Cosmic strings

Type of physical science: Cosmic Strings, Gravity, Astronomy and astrophysics

Field of study: Cosmology

Cosmics strings are objects that, though they have not been proved to exist, could explain the origin of stars, galaxies, and supergalaxies.

Overview

A cosmic string, if such a thing exists, is an endlessly long, infinitesimally thin object that was created at the birth of the universe. These strings would have had a tremendous influence on the formation of large-scale objects such as the stars, galaxies, and clusters of galaxies that make up the physical universe. If they are real, cosmic strings can help answer a problem that has long puzzled scientists: how galaxies of billions of stars and galaxy clusters containing millions of galaxies were formed. What is amazing is that these large-scale structures are not randomly distributed throughout the universe but seem to be organized in some sort of orderly pattern. The fact that large-scale structures are found in orderly clusters throughout the cosmos has long interested astronomers and cosmologists. A possible answer to this mystery may lie in the existence of cosmic strings.

Cosmic strings were first described by the British physicist T. W. B. Kibble in 1976. According to some grand unification theories (GUTs), which are theories that try to describe the origins of everything in the universe as part of a single process, these strings were formed in the first billionth of a second after the big bang. In this first fraction of a second, when temperatures were in the hundreds of billions of degrees, all matter was combined in an object the size of a grapefruit. At that time, all the forces of nature, including gravity, the strong and weak nuclear forces, and the electromagnetic force, were unified in a single, primal force. This symmetry, or unity, of forces was broken by a gigantic explosion that blasted apart this primal force and grapefruit-sized solid mass into all the constituent parts of the universe, including the hundreds of elementary particles that make up atoms, nuclei, and all matter.

As the cosmos expanded, it cooled. Eventually, some of the atoms and particles released by the big bang began to freeze, and matter began to solidify. Among the very first objects formed were cosmic strings, at least according to the model described by Kibble. The creation of strings can be likened to the formation of cracks in ice in a lake as it freezes, or to defects in a flawed diamond as it solidifies under immense pressure.

To understand the process of string formation, imagine water freezing into ice in a pond or a lake. As water solidifies into ice, its weight and atomic structure change, and it becomes more orderly. Ice is made of long, regularly arranged atoms and molecules all facing in a specific direction, whereas in water, these particles are flowing in several directions at the same time. The entire lake does not turn into ice at exactly the same time, however. Water transforms into ice through several phases as the temperature approaches the freezing point; in a large body of water such as a lake, different sections turn into ice at varying rates. As these parts freeze at different intervals, defects or cracks are formed along the lines where solid ice and not-quite-frozen water meet. Much the same type of defect-and-crack formation took place as the early universe cooled and matter literally froze into existence.

The temperature continued to fall after the original unity of all forces was shattered and the primal force exploded into subforces, each becoming a clear and separate force of nature. Gravity, the force that attracts bodies of matter to one another, separated out a billionth of a second before the others. The strong nuclear force was the next to separate, followed by electromagnetism and the weak nuclear force. These separations and creations spread across the universe at almost the speed of light, but defects and cracks followed because of the vast distances involved. The universe is billions and billions of light-years from one end to the other. Thus, the unity and symmetry were broken, but in different places and at slightly different times. Again, as in a lake, different parts froze and crystallized at different times, and just as with the ice, different parts did not link up perfectly with the adjacent, not-yet-frozen areas of the cosmos. These lines of disunity are the areas of cosmic-string formation.

If one looks closely at ice in a lake, one can see that it is crisscrossed with thin white lines, fractures that mark where ice crystals have matched up imprecisely with others because they completed their phase transition, or froze, at different times. Another example of this kind of defect can be seen in a flawed diamond, the parts of which crystallized at slightly different times. The universe may have these same kinds of line defects and flaws: cosmic strings.

The cosmic strings form either straight lines or closed loops, billions of light-years long or around but less than a billionth of a billionth of a billionth of a centimeter wide—essentially one-dimensional. They are enormously heavy, each one weighing about ten million billion tons. One centimeter of cosmic string would weigh as much as the entire range of the Rocky Mountains. The strings contain an immense amount of energy, trapped within them when they were formed in the earliest fractions of a second after the cosmos began expanding. They are so huge and stretch so far into the endless reaches of space that the gravity radiating from them attracts massive quantities of matter and actually bends light waves around them.

