Cosmic string

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.

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

Observational evidence

Cosmic strings were once thought to be an explanation of the large scale structure of 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 precison observations therefore tend to rule out a significant role of cosmic strings.

A recent discovery of a "double galaxy" Capodimonte-Sternberg lens candidate 1 (CSL-1) 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 in the sky.

The cosmic string producing this double image could have been detectable in precision measurements of the cosmic microwave background but at the spatial resolution of WMAP, the result was inconclusive. Re-examination with the PLANCK detector could decide the issue. However, observations of the Hubble Space Telescope in January 2005 have shown that CSL-1 is a pair of distinct galaxies rather than a pair of images of the same galaxy.

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. Their findings were published in Volume 422 of Astronomy and Astrophysics in August 2004.

String theory and cosmic strings

In the early 2000s, string theorists revived interest in cosmic strings. It was pointed out by 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.

While intriguing, these cosmological proposals fall short in one respect: testing a theory requires that the test be capable, at least in principle, of falsifying the theory. For example, if observing the Sun during a solar eclipse had not shown that the Sun's gravity deflected light, Einstein's general relativity theory would have been proven wrong. Not finding cosmic strings would not demonstrate that string theory is fundamentally wrong — merely that the particular idea of highly stretched strings acting "cosmic" is in error. While many measurements could in principle be made that would suggest that string theory is on the right track, scientists have not at present devised a stringent "test".

Observational evidence

String theory and cosmic strings

External links

See also

Gourt Home::Gourt :: Cosmic string

Observational evidence

String theory and cosmic strings

External links

See also