中微子味混合打破了早期宇宙的各向同性
Neutrino flavor mixing breaks isotropy in the early universe
摘要：
在早期宇宙中，中微子场通常被认为是各向同性和均匀的。然而，由于大的中微子密度，中微子场的各向同性的小扰动可能会被中微子自相互作用引起的非线性味道混合放大。我们进行了第一次数值模拟的中微子味的演变在一个多角度的各向异性设置。由于所涉及的计算挑战，我们采用了一个简化的框架，包括一个均匀的宇宙有两个角度仓-左，右移动模式-中微子和反中微子，以及碰撞项的近似形式，超出了通常采用的阻尼近似。通过假设一个小的初始左右不对称的$\mathcal{O}(10^{-15})$，我们令人信服地证明，味道演变可以在两个质量排序的影响，与热激发中微子物种（$N_{\mathrm{eff}}$）的有效数量的影响。值得注意的是，对$N_{\rm eff}$的修正可以与正常顺序下有限温度QED效应的高阶修正相媲美。此外，通过假设中微子扇区中的轻子初始不对称性与重子扇区[ $\mathcal{O}(10^{-9})$ ]中的轻子初始不对称性相同，我们发现中微子-反中微子不对称性在各向同性和各向异性初始条件下增长了几个数量级。这项工作清楚地表明，有必要批判性地重新审视关于早期宇宙中中微子味道混合的标准假设，特别是考虑到对宇宙学可观测量的可能影响。
英文摘要：
The neutrino field is commonly assumed to be isotropic and homogeneous in the early universe. However, due to the large neutrino density, a small perturbation of the isotropy of the neutrino field could potentially be amplified by the non-linear flavor mixing caused by neutrino self-interactions. We carry out the first numerical simulations of the neutrino flavor evolution in a multi-angle anisotropic setting. Due to the computational challenges involved, we adopt a simplified framework consisting of a homogeneous universe with two angle bins -- left and right moving modes -- for neutrinos and antineutrinos, together with an approximate form for the collision term which goes beyond the commonly adopted damping approximation. By assuming a small initial left-right asymmetry of $\mathcal{O}(10^{-15})$, we convincingly demonstrate that flavor evolution can be affected in both mass orderings, with implications on the effective number of thermally excited neutrino species ($N_{\mathrm{eff}}$). Notably, the correction to $N_{\rm eff}$ is comparable to higher order corrections from finite temperature QED effects in normal ordering. In addition, by assuming an initial lepton asymmetry in the neutrino sector of the same order as the baryon one [$\mathcal{O}(10^{-9})$], we find that the neutrino-antineutrino asymmetry grows by several orders of magnitude for isotropic as well as anisotropic initial conditions. This work clearly shows that it is imperative to critically revisit standard assumptions concerning neutrino flavor mixing in the early universe, especially in the light of possible implications on the cosmological observables.
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