CMB spectral distortions
What are CMB spectral distortion? One of the important properties of the CMB is the distribution of photons in frequency or energy. This is referred to as the CMB energy spectrum or short the CMB spectrum, and should not be confused with the power spectrum of the CMB temperature and polarization anisotropies, which concerns the spatial variations of the photon flux in different directions. From the measurements with COBE/FIRAS in the 1990's we know that the CMB spectrum is represented by that of the blackbody at a temperature of T0 = 2.7255 K to extremely high precision . In recognition for this fundamental observation, John C. Mather was awarded the Nobel Prize in Physics 2006. This finding tells us that the Universe initially must have been extremely hot and dense, thus allowing thermalization processes to create the CMB equilibrium radiation field. However, several processes, both standard and non-standard, can actually cause departures from a perfect blackbody spectrum at a level below the sensitivity of COBE/FIRAS. These signals are referred to as CMB spectral distortions and they tell us about interactions between matter and radiation across cosmic time. Within the Voyage 2050 program, cosmologists have the unique opportunity to access this new source of information about fundamental physics of the Universe.
Figure 1: Evolution of spectral distortions across time. Distortions probe the thermal history over long periods deep into the primordial Universe that are inaccessible by other means. The distortion shape contains valuable epoch-dependent information that allows distinguishing different sources of distortions. Line-emission is created during the cosmological recombination eras leaving a detailed ’fingerprint’ of the recombination process.
Thermalization physics primer: The precise shape of the CMB energy spectrum encodes new information that can be extracted using absolute CMB spectroscopy. At redshifts z > 2 x 10^6, thermalization processes are extremely efficient and promptly restore a near perfect blackbody spectrum of the CMB if full thermal equilibrium was perturbed. However, at later epochs, starting a few months after the Big Bang, traces of energy-releasing or photon-injecting processes can be found by measurements of CMB spectral distortions (see Fig. 1).
While classically spectral distortions are described as a sum of μ- and y-type distortion signals [2–5], modern treatments of the problem have demonstrated that far more than just two numbers can be extracted [e.g, 6–10]. Measurements can constrain processes expected within ΛCDM including the damping of primordial perturbations and the cosmological recombination radiation, and thereby open the discovery space to the pre-recombination Universe, which cannot be accessed directly in any other way. The measurements with COBE/FIRAS still define the long-standing benchmark for CMB spectral distortions, but several orders of magnitude of sensitivity improvements are in principle possible, as envisioned for PIXIE [e.g., 11], the spectrometer of PRISM  and Super-PIXIE . These experimental concepts form the starting point for the spectrometer design put forward in the Voyage 2050 proposals.
What can we learn from CMB spectral distortions? The CMB energy spectrum is a probe of the thermal history, starting a few month (~10^6 second) after the Big Bang until the present. It is clear that the structure formation and reionization process a low redshifts leaves the largest expected imprint (Fig. 4). This signal is composed of the cumulative SZ flux from all clusters and hot gas in the Universe. Future spectrometers are so sensitive that they will also be able to constrain the average gas temperature, thus providing tight constraints on feedback models .
While the average low-redshift y-distortion provides a clear target for future distortion missions, the probably most exciting guaranteed signal withing reach arises from the damping of primordial acoustic modes at small scales through Silk-damping (Fig. 3). This distortion allows us to constrain the amplitude of perturbations at scales that are otherwise inaccessible by other means, and thus could provide very strong tests for the standard slow-roll inflation model [e.g., 15].
With a CMB spectrometer, we could furthermore probe new physics relating to primordial black holes, the nature of dark matter and dark energy, decaying particles, primordial magnetic fields, axions, gravitons and many other exciting processes, as outlined in the spectrometer white paper 1909.01593.
Figure 2: Forecast constraints on the primordial curvature power spectrum for features with a steep (i.e., k^4) profile that cuts off sharply at some larger wavenumber. The μ-distortions constrain perturbations at scales and levels inaccessible to other probes. Early-universe models with enhanced small-scale power at k ≃ 10 − 10^4 Mpc^−1 will be immediately ruled out if no distortion with μ > 2 × 10^−8 is detected.
Figure 3: Level of the expected CMB spectral distortion signals and cumulative astrophysical foregrounds. The estimated sensitivities for various mission concepts are illustrated as well as their channel distribution. The Super-PIXIE high and mid-frequency bands merged around ν ≃ 600 GHz. Within the ESA Voyage 2050 program, the ≃ 0.01 − 0.1 Jy/sr level could be targeted, yielding clear detections of μ ≃ 2 × 10^−8 and also the Cosmological Recombination Radiation.
Foreground Challenge: One of the biggest challenges for future CMB spectral distortion measurements stems from galactic and extra-galactic foregrounds (Fig. 3). The valuable spectral distortion signals are buried under many orders of magnitudes larger emission from thermal dust, synchrotron, free-free, spinning dust and other foregrounds. To tackle the component separation problem therefore calls for unprecedented sensitivity and broad spectral coverage. In addition, the combination of spatial and spectral information will be vital. The unique capabilities of modern Fourier-Transform Spectrometers provide the required control of systematics and flexibility to in principle meet these high experimental demands. However, many obstacles still lie in the way but we will learn a lot about galactic and extra-galactic science in combination with existing and proposed CMB imaging approaches. This could be realized in the Voyage 2050 program.