The cosmic microwave background (CMB) has been an immense source of information about the Universe we live in. Not only has its detection in 1964 forged one of the main pillars of Big Bang Cosmology, but it also stimulated the first theoretical works on the CMB anisotropies in the early 1970's, which then defined the activities of the CMB community for the last few decades. The CMB, to leading order, is a uniform radiation field coming from the early phases of the expanding Universe that today is visible at microwave frequencies. Fluctuations in the density of the matter imprint tiny temperature and polarization anisotropies, which have been extensively studied, most recently using ESA's Planck Satellite. These observations clearly cemented the standard LCDM concordance model, which successfully describes cosmological observations over a vast range of scales.
However, another invaluable piece of information is provided by the energy distribution of the CMB photons, the so-called CMB spectrum. With COBE/FIRAS it was shown that the CMB spectrum is extremely close to that of a perfect blackbody at a temperature of 2.725 K, an observation that was awarded the Nobel Prize in Physics in 2006. This provided additional very strong support of the Big Bang picture, which naturally starts in a hot and dense phase that allows blackbody radiation to form. Importantly, even within LCDM we expect tiny departures of the CMB spectrum from that of a blackbody. These signals are commonly referred to as CMB spectral distortions and teach us about the thermal history of the Universe.
The main objective of CMBSPEC is to study CMB spectral distortion in the early Universe. Since COBE/FIRAS in the early 1990's, no serious observational attempt was made to measure the CMB energy spectrum more precisely. However, technologically we have now reached a stage that should allow us to detect some of the predicted LCDM distortion signals. In addition, our theoretical understanding of how CMB spectral distortions form has evolved significantly since the earliest predictions in the 1970's. The CMBSPEC team is thus working hard to facilitate new observations of the CMB spectrum by answering important questions about
1) the formation of distortions in the early Universe
2) how to analyze and interpret future distortion measurements
3) which experimental concepts will provide the most stringent limits
4) how can we overcome and deal with spectral distortions foregrounds
CMB spectral distortions are extremely small and heavily obscured by galactic and extra-galactic foregrounds. We have extensive experience with CMB foregrounds from observations of the CMB anisotropies, and several aspects directly translate to CMB spectral distortions. However, the dynamic range encountered in signal to foreground is huge and innovative methods have to be developed to extract the distortions. In addition, systematic effects are very different given that for CMB spectral distortion measurements an absolute calibration or at least pristine channel-intercalibration is needed. The work of the CMBSPEC team provides important input for the experimental design and optimization of CMB spectrometers and how these can be combined with planned CMB imagers to overcome the challenges.
Another important question is how spectral distortions form and evolve in the early Universe. The CMBSPEC team has shown that the distortion signals directly depend on when in cosmic history the distortion was imprinted. In addition, there are differences in distortion signals that are created by heating and photon injection. The details of how to link the shape and amplitude of the distortion to specific processes are crucial for the interpretation of future distortion data and my team is working intensively on understanding how to model and analyze distortions from various mechanism. One of the big hopes and drivers ultimately is to learn about primordial inflation and dark matter by studying CMB spectral distortion, thus targeting highly fundamental questions about the Universe we live in.
Through CMBSPEC, the team has achieved several important milestones. CMB spectral distortions are now widely recognized as an important target for the future. BISOU, a french-led balloon-borne CMB spectrometer is set to start Phase A in 2024, promising the first new measurements of the CMB spectrum since COBE/FIRAS by the end of this decade. A number of ground-based spectrometers (COSMO and TMS) are also progressing towards measurements of distortions and the long-term prospects for a CMB space mission are extremely promising. The team has furthermore opened a novel direction in CMB distortion science, for the first time treating the full spectro-spatial thermalization problem in the perturbed universe, defining new spectral distortion targets for Litebird and CMB-S4.
