) 353; M. Sazhin et al., astro-ph/0406516.
[11] R. Schild et al., astro-ph/0406434.

Jorge Pullin 2005-03-10