  We present the various science cases for building Band 1 receivers as part of
ALMA's ongoing Development Program. We describe the new frequency range for
Band 1 of 35-52 GHz, a range chosen to maximize the receiver suite's scientific
impact. We first describe two key science drivers: 1) the evolution of grains
in protoplanetary disks and debris disks, and 2) molecular gas in galaxies
during the era of re-ionization. Studies of these topics with Band 1 receivers
will significantly expand ALMA's Level 1 Science Goals. In addition, we
describe a host of other exciting continuum and line science cases that require
ALMA's high sensitivity and angular resolution. For example, ALMA Band 1
continuum data will probe the Sunyaev-Zel'dovich Effect in galaxy clusters,
Very Small Grains and spinning dust, ionized jets from young stars, spatial and
flaring studies of Sgr A*, the acceleration sites of solar flares, pulsar wind
nebulae, radio supernovae, and X-ray binaries. Furthermore, ALMA Band 1 line
data will probe chemical differentiation in cloud cores, complex carbon chain
molecules, extragalactic radio recombination lines, masers, magnetic fields
through Zeeman effect measurements, molecular outflows from young stars, the
co-evolution of star formation and active galactic nuclei, and the molecular
content of galaxies at z ~ 3. ALMA provides similar to better sensitivities
than the JVLA over 35-50 GHz, with differences increasing with frequency.
ALMA's smaller antennas and shorter baselines, greater number of baselines, and
single-dish capabilities, however, give it a significant edge for observing
extended emission, making wide-field maps (mosaics), or attaining high image
fidelity, as required by the described science cases.
