In this manuscript, we investigate the oscillatory behaviour of the anisotropy in the diagonal Bianchi-I spacetimes. Our starting point is a simplification of Einstein’s equations using only observable or physical variables. As a consequence, we are able to: (a) Prove general results concerning the existence of oscillations of the anisotropy in the primordial and the late-time universe. For instance, in the expanding scenario, we show that a past weakly mixmaster behaviour (oscillations as we approach the Kasner solutions) might appear even with no violation of the usual energy conditions, while in the future, the pulsation (oscillations around isotropic solutions) seems to be most favoured; (b) Determine a large scheme for deriving classes of physically motivated exact solutions, and we give some (including the general barotropic perfect fluid and the magnetic one); (c) Understand the physical conditions for the occurrence of the isotropization or anisotropization during the cosmological evolution; (d) Understand how anisotropy and energy density are converted one into another. In particular, we call attention to the presence of a residue in the energy density in a late-time isotropic universe coming from its past anisotropic behaviour.

在这份手稿中，我们研究了对角线 Bianchi-I 时空中各向异性的振荡行为。我们的出发点是仅使用可观察变量或物理变量来简化爱因斯坦方程。因此，我们能够： (a) 证明关于原始宇宙和晚期宇宙中各向异性振荡存在的一般结果。例如，在扩展场景中，我们表明，即使不违反通常的能量条件，也可能会出现过去的弱混合主行为（接近 Kasner 解时的振荡），而在未来，脉动（围绕各向同性解的振荡）似乎最受青睐；(b) 确定用于推导物理驱动的精确解类的大型方案，我们给出了一些（包括一般的正压完美流体和磁性流体）；(c) 了解宇宙演化过程中发生各向同性或各向异性的物理条件；(d) 了解各向异性和能量密度如何相互转换。特别是，我们提请注意来自过去各向异性行为的晚期各向同性宇宙中能量密度中残留物的存在。