Steady State Universe:
A cosmological model developed by Fred Hoyle, Thomas Gold and Hermann Bondi in 1948 as the main alternative to the standard Big Bang theory of the universe. Steady state theory holds that the universe is expanding but that new matter and new galaxies are continuously created in order to maintain the perfect cosmological principle (the idea that, on the large scale, the universe is essentially homogenous and isotropic in both space and time), and therefore has no beginning and no end. The theory was quite popular in the 1950s and 1960s, but fell out of favour with the discovery of distant quasars and cosmic background radiation in the 1960s.
String:
An object with a one-dimensional spatial extent, length (unlike an elementary particle which is zero-dimensional, or point-like). According to string theory, the different fundamental particles of the standard model can be considered to be just different manifestations of one basic object, a string, with different vibrational modes. The characteristic length scale of strings is thought to be on the order of the Planck Length (about 10-35 metres, still too small to be visible in current physical laboratories), the scale at which the effects of quantum gravity are believed to become significant.
Cosmic string is a similar but separate concept which refers to one-dimensional topological defects, extremely thin but immensely dense, which are hypothesized to have formed as a result of phase changes soon after the Big Bang (analogous to the imperfections that form between crystal grains in solidifying liquids or the cracks that form when water freezes into ice). According to some theories, such cosmic strings grew as the universe expanded and were instrumental in the accretion of matterand the formation of galaxy clusters and large-scale structures in the universe.
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