Bridging the gap: spectral distortions meet gravitational waves
  Gravitational waves (GWs) have the potential to probe the entirety of
cosmological history due to their nearly perfect decoupling from the thermal
bath and any intervening matter after emission. In recent years, GW cosmology
has evolved from merely being an exciting prospect to an actively pursued
avenue for discovery, and the early results are very promising. As we highlight
in this paper, spectral distortions (SDs) of the cosmic microwave background
(CMB) uniquely probe GWs over six decades in frequency, bridging the gap
between astrophysical high- and cosmological low-frequency measurements. This
means SDs will not only complement other GW observations, but will be the sole
probe of physical processes at certain scales. To illustrate this point, we
explore the constraining power of various proposed SD missions on a number of
phenomenological scenarios: early-universe phase transitions (PTs), GW
production via the dynamics of SU(2) and ultra-light U(1) axions, and cosmic
string (CS) network collapse. We highlight how some regions of parameter space
were already excluded with data from COBE/FIRAS, taken over two decades ago. To
facilitate the implementation of SD constraints in arbitrary models we provide
GW2SD. This tool calculates the window function, which easily maps a GW
spectrum to a SD amplitude, thus opening another portal for GW cosmology with
SDs, with wide reaching implications for particle physics phenomenology.
