*Cosmological and Extragalactic Science Cases for Incompatible Versions of
the ngVLA*
We present three science cases and their associated radio array
requirements:
(1) Extragalactic proper motions enable measurements of the secular
extragalactic parallax, transverse peculiar velocities of galaxies, the
evolution of the baryon acoustic oscillation, and the primordial
gravitational wave background. This science requires 8000 km baselines and
50,000 sq. m collecting area (10x the VLBA) at 8 GHz.
(2) A formaldehyde deep field provides a mass-limited survey of molecular
gas across the history of star formation and galaxy evolution (*z* = 0–7).
This requires a 100 km array with at least 0.1 K brightness temperature
sensitivity at 2–10 GHz (10x the VLA collecting area).
(3) The cosmological acceleration can be measured directly by monitoring
the drift of molecular absorption lines over time. This is “light bucket”
science that needs to maximize collecting area while minimizing atmospheric
and array-based spectroscopic systematics. Measuring *ż* would best be
done with a compact large-area array at 40 or 100 GHz.
The arrays required by these science goals are mutually incompatible. The
reference design ngVLA that includes long baselines may be a reasonable
compromise for extragalactic proper motions and a formaldehyde deep field,
but it may not be capable of the parts-per-trillion precision needed to
measure the redshift drift.
