The Impossibly Early Galaxy Problem
  The current hierarchical merging paradigm and $\Lambda$CDM predict that the
$z \sim 4-8$ universe should be a time in which the most massive galaxies are
transitioning from their initial halo assembly to the later baryonic evolution
seen in star-forming galaxies and quasars. However, no evidence of this
transition has been found in many high redshift galaxy surveys including
CFHTLS, CANDELS and SPLASH, the first studies to probe the high-mass end at
these redshifts. Indeed, if halo mass to stellar mass ratios estimated at
lower-redshift continue to $z \sim 6-8$, CANDELS and SPLASH report several
orders of magnitude more $M \sim 10^{12-13} M_\odot$ halos than are possible to
have formed by those redshifts, implying these massive galaxies formed
impossibly early. We consider various systematics in the stellar synthesis
models used to estimate physical parameters and possible galaxy formation
scenarios in an effort to reconcile observation with theory. Although known
uncertainties can greatly reduce the disparity between recent observations and
cold dark matter merger simulations, even taking the most conservative view of
the observations, there remains considerable tension with current theory.
