Mapping Large-Scale-Structure Evolution over Cosmic Times
  This paper outlines the science case for line-intensity mapping with a
space-borne instrument targeting the sub-millimeter (microwaves) to the
far-infrared (FIR) wavelength range. Our goal is to observe and characterize
the large-scale structure in the Universe from present times to the high
redshift Epoch of Reionization. This is essential to constrain the cosmology of
our Universe and form a better understanding of various mechanisms that drive
galaxy formation and evolution. We argue that the proposed frequency range
would make it possible to probe important metal cooling lines such as [CII] up
to very high redshift as well as a large number of rotational lines of the CO
molecule. These can be used to trace molecular gas and dust evolution and
constrain the buildup in both the cosmic star formation rate density and the
cosmic infrared background (CIB). Moreover, surveys at the highest frequencies
will detect FIR lines which are used as diagnostics of galaxies and AGN.
Tomography of these lines over a wide redshift range will enable invaluable
measurements of the cosmic expansion history at epochs inaccessible to other
methods, competitive constraints on the parameters of the standard model of
cosmology, and numerous tests of dark matter, dark energy, modified gravity and
inflation. To reach these goals, large-scale structure must be mapped over a
wide range in frequency to trace its time evolution over a reasonable fraction
of the volume of the observable Universe. In addition, the surveyed area needs
to be very large to beat cosmic variance and to probe the largest scales where
its easier to separate the astrophysical and cosmological contributions to the
observed signal. Only, a space-borne mission can properly meet these
requirements.
