The evolution of CMB spectral distortions in the early Universe (1109.6552v1)

Abstract: The energy spectrum of the cosmic microwave background (CMB) allows constraining episodes of energy release in the early Universe. In this paper we revisit and refine the computations of the cosmological thermalization problem. For this purpose a new code, called CosmoTherm, was developed that allows solving the coupled photon-electron Boltzmann equation in the expanding, isotropic Universe for small spectral distortion in the CMB. We explicitly compute the shape of the spectral distortions caused by energy release due to (i) annihilating dark matter; (ii) decaying relict particles; (iii) dissipation of acoustic waves; and (iv) quasi-instantaneous heating. We also demonstrate that (v) the continuous interaction of CMB photons with adiabatically cooling non-relativistic electrons and baryons causes a negative mu-type CMB spectral distortion of DI_nu/I_nu ~ 10^{-8} in the GHz spectral band. We solve the thermalization problem including improved approximations for the double Compton and Bremsstrahlung emissivities, as well as the latest treatment of the cosmological recombination process. At redshifts z <~ 10^3 the matter starts to cool significantly below the temperature of the CMB so that at very low frequencies free-free absorption alters the shape of primordial distortions significantly. In addition, the cooling electrons down-scatter CMB photons introducing a small late negative y-type distortion at high frequencies. We also discuss our results in the light of the recently proposed CMB experiment Pixie, for which CosmoTherm should allow detailed forecasting. Our current computations show that for energy injection because of (ii) and (iv) Pixie should allow to improve existing limits, while the CMB distortions caused by the other processes seem to remain unobservable with the currently proposed sensitivities and spectral bands of Pixie.

• The paper introduces CosmoTherm, a tool that solves the photon-electron Boltzmann equation to enhance predictions of minute CMB distortions.
• It rigorously quantifies energy release scenarios, including annihilating dark matter, decaying particles, and acoustic wave dissipation leading to negative μ-type distortions.
• It evaluates the observational prospects for missions like Pixie, providing sharper constraints on early Universe processes and particle properties.

Analyzing CMB Spectral Distortions in the Early Universe

The paper at hand, authored by J. Chluba and R. A. Sunyaev, revolves around a comprehensive exploration of the spectral distortions in the Cosmic Microwave Background (CMB) within the early Universe. The primary focus is on refining computations of the cosmological thermalization problem through a new computational tool, named {\sc CosmoTherm}, designed to solve the coupled photon-electron Boltzmann equation in the framework of an isotropic, expanding Universe. This investigation addresses the intricacies of small spectral distortions that may arise due to varying mechanisms of energy release at specific cosmological epochs.

Several scenarios for energy release are rigorously quantified in terms of the resultant spectral distortions on the CMB; these include annihilating dark matter, decaying relic particles, and the dissipative effects of acoustic wave dissipation. Notably, the paper also explores the negative distortions caused by the continuous interaction of CMB photons with adiabatically cooling electrons and baryons, as well as the implications of quasi-instantaneous heating events.

A significant result of the paper is the assessment of a {\it negative} $\mu$-type CMB spectral distortion at the order of $\Delta I_\nu/I_\nu\sim 10^{-8}$, occurring in the GHz spectral band. This distortion originates from the cooling matter processes and is of potential interest for upcoming CMB spectral measurements, such as those proposed by the {\sc Pixie} mission. The computations indicate that much of the CMB cooling induced distortion occurs well below the current observable limits.

The authors also provide detailed insight into potential observational impacts of spectral distortions due to dissipation of acoustic waves. The claimed sensitivities for the {\sc Pixie} experiment propose the possibility of enhancing current limits, particularly for cases arising due to decaying particles or sudden heating. However, scenarios involving annihilating dark matter are presented as less observable within the presently proposed capabilities of {\sc Pixie}.

The transition from $\mu$-type to $y$-type distortions is systematically traced through redshifts $z\lesssim 10^3$, when matter cooling deviates significantly from the CMB temperature. This deviation alters the primordial distortion's shape appreciably, especially at low frequencies due to free-free absorption and leads to a small late {\it negative} $y$-type distortion at high frequencies.

The paper's implications span both theoretical and practical frameworks within cosmology and astrophysics. The refined computation approach opens pathways for precise forecasting of CMB spectral distortions, potentially enabling the differentiation between competing models of early Universe phenomena and providing sharper constraints on parameters such as dark matter properties or the inflationary mechanism's characteristics.

In summary, while this paper does not claim breakthroughs in the observation of CMB distortions, it clearly advances the theoretical groundwork necessary for interpreting future spectral data and predicting minor spectral anomalies in the CMB. The paper's analytic approach coupled with numerical implementations provides a robust foundation for evaluating minute CMB distortion phenomena that may become detectable with the advancement of observational technology and methodologies in the future.