arXiv 2401.097842024-01gr-qc

几何完美流体与宇宙的暗面

Geometric Perfect Fluids and Dark Side of the Universe

Metin Gürses, Yaghoub Heydarzade, Çetin Şentürk

备注：: 27 pages, 4 figures, published version
发表信息：: Phys. Rev. D 110, 024073 (2024)

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AI总结：

本文提出宇宙晚期加速膨胀可由高阶曲率项形成的几何完美流体驱动，并通过D维二次引力场方程分析减速参数演化，证明在合适参数下时空曲率足以解释当前加速膨胀。

摘要：

最近我们证明了在FLRW宇宙学中，任何一般度规引力理论中的高阶曲率项对能量-动量张量的贡献都具有完美流体形式。这种几何完美流体可以被解释为从宇宙开端遗留下来的流体，而在宇宙开端时期弦理论被认为是有效的。已知在宇宙开端之后不久，爱因斯坦-希尔伯特作用量被假定为通过添加所有可能的曲率不变量而被修改。我们提出，观测到的宇宙晚期加速膨胀可以完全由这种几何流体驱动。为了支持我们的主张，我们专门研究了D维二次引力场方程。我们证明，该理论在FLRW度规下的场方程具有一个包含两个关键参数σ₁和σ₂的几何完美流体源。为了分析该理论关于其参数空间(σ₁, σ₂)的性质，我们得到了支配减速参数q晚期动力学的一般二阶非线性微分方程。因此，利用一些当今的宇宙学数据作为我们的初始条件，我们对σ₂=0情形的研究结果如下：(i)为了使几何流体具有正能量密度ρ_g，对于所有维度直到D=11，参数σ₁必须为负；(ii)对于合适的σ₁选择，减速参数在过去和未来经历符号变化，同时它处于一个负值范围内，这意味着当前观测到的宇宙加速膨胀阶段可以完全由时空曲率驱动；(iii) q经历符号变化，并且随着时空维度D的增加，这种符号变化分别在过去和未来发生在更早和更晚的时刻。

英文摘要：

Recently we showed that in FLRW cosmology, the contribution from higher curvature terms in any generic metric gravity theory to the energy-momentum tensor is of the perfect fluid form. Such a geometric perfect fluid can be interpreted as a fluid remaining from the beginning of the universe where the string theory is thought to be effective. Just a short time after the beginning of the Universe, it is known that the Einstein-Hilbert action is assumed to be modified by adding all possible curvature invariants. We propose that the observed late-time accelerating expansion of the Universe can be solely driven by this geometric fluid. To support our claim, we specifically study the quadratic gravity field equations in $D$-dimensions. We show that the field equations of this theory for the FLRW metric possess a geometric perfect fluid source containing two critical parameters $σ_1$ and $σ_2$. To analyze this theory concerning its parameter space $(σ_1, σ_2)$, we obtain the general second-order nonlinear differential equation governing the late-time dynamics of the deceleration parameter $q$. Hence using some present-day cosmological data as our initial conditions, our findings for the $σ_2=0$ case are as follows: $ (i)$ In order to have a positive