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Dark Radiation and Decaying Matter

M. C. Gonzalez-Garcia, V. Niro, Jordi Salvado

TL;DR

The paper investigates whether the observed hints of extra relativistic energy density, usually parametrized by $N_{\rm eff}$, can be explained without new light species by decaying a heavy thermally produced matter state into SM neutrinos, effectively generating dark radiation. The authors implement the decaying-matter framework in the CLASS Boltzmann code and perform a global cosmological analysis using Markov Chain Monte Carlo methods, incorporating WMAP7, SPT, ACT, BAO, SN Ia, $H_0$ priors, and BBN constraints on $N_{\rm eff}^{\rm BBN}$. They find that a decay lifetime around $\tau_{\rm dec} \sim 10^3$ s is favored, with the decaying-density today being negligible and $N_{\rm eff}$ at BBN and CMB times becoming similar under the BBN prior. The work highlights that this decaying-matter scenario can mimic additional radiation without sterile neutrinos and emphasizes how priors and timing of decay shape the inferred $N_{\rm eff}$ evolution. Overall, the study provides a viable alternative explanation for hints of dark radiation and demonstrates the potential of early-universe decays to reconcile cosmological data without introducing new relativistic particle species.

Abstract

Recent cosmological measurements favour additional relativistic energy density beyond the one provided by the three active neutrinos and photons of the Standard Model (SM). This is often referred to as "dark radiation", suggesting the need of new light states in the theory beyond those of the SM. In this paper, we study and numerically explore the alternative possibility that this increase comes from the decay of some new form of heavy matter into the SM neutrinos. We study the constraints on the decaying matter density and its lifetime, using data from the Wilkinson Microwave Anisotropy Probe, the South Pole Telescope, measurements of the Hubble constant at present time, the results from high-redshift Type-I supernovae and the information on the Baryon Acoustic Oscillation scale. We, moreover, include in our analysis the information on the presence of additional contributions to the expansion rate of the Universe at the time of Big Bang Nucleosynthesis. We compare the results obtained in this decaying matter scenario with those obtained with the standard analysis in terms of a constant $N_{\rm eff}$.

Dark Radiation and Decaying Matter

TL;DR

The paper investigates whether the observed hints of extra relativistic energy density, usually parametrized by , can be explained without new light species by decaying a heavy thermally produced matter state into SM neutrinos, effectively generating dark radiation. The authors implement the decaying-matter framework in the CLASS Boltzmann code and perform a global cosmological analysis using Markov Chain Monte Carlo methods, incorporating WMAP7, SPT, ACT, BAO, SN Ia, priors, and BBN constraints on . They find that a decay lifetime around s is favored, with the decaying-density today being negligible and at BBN and CMB times becoming similar under the BBN prior. The work highlights that this decaying-matter scenario can mimic additional radiation without sterile neutrinos and emphasizes how priors and timing of decay shape the inferred evolution. Overall, the study provides a viable alternative explanation for hints of dark radiation and demonstrates the potential of early-universe decays to reconcile cosmological data without introducing new relativistic particle species.

Abstract

Recent cosmological measurements favour additional relativistic energy density beyond the one provided by the three active neutrinos and photons of the Standard Model (SM). This is often referred to as "dark radiation", suggesting the need of new light states in the theory beyond those of the SM. In this paper, we study and numerically explore the alternative possibility that this increase comes from the decay of some new form of heavy matter into the SM neutrinos. We study the constraints on the decaying matter density and its lifetime, using data from the Wilkinson Microwave Anisotropy Probe, the South Pole Telescope, measurements of the Hubble constant at present time, the results from high-redshift Type-I supernovae and the information on the Baryon Acoustic Oscillation scale. We, moreover, include in our analysis the information on the presence of additional contributions to the expansion rate of the Universe at the time of Big Bang Nucleosynthesis. We compare the results obtained in this decaying matter scenario with those obtained with the standard analysis in terms of a constant .

Paper Structure

This paper contains 7 sections, 16 equations, 2 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Decaying Matter
  3. Boltzmann Equations for Decaying Matter
  4. Numerical Analysis
  5. Cosmological Inputs
  6. Results
  7. Conclusions

Figures (2)

  • Figure 1: Marginalized one-dimensional probability distributions for the analysis parameters. We present the results for the case of $dec$M+BBN with solid lines. The dashed line shows the results for $\Lambda$CDM+$N_{\rm eff}$+BBN.
  • Figure 2: Two-dimensional probability distribution at the 68% and 95% C.L. for the $dec$M parameters $\{\log(\rho_{dec}/\rho_\gamma)$, $\log(\tau_{dec}/\rm{s}) \}$, and $\{\log(\tau_{dec}/\rm{s})$, $\Delta N_{\rm eff}^{\rm BBN}\}$.