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Register a new userFuture Steps in Cosmology using Spectral Distortions of the Cosmic Microwave Background
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Jens Chluba
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Since the measurements of COBE/FIRAS in the mid-90s we know that the energy spectrum of the cosmic microwave background (CMB) is extremely close to that of a perfect blackbody at an average temperature T0~2.726K. However, a number of early-universe processes are expected to create CMB spectral distortions - departures of the average CMB energy spectrum from a blackbody - at a level that is within reach of present-day technology. This provides strong motivation to study the physics of CMB spectral distortions and ask what these small signals might be able to tell us about the Universe we live in. In this lecture, I will give a broad-brush overview of recent theoretical and experimental developments, explaining why future spectroscopic measurements of the CMB will open an unexplored new window to early-universe and particle physics. I will give an introduction about the different types of distortions, how they evolve and thermalize and highlight some of the physical processes that can cause them. I hope to be able to convince you that CMB spectral distortions could open an exciting new path forward in CMB cosmology, which is complementary to planned and ongoing searches for primordial B-mode polarization signals. Spectral distortions should thus be considered very seriously as part of the activities in the next decades.
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Voyage 2050 White Paper highlighting the unique science opportunities using spectral distortions of the cosmic microwave background (CMB). CMB spectral distortions probe many processes throughout the history of the Universe. Precision spectroscopy, possible with existing technology, would provide key tests for processes expected within the cosmological standard model and open an enormous discovery space to new physics. This offers unique scientific opportunities for furthering our understanding of inflation, recombination, reionization and structure formation as well as dark matter and particle physics. A dedicated experimental approach could open this new window to the early Universe in the decades to come, allowing us to turn the long-standing upper distortion limits obtained with COBE/FIRAS some 25 years ago into clear detections of the expected standard distortion signals.
We compute the spectral distortions of the Cosmic Microwave Background (CMB) polarization induced by non-linear effects in the Compton interactions between CMB photons and cold intergalactic electrons. This signal is of the $y$-type and is dominated by contributions arising from the reionized era. We stress that it is not shadowed by the thermal SZ effect which has no equivalent for polarization. We decompose its angular dependence into $E$- and $B$-modes, and we calculate the corresponding power spectra, both exactly and using a suitable Limber approximation that allows a simpler numerical evaluation. We find that $B$-modes are of the same order of magnitude as $E$-modes. Both spectra are relatively flat, peaking around $ell=280$, and their overall amplitude is directly related to the optical depth to reionization. Moreover, we find this effect to be one order of magnitude larger than the non-linear kinetic Sunyaev-Zeldovich effect in galaxy clusters. Finally, we discuss how to improve the detectability of our signal by cross-correlating it with other quantities sourced by the flow of intergalactic electrons.
Many inflation models predict that primordial density perturbations have a nonzero three-point correlation function, or bispectrum in Fourier space. Of the several possibilities for this bispectrum, the most commmon is the local-model bispectrum, which can be described as a spatial modulation of the small-scale (large-wavenumber) power spectrum by long-wavelength density fluctuations. While the local model predicts this spatial modulation to be scale-independent, many variants have some scale-dependence. Here we note that this scale dependence can be probed with measurements of frequency-spectrum distortions in the cosmic microwave background (CMB), in particular highlighting Compton-$y$ distortions. Dissipation of primordial perturbations with wavenumbers $50,{rm Mpc}^{-1} lesssim k lesssim 10^4,{rm Mpc}^{-1}$ give rise to chemical-potential ($mu$) distortions, while those with wavenumbers $1,{rm Mpc}^{-1} lesssim k lesssim 50,{rm Mpc}^{-1}$ give rise to Compton-$y$ distortions. With local-model non-Gaussianity, the distortions induced by this dissipation can be distinguished from those due to other sources via their cross-correlation with the CMB temperature $T$. We show that the relative strengths of the $mu T$ and $yT$ correlations thus probe the scale-dependence of non-Gaussianity and estimate the magnitude of possible signals relative to sensitivities of future experiments. We discuss the complementarity of these measurements with other probes of squeezed-limit non-Gaussianity.
The cosmic microwave background (CMB) has been a treasure trove for cosmology. Over the next decade, current and planned CMB experiments are expected to exhaust nearly all primary CMB information. To further constrain cosmological models, there is a great benefit to measuring signals beyond the primary modes. Rayleigh scattering of the CMB is one source of additional cosmological information. It is caused by the additional scattering of CMB photons by neutral species formed during recombination and exhibits a strong and unique frequency scaling ($propto u^4$). We will show that with sufficient sensitivity across frequency channels, the Rayleigh scattering signal should not only be detectable but can significantly improve constraining power for cosmological parameters, with limited or no additional modifications to planned experiments. We will provide heuristic explanations for why certain cosmological parameters benefit from measurement of the Rayleigh scattering signal, and confirm these intuitions using the Fisher formalism. In particular, observation of Rayleigh scattering allows significant improvements on measurements of $N_{rm eff}$ and $sum m_ u$.
Delensing is an increasingly important technique to reverse the gravitational lensing of the cosmic microwave background (CMB) and thus reveal primordial signals the lensing may obscure. We present a first demonstration of delensing on Planck temperature maps using the cosmic infrared background (CIB). Reversing the lensing deflections in Planck CMB temperature maps using a linear combination of the 545 and 857GHz maps as a lensing tracer, we find that the lensing effects in the temperature power spectrum are reduced in a manner consistent with theoretical expectations. In particular, the characteristic sharpening of the acoustic peaks of the temperature power spectrum resulting from successful delensing is detected at a significance of 16$rm{sigma}$, with an amplitude of $A_{rm{delens}} = 1.12 pm 0.07$ relative to the expected value of unity. This first demonstration on data of CIB delensing, and of delensing techniques in general, is significant because lensing removal will soon be essential for achieving high-precision constraints on inflationary B-mode polarization.
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