Unlocking the synergy between CMB spectral distortions and anisotropies
  Measurements of the cosmic microwave background (CMB) spectral distortions
(SDs) will open a new window on the very early universe, providing new
information complementary to that gathered from CMB temperature and
polarization anisotropies. In this paper, we study their synergy as a function
of the characteristics of the considered experiments. In particular, we examine
a wide range of sensitivities for possible SD measurements, spanning from FIRAS
up to noise levels 1000 times better than PIXIE, and study their constraining
power when combined with current or future CMB anisotropy experiments such as
Planck or LiteBIRD plus CMB-S4. We consider a number of different cosmological
models such as the $\Lambda$CDM, as well as its extensions with the running of
the scalar spectral index, the decay or the annihilation of dark matter (DM)
particles. While upcoming CMB anisotropy experiments will be able to decrease
the uncertainties on inflationary parameters such as $A_s$ and $n_s$ by about a
factor 2 in the $\Lambda$CDM case, we find that an SD experiment 100 times more
sensitive than PIXIE (comparable to the proposed Super-PIXIE satellite) could
potentially further contribute to constrain these parameters. This is even more
significant in the case of the running of the scalar spectral index.
Furthermore, as expected, constraints on DM particles decaying at redshifts
probed by SDs will improve by orders of magnitude even with an experiment 10
times worse than PIXIE as compared to CMB anisotropies or Big Bang
Nucleosynthesis bounds. On the contrary, DM annihilation constraints will not
significantly improve over CMB anisotropy measurements. Finally, we forecast
the constraints obtainable with sensitivities achievable either from the ground
or from a balloon.