In the early 1980s, Yakov B. Zel'dovich, a physicist at the Institute for Physical Problems in what was then the Soviet Academy of Sciences in Moscow, suggested that cosmic strings could explain the clumping of matter throughout the cosmos. The most important moment in the formation of galaxies was when the universe was only a few tens of thousands of years old. At this period of early history, electrons and photons began to combine to form atomic hydrogen. This happened as radiation pressure was reduced enough to allow gravity to pull the elementary particles together. According to Zel'dovich's proposal, the loops of cosmic strings present during this time would have acted as seeds around which matter—first only hydrogen atoms, then much more complex combinations—began to condense. This condensation would have led to the birth of matter, large scale and small, and then the consolidation of that matter into billions of large-scale structures. However, calculations have since determined that cosmic strings most likely contributed to the formation of less than 10 percent of such structures.

Applications

Cosmic strings might provide the answer to a question that has puzzled scientists since it was first raised in the 1930s by the American astronomer Fritz Zwieback. According to his calculations, the clusters at the edge of the known universe were moving so fast that gravity alone could not possibly hold them together. Some investigators speculated that only large amounts of unseen matter, which they called dark matter, could account for holding these clusters together. Dark matter, if it exists, might account for around 27 percent of the mass-energy of the universe, while dark energy—a hypothetical form of energy that accelerates the expansion of the universe—is believed to account for another 68 percent. On the other hand, the power might come from cosmic strings. Either way, something must be holding large-scale objects together, or the stars would be crashing into each other with wild abandon. The mystery has yet to be solved, but the theory of cosmic strings provides one possibility for describing the visible orderliness in the location of matter and large-scale objects in the cosmos.

As the universe continues to expand, cosmic strings race through space at almost the speed of light. According to scientists, if the strings really do exist, the friction of their movement through space produces gravitational radiation in the form of gravitational waves, which can be measured by their impact on the timing of pulsar emissions of radioactivity. Pulsars are the remains of stars that have collapsed into extremely bright and very dense objects called neutron stars; they rotate at very high speeds and give off radiation in regularly timed beams, or pulses, with intervals ranging from less than 0.01 second to 8.5 seconds. As a cosmic string passes by a pulsar, the resulting gravitational waves should create fluctuations in the rhythm of the pulse.

Cosmic strings were originally thought to be unrelated to string theory due to the vast differences in scale; cosmic strings are massive, while the strings described by string theory are presumed to be in the neighborhood of the Planck length, approximately 1.6 x 10^-35 meters. While the fundamental strings of string theory could have been stretched to such immense proportions during the inflationary period of the early universe, physicists determined that they would have disintegrated, diluted, or collapsed as a result of this expansion.

However, following the so-called second superstring revolution of 1995, which unified the various existing string theories into one overarching model known as M-theory, physicists began to consider anew whether cosmic strings and M-theory could be similarly unified. One reason for this was the introduction into string theory of the concept of branes (short for "membranes," from which the M of M-theory was originally derived). Branes are theoretical physical objects that exist throughout space-time and may manifest in various forms, including points, lines (strings), and planes, depending on how many dimensions they occupy—potentially as many as nine. M-theory allows for branes occupying the higher dimensions to be spatially extended in one dimension and compacted in the others; such branes could theoretically manifest as cosmic strings.

Context

When the theory of cosmic strings was first proposed, it provided scientists with one possible explanation for the emergence of the orderly collection of stars, galaxies, and superclusters that make up the known universe. Cosmic strings, if they exist, were created in the earliest fraction of a second of the universe and contain gigantic quantities of matter and energy, perhaps much of the matter that seems to be "missing." The large-scale structure of the universe somewhat resembles a giant sponge, with solid parts mixed together with empty space, though on a huge scale. Most of space, perhaps as much as 95 percent of it, is empty—but the part that is matter, the remaining 5 percent, is arranged in fairly regularly spaced structures.

Galaxies are evidently not just thrown together in a haphazard manner. Galaxies of billions of stars clump together to form galaxy clusters, which clump together to form superclusters. Researchers have concluded that these clusters are moving too fast to be held together simply by gravity. There must be another force holding them together, because the clusters do not have enough mass to account for this fact by themselves. The immense mass of a cosmic string provides one possible answer to this mystery.

