[Paper Review] Cosmological forecasts on thermal axions, relic neutrinos and light elements

William Giarè, Fabrizio Renzi+3 more|arXiv (Cornell University)|Oct 1, 2021

Dark Matter and Cosmic Phenomena93 references17 citations

TL;DR

This paper forecasts cosmological constraints on thermally produced QCD axions and massive neutrinos using simulated Stage-IV CMB and DESI-like BAO data. It demonstrates that future observations can improve bounds on axion and neutrino masses to 𝑚𝑎 ≲ 0.92 eV and ∑𝑚𝜈 ≲ 0.12 eV at 95% CL, respectively, while confirming robustness against BBN uncertainty assumptions.

ABSTRACT

One of the targets of future Cosmic Microwave Background and Baryon Acoustic Oscillation measurements is to improve the current accuracy in the neutrino sector and reach a much better sensitivity on extra dark radiation in the Early Universe. In this paper we study how these improvements can be translated into constraining power for well motivated extensions of the Standard Model of elementary particles that involve axions thermalized before the quantum chromodynamics (QCD) phase transition by scatterings with gluons. Assuming a fiducial $\Lambda$CDM cosmological model, we simulate future data for Stage-IV CMB-like and Dark Energy Spectroscopic Instrument (DESI)-like surveys and analyze a mixed scenario of axion and neutrino hot dark matter. We further account also for the effects of these QCD axions on the light element abundances predicted by Big Bang Nucleosynthesis. The most constraining forecasted limits on the hot relic masses are $m_{ m a} \lesssim 0.92$ eV and $\sum m_ u\lesssim 0.12$ eV at 95 per cent Confidence Level, showing that future cosmic observations can substantially improve the current bounds, supporting multi-messenger analyses of axion, neutrino and primordial light element properties.

Motivation & Objective

• To forecast cosmological constraints on thermally produced QCD axions and massive neutrinos using next-generation CMB and BAO data.
• To assess the impact of axions on early Universe observables, including extra dark radiation (𝑁eff) and Big Bang Nucleosynthesis (BBN) light element abundances.
• To evaluate the robustness of constraints against uncertainties in BBN predictions, particularly for beryllium-7 and other light isotopes.
• To investigate the complementarity between CMB, BAO, and BBN data in constraining hot relic masses.
• To determine whether future observations can probe the sub-eV axion mass range and test the normal neutrino mass ordering.

Proposed method

• Simulated future data for Stage-IV CMB (CMB-S4-like) and DESI-like BAO surveys under a fiducial ΛCDM cosmological model.
• Used Markov Chain Monte Carlo (MCMC) sampling with GetDist and NumPy to analyze cosmological parameter constraints.
• Incorporated the PArthENoPE BBN code to compute theoretical primordial light element abundances, including effects of extra relativistic species.
• Applied the effective number of relativistic degrees of freedom (𝑁eff) formalism to model axion contributions during the radiation-dominated era.
• Accounted for the early integrated Sachs-Wolfe effect and free-streaming suppression of small-scale structure due to hot dark matter.
• Compared results with and without BBN code to test sensitivity to nuclear physics assumptions.

Experimental results

Research questions

• RQ1 What are the projected constraints on the mass of thermally produced QCD axions using future CMB-S4 and DESI-like BAO data?
• RQ2 How do axions affect the effective number of relativistic species (𝑁eff) and the CMB power spectrum, particularly the damping tail?
• RQ3 To what extent do uncertainties in BBN predictions—especially for beryllium-7 and helium-4—affect the derived bounds on axion and neutrino masses?
• RQ4 Can future observations distinguish between normal and inverted neutrino mass orderings through hot dark matter suppression?
• RQ5 How does combining CMB and BAO data improve sensitivity to hot relic masses compared to CMB alone?

Key findings

• Future CMB-S4 and DESI-like BAO data are forecast to constrain the axion mass to 𝑚𝑎 < 0.92 eV at 95% confidence level, representing a ~5-fold improvement over current bounds.
• The sum of neutrino masses is expected to be constrained to ∑𝑚𝜈 < 0.12 eV at 95% CL when combining CMB-S4 and BAO data, a ~2-fold improvement over CMB-only forecasts.
• Including BBN effects via the PArthENoPE code does not significantly alter the constraints, confirming robustness against uncertainties in primordial nucleosynthesis predictions.
• The bounds on axion and neutrino masses remain stable even when neutron lifetime assumptions are varied, indicating insensitivity to BBN input uncertainties.
• A two-sigma detection of ∑𝑚𝜈 = 0.06 eV is excluded with current data, meaning only upper limits (near the inverted ordering prediction) can be derived.
• The results support the potential of multi-messenger cosmology: combining CMB, BAO, and precise light element measurements can probe axion masses in the sub-eV range, favoring normal neutrino mass ordering.