Tom W. B. Kibble introduced the concept of cosmic strings, infinitesimally thin filaments of primordial material left over from the early moments of cosmic history, as a possible mechanism for explaining how the macroscopic structure of the universe, from stars to galaxies to clusters of galaxies, evolved.

In 1915, Albert Einstein published his formulation of the general theory of relativity, General relativity
Relativity, general
Physics;general relativity a new theory of gravity, in which he proposed that massive objects cause space to curve and time to slow down. These space-time distortions are most noticeable near large masses or compact objects. Over the next four decades, Einstein sought a single theory that would unify the four basic forces—gravity, electromagnetism, the strong nuclear force, and the weak nuclear force—into one superforce that governs all of the interactions observed in the universe. Cosmic string theory
Physics;cosmic string theory
Universe;structure
[kw]Kibble Proposes the Theory of Cosmic Strings (1976)
[kw]Theory of Cosmic Strings, Kibble Proposes the (1976)
[kw]Cosmic Strings, Kibble Proposes the Theory of (1976)
Cosmic string theory
Physics;cosmic string theory
Universe;structure
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Kibble, Tom W. B.
Einstein, Albert
Witten, Edward
Zurek, Wojciech H.
Polchinski, Joseph
Hindmarsh, Mark Bernard

Many physicists, including Tom W. B. Kibble, continued Einstein’s search in an effort to find some description or mechanism that might underlie a grand unified theory Grand unified theory of all things. By the 1970’s, this pursuit had led Kibble to the concept of cosmic strings, linear concentrations of mass-energy that formed in the very early universe. Suggesting that cosmic strings were a viable source for seeding galaxy and cluster formation in the universe, Kibble’s theory provided an alternative explanation to the theory of cosmic inflation, which proposed an incredibly short-lived rate of expansion in the early universe.

According to the big bang theory, Big bang theory when the universe began, the four basic forces were a single unified force, the superforce. Within 10^-10 seconds, this symmetry was broken, and the full complement of four fundamental forces was manifest. Before one million years had transpired, atoms were formed, the universe became transparent, and radiation was released. This radiation, which cooled and stretched out in space, is referred to as the cosmic microwave background Cosmic microwave background radiation (CMB). Initially discovered in 1965, the CMB, which represents a picture of the early universe, seems to be pretty much the same in any direction in the universe. This uniformity suggests that the universe had a smooth, regular beginning. Kibble as well as other physicists and astrophysicists wondered how this could be reconciled with the fact that the distribution of matter in the universe has an irregular, “lumpy” structure.

By 1976, Kibble had developed some mathematical models to describe the fraction of a second in time when the superforce should have existed. His best model suggested that the rapid expansion and cooling of the universe after the big bang had generated stringlike defects in space and time similar to what happens when cracks form as water undergoes a phase transition and freezes into ice. Kibble used the term “cosmic strings” for the topological defects produced by cosmological phase transitions that occurred in the early universe. These strands of mass-energy were thinner than a proton, were extremely dense, and could stretch the length of the universe. A 10-kilometer length of string might weigh as much as Earth itself but would emit little or no electromagnetic radiation.

According to Kibble’s model, cosmic strings possess immense energies, move at velocities that approach the speed of light, and curve space around themselves. Along a cosmic string, the strong nuclear, weak nuclear, and electromagnetic forces remained unified as one force. As cosmic strings vibrate back and forth, it is possible for them to form into loops that might encircle an entire galaxy. Although other topological defects in addition to cosmic strings might exist in the fabric of space-time, cosmic strings were the only candidates that predicted simple cosmological evolution, had reasonable mathematical behavior, and were consistent with the scale of cosmic time.

After careful analysis of his model, Kibble suggested that cosmic strings might be the basic seed for cosmic structure formation, from stars to galaxies. Over a period of several years after Kibble’s theory was introduced, Kibble, Edward Witten, Mark Bernard Hindmarsh, Wojciech H. Zurek, Joseph Polchinski, and other theoretical physicists and astrophysicists developed ideas about the evolution of cosmic strings and their likely cosmological effects. Because of the strong gravitational fields that could be produced by cosmic strings, matter might have collected around them billions of years ago. This could form large-scale, long-chained galaxies or clusters of galaxies along the length of a cosmic string, giving rise to the “lumpiness” of space. The cosmic strings could also push matter around, giving the resulting cosmological structures bulk drift velocities.

