用物质不均匀性模拟宇宙加速膨胀--对某些误解的纠正
Imitating accelerated expansion of the Universe by matter inhomogeneities - corrections of some misunderstandings
摘要：
一些误解建模的明显加速膨胀的宇宙，并对“弱奇点”澄清：1。在Hirata和Seljak（HS）给出的减速参数的五种定义中，只有$q_1$是膨胀加减速的正确不变测度。在非均匀模型中，$q_3$和$q_4$与加速度无关。2. $q_4$定义中涉及的方向平均与观测天文学中所做的并不对应。3. HS的方程（38）将$q_4$与流动不变量连接起来，当应用于Lemartre-Tolman（L-T）模型的对称中心时，给出了自相矛盾的结果。确定$q_{3'}$的中间方程（31）是正确的，但是近似的，因此它不能用于确定减速参数的符号。即便如此，在L-T模型的对称中心，它对$q_{3'}(0)$的符号没有限制。4. Vanderveld { \it et al.}是一个中心质量密度的圆锥形轮廓--一个完全可以接受的配置。5.在L-T模型中，中心观测者的“逆问题”方程中的所谓“临界点”是视视界的一种表现--这是零λ L-T模型中过去光锥的一个共同特性，如果方程被正确积分，就完全可以处理。
英文摘要：
A number of misunderstandings about modeling the apparent accelerated expansion of the Universe, and about the `weak singularity' are clarified: 1. Of the five definitions of the deceleration parameter given by Hirata and Seljak (HS), only $q_1$ is a correct invariant measure of acceleration/deceleration of expansion. The $q_3$ and $q_4$ are unrelated to acceleration in an inhomogeneous model. 2. The averaging over directions involved in the definition of $q_4$ does not correspond to what is done in observational astronomy. 3. HS's equation (38) connecting $q_4$ to the flow invariants gives self-contradictory results when applied at the centre of symmetry of the Lema\^ıtre-Tolman (L-T) model. The intermediate equation (31) that determines $q_{3'}$ is correct, but approximate, so it cannot be used for determining the sign of the deceleration parameter. Even so, at the centre of symmetry of the L-T model, it puts no limitation on the sign of $q_{3'}(0)$. 4. The `weak singularity' of Vanderveld {\it et al.} is a conical profile of mass density at the centre - a perfectly acceptable configuration. 5. The so-called `critical point' in the equations of the `inverse problem' for a central observer in an L-T model is a manifestation of the apparent horizon - a common property of the past light cones in zero-lambda L-T models, perfectly manageable if the equations are correctly integrated.
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