The cosmic microwave background (CMB) is microwave radiation from the Big Bang which permeates the entire observable universe. Inflation, a brief period of exponential expansion in the early universe, is predicted to have imprinted an extremely faint polarization signature, known as the primordial B-modes, on the CMB. Detecting these B-modes would provide observational evidence for the theory of inflation. The Q and U Bolometric Interferometer for Cosmology (QUBIC) is a telescope which is observing the CMB with the goal of detecting the primordial B-modes. In QUBIC, a cold reflective optical combiner superimposes the re-emitted beams from an array of aperture feedhorns onto an array of detectors, forming a multi-peaked beam pattern which varies with frequency. This allows QUBIC to have precise control over instrument systematics and to remove signals from astronomical foregrounds which would otherwise obscure the faint polarization signal. This work investigates how uncertainties in the beam pattern affect the analysis of observations made by QUBIC, and to what precision the beam must be known to measure the B-modes to a given accuracy. We have modelled the QUBIC beam pattern at multiple frequencies, centered on 150 GHz, using precise electromagnetic mode matching and physical optics (PO) simulations that include the effects of optical aberrations. These simulations are then used in QUBIC’s simulation software to observe a CMB sky and obtain the time ordered data produced. QUBIC’s data processing pipeline is then used to reconstruct the sky from this data. For the reconstruction a different beam may be used to investigate the effect on the mapmaking process. The beams used in reconstruction include beams without optical aberrations and beams with slightly changed peak positions, as well as an idealised beam from an analytical formula. Finally, we can produce the power spectra for both the original sky and the reconstructed sky to assess the differences.
Flood, A., O'Sullivan, C., Gradziel, M., Gayer, D., Marwede, S., Scóccola, C., et al. (2026). The effect of QUBIC beam uncertainty on the reconstruction of the CMB B-mode power spectrum. In Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy XIII SPIE Astronomical Telescopes + Instrumentation [10.1117/12.3101037].
The effect of QUBIC beam uncertainty on the reconstruction of the CMB B-mode power spectrum
Gervasi, M.;Zannoni, M.
2026
Abstract
The cosmic microwave background (CMB) is microwave radiation from the Big Bang which permeates the entire observable universe. Inflation, a brief period of exponential expansion in the early universe, is predicted to have imprinted an extremely faint polarization signature, known as the primordial B-modes, on the CMB. Detecting these B-modes would provide observational evidence for the theory of inflation. The Q and U Bolometric Interferometer for Cosmology (QUBIC) is a telescope which is observing the CMB with the goal of detecting the primordial B-modes. In QUBIC, a cold reflective optical combiner superimposes the re-emitted beams from an array of aperture feedhorns onto an array of detectors, forming a multi-peaked beam pattern which varies with frequency. This allows QUBIC to have precise control over instrument systematics and to remove signals from astronomical foregrounds which would otherwise obscure the faint polarization signal. This work investigates how uncertainties in the beam pattern affect the analysis of observations made by QUBIC, and to what precision the beam must be known to measure the B-modes to a given accuracy. We have modelled the QUBIC beam pattern at multiple frequencies, centered on 150 GHz, using precise electromagnetic mode matching and physical optics (PO) simulations that include the effects of optical aberrations. These simulations are then used in QUBIC’s simulation software to observe a CMB sky and obtain the time ordered data produced. QUBIC’s data processing pipeline is then used to reconstruct the sky from this data. For the reconstruction a different beam may be used to investigate the effect on the mapmaking process. The beams used in reconstruction include beams without optical aberrations and beams with slightly changed peak positions, as well as an idealised beam from an analytical formula. Finally, we can produce the power spectra for both the original sky and the reconstructed sky to assess the differences.
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