CMB Spectral Distortions: Status and Prospects

Abstract: Departures of the energy spectrum of the cosmic microwave background (CMB) from a perfect blackbody probe a fundamental property of the universe -- its thermal history. Current upper limits, dating back some 25 years, limit such spectral distortions to 50 parts per million and provide a foundation for the Hot Big Bang model of the early universe. Modern upgrades to the 1980's-era technology behind these limits enable three orders of magnitude or greater improvement in sensitivity. The standard cosmological model provides compelling targets at this sensitivity, spanning cosmic history from the decay of primordial density perturbations to the role of baryonic feedback in structure formation. Fully utilizing this sensitivity requires concurrent improvements in our understanding of competing astrophysical foregrounds. We outline a program using proven technologies capable of detecting the minimal predicted distortions even for worst-case foreground scenarios.

CMB Spectral Distortions: Status and Prospects

The white paper on Cosmic Microwave Background (CMB) spectral distortions presents a detailed examination of the current understanding and future possibilities around the spectral distortions of the CMB, which serve as fundamental probes into the universe's thermal history. These distortions offer crucial insights into the various phases of cosmic evolution, from the decay of primordial density fluctuations to the processes governing galaxy formation. The paper underscores the limitations and advancements required in observational technology to improve sensitivity to these spectral deviations.

Numerical Results and Key Findings

The current upper limit on CMB spectral distortions dates back approximately 25 years to the measurements made by the Far Infrared Absolute Spectrophotometer (FIRAS). These constraints are set at $|y| < 15 \times 10^{-6}$ and $|\mu| < 9 \times 10^{-5}$, correlating to deviations of less than 50 parts per million. The paper argues that with modern technologies, specifically detectors operating at lower temperatures, it is possible to achieve improvements in sensitivity by three orders of magnitude. Preliminary designs, like the Primordial Inflation Explorer (PIXIE), propose instrument modifications capable of detecting spectral distortions with unprecedented precision.

Implications of Advances in Sensitivity

Enhanced sensitivity to CMB spectral distortions would provide powerful tests for the standard cosmological model. It could probe a range of phenomena including:

• Long-lived dark matter decay, axion-photon interactions, and gravitino decay.
• The role of baryonic feedback in structure formation.
• Primordial non-Gaussianities and small-scale perturbations leading to scenarios like those producing LIGO mass primordial black holes.

Such advancements could transform the observational landscape of cosmology, offering insights into new physics beyond the standard model framework.

Challenges and Solutions for Improved Observational Techniques

The paper outlines how Fourier Transform Spectroscopy (FTS) is optimally suited for detecting CMB spectral distortions. It emphasizes the need for comprehensive foreground subtraction, as astrophysical foregrounds currently overshadow the sensitivity required to detect minimal spectral distortions. Here are some strategies discussed:

• Utilizing data below 100 GHz to resolve foreground ambiguities related to synchrotron, free-free, and anomalous microwave emissions, which overlap significantly with CMB signals.
• Implementing improvements in data modeling that leverage spatial-frequency correlations along with spectral information to enhance foreground separation.

Future Developments and Mission Outlook

The white paper suggests that order-of-magnitude improvements necessitate space missions, as terrestrial observatories are limited by atmospheric interference. Concepts like the PIXIE mission architecture provide a feasible blueprint for these high-sensitivity observations. The mission design includes several modules, each optimized with specific frequency binners for targeted observations, to isolate CMB distortions from strong foreground signals.

Conclusion

The potential to differentiate and measure CMB spectral distortions with high precision is an exciting frontier in cosmological research. By leveraging modern detector technology and improved foreground subtraction methodologies, future missions can significantly advance our understanding of cosmic evolution. There is a tangible opportunity to test theoretical predictions on inflation and cosmic structure formation across vastly unexplored physical scales, paving the way for substantial discoveries in fundamental physics.