$\mu$-Distortions or Running: A Guaranteed Discovery from CMB Spectrometry

Abstract

We discuss the implications of a PIXIE-like experiment, which would measure $\mu$-type spectral distortions of the CMB at a level of ${\sigma }_{\mu }=(1/n)\times 10^{-8}$, with $n\ge 1$ representing an improved sensitivity (e.g. $n=10$ corresponds to PRISM). Using Planck data and considering the six-parameter $\Lambda$CDM model, we compute the posterior for ${\mu }_8\equiv \mu \times 10^8$ and find ${\mu }_8=1.57_{-0.13}^{+0.11}$ ($68\%\mathrm{CL}$). This becomes ${\mu }_8=1.28_{-0.52}^{+0.30}$ ($68\%\mathrm{CL}$) when the running ${\alpha }_{\mathrm{s}}$ of the spectral index is included. We point out that a sensitivity of about $3\times$ PIXIE implies a guaranteed discovery: $\mu$-distortion is detected or ${\alpha }_{\mathrm{s}}\ge 0$ is excluded (both at $95\%\mathrm{CL}$ or higher). This threshold sensitivity sets a clear benchmark for CMB spectrometry. For a combined analysis of PIXIE and current Planck data, we discuss the improvement on measurements of the tilt $n_{\mathrm{s}}$ and the running ${\alpha }_{\mathrm{s}}$ and the dependence on the choice of the pivot. A fiducial running of ${\alpha }_{\mathrm{s}}=-0.01$ (close to the Planck best-fit) leads to a detection of negative running at $2\sigma$ for $5\times$ PIXIE. A fiducial running of ${\alpha }_{\mathrm{s}}=-0.02$, still compatible with Planck, requires $3\times$ PIXIE to rule out ${\alpha }_{\mathrm{s}}=0$ (at $95\%\mathrm{CL}$). We propose a convenient and compact visualization of the improving constraints on the tilt, running and tensor-to-scalar ratio.

10+2 pages, 8 figures, 2 tables. Matches published version

References in corpus (13)

• Beyond y and μ: the shape of the CMB spectral distortions in the intermediate epoch, 1.5x10^4 < z < 2x10^5
• Lines in the Cosmic Microwave Background Spectrum from the Epoch of Cosmological Hydrogen Recombination
• CMB Polarization can constrain cosmology better than CMB temperature
• Cosmological hydrogen recombination: populations of the high level sub-states
• Mixing of blackbodies: entropy production and dissipation of sound waves in the early Universe
• Creation of the CMB spectrum: precise analytic solutions for the blackbody photosphere
• Updated Constraints and Forecasts on Primordial Tensor Modes
• On what scale should inflationary observables be constrained?
• CosmoSpec: Fast and detailed computation of the cosmological recombination radiation from hydrogen and helium
• Another look to distortions of the CMB spectrum
• Radiative Corrections from Heavy Fast-Roll Fields during Inflation
• Cosmic Microwave Background Spectral Distortions from Cosmic String Loops
• The Sunyaev-Zel'dovich Effect and Large-Scale Structure

Cited by in corpus (33)

• Gravitational probes of dark matter physics
• Exploring Cosmic Origins with CORE: Inflation
• Which spectral distortions does $\Lambda$CDM actually predict?
• The synergy between CMB spectral distortions and anisotropies
• Prospects for Measuring Cosmic Microwave Background Spectral Distortions in the Presence of Foregrounds
• Towards a measurement of the spectral runnings
• CMB spectral distortions revisited: a new take on $\mu$ distortions and primordial non-Gaussianities from FIRAS data
• CMB Spectral Distortions: Status and Prospects
• Shedding light on the small-scale crisis with CMB spectral distortions
• On CMB B-Mode Non-Gaussianity
• Running the running
• Bridging the gap: spectral distortions meet gravitational waves
• Critical point Higgs inflation in the Palatini formulation
• Evolution of CMB spectral distortion anisotropies and tests of primordial non-Gaussianity
• Spectral distortion anisotropies from single-field inflation
• The role of CMB spectral distortions in the Hubble tension: a proof of principle
• Sensitivity forecasts for the cosmological recombination radiation in the presence of foregrounds
• Running of the Running and Entropy Perturbations During Inflation
• Unlocking the synergy between CMB spectral distortions and anisotropies
• 21 cm Angular Power Spectrum from Minihalos as a Probe of Primordial Spectral Runnings
• Impact of EDGES 21cm Global Signal on Primordial Power Spectrum
• $\mu$-distortion around stupendously large primordial black holes
• CMB Spectral Distortion Constraints on Thermal Inflation
• Probing the primordial Universe with 21-cm line from cosmic dawn/epoch of reionization
• The impact of theoretical assumptions in the determination of the neutrino effective number from future CMB measurements
• New Horizons in Cosmology with Spectral Distortions of the Cosmic Microwave Background
• CMB spectral distortions in generic two-field models
• Can power spectrum observations rule out slow-roll inflation?
• Spectral Distortions from Axion Monodromy Inflation
• Non-standard hierarchies of the runnings of the spectral index in inflation
• SPECTER: An Instrument Concept for CMB Spectral Distortion Measurements with Enhanced Sensitivity
• Fundamental Physics in 2025: Status, Decisive Targets, and Path Forward
• Cosmic microwave background spectral distortions from Rayleigh scattering at second order