  If the Universe is dominated by cold dark matter and dark energy as in the
currently popular LCDM cosmology, it is expected that large scale structures
form gradually, with galaxy clusters of mass M > ~10^14 Msun appearing at
around 6 Gyrs after the Big Bang (z ~ 1). Here, we report the discovery of 59
massive structures of galaxies with masses greater than a few x 10^13 Msun at
redshifts between z=0.6 and 4.5 in the Great Observatories Origins Deep Survey
fields. The massive structures are identified by running top-hat filters on the
two dimensional spatial distribution of magnitude-limited samples of galaxies
using a combination of spectroscopic and photometric redshifts. We analyze the
Millennium simulation data in a similar way to the analysis of the
observational data in order to test the LCDM cosmology. We find that there are
too many massive structures (M > 7 x 10^13 Msun) observed at z > 2 in
comparison with the simulation predictions by a factor of a few, giving a
probability of < 1/2500 of the observed data being consistent with the
simulation. Our result suggests that massive structures have emerged early, but
the reason for the discrepancy with the simulation is unclear. It could be due
to the limitation of the simulation such as the lack of key, unrecognized
ingredients (strong non-Gaussianity or other baryonic physics), or simply a
difficulty in the halo mass estimation from observation, or a fundamental
problem of the LCDM cosmology. On the other hand, the over-abundance of massive
structures at high redshifts does not favor heavy neutrino mass of ~ 0.3 eV or
larger, as heavy neutrinos make the discrepancy between the observation and the
simulation more pronounced by a factor of 3 or more.