Kibble’s cosmic string theory led to the calculation of the correct magnitude of cosmological density perturbations to seed the formation of galaxies and clusters. According to Kibble’s theory, cosmic strings should be widely dispersed throughout the universe, but because they are vibrating and losing energy, cosmic strings can decay, leaving the universe filled with a background of gravitational waves.

During the late 1990’s, the popularity of Kibble’s theory of cosmic strings waned as CMB measurements showed that cosmic strings could not provide a significant contribution to the density inhomogeneities in the cosmos. In 1999, physicists—including Hindmarsh, Carlo Contadi, Contadi, Carlo and João Magueijo Magueijo, João —determined that CMB irregularities are best described by an early period of cosmological inflation combined with perturbations seeded by cosmic strings produced at the end of inflation. Based on this explanation, the analysis of other cosmological data, and the possible connection of cosmic strings to fundamental string theory, the usefulness of Kibble’s theory was revived during the early years of the twenty-first century. Many prominent physicists and astrophysicists once again engaged in the search for cosmic strings.

Kibble’s theory of cosmic strings led to a number of important discoveries. In 2000, Alexander Vilenkin Vilenkin, Alexander and Thibault Damour Damour, Thibault discovered that cosmic strings could generate sizable bursts of gravitational waves. They determined that the values of cosmic string tension that result in measurable signals that can be recorded by gravitational wave detectors are below those suggested by CMB observations. Many astrophysicists believe that such signals will soon be detected by the Laser Interferometric Gravitational-Wave Observatory (LIGO) detector in the United States, the European Science Foundation Gravitational-Wave Observatory in Italy, the Japanese Gravitational-Wave Observatory, or the China Einstein Gravitational-Wave Observatory.

By 2002, Kibble’s cosmic string theory had been linked to superstring theory, the theory that all of the elementary particles are produced by fundamental, vibrating microscopic strings. The existence of macroscopic defects such as cosmic strings is predicated by many superstring theory scenarios. If superstrings, the proposed fundamental constituents of matter, can have astronomical dimensions and form cosmic strings, the best insight into the popular superstring theory might be provided through the observation of cosmic strings. Witten has argued that a geometric theory describing the four fundamental forces in nature works best mathematically with eleven dimensions. The extra dimensions must be so tightly curled up in cosmic strings that they are unobservable. This leads to the possible existence of massive particles that have not yet been discovered. These particles are possible manifestations of cosmic strings that may constitute the dark matter that pervades the universe and holds clusters of galaxies together. Observations indicate that about 90 percent of the universe consists of cold dark matter.

In 2004, Rudolph E. Schild Schild, Rudolph E. and his research team at the Harvard-Smithsonian Center for Astrophysics reported that the behavior of a quasar located near the Big Dipper and identified as Q0957+561A,B was best explained by the existence of a cosmic string. The double image of this quasar is produced by a massive galaxy located between the quasar and Earth. After careful examination of the images, Schild and his colleagues concluded that the data could be explained only by the gravitational bending of light caused by the presence of a moving cosmic string that exists near the Milky Way galaxy. Astrophysicists discovered an even stronger candidate for a cosmic string acting as a gravitational lens in the constellation Corvus, where a galaxy is split into two similar, undistorted images. Cosmic string theory
Physics;cosmic string theory
Universe;structure

• Krauss, Lawrence Maxwell. Hiding in the Mirror: The Mysterious Allure of Extra Dimensions, from Plato to String Theory and Beyond. New York: Viking Press, 2005. Explores historical developments leading to the idea of a multidimensional universe generated by the evolution and dynamics of cosmic strings.
• Susskind, Leonard. The Cosmic Landscape: String Theory and the Illusion of Intelligent Design. New York: Little, Brown, 2006. Presents an investigation of symmetry breaking and possible energy states associated with the early universe that could generate cosmic strings.
• Szabo, Richard J. An Introduction to String Theory and D-Brane Dynamics. London: Imperial College Press, 2004. Discusses possible connections between microscopic string theory and cosmic string theory in terms of multidimensional membranes that might provide the links among observations of quantum mechanics, cosmology, and gravity.
• Vilenkin, Alexander, and E. Paul Shellard. Cosmic Strings and Other Topological Defects. 2d ed. New York: Cambridge University Press, 2000. Describes the properties of cosmic strings and their possible evolution in the formation of cosmological structure.
• Zwiebach, Barton. A First Course in String Theory. New York: Cambridge University Press, 2004. Presents an overview of how vibrating strings might produce the fundamental particles of nature and provide the makeup of cosmic strings.

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