  Recent hydrodynamic simulations of galaxy formation reveal streams of cold (T
~ 1e4 K) gas flowing into the centers of dark matter halos as massive as
1e12-1e13.5 M_sun at redshifts z~1-3. In this paper we show that if > 20% of
the gravitational binding energy of the gas is radiated away, then the
simulated cold flows are spatially extended Lyman Alpha (Lya) sources with
luminosities, Lya line widths, and number densities that are comparable to
those of observed Lya blobs. Furthermore, the filamentary structure of the cold
flows can explain the wide range of observed Lya blob morphologies. Since the
most massive halos form in dense environments, the association of Lya blobs
with overdense regions arise naturally. We argue that Lya blobs - even those
which are clearly associated with starburst galaxies or quasars - provide
direct observational support for the cold accretion mode of galaxies. We
discuss various testable predictions of this association.
