Understanding the observed astrophysical phenomena is among the problems of modern cosmology. Surveys of galaxies and clusters of galaxies show the visible matter distribution in our neighbouhood. Other observations suggest the Universe should be filled with another, unvisible and yet-unknown (non-baryonic), type of matter. Flashes of very high energy photons suddenly appear in the sky on a daily basis in the form of gamma ray bursts, coming from unidentified source of cosmological distribution. Detection of the cosmological microwave background anisotropies over very different angular scales seem to imply peculiar properties of the relic photon trajectories from the time they were emitted until they reached us, although the origin of these primordial perturbations is not clear. Weird particles hit the Earth atmosphere with considerable energies, yielding secondary particle showers that have been known as the Ultra Hight Energy Cosmic Ray (UHECR) enigma.
For all those observations, to mention only a few, conventional explainations just don't work. Standard mechanisms are not always sufficient to explain whatever the Universe has to show, and in some cases, they are simply unable to provide even a hint on what the sources could be. Moreover, there are fundamental problems standard cosmology can't handle: for instance, why is the microwave background, whose very existence is a pillar of cosmology, appears so isotropic in opposite directions in the sky, and this even though the regions they map have never been in causal contact ? Another problem: how is it that the density of our Universe is so close to the critical density ? Slightly more, and expansion would stop rapidely and the Universe would enter a crunsh; slightly less, and it would be expanding forever. Besides, evolution is unstable near this critical value ! This means that in order that it has the observed value now, which is reasonnably close to critical, it must have been much closer when the Universe was a second young: a relative precision of some 10-15 is required. How could it reach that precision level ? Just a few more questions awaiting answers.
The inflationary paradigm, first proposed by Alan Guth in 1980, attempts to answers these questions. It is based on a very rapid, quasi exponential, expansion during the first moments of the Universe, some 10-35-10-32 seconds after the Big-Bang. This very attractive idea, elegantly explaining many of the abovementionned difficulties, does not however explain everything. In particular, some of the parameters need be quite unnaturally fine-tuned in order for the subsequent structure formation to be compatible with observations.
Cosmic topological defects make the link between particle physics, cosmology, and daily physics that can be achieved in the lab: condensed matter physics. They provide explainations concerning the very early Universe phenomena in terms of analogies with systems well studied in various experiments. In fact, if present ideas based on extension of the standard particle model (whose success needs not be recalled) are correct, then topological defects are an unavoidable consequence. Besides, they possess interesting astrophysical and cosmological properties which motivated investigations on their capabilities to cure some of the previously discussed problems in a more economical fashion.
Among these topological defects, cosmic strings are presumably the most fashionable. It should be remarked here that they are not to be confused with fundamental strings some theories propose to describe elementary particles. Cosmic strings would be very thin lines of primordial matter, not existing anymore anywhere else, containing incredible amounts of energy, and moving in the Universe at velocities close to that of light, curving space as they do so. Because of their peculiar gravitational properties, they can in particular accrete the surrounding matter until they form a structure as observed today. Moreover, they would bend light trajectories as galaxies and quasars do. Besides, they could perturb the microwave background radiation at both the time of decoupling between matter and radiation, some tens of thousands of years after the Big-Bang, and still now, thereby affecting the photon motion all the way through. The rapid motion of the string network yields a very complicated structure consisting of loops whose decay could have filled the Universe with a gravitational radiation background whose detection might be someday feasible.
And if such a phenomenology was not interesting enough, it turns out that there are very well motivated models in which, at some stage of their evolution, cosmic strings can develop enormous current. They therefore become like gigantic superconducting wires that, again, could help understanding the various phenomena yet unexplained. In particular, those currents would be of primary importance for generating primordial magnetic fields (another mystery !) over protogalactic scales. Moreover, the motion of the charge carriers in the loops could allow them, thanks to centrifugal forces, not to decay completely, leaving behind a remnant density of small charged loop that have been called vortons. If they are stable (a point currently under great debate), one could envisage that they are still around, forming part of the dark matter.
Topological defects in cosmology, D.A.M.T.P., Cambridge University.