Cosmic microwave background spectral distortions

One important scenario relates to spectral distortions from early energy injection, for instance, by decaying particles, primordial black hole evaporation or the dissipation of acoustic waves set up by inflation.

In this process, the baryons heat up and transfer some of their excess energy to the ambient CMB photon bath via Compton scattering.

CMB spectral distortions therefore provide a powerful probe of early-universe physics and even deliver crude estimates for the epoch at which the injection occurred.

distortions, can be created by photon injection processes, relativistic electron distributions and during the gradual transition between the

[6][7] These findings constitute one of the most important pillars of Big Bang cosmology, which predicts the blackbody nature of the CMB.

However, as shown by Zeldovich and Sunyaev, energy exchange with moving electrons can cause spectral distortions.

These treated the thermalization problem including Compton scattering and the Bremsstrahlung process for a single release of energy.

In 1982, the importance of double Compton emission as a source of photons at high redshifts was recognized by Danese and de Zotti.

Modern considerations of CMB spectral distortions started with the works of Burigana, Danese and de Zotti and Hu, Silk and Scott in the early 1990s.

After COBE/FIRAS provided stringent limits on the CMB spectrum, essentially ruling out distortions at the level

In 2011, PIXIE[8] was proposed to NASA as a mid-Ex satellite mission, providing first strong motivation to revisit the theory of spectral distortions.

Although no successor of COBE/FIRAS has been funded so far, this led to a renaissance of CMB spectral distortions with numerous theoretical studies and the design of novel experimental concepts [9] In the cosmological 'thermalization problem', three main eras are distinguished: the thermalization or temperature-era, the

-era, each with slightly different physical conditions due to the change in the density and temperature of particles caused by the Hubble expansion.

In the very early stages of cosmic history (up until a few months after the Big Bang), photons and baryons[10] are efficiently coupled by scattering processes and, therefore, are in full thermodynamic equilibrium.

This allows the photon field to quickly relax back to a Planckian distribution, even if for a very short phase a spectral distortion appears.

Observations today cannot tell the difference in this case, as there is no independent cosmological prediction for the CMB monopole temperature.

CDM cosmology, the adiabatic cooling of matter and dissipation of acoustic waves set up by inflation cause a

-distortion constraint, the first limits on the small-scale power spectrum could be obtained well-before direct measurements became possible [14] At redshifts

-distortion stems from the cumulative cluster SZ signal, which provides a way to constrain the amount of hot gas in the Universe.

[16] The classical studies mainly considered energy release (i.e., heating) as a source of distortions.

However, recent work has shown that richer signals can be created by direct photon injection and non-thermal electron populations, both processes that appear in connection with decaying or annihilating particles.

All these effects could allow us to differentiate observationally between a wide range of scenarios, as additional time-dependent information can be extracted.

About 280,000 years after the Big Bang, electrons and protons became bound into electrically neutral atoms as the Universe expanded.

In cosmology, this is known as recombination and preludes the decoupling of the CMB photons from matter before they free stream throughout the Universe around 380,000 years after the Big Bang.

Within the energy levels of hydrogen and helium atoms, various interactions take place, both collisional and radiative.

Since the distortion signal arises from the hydrogen and two helium recombination eras, this gives us a unique probe of the pre-recombination Universe that allows us to peek behind the last scattering surface that we observe using the CMB anisotropies.

While this is considered to be an ‘easy’ target, the cosmological recombination radiation (CRR), as the smallest expected signal, has an amplitude that is another factor of

A detection of the LCDM distortions therefore requires novel experimental approaches that provide unprecedented sensitivity, spectral coverage, control of systematics and the capabilities to accurately remove foregrounds.

Building on the design of FIRAS and experience with ARCADE, this has led to several spectrometer concepts to observe from space (PIXIE, PRISM, PRISTINE, SuperPIXIE and Voyage2050),[8][2] balloon (BISOU) and the ground (APSERa and Cosmo at Dome-C, TMS at Teide Observatory).

As an ultimate frontier, a full characterization and exploitation of the cosmological recombination signal could be achieved by using a coordinated international experimental campaign, potentially including an observatory on the moon [17] In June 2021, the European Space Agency unveiled its plans for the future L-class missions as part of Voyage 2050 with a chance for `high precision spectroscopy` for the new early universe part of their strategy, opening the door for spectral distortions telescopes for the future.

Image to show how the distortions transition from a temperature redistribution to the mu distortion to the y distortion as time carries on, with the recombination radiation appearing around 280,000 years after the Big Bang.
Spectral distortions at different cosmological epochs. At very early times, with redshift , any injection of energy emerges as a temperature shift in the black body. As the age of the Universe increases, the processes that lead to thermalization of CMB distortions to a blackbody become less efficient (bremsstrahlung and double Compton scattering when , Compton scattering when ). The spectral distortions also interplay with distinguished epochs of cosmic history such as reionization , recombination and Big Bang nucleosynthesis as shown. Specifically, during the recombination epoch ( years after the Big Bang), the cosmological recombination lines are imprinted on the CMB as a result of non-equilibrium atomic processes during that era [ 2 ]
Movie showing how a smooth almost-inverse parabolic shape at very early redshifts bigger than 1 million compare to a distortion with a unique bumpy shape known as a mu distortion as redshifts get lower, until a redshift of 10 thousand, where this spline gets much sharper and we call this a y distortion. Over time, this shape becomes more pronounced as the energy injection epoch changes from early times to late times in the Universe's growth
The spectral distortion in the cosmic microwave background (CMB) looks different depending on the moment in the universe's history where this black body was modified. At very early times where , any injection of energy emerges as a temperature shift in the black body. If the energy injection is later (still very early in the Universe's history), we see the shape of the - distortion, whereas we can see a sharper fluctuation at later times, associated with -distortion. Here some energy is injected into the CMB at time defined by the redshift with the resultant distortion being plotted.