Cosmic string theory introduced
According to big bang theory, for the first 10 - 43 seconds of the universe's life, all the forces of nature existed as one superforce. The universe was all energy, no matter. Physicists call this symmetry. In the next tiny ticks of time individual forces appeared -- first gravity, then the strong nuclear force -- ending the symmetry state. Within minutes elementary particles and atomic nuclei had formed. But it took another 700,000 years for the first atoms to form.
Early theories proposed that matter began to lump together as the universe cooled, ultimately forming stars and galaxies. But this didn't seem right to some scientists. There hadn't really been enough time for all that to happen. Also, the cosmic background radiation discovered in 1965, considered a virtual echo of the big bang, was completely uniform throughout the sky, ruling out the notion that irregularities in the initial bang could account for the distribution of matter.Why then is the universe so inconsistent, with some places jammed with matter and others apparently empty?
In 1976, physicist Thomas Kibble was working on mathematical models of that fraction of a second when individual forces were taking shape out of the "superforce." His model suggested that the rapid cooling after the explosion of the universe caused flaws that were stringlike -- not unlike the cracks formed when water freezes into ice. Kibble described these as slender strands (skinnier than a proton) of very concentrated mass-energy. These cosmic strings could stretch the length of the universe. A piece of this string only 1.6 kilometers long would weigh more than the earth.
Others have added to the theory: That symmetry still exists within strings. That strings evolve when vibrations cause part of a string to snap off, so that now strings of any size may exist. It is thought that strings oscillate near the speed of light and give off gravitational waves (predicted by Einstein's theory of general relativity), ripples in space-time. They have a finite lifespan, since by vibrating and giving off energy, they ultimately disappear. Perhaps there are none left. In any case, they would be widely dispersed through the universe.
Wild as they seem, cosmic strings could solve some of the questions about how an irregular universe came from a uniformly exploding plasma. Strings could be a form of dark matter which attracted matter to cohere. Other weird things in the universe, such as long sheetlike groups of galaxies or the "Great Attractor" toward which the Milky Way and other galaxies are drawn, as well as vast expanses of emptiness, may be explained by gravitational or magnetic fields caused by cosmic strings.