2 research outputs found
Characterizing the non-linear growth of large-scale structure in the Universe
The local Universe displays a rich hierarchical pattern of galaxy clusters
and superclusters. The early Universe, however, was almost smooth, with only
slight 'ripples' seen in the cosmic microwave background radiation. Models of
the evolution of structure link these observations through the effect of
gravity, because the small initially overdense fluctuations attract additional
mass as the Universe expands. During the early stages, the ripples evolve
independently, like linear waves on the surface of deep water. As the
structures grow in mass, they interact with other in non-linear ways, more like
waves breaking in shallow water. We have recently shown how cosmic structure
can be characterized by phase correlations associated with these non-linear
interactions, but hitherto there was no way to use that information to reach
quantitative insights into the growth of structures. Here we report a method of
revealing phase information, and quantify how this relates to the formation of
a filaments, sheets and clusters of galaxies by non-linear collapse. We use a
new statistic based on information entropy to separate linear from non-linear
effects and thereby are able to disentangle those aspects of galaxy clustering
that arise from initial conditions (the ripples) from the subsequent dynamical
evolution.Comment: Accepted for publication in Nature. For high-resolution Figure 3,
please see http://www.nottingham.ac.uk/~ppzpc/phases/n0colorphase.html, For
the animations and the idea of this paper please see
http://www.nottingham.ac.uk/~ppzpc/phases/index.htm
A Critical Study of Daniel Defoe's Verse
I review the standard paradigm for understanding the formation and evolution
of cosmic structure, based on the gravitational instability of dark matter, but
many variations on this basic theme are viable. Despite the great progress that
has undoubtedly been made, steps are difficult because of uncertainties in the
cosmological parameters, in the modelling of relevant physical processes
involved in galaxy formation, and perhaps most fundamentally in the
relationship between galaxies and the underlying distribution of matter. For
the foreseeable future, therefore, this field will be led by observational
developments allowing model parameters to be tuned and, hopefully, particular
scenarios falsified. In these lectures I focus on two ingredients in this class
of models: (i) the role of galaxy bias in interpreting clustering data; and
(ii) the statistical properties of the initial fluctuations. In the later case,
I discuss some ideas as to how the standard assumption - that the primordial
density fluctuations constitute a Gaussian random field - can be tested using
measurements galaxy clustering and the cosmic microwave background.Comment: 30 pages, 2 figures. To appear in Proceedings of International School
of Physics `D. Chalonge' "Phase Transitions in the Early Universe: Theory and
Observations", Erice, December 2000, Eds. H.J.de Vega, I.Khalatnikov,
N.Sanchez. Uses kluwer.cls; igh resolution Figure 1 courtesy of the Virgo
Consortium can be found at
http://www.mpa-garching.mpg.de/Virgo/virgopics.htm