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Measuring the cosmological background of relativistic particles with WMAP

Patrick Crotty, Julien Lesgourgues, Sergio Pastor

TL;DR

Measuring the radiation content of the early universe, the paper constrains the excess relativistic energy density through the parameter $\\Delta N_{\\rm eff}$ using the first-year WMAP data. The authors compute CMB and matter perturbations with the code cmbfast across a grid of flat-LCDM models including $\\Delta N_{\\rm eff}$ and other parameters, and they perform a Bayesian likelihood analysis with priors on $h$ and supplementary CMB/LSS data. Their results show $\\Delta N_{\\rm eff}$ is not required by WMAP alone ($\\Delta N_{\\rm eff} < 6$ at 95% CL), and with additional data the allowed window tightens to $-1.6 <\\Delta N_{\\rm eff} < 3.8$, with no-neutrino case excluded at 99.9% CL. The findings are consistent with standard BBN and place strong constraints on non-minimal relativistic content in the early universe, highlighting the importance of precise $h$ measurements for closing any remaining degeneracy.

Abstract

We show that the first year results of the Wilkinson Microwave Anisotropy Probe (WMAP) constrain very efficiently the energy density in relativistic particles in the universe. We derive new bounds on additional relativistic degrees of freedom expressed in terms of an excess in the effective number of light neutrinos Delta N_eff. Within the flat LambdaCDM scenario, the allowed range is Delta N_eff < 6 (95% CL) using WMAP data only, or -2.6 < Delta N_eff < 4 with the prior H_0= 72 \pm 8 km/s/Mpc. When other cosmic microwave background and large scale structure experiments are taken into account, the window shrinks to -1.6 < Delta N_eff < 3.8. These results are in perfect agreement with the bounds from primordial nucleosynthesis. Non-minimal cosmological models with extra relativistic degrees of freedom are now severely restricted.

Measuring the cosmological background of relativistic particles with WMAP

TL;DR

Measuring the radiation content of the early universe, the paper constrains the excess relativistic energy density through the parameter using the first-year WMAP data. The authors compute CMB and matter perturbations with the code cmbfast across a grid of flat-LCDM models including and other parameters, and they perform a Bayesian likelihood analysis with priors on and supplementary CMB/LSS data. Their results show is not required by WMAP alone ( at 95% CL), and with additional data the allowed window tightens to , with no-neutrino case excluded at 99.9% CL. The findings are consistent with standard BBN and place strong constraints on non-minimal relativistic content in the early universe, highlighting the importance of precise measurements for closing any remaining degeneracy.

Abstract

We show that the first year results of the Wilkinson Microwave Anisotropy Probe (WMAP) constrain very efficiently the energy density in relativistic particles in the universe. We derive new bounds on additional relativistic degrees of freedom expressed in terms of an excess in the effective number of light neutrinos Delta N_eff. Within the flat LambdaCDM scenario, the allowed range is Delta N_eff < 6 (95% CL) using WMAP data only, or -2.6 < Delta N_eff < 4 with the prior H_0= 72 \pm 8 km/s/Mpc. When other cosmic microwave background and large scale structure experiments are taken into account, the window shrinks to -1.6 < Delta N_eff < 3.8. These results are in perfect agreement with the bounds from primordial nucleosynthesis. Non-minimal cosmological models with extra relativistic degrees of freedom are now severely restricted.

Paper Structure

This paper contains 4 sections, 4 equations, 2 figures, 1 table.

Table of Contents

  1. Introduction
  2. The effective number of relativistic neutrinos
  3. Method
  4. Results and discussion

Figures (2)

  • Figure 1: The $\Delta N_{\rm eff}$ likelihood for WMAP + weak $h$ prior (green), WMAP + strong $h$ prior (red), and the same plus other CMB experiments and the 2dF redshift survey (blue). The horizontal lines show the 68% (resp. 95%) confidence levels. The step of $\Delta N_{\rm eff}$ in our grid of models is $0.5$.
  • Figure 2: The two-dimensional confidence limits on ($\Delta N_{\rm eff}$, $h$), based on CMB and LSS data, at the 1-$\sigma$ (dark blue) and 2-$\sigma$ (light blue) levels. The superimposed yellow stripe shows the HST Key Project measurement of $h$ (1-$\sigma$ level).