The Cosmic Microwave Background Spectrum from the Full COBE/FIRAS Data Set

TL;DR

This work reports a refined FIRAS analysis of the full COBE dataset to tightly constrain spectral distortions of the cosmic microwave background. By enhancing calibration, deglitching, destriping, and Galactic contamination handling, the authors extract monopole and dipole spectra with high fidelity, confirming a blackbody CMBR at $T_0 = 2.728\pm0.004$ K and a Doppler-consistent dipole toward $(\ell,b)=(264.14^\circ,48.26^\circ)$. They place stringent 95% CL limits on Bose-Einstein ($|\mu|<9\times10^{-5}$) and Compton ($|y|<15\times10^{-6}$) distortions, thereby constraining energy release in the early universe. The results strengthen the standard cosmological model by showing the CMBR spectrum is exquisitely close to a blackbody across 2–21 cm$^{-1}$ and by quantifying the level of permissible deviations due to early-universe processes. Collectively, these findings provide critical benchmarks for theories of cosmic energy injection and the thermal history of the universe.

Abstract

We have refined the analysis of the data from the FIRAS (Far InfraRed Absolute Spectrophotometer) on board the COBE (COsmic Background Explorer). The FIRAS measures the difference between the cosmic microwave background and a precise blackbody spectrum. We find new tighter upper limits on general deviations from a blackbody spectrum. The RMS deviations are less than 50 parts per million of the peak of the CMBR. For the Comptonization and chemical potential we find $|y| < 15\times10^{-6}$ and $|μ| < 9\times10^{-5}$ (95\% CL). There are also refinements in the absolute temperature, 2.728 $\pm$ 0.004 K (95\% CL), and dipole direction, $(\ell,b)=(264.14^\circ\pm0.30, 48.26^\circ\pm0.30)$ (95\% CL), and amplitude, $3.372 \pm 0.007$ mK (95\% CL). All of these results agree with our previous publications.

Figures (6)

• Figure 1: $\chi^2$/DOF in the destriping process. a) $\chi^2$/DOF as a function of frequency for low frequency channels. b) $\chi^2$ distribution for pixels for low frequency data.
• Figure 2: Frequency of $C^+$ line as a function of Galactic longitude. Galactic rotation is clearly evident. Diamond shows low frequency measurement, which was used to set the frequency scale.
• Figure 3: Dipole spectrum and fit to $\frac{dB}{dT}$. Vertical lines indicate plus and minus one $\sigma$ uncertainties. Peak of uniform CMBR is approximately 400 MJy/sr.
• Figure 4: Uniform spectrum and fit to $Planck(T)$. Uncertainties are a small fraction of the line thickness.
• Figure 5: FIRAS measured CMBR residuals,(------) $I_0 - B_\nu(T_0) - \Delta T \frac{dB}{dT} - G_0 g(\nu)$. Spectrum model components: the maximum allowed distortions (95% CL) $y =$$15\times10^{-6}$ (-- -- --) and $|\mu| =$$9\times10^{-5}$ ($\cdots$); the Galaxy spectrum $g(\nu)$ scaled to one fourth the flux at the Galactic pole ($\cdot\;$--), and the effects of a 100 $\mu$K temperature shift in $T_0$, $0.0001\* \frac{dB}{dT}$, ($\cdots\;$--).