The discovery of primordial gravitational waves through cosmic microwave background (CMB) polarization B-modes remains one of the most significant objectives in cosmology, as it would provide the missing piece of evidence predicted by cosmic inflation. Given the intrinsic weakness of the B-mode signal, precise calibration and detailed knowledge of the instruments are crucial to achieving the ambitious scientific goals set by current and next-generation CMB experiments. To address such calibration challenges, the Dept. of Physics of the University of Milano-Bicocca hosts the CMB-CAL 2024 Workshop from November 4 to 8, 2024, fully supported by the ERC POLOCALC project.

The CMB, the relic radiation that permeates the observable universe, serves as key evidence for the Big Bang theory and carries invaluable information about the evolution of the cosmos. In the early universe, a dense, hot plasma of subatomic particles created an opaque environment. As the universe expanded and cooled, protons and electrons combined into neutral atoms. Unlike plasma, these atoms could no longer scatter thermal radiation through Thomson scattering, rendering the universe transparent in a phase known as the recombination epoch. This marked the release of photons that could travel freely through space. Over time, these photons were stretched by the universe’s expansion, leading to the cosmological redshift observed in the CMB today—a near-perfect blackbody emission at 2.725 K.

While the CMB appears remarkably uniform, it exhibits faint temperature and polarization anisotropies, which can be detected by highly sensitive instruments. Numerous experiments over the past decades have mapped these variations in great detail. These anisotropies arise from interactions between matter and photons before the universe became transparent, forming a distinctive pattern that varies across different angular scales. The angular power spectrum of these patterns provides a powerful statistical tool to measure fundamental cosmological parameters.

Although the power spectrum of temperature anisotropies is now well characterized, extracting the weak signals of polarization anisotropies remains a significant experimental challenge. Polarization maps can be divided into two components: E-modes and B-modes. E-modes are associated with the same density perturbations that caused temperature anisotropies, while B-modes, which are far weaker, are generated by gravitational waves from the inflationary epoch. Additionally, gravitational lensing of E-modes can create secondary B-mode polarization. Other astrophysical effects, such as diffuse galactic emissions, Cosmic Birefringence, and Faraday rotation, can also contribute to B-modes. Moreover, spurious B-modes can be introduced by systematic effects in the complex instrumentation used in CMB telescopes.

To detect the primordial B-modes, it is crucial to carefully account for contamination from astrophysical sources and instrumental systematics, which currently dominate these faint signals. Accurate calibration is therefore essential in many areas, including gain calibration, beam characterization, polarization angle determination, telescope pointing, detector performance, and bandpass calibration. Furthermore, understanding and mitigating galactic foregrounds and instrumental systematics are vital for isolating the primordial signal.

The CMB-CAL 2024 Workshop at the University of Milano-Bicocca focuses on the current state and future directions of calibration methods in CMB experiments. Topics will include gain calibration, telescope pointing, beam characterization, polarization angle determination, detector calibration, and bandpass calibration, with particular attention to the challenges posed by galactic foregrounds and the potential measurement of Cosmic Birefringence. Special emphasis will be placed on advanced calibration technologies, such as far-field sources, wire grids, and holography setups, and how these methods interrelate to achieve unprecedented precision. The workshop features a mix of presentations and hands-on sessions, fostering discussion on these critical topics. Contributions from workshop participants will be compiled into this Research Topic, which welcomes all manuscript types accepted by Frontiers in Astronomy and Space Sciences, reflecting the diverse approaches and topics covered during the event. For more information please contact spacesciences@frontiersin.org.

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