Beyond one billion light-years, when
averaged over 30 Mpc, the visible
Universe can be seen as a gas of
galaxies, uniformly distributed. At
smaller spatial scales, astronomical
observations and dedicated numerical
simulations have shown that the repartition
of the luminous matter is not so
homogeneous.
The three-dimensional distribution of
galaxies in the today Universe is indeed
characterised by a complex network of
filamentary structures which delineates
spherical regions of about 100 million
light-years in diameter devoid of objects,
suggesting a sponge-like or cell-like
topology for the underlying matter density
field. The finite age of the Universe
and the low enough peculiar velocity of
galaxies imply that information about
the initial conditions are preserved at
such large spatial scales. Characterising
the properties of the galaxy clustering
puts therefore strong constraints on the
theoretical models for the formation of
structures under the influence of gravity
in an expanding Universe known to be
dominated by dark matter and energy
components. Identifying elongated
structures like filaments, which might
only occupy 10% of the volume, and
measuring their statistical properties
would allow one to go beyond the information
provided by the usual two-point
correlation function measurements
which suffer from degeneracies with
respect to the topology.
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