As an alternative to solving
the computationally demanding lens equations,
Cen et al. [49]
developed an efficient scheme to
identify regions with surface densities capable of
generating multiple images accurately for
splittings larger than three arcseconds.
They applied this technique to a standard CDM model
with 
, and found that this model predicts
more large angle splittings (> 8'')
than are known to exist in the observed Universe.
Their results suggest that the standard CDM
model should be excluded as a viable model of our Universe.
A similar analysis for a flat low density CDM model
with a cosmological constant
( 
, 
) suggests a
lower and perhaps acceptable number of lensing events.
However, an uncertainty in their studies is the nature of
the lenses at and below the resolution of the numerical grid.
They model the lensing structures as simplified
Singular Isothermal Spheres (SIS) with no
distinctive cores.
Large angle
splittings are generally attributed to larger structures
such as clusters of galaxies, and there are indications
that clusters have small but finite core radii
kpc. Core radii of
this size can have an important effect on the
probability of multiple imaging.
Flores and Primack [66]
considered the effects of assuming two
different kinds of splitting sources:
isothermal spheres with small but finite
core radii 
,
and spheres with a Hernquist density
profile 
,
where kpc and
kpc.
They find that the computed frequency
of large-angle splittings, when using the
nonsingular profiles, can potentially decrease by
more than an order of
magnitude relative to the SIS case and can bring
the standard CDM model into better agreement
with observations.
They stress that lensing events are sensitive
to both the cosmological model (essentially the number
density of lenses) and to the inner lens structure,
making it difficult to probe the models until the
structure of the lenses, both observationally
and numerically, is better known.
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Computational Cosmology: from the Early Universe to the Large Scale Structure Peter Anninos http://www.livingreviews.org/lrr-2001-2 © Max-Planck-Gesellschaft. ISSN 1433-8351 Problems/Comments to livrev@aei-potsdam.mpg.de |