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A cosmic string is a hypothetical 1-dimensional topological defect in the fabric of spacetime. Cosmic strings are hypothesized to form when different regions of spacetime undergo phase changes, resulting in domain boundaries between the two regions when they meet. This is somewhat analogous to the boundaries that form between crystal grains in solidifying liquids, or the cracks that form when water freezes into ice. In the case of our universe, such phase changes may have occurred in the early days as the universe formed.

Cosmic strings, if they exist, would be extremely thin with diameters on the same order as a proton. They would have immense density, however, and so would represent significant gravitational sources. A cosmic string 1.6 kilometers in length would exert more gravity than the Earth. Cosmic strings would form a network of loops in the early universe, and their gravity could have been responsible for the original clumping of matter into galactic superclusters.

A cosmic string's vibrations, which are thought to oscillate near the speed of light, can cause part of the string to pinch off into an isolated loop. These loops have a finite lifespan due to decay via gravitational radiation.

Other types of topological defects in spacetime are domain walls, monopoles, and Textures.

It was once thought that the gravitational influence of cosmic strings might contribute to the large-scale clumping of matter in the universe, but all that is known today through galaxy surveys and precision measurements of the cosmic microwave background fits an evolution out of random, gaussian fluctuations. These precise observations therefore tend to rule out a significant role for cosmic strings.

Gravitational lensing of a galaxy by a straight section of a cosmic string would produce two identical, undistorted images of the galaxy. In 2003 a group led by Mikhail Sazhin reported the accidental discovery of two seemingly identical galaxies very close together in the sky, leading to speculation that a cosmic string had been found.^[1] However, observations by the Hubble Space Telescope in January 2005 showed them to be a pair of similar galaxies, not two images of the same galaxy.^[2][3] A cosmic string would produce a similar duplicate image of fluctuations in the cosmic microwave background, which might be detectable by the upcoming Planck Surveyor mission.

A second piece of evidence supporting cosmic string theory is a phenomenon observed in observations of the "double quasar" called Q0957+561A,B. Originally discovered by Dennis Walsh, Bob Carswell, and Ray Weymann in 1979, the double image of this quasar is caused by a galaxy positioned between it and the Earth. The gravitational lens effect of this intermediate galaxy bends the quasar's light so that it follows two paths of different lengths to Earth. The result is that we see two images of the same quasar, one arriving a short time after the other (about 417.1 days later).

However, a team of astronomers at the Harvard-Smithsonian Center for Astrophysics led by Rudolph Schild studied the quasar and found that during the period between September 1994 and July 1995 the two images appeared to have no time delay; changes in the brightness of the two images occurred simultaneously on four separate occasions. Schild and his team believe that the only explanation for this observation is that a cosmic string passed between the Earth and the quasar during that time period traveling at very high speed and oscillating with a period of about 100 days.^[4]

The Laser Interferometer Gravitational-Wave Observatory (LIGO) and upcoming gravitational wave observatories will search for cosmic strings as well as other phenomenon with the byproduct of gravitational waves.

There is no direct connection between string theory and the theory of cosmic strings (the names were chosen independently by analogy with ordinary string). However, work in string theory revived interest in cosmic strings in the early 2000s. It was pointed out by string theorist Joseph Polchinski that the expanding Universe could have stretched a "fundamental" string (the sort which superstring theory considers) until it was of intergalactic size. Such a stretched string would exhibit many of the properties of the old "cosmic" string variety, making the older calculations useful again. Furthermore, modern superstring theories offer other objects which could feasibly resemble cosmic strings, such as highly elongated one-dimensional D-branes (known as "D-strings"). As theorist Tom Kibble remarks, "string theory cosmologists have discovered cosmic strings lurking everywhere in the undergrowth". Older proposals for detecting cosmic strings could now be used to investigate superstring theory.

Superstrings, D-strings or other stringy objects stretched to intergalactic scales would radiate gravitational waves, which could presumably be detected using experiments like LIGO. They might also cause slight irregularities in the cosmic microwave background, too subtle to have been detected yet but possibly within the realm of future observability.

Note that most of these proposals depend, however, on the appropriate cosmological fundamentals (strings, branes, etc.), and no convincing experimental verification of these has been performed.

1. ^ Sazhin, Mikhail; et al. (August 2003). "CSL-1: chance projection effect or serendipitous discovery of a gravitational lens induced by a cosmic string?". Monthly Notices of the Royal Astronomical Society 343 (2). 
2. ^ Hubble imaging excludes cosmic string lens, Physical Review D, volume 73 page 087302, 2006
3. ^ 2 Feb 2006, The true nature of CSL-1, M. V. Sazhin, M. Capaccioli, G. Longo, M. Paolillo, O. S. Khovanskaya, N. A. Grogin, E. J. Schreier, G. Covone arXiv:astro-ph/0601494
4. ^ Anomalous fluctuations in observations of Q0957+561 A,B: Smoking gun of a cosmic string?, R. Schild, I. S. Masnyak, B. I. Hnatyk and V. I. Zhdanov, Astronomy and Astrophysics, volume 422, pages 477-482, August 2004

• Dr. Kip Thorne, ITP & CalTech. Spacetime Warps and the Quantum: A Glimpse of the Future. Lecture slides and audio

• 0-dimensional singularity: gravitational singularity
• 1-dimensional singularity:
• 2-dimensional singularity: domain wall

• http://www.damtp.cam.ac.uk/user/gr/public/cs_interact.html
• astro-ph/0302547 CSL-1: a chance projection effect or serendipitous discovery of a gravitational lens induced by a cosmic string?
• astro-ph/0406434 Anomalous Fluctuations in Observations of Q0957+561 A,B: Smoking Gun of a Cosmic String?
• astro-ph/0410073 Cosmic strings reborn?
• astro-ph/0503120 Signatures of Cosmic Strings in the Cosmic Microwave Background
• astro-ph/0506400 Further spectroscopic observations of the CSL-1 object
• astro-ph/0603838 Hubble Imaging Excludes Cosmic String Lens