SUPERCONDUCTING COSMIC
STRINGS.
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.
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