Spectral Distortions of the CMB as a Probe of Inflation, Recombination, Structure Formation and Particle Physics (1903.04218v2)

Abstract: Following the pioneering observations with COBE in the early 1990s, studies of the cosmic microwave background (CMB) have focused on temperature and polarization anisotropies. CMB spectral distortions - tiny departures of the CMB energy spectrum from that of a perfect blackbody - provide a second, independent probe of fundamental physics, with a reach deep into the primordial Universe. The theoretical foundation of spectral distortions has seen major advances in recent years, which highlight the immense potential of this emerging field. Spectral distortions probe a fundamental property of the Universe - its thermal history - thereby providing additional insight into processes within the cosmological standard model (CSM) as well as new physics beyond. Spectral distortions are an important tool for understanding inflation and the nature of dark matter. They shed new light on the physics of recombination and reionization, both prominent stages in the evolution of our Universe, and furnish critical information on baryonic feedback processes, in addition to probing primordial correlation functions at scales inaccessible to other tracers. In principle the range of signals is vast: many orders of magnitude of discovery space could be explored by detailed observations of the CMB energy spectrum. Several CSM signals are predicted and provide clear experimental targets, some of which are already observable with present-day technology. Confirmation of these signals would extend the reach of the CSM by orders of magnitude in physical scale as the Universe evolves from the initial stages to its present form. The absence of these signals would pose a huge theoretical challenge, immediately pointing to new physics.

Analysis of CMB Spectral Distortions as Probes in Cosmology

The paper presents a comprehensive exploration of Cosmic Microwave Background (CMB) spectral distortions as a novel probe in understanding the Universe's thermal history and fundamental physics. By examining deviations from the CMB's ideal blackbody spectrum, the paper advances our knowledge beyond traditional anisotropy analyses, revealing pivotal information on early Universe phenomena, including inflation, recombination, and structure formation.

Theoretical Insights and Observational Framework

This white paper emphasizes the role of CMB spectral distortions in extending the reach of cosmological models. Spectral distortions offer insights into the thermal history of the Universe, providing new avenues to explore the standard cosmological model (CSM) and potential physics beyond it. The paper suggests that analyses of CMB spectra can offer additional constraints on parameters such as the Universe's density, expansion rate, and baryon content.

Contrary to purely temperature-based anisotropy studies, spectral distortions, including $\mu$-type (chemical potential) and $y$-type (Compton y-distortion) models, probe different epochs. These distortions capture energy releases from mechanisms like photon injections and baryonic matter heating, indicators of crucial cosmological processes that occur post-recombination.

Probing Physics Beyond the Standard Model

Significantly, the paper postulates that spectral distortions could illuminate the origins of primordial density perturbations, thereby providing evidence for or against inflationary scenarios. Historically, primordial perturbations underpin modern structure formation; however, their origins remain speculative. CMB analyses, particularly the constrained amplitude of density perturbations, could thus highlight the shortcomings or extensions required in the $\Lambda$CDM model.

Additionally, distortions provide methodologies to probe new physics. For instance, extensions beyond the Standard Model of particle physics and candidates for dark matter, such as axions or primordial black holes, are addressable within this framework. This exploration elevates spectral distortions from mere theoretical constructs to practical strategies for unraveling unexplained cosmological features.

Implications and Future Prospects

Given the current technological advancements and proposed instruments like PIXIE, the paper outlines a promising future for CMB spectral analyses. Modern detectors could significantly surpass past sensitivities, enabling the detection of weak distortions that were previously undetectable. Such advancements hold implications for forthcoming discoveries in particle physics, dark matter investigations, and understanding large-scale cosmic structures.

By progressing beyond the COBE measurements of the early '90s, researchers could achieve four orders of magnitude improvement in detecting energy releases, a leap that underscores the untapped potential of spectral distortion studies. The observed need for broad frequency coverage, particularly in space-borne observations, further highlights areas for future technological and methodological development.

Conclusion

The paper serves as a call to action for the cosmology community to leverage CMB spectral distortions as a new observational horizon. While theoretical challenges and technological hurdles remain, the prospects of significant contributions to our understanding of the universe affirm the importance of pursuing this line of inquiry. By doing so, spectral distortions might soon occupy a central role in cosmological investigations, complementing traditional paradigms and probing realms that remain shrouded in theoretical uncertainties.