Which spectral distortions does $\Lambda$CDM actually predict?
  Ever refined cosmological measurements have established the $\Lambda$CDM
concordance model, with the key cosmological parameters being determined to
percent-level precision today. This allows us to make explicit predictions for
the spectral distortions of the cosmic microwave background (CMB) created by
various processes occurring in the early Universe. Here, we summarize all
guaranteed CMB distortions and assess their total uncertainty within
$\Lambda$CDM. We also compare simple methods for approximating them,
highlighting some of the subtle aspects when it comes to interpreting future
distortion measurements. Under simplified assumptions, we briefly study how
well a PIXIE-like experiment may measure the main distortion parameters (i.e.,
$\mu$ and $y$). Next generation CMB spectrometers are expected to detect the
distortion caused by reionization and structure formation at extremely high
significance. They will also be able to constrain the small-scale power
spectrum through the associated $\mu$-distortion, improving limits on running
of the spectral index. Distortions from the recombination era, adiabatic
cooling of matter relative to the CMB and dark matter annihilation require a
higher sensitivity than PIXIE in its current design. The crucial next step is
an improved modeling of foregrounds and instrumental aspects, as we briefly
discuss here.
