How Massless Neutrinos Affect the Cosmic Microwave Background Damping Tail (1104.2333v2)

Abstract: We explore the physical origin and robustness of constraints on the energy density in relativistic species prior to and during recombination, often expressed as constraints on an effective number of neutrino species, Neff. Constraints from current data combination of Wilkinson Microwave Anisotropy Probe (WMAP) and South Pole Telescope (SPT) are almost entirely due to the impact of the neutrinos on the expansion rate, and how those changes to the expansion rate alter the ratio of the photon diffusion scale to the sound horizon scale at recombination. We demonstrate that very little of the constraining power comes from the early Integrated Sachs-Wolfe (ISW) effect, and also provide a first determination of the amplitude of the early ISW effect. Varying the fraction of baryonic mass in Helium, Yp, also changes the ratio of damping to sound-horizon scales. We discuss the physical effects that prevent the resulting near-degeneracy between Neff and Yp from being a complete one. Examining light element abundance measurements, we see no significant evidence for evolution of Neff and the baryon-to-photon ratio from the epoch of big bang nucleosynthesis to decoupling. Finally, we consider measurements of the distance-redshift relation at low to intermediate redshifts and their implications for the value of Neff.

• The paper demonstrates that modifications in Nₑff alter the CMB damping tail through changes in the early universe's expansion rate.
• It employs WMAP and SPT observations to link Nₑff with helium abundance, resolving degeneracies in light element predictions.
• The study reveals that even small variations in relativistic species can significantly affect Silk damping, prompting refinements in cosmological models.

Analysis of "How Massless Neutrinos Affect the Cosmic Microwave Background Damping Tail"

This paper presents a comprehensive examination of how massless neutrinos influence the cosmic microwave background (CMB), particularly the damping tail of the CMB power spectrum. The primary focus is on the effective number of neutrino species, $N_{\rm eff}$, a parameter that encapsulates the contribution of relativistic species, including potential additional neutrinos, to the energy density of the universe at the time of recombination.

Main Findings

The analysis is based on data from the Wilkinson Microwave Anisotropy Probe (WMAP) and the South Pole Telescope (SPT). It concludes that the constraints on $N_{\rm eff}$ currently arise primarily from modifications to the early universe's expansion rate. These changes, in turn, impact critical cosmological quantities, such as the photon diffusion scale and the sound horizon scale at recombination. The paper specifically highlights that the constraint does not significantly rely on the early Integrated Sachs-Wolfe (ISW) effect, which the authors determine for the first time.

The paper details the correlation between $N_{\rm eff}$ and parameters like the baryonic mass fraction in Helium, showing that variations in these parameters prevent a complete degeneracy between $N_{\rm eff}$ and the helium fraction when considering light element abundance measurements and predictions from big bang nucleosynthesis (BBN).

Theoretical Implications

The paper advances our theoretical understanding of the CMB's sensitivity to relativistic species, suggesting that even minute changes in $N_{\rm eff}$ could have significant implications for the early universe’s dynamics. The authors show that while altering $N_{\rm eff}$ affects the overall damping seen in the CMB power spectrum, it is particularly the Silk damping effect, influenced by changes in the expansion rate due to increased relativistic energy density, that plays a major role.

Practical Implications and Future Research Directions

The implications for cosmological models are profound, as the findings support the presence of either additional neutrinos or other weakly interacting components that were relativistic during recombination. The results also suggest modest yet important refinements to the standard cosmological model, particularly around the estimates of matter density parameters and their relationships with the cosmic background radiation.

The authors also speculate on potential developments in observational cosmology. Future improvements in CMB measurements, such as those anticipated from the Planck satellite, are expected to refine the constraints on $N_{\rm eff}$ further, also providing a better understanding of dark energy parameters and the overall curvature of the universe.

Conclusion

This paper provides a technical and nuanced comprehension of how massless neutrinos can be studied through the cosmic microwave background. The integration of current CMB observations with theoretical predictions offers a robust framework for examining relativistic species' effects in the early universe, pointing the way toward resolving some of cosmology's intricacies and hinted-at anomalies. The work pairs theoretical insights with observational data, creating a detailed picture that enhances our understanding of the universe's formative stages and strengthens the empirical basis for exploring new physics beyond the standard model.