Principal terms

BIG BANG THEORY: the theory that the universe began with a giant explosion about fourteen billion years ago

COSMOLOGY: the study of the origins and evolution of the universe as an orderly process

ELEMENTARY PARTICLES: the fundamental particles out of which all matter is made

GALAXY: a huge collection of stars, numbering in the billions or hundreds of billions

GALAXY CLUSTER: a group of hundreds or thousands of galaxies extending over an area of millions of light-years

GRAND UNIFIED THEORY (GUT): an attempt to explain the electromagnetic, weak nuclear, and strong nuclear interactions with consistent model

LARGE-SCALE STRUCTURES: giant accumulations of matter that make up galaxies and clusters of galaxies

PRIMAL FORCE: the original unified force out of which the elementary particles and the four interactions of nature exploded at the big bang

PULSAR TIME: the rhythmical beat of a star that sends out radiation at regular intervals

SUPERCLUSTER: a group of perhaps one hundred or more galaxy clusters

SYMMETRY: the original state of the cosmos, in which all forces and powers were unified and in balance

Bibliography

Cornell, James. Bubbles, Voids, and Bumps in Time: The New Cosmology. New York: Cambridge UP, 1989. Print. A popularly written, scientifically accurate work that describes the amazing facts being discovered and speculated upon by scientists looking into the basic questions about the meaning and the origins of the universe. Well illustrated. Index.

Erdmenger, Johanna, ed. String Cosmology: Modern String Theory Concepts from the Big Bang to Cosmic Structure. Weinheim: Wiley, 2009. Print.

Gibbons, G. W., S. W. Hawking, and T. Vachaspati, eds. The Formation and Evolution of Cosmic Strings. New York: Cambridge UP, 1990. Print. Contains the sometimes-technical proceedings of a conference on the formation of cosmic strings. Thirty experts present their views on how strings came into being, changed, and influenced the large-scale structures of the cosmos. Drawings and an index.

Gorbunov, Dmitry S., and Valery A. Rubakov. Introduction to the Theory of the Early Universe: Hot Big Bang Theory. Singapore: World Scientific, 2011. Print.

Kaku, Michio. Hyperspace: A Scientific Odyssey through Parallel Universes, Time Warps, and the Tenth Dimension. New York: Oxford UP, 1994. Print. A relatively easy-to-understand discussion of the physics of superstrings, with a brief review of the history of cosmic strings. Some useful illustrations and drawings. Contains interviews with many of the scientists who developed the new view of cosmology and the formation of large-scale structures.

Khlopov, Maxim. Fundamentals of Cosmic Particle Physics. Cambridge: Cambridge Intl. Sci., 2012. Print.

Layzer, David. Cosmogenesis: The Growth of Order in the Universe. New York: Oxford UP, 1990. Print. An analysis of most of the grand unification theories (GUTs) that contains brief discussion of the possible existence and significance of cosmic strings. Well illustrated, though some of the language is technical. Includes a useful bibliography and an index.

Peat, F. David. Superstrings and the Search for the Theory of Everything. Chicago: Contemporary, 1988. Print. Includes a brief review of the most important developments in cosmology, astronomy, and string theory and a chapter devoted to cosmic strings. Notes and index.

Shlaer, Benjamin, Alexander Vilenkin, and Abraham Loeb. "Early Structure Formation from Cosmic String Loops." Journal of Cosmology and Astroparticle Physics 2012.5 (2012): 1–26. Web. 16 Jan. 2014.

Silk, Joseph. The Big Bang. Rev. ed. New York: Freeman, 1989. Print. An excellent volume on the origins of the universe, with many drawings, photographs, and nontechnical descriptions of the processes at work in the earliest milliseconds of the universe. Contains a brief description of the theory of cosmic strings. Index.

Silk, Joseph. A Short History of the Universe. New York: Scientific Amer. Lib., 1994. Print. A well-illustrated account of the origins and history of the universe. Traces the physical history of the cosmos and describes the rules and laws that govern matter and the expansion of the universe. Includes an index and some marvelous photographs and drawings.

Smoot, George, and Keay Davidson. Wrinkles in Time. New York: Morrow, 1993. Print. A description of the discoveries made by the COBE satellite and how they have confirmed some theories of the origins of the cosmos. Written in a nontechnical language and meant for students and nonexperts with an interest in cosmology and astronomy. Briefly discusses cosmic strings. Many drawings, photographs, charts, and illustrations. Index and bibliography.

Trefil, James. The Moment of Creation: Big Bang Physics from before the First Millisecond to the Present Universe. New York: Macmillan, 1984. Print. An older but still useful depiction of the physical processes involved in the grand explosion out of which everything came. Written for a popular audience by an eminent scientist known for his ability to make great ideas of science easily understood. Well illustrated. Index.

Weinberg, Steven. The First Three Minutes: A Modern View of the Origin of the Universe. New York: Basic, 1977. Print. A book by a nonscientist that describes the fantastic chain of events following the big bang. A millisecond-by-millisecond account of the creation of gravity, matter, and energy. Very well written and understandable.