标准$Λ$ CDM模型Bianchi I型时空扩张的约束
Constraints on a Bianchi type I spacetime extension of the standard $Λ$CDM model
摘要：
我们认为最简单的各向异性推广，作为一个修正，标准的$Λ$ CDM模型，通过取代空间平坦的罗伯逊-沃克度规的比安奇I型度量，这带来了一个新的术语$Ω_{σ0}a^{-6}$（模仿刚性流体）的平均膨胀率$H(a)$的宇宙。从哈勃和万神殿的数据，有关的晚期宇宙（$z\lesssim 2.4$），我们得到的约束$Ω_{\sigma0}\lesssim10^{-3}$，在模型无关的约束。当包括重子声学振荡和宇宙微波背景（CMB）数据时，约束改善了12个数量级，即，$Ω_{\sigma0}\lesssim10^{-15}$。我们发现，这种约束既不能改变物质辐射等性红移，也不能改变物质微扰的峰值。在要求膨胀各向异性对标准大爆炸核合成（BBN）没有显著影响的条件下，我们找到了约束$Ω_{\sigma0}\lesssim10^{-23}$ .我们明确地表明，从BBN的约束使膨胀各向异性无关，使一个显着的变化，在CMB四极温度，而从宇宙学数据在我们的模型中的约束提供的温度变化高达$\sim11\, \rm mK$，虽然它是远远超过CMB四极温度。
英文摘要：
We consider the simplest anisotropic generalization, as a correction, to the standard $Λ$CDM model, by replacing the spatially flat Robertson-Walker metric by the Bianchi type-I metric, which brings in a new term $Ω_{σ0}a^{-6}$ (mimicking the stiff fluid) in the average expansion rate $H(a)$ of the Universe. From Hubble and Pantheon data, relevant to the late Universe ($z\lesssim 2.4$), we obtain the constraint $Ω_{\sigma0}\lesssim10^{-3}$, in line with the model-independent constraints. When the baryonic acoustic oscillations and cosmic microwave background (CMB) data are included, the constraint improves by 12 orders of magnitude, i.e., $Ω_{\sigma0}\lesssim10^{-15}$. We find that this constraint could alter neither the matter-radiation equality redshift nor the peak of the matter perturbations. Demanding that the expansion anisotropy has no significant effect on the standard big bang nucleosynthesis (BBN), we find the constraint $Ω_{\sigma0}\lesssim10^{-23}$. We show explicitly that the constraint from BBN renders the expansion anisotropy irrelevant to make a significant change in the CMB quadrupole temperature, whereas the constraint from the cosmological data in our model provides the temperature change up to $\sim11\, \rm mK$, though it is much beyond the CMB quadrupole temperature.
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