Unveiling the Cosmic History of Gas and Dust with Broad ALMA Bandwidths

In search of our Cosmic Origins, molecular deep fields have provided an unprecedented view on the evolution of the cosmic molecular gas through time. These spectral-scan mosaics, conducted on the worlds largest interferometers (e.g., the 200h ALMA/ASPECS large program), map the total gas and dust content of the universe and have, in the last decade, revealed its evolution similar to the cosmic density of star formation for the first time. Yet, it has also become clear that further progress is limited with current facilities, unless significantly larger time investments are made. The 2-4x increase in total bandwidth from ALMA Wideband Sensitivity Upgrade is an critical development in this regard: as the efficiency of spectral scan surveys scales linearly with bandwidth, a 4x increase in bandwidth will be instrumental to enable the next-generation of molecular deep fields and spectral scan surveys. The large instantaneous bandwidths will also critically improve the dust continuum sensitivity. For both nearby and distant galaxies, a 4x larger bandwidth will enable a near complete spectroscopic inventory in a single tuning. Including the overall gains in sensitivity from upgrading the ALMA signal-chain, the WSU will greatly enhance the efficiency ALMA in unveiling gas and dust in galaxies through cosmic time.