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New constraint on the cosmological background of relativistic particles

Steen Hannestad

TL;DR

This paper constrains the cosmological background of relativistic particles by performing a joint likelihood analysis of CMB, LSS, and Type Ia supernova data within a flat cosmology, parameterizing extra relativistic energy density with the effective number of neutrino species $N_\nu$ and allowing neutrino masses to vary. The analysis yields $N_\nu = 4.2^{+1.2}_{-1.7}$ (95% C.L.) with $N_\nu = 0$ excluded at $5.4\sigma$, demonstrating a clear cosmological background of relativistic species and showing compatibility with BBN predictions. It also provides a bound on the primordial helium abundance $0.240<Y<0.281$ and finds that the current data set constrains $N_\nu$ as tightly as BBN, while highlighting potential future improvements from CMB-only measurements to about $\Delta N_\nu\sim0.1$ precision. These results reinforce the standard cosmological model and inform ongoing searches for additional light particle species in the early universe.

Abstract

We have derived new bounds on the relativistic energy density in the Universe from cosmic microwave background (CMB), large scale structure (LSS), and type Ia supernova (SNI-a) observations. In terms of the effective number of neutrino species a bound of N_ν= 4.2^{+1.2}_{-1.7} is derived at 95% confidence. This bound is significantly stronger than previous determinations, mainly due to inclusion of new CMB and SNI-a observations. The absence of a cosmological neutrino background (N_ν= 0) is now excluded at 5.4 σ. The value of N_νis compatible with the value derived from big bang nucleosynthesis considerations, marking one of the most remarkable successes of the standard cosmological model. In terms of the cosmological helium abundance, the CMB, LSS, and SNI-a observations predict a value of 0.240 < Y < 0.281.

New constraint on the cosmological background of relativistic particles

TL;DR

This paper constrains the cosmological background of relativistic particles by performing a joint likelihood analysis of CMB, LSS, and Type Ia supernova data within a flat cosmology, parameterizing extra relativistic energy density with the effective number of neutrino species and allowing neutrino masses to vary. The analysis yields (95% C.L.) with excluded at , demonstrating a clear cosmological background of relativistic species and showing compatibility with BBN predictions. It also provides a bound on the primordial helium abundance and finds that the current data set constrains as tightly as BBN, while highlighting potential future improvements from CMB-only measurements to about precision. These results reinforce the standard cosmological model and inform ongoing searches for additional light particle species in the early universe.

Abstract

We have derived new bounds on the relativistic energy density in the Universe from cosmic microwave background (CMB), large scale structure (LSS), and type Ia supernova (SNI-a) observations. In terms of the effective number of neutrino species a bound of N_ν= 4.2^{+1.2}_{-1.7} is derived at 95% confidence. This bound is significantly stronger than previous determinations, mainly due to inclusion of new CMB and SNI-a observations. The absence of a cosmological neutrino background (N_ν= 0) is now excluded at 5.4 σ. The value of N_νis compatible with the value derived from big bang nucleosynthesis considerations, marking one of the most remarkable successes of the standard cosmological model. In terms of the cosmological helium abundance, the CMB, LSS, and SNI-a observations predict a value of 0.240 < Y < 0.281.

Paper Structure

This paper contains 9 sections, 7 equations, 3 figures, 1 table.

Table of Contents

  1. Introduction
  2. Current constraints
  3. Type Ia supernovae (SNI-a).
  4. Large Scale Structure (LSS).
  5. Cosmic Microwave Background (CMB).
  6. Results
  7. Comparison with Big Bang Nucleosynthesis
  8. What are the predictions for primordial $^4$He?
  9. Discussion

Figures (3)

  • Figure 1: $\Delta \chi^2$ values as a function of $N_\nu$ for various data sets. The full line includes all available data, and the dashed line is for WMAP and LSS data only.
  • Figure 2: The 68% (dark) and 95% (light) likelihood contours for $\Omega_b h^2$ and $N_\nu$ for all available data. The other contours are 68% and 95% regions for BBN, assuming the $^4$He and D values given in Cyburt:2004yc.
  • Figure 3: Isocontours for $Y$ as a function of $\Omega_b h^2$ and $N_\nu$. The likelihood contours are as in Fig. 2.