  While the upcoming telescopes will reveal correspondingly fainter, more
distant galaxies, a question will persist: what more is there that these
telescopes cannot see? One answer is the source-subtracted Cosmic Infrared
Background (CIB). The CIB is comprised of the collective light from all sources
remaining after known, resolved sources are accounted for. Ever-more-sensitive
surveys will identify the brightest of these, allowing them to be removed, and
- like peeling layers off an onion - reveal deeper layers of the CIB. In this
way it is possible to measure the contributions from populations not accessible
to direct telescopic observation. Measurement of fluctuations in the
source-subtracted CIB, i.e., the spatial power spectrum of the CIB after
subtracting resolved sources, provides a robust means of characterizing its
faint, and potentially new, populations. Studies over the past 15 years have
revealed source-subtracted CIB fluctuations on scales out to ~100' which cannot
be explained by extrapolating from known galaxy populations. Moreover, they
appear highly coherent with the unresolved Cosmic X-ray Background, hinting at
a significant population of accreting black holes among the CIB sources.
Characterizing the source-subtracted CIB with high accuracy, and thereby
constraining the nature of the new populations, is feasible with upcoming
instruments and would produce critically important cosmological information in
the next decade. New coextensive deep and wide-area near-infrared, X-ray, and
microwave surveys will bring decisive opportunities to examine, with high
fidelity, the spatial spectrum and origin of the CIB fluctuations and their
cross-correlations with cosmic microwave and X-ray backgrounds, and determine
the formation epochs and the nature of the new sources (stellar nucleosynthetic
or accreting black holes).
