The role of $T_0$ in CMB anisotropy measurements

Abstract

The quantity $T_0$, the cosmic microwave background (CMB) monopole, is an often neglected seventh parameter of the standard cosmological model. As well as its variation affecting the physics of the CMB, the measurement of $T_0$ is also used to calibrate the anisotropies, via the orbital dipole. We point out that it is easy to misestimate the effect of $T_0$ because the CMB anisotropies are conventionally provided in temperature units. In fact the anisotropies are most naturally described as dimensionless and we argue for restoring the convention of working with $\Delta T/T$ rather than $\Delta T$. As a free cosmological parameter, $T_0$ most naturally only impacts the CMB power spectra through late-time effects. Thus if we ignore the COBE-FIRAS measurement, current CMB data only weakly constrain $T_0$. Even ideal future CMB data can at best provide a percent-level constraint on $T_0$, although adding large-scale structure data will lead to further improvement. The FIRAS measurement is so precise that its uncertainty negligibly effects most, but not all, cosmological parameter inferences for current CMB experiments. However, if we eventually want to extract all available information from CMB power spectra measured to multipoles $\ell \simeq 5000$, then we will need a better determination of $T_0$ than is currently available.

18 pages, 5 figures, 7 tables. Revised to match version accepted by PRD

References in corpus (20)

• Large Magellanic Cloud Cepheid Standards Provide a 1% Foundation for the Determination of the Hubble Constant and Stronger Evidence for Physics Beyond LambdaCDM
• The DESI Experiment Part I: Science,Targeting, and Survey Design
• Cobaya: Code for Bayesian Analysis of hierarchical physical models
• The Atacama Cosmology Telescope: DR4 Maps and Cosmological Parameters
• Planck intermediate results. LVII. Joint Planck LFI and HFI data processing
• Precision cosmology defeats void models for acceleration
• Cosmological Information from Lensed CMB Power Spectra
• H0 tension or T0 tension?
• $T_0$ censorship of early dark energy and AdS vacua
• Probing the neutrino mass hierarchy with CMB weak lensing
• Gauging the cosmic microwave background
• Is there a need and another way to measure the Cosmic Microwave Background temperature more accurately?
• Easing cosmic tensions with an open and hotter universe
• Effects of CMB temperature uncertainties on cosmological parameter estimation
• The Evolution of the Cosmic Microwave Background
• Calibration Method and Uncertainty for the Primordial Inflation Explorer (PIXIE)
• Monopole Fluctuation of the CMB and its Gauge Invariance
• Interplay of CMB Temperature, Space Curvature, and Expansion Rate Parameters
• Observing the evolution of the CMB
• Construction of a Database of CMB Spectrum Observations

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• Cosmology with the SZ spectrum: measuring the Universe's temperature with galaxy clusters