SPECTER: An Instrument Concept for CMB Spectral Distortion Measurements with Enhanced Sensitivity

TL;DR

SPECTER introduces a novel, absolutely-calibrated, multi-band photometric instrument concept to measure CMB spectral distortions with a focus on robust detection of the ΛCDM μ-distortion. The approach uses Fisher forecasting to optimize independent frequency bands and detector counts, marginalizing foregrounds, and leveraging a flexible 1–2000 GHz band set across low-frequency HEMT radiometers, mid-frequency bolometers, and high-frequency bolometers. The study predicts μ detection at about 5σ after foreground marginalization for 1 year (10σ for 4 years), and sub-percent precision on the y-distortion with relativistic corrections, enabling strong constraints on Silk damping and the thermal history of baryons. A key contribution is the demonstration that an absolute-calibration strategy with configurable bands can outperform traditional Fourier-transform spectrometers in achieving high μ-sensitivity, while also outlining calibration, sky-model robustness, and design considerations for a feasible future mission. The work provides open-source tools for optimization and highlights calibration and foreground-modeling challenges as critical pathways for realizing μ-distortion science in the coming decades.

Abstract

Deviations of the cosmic microwave background (CMB) energy spectrum from a perfect blackbody uniquely probe a wide range of physics, ranging from fundamental physics in the primordial Universe ($μ$-distortion) to late-time baryonic feedback processes ($y$-distortion). While the $y$-distortion can be detected with a moderate increase in sensitivity over that of COBE/FIRAS, the $Λ$CDM-predicted $μ$-distortion is roughly two orders of magnitude smaller and requires substantial improvements, with foregrounds presenting a serious obstacle. Within the standard model, the dominant contribution to $μ$ arises from energy injected via Silk damping, yielding sensitivity to the primordial power spectrum at wavenumbers $k \approx 1-10^{4}$ Mpc$^{-1}$. Here, we present a new instrument concept, SPECTER, with the goal of robustly detecting $μ$. The instrument technology is similar to that of LiteBIRD, but with an absolute temperature calibration system. Using a Fisher approach, we optimize the instrument's configuration to target $μ$ while marginalizing over foreground contaminants. Unlike Fourier-transform-spectrometer-based designs, the specific bands and their individual sensitivities can be independently set in this instrument, allowing significant flexibility. We forecast SPECTER to observe the $Λ$CDM-predicted $μ$-distortion at $\approx 5σ$ (10$σ$) assuming an observation time of 1 (4) year(s) (corresponding to mission duration of 2 (8) years), after foreground marginalization. Our optimized configuration includes 16 bands spanning 1-2000 GHz with $\sim$degree-scale angular resolution at $\sim150$ GHz and 1100 total detectors. SPECTER will additionally measure the $y$-distortion at sub-percent precision and its relativistic correction at percent-level precision, yielding tight constraints on the total thermal energy and mean temperature of ionized gas.

Figures (15)

• Figure 1: Sky signals used in our Fisher forecasts. Foregrounds are shown with blue curves. Galactic dust and CIB (dotted) dominate at high frequencies, while synchrotron (dot-dashed) dominates at low frequencies, with additional contributions from free-free, AME, and CO emission (dot-dashed). CMB spectral distortions are also plotted as labeled in the legend, with negative (positive) values indicated by dashed (solid) curves.
• Figure 2: Visualization of the band optimization process. The SNR$_{\mu}$ and sensitivities in this plot are computed assuming 1 detector per band for bolometers and 4 detector per band for HEMT amplifiers and $t_{\rm obs}=6$ months. Top: The SNR$_{\mu}$ at each iteration step in the band-optimization process for several example set-ups using different minimum frequencies. We start with a set of narrow initial bands and at each step combine a pair that will have the most optimal effect on SNR$_{\mu}$. The sensitivity increases with increasing bandwidth, but once the frequency resolution is too low, the SNR$_{\mu}$ drops. Bottom: Comparison between the initial bands $\nu_{i}$ and the optimized configuration, $\nu_{f}$. Note the scale on the y-axis: the noise is many orders of magnitude larger at the highest frequencies due to the photon loading from the high-frequency foregrounds.
• Figure 3: Visualization of the detector optimization: SNR$_{\mu}$ versus focal plane area for a grid of 5,062,500 instrument configurations. The points are colored by the sum of the number of detectors in the six lowest-frequency bands ($<43$ GHz, see Table \ref{['tab:fid_setup']}), since the low-frequency detectors are the largest in area and therefore drive the size and cost of the instrument. The dotted line marks SNR$_{\mu}= 5$. Our chosen configuration is marked with a diamond marker (red).
• Figure 4: SPECTER sensitivity (blue/cyan circles with horizontal bars for optimized 16-band/34-band multichroic configuration assuming $t_{\rm obs} = 1$ year) plotted in comparison to the total foregrounds (dotted blue), CMB spectral distortions (green) including $\mu$ (darkest shade), relativistic tSZ (intermediate shade), and $y$ (lightest shade), with positive (negative) values shown in solid (dashed), and the sensitivities for several other missions: PIXIE (dot-dashed black), Voyage 2050 (magenta), and COBE/FIRAS (purple). We use the PIXIE noise from A17 scaled to a 12-month duration. The SPECTER and PIXIE sensitivities assume a full-sky observation here. For Voyage 2050, we use the nominal sensitivity from Ref. Voyage2050.
• Figure 5: Left: Ratio between the total CMB signal (including the distortions) and the total foreground signal. The CMB clearly dominates at $\sim10-800$ GHz, while foregrounds take over at $>800$ GHz. The CMB signal is still greater than the foregrounds at the lowest frequencies, but to a lesser extent than at the intermediate frequencies. Right: The ratio of the total foregrounds between 1-40 GHz to the total foregrounds at the reference frequencies labeled in the legend. The largest difference is a factor of $\sim 10$, which suggests that calibration requirements could potentially be relaxed at the lowest frequencies as compared to the CMB-dominated frequencies, by roughly this amount.