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arXiv 2401.08795astro-ph.COhep-ph
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大爆炸核合成期间非相对论性物质的限制

Limits on Non-Relativistic Matter During Big-Bang Nucleosynthesis

Tsung-Han Yeh, Keith A. Olive, Brian D. Fields

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24 pages, 22 figures
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AI总结：

本文研究大爆炸核合成期间非相对论无压物质的衰变效应，利用轻元素丰度和CMB数据限制其寿命与密度，发现电磁衰变物质可更好地拟合Planck给出的N_ν偏低结果，并给出参数限制。

摘要：

大爆炸核合成（BBN）在温度约10 MeV至约100 keV范围内探测宇宙质能密度。本文考虑在BBN期间存在一种非相对论且无压强的类物质宇宙组分的影响。这类组分必须发生衰变；在BBN期间衰变会改变重子-光子比η和有效中微子物种数。我们利用轻元素丰度以及宇宙微波背景（CMB）对η和N_ν的限制来约束这类物质组分。我们发现电磁衰变会相对于中微子加热光子，从而在三种标准模型中微子物种的情况下将有效相对论性物种数稀释至N_eff < 3。有趣的是，仅基于Planck CMB数据的似然结果给出N_ν = 2.800 ± 0.294，而当与标准BBN以及D和He4的观测相结合时给出N_ν = 2.898 ± 0.141。尽管这两个结果都与标准模型一致，但我们发现非零丰度的电磁衰变物质能更好地拟合这些结果。我们的最佳拟合结果对应一种完全以电磁方式衰变的物质组分，其寿命为τ_X = 0.89秒，衰变前密度在10 MeV时占辐射能量密度的比例为ξ = (ρ_X/ρ_rad)|_{10 MeV} = 0.0026；在ξτ_X^{1/2}保持恒定的范围内也能找到同样良好的拟合。另一方面，衰变物质常常破坏BBN与CMB的一致性，我们在(τ_X, ξ)平面上给出了电磁衰变和不可见衰变两种情形的限制。对于暗（不可见）衰变，标准BBN（即ξ=0）提供最佳拟合。最后我们简要讨论了包括CMB-S4在内的未来测量的影响。

英文摘要：

Big-bang nucleosynthesis (BBN) probes the cosmic mass-energy density at temperatures $\sim 10$ MeV to $\sim 100$ keV. Here, we consider the effect of a cosmic matter-like species that is non-relativistic and pressureless during BBN. Such a component must decay; doing so during BBN can alter the baryon-to-photon ratio, $η$, and the effective number of neutrino species. We use light element abundances and the cosmic microwave background (CMB) constraints on $η$ and $N_ν$ to place constraints on such a matter component. We find that electromagnetic decays heat the photons relative to neutrinos, and thus dilute the effective number of relativistic species to $N_{\rm eff} < 3$ for the case of three Standard Model neutrino species. Intriguingly, likelihood results based on {\em Planck} CMB data alone find $N_ν = 2.800 \pm 0.294$, and when combined with standard BBN and the observations of D and \he4 give $N_ν = 2.898 \pm 0.141$. While both results are consistent with the Standard Model, we find that a nonzero abundance of electromagnetically decaying matter gives a better fit to these results. Our best-fit results are for a matter species that decays entirely electromagnetically with a lifetime $τ_X = 0.89 \ \rm sec$ and pre-decay density that is a fraction $ξ= (ρ_X/ρ_{\rm rad})|_{10 \ \rm MeV} = 0.0026$ of the radiation energy density at 10 MeV; similarly good fits are found over a range where $ξτ_X^{1/2}$ is constant. On the other hand, decaying matter often spoils the BBN+CMB concordance, and we present limits in the $(τ_X,ξ)$ plane for both electromagnetic and invisible decays. For dark (invisible) decays, standard BBN (i.e. $ξ=0$) supplies the best fit. We end with a brief discussion of the impact of future measurements including CMB-S4.

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