Cosmic string

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A cosmic string (English "thread", "string") is a hypothetical, almost one-dimensional topological defect in space . So far, cosmic strings are only known in theory, there are no observations to prove their existence. From the theories about the evolution of the universe , however, one can at least infer the possibility of its existence.


Cosmic strings could have been formed in the short time after the Big Bang , when a GUT interaction split into the electroweak interaction and the strong interaction as a result of a symmetry break due to the cooling of the universe . This process does not have to take place uniformly in the entire universe, but could also have spread independently of one another in separate areas (bubbles). Cosmic strings and other defects arise when areas with different field states meet - a comparable model for this would be crystallization . The edge remains stable, while the rest of the phase transitiontakes place to the appropriate forces. The cosmic string is a one-dimensional edge, it consists of a field state that did not go through the phase transition.

A cosmic string would be shaped like an extremely thin, long, heavy thread. According to GUT models, the diameter of a string corresponds to only about a trillionth of a hydrogen atom , while its length can assume cosmological standards. A string only 6 kilometers long would have a mass roughly equivalent to the total mass of the earth.

In the early universe, cosmic strings could have been the seeds for the density fluctuations from which the large-scale structures (such as galaxy clusters and superclusters ) later formed. Today's more precise observations of the structure of the distribution of matter in the universe - directly through sky surveys, indirectly through observation of the background radiation - speak against a significant participation of cosmic strings in the structure formation.

The discovery of the double galaxy CSL-1 ("Capodimonte-Sternberg Lens candidate 1") had given new impetus to the acceptance of cosmic strings. The agreement of the spectra seemed to contradict the assumption of two independent galaxies , the undistorted shape excluded gravitational lensing effects from foreground galaxies . Subsequent observations by the Hubble Space Telescope in 2006 showed, however, that the two galaxies are actually a double galaxy and not, as initially assumed, a double image of the same galaxy.

Another possibility for the direct detection of cosmic strings are precision measurements of the cosmic background radiation. With the spatial resolution of the WMAP data, however, no clear assessment is possible; only future measurement results with the Planck satellite will be able to decide this question.

Cosmic Strings and String Theory

There is no direct connection between the theory of cosmic strings and string theory , the names form independent analogies to the thread (English "string"). However, work in string theory in the 2000s renewed interest in cosmic strings. In 2002 Henry Tye and co-workers discovered that in the model of brane inflation, cosmic strings are formed at the end of development. Joseph Polchinski showed that the expansion of the universe can stretch a fundamental string, i.e. an object of string theory, to intergalactic size. Such a “stretched” string would have many properties that the cosmic strings already examined have, which renewed interest in this work. Other objects from string theory, such as greatly elongated D-branes , so-called "D-strings", can also have properties similar to cosmic strings. The discovery of cosmic strings can therefore also provide information about string theory.

The cosmic strings from string theory would emit gravitational waves that could likely be observed by experiments like LIGO . They could also lead to small irregularities in the background radiation that have not yet been discovered but may be seen with future studies such as the Planck Space Telescope .

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  1. Sazhin, M. et al. (2003): CSL-1: chance projection effect or serendipitous discovery of a gravitational lens induced by a cosmic string? , Mon. Not. Roy. Astr. Soc. 343, 353 ( online )
  2. Sazhin, M. et al. (2006), The true nature of CSL-1 , astro-ph / 0601494
  3. Agol, E., Hogan, CJ & Plotkin, RM (2006), Hubble imaging excludes cosmic string lens , Phys. Rev. D 73, 087302 ( online )