Cosmic string
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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.
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. Today precison observations of the large scale structure of the universe, directly using sky survey, or indirectly from observing the cosmic microwave background tend to rule out a significant role of cosmic strings.
Other types of topological defects in spacetime are domain walls, monopoles, and textures.
Evidence of Cosmic Strings
A recent discovery of a "double galaxy" has some interesting implications for cosmic string theory. In an article entitled "CSL-1: chance projection effect or serendipitous discovery of a gravitational lens induced by a cosmic string?" published in August 2003 in Volume 343, Issue 2 of "Monthly Notices of the Royal Astronomical Society," a group led by Mikhail Sazhin reported the accidental discovery of two seemingly identical galaxies very close together on the sky.
From the abstract: "The Capodimonte–Sternberg–Lens candidate no. 1 is an extragalactic double source detected in the Osservatorio Astronomico di Capodimonte – Deep Field. It can be interpreted either as the chance alignment of two identical galaxies at z= 0.46 or as the first case of gravitational lensing by a cosmic string. Extensive modelling shows in fact that cosmic strings are the only type of lens which (at least at low angular resolution) can produce undistorted double images of a background source. We propose an experimentum crucis to disentangle these two possible explanations. If the lensing by a cosmic string should be confirmed, it would provide the first measurements of energy scale of symmetry breaking and of the energy scale of grand unified theory."
Authors of the paper also included: J. M. Alcalá, M. Capaccioli, G. Covone, O. Khovanskaya, G. Longo, M. Pannella, M. Pavlov, M. Radovich, R. Silvotti, and V. Testa.
A second piece of evidence supporting 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 Centre 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. Their findings were published in Volume 422 of Astronomy and Astrophysics in August 2004.
From the abstract: "We report the detection of anomalous brightness fluctuations in the multiple image Q0957 + 561 A,B gravitational lens system, and consider whether such anomalies have a plausible interpretation within the framework of cosmic string theory. We study a simple model of gravitational lensing by an asymmetric rotating string. An explicit form of the lens equation is obtained and approximate relations for magnification are derived. We show that such a model with typical parameters of the GUT string can quantitatively reproduce the observed pattern of brightness fluctuations. On the other hand explanation involving a binary star system as an alternative cause requires an unacceptably large massive object at a small distance. We also discuss possible observational manifestations of cosmic strings within our lens model."
Authors of the paper also included: B. I. Hnatyk, I. S. Masnyak, and V.I. Zhdanov