  Large galaxy redshift surveys have long been used to constrain cosmological
models and structure formation scenarios. In particular, the largest structures
discovered observationally are thought to carry critical information on the
amplitude of large-scale density fluctuations or homogeneity of the universe,
and have often challenged the standard cosmological framework. The Sloan Great
Wall (SGW) recently found in the Sloan Digital Sky Survey (SDSS) region casts
doubt on the concordance cosmological model with a cosmological constant (i.e.
the flat LCDM model). Here we show that the existence of the SGW is perfectly
consistent with the LCDM model, a result that only our very large cosmological
N-body simulation (the Horizon Run 2, HR2) could supply. In addition, we report
on the discovery of a void complex in the SDSS much larger than the SGW, and
show that such size of the largest void is also predicted in the LCDM paradigm.
Our results demonstrate that an initially homogeneous isotropic universe with
primordial Gaussian random phase density fluctuations growing in accordance
with the General Relativity, can explain the richness and size of the observed
large-scale structures in the SDSS. Using the HR2 simulation we predict that a
future galaxy redshift survey about four times deeper or with 3 magnitude
fainter limit than the SDSS should reveal a largest structure of bright
galaxies about twice as big as the SGW.
