  We present a new model for the observed Lyman alpha blobs (LABs) within the
context of the standard cold dark matter model. In this model, LABs are the
most massive halos with the strongest clustering (proto-clusters) undergoing
extreme starbursts in the high-z universe. Aided by calculations of detailed
radiative transfer of Lya photons through ultra-high resolution (159pc)
large-scale (>30Mpc) adaptive mesh-refinement cosmological hydrodynamic
simulations with galaxy formation, this model is shown to be able to, for the
first time, reproduce simultaneously the global Lya luminosity function and
luminosity-size relation of the observed LABs. Physically, a combination of
dust attenuation of Lya photons within galaxies, clustering of galaxies, and
complex propagation of Lya photons through circumgalactic and intergalactic
medium gives rise to the large sizes and frequently irregular isophotal shapes
of LABs that are observed. A generic and unique prediction of this model is
that there should be strong far-infrared (FIR) sources within each LAB, with
the most luminous FIR source likely representing the gravitational center of
the proto-cluster, not necessarily the apparent center of the Lya emission of
the LAB or the most luminous optical source. Upcoming ALMA observations should
unambiguously test this prediction. If verified, LABs will provide very
valuable laboratories for studying formation of galaxies in the most overdense
regions of the universe at a time when global star formation is most vigorous.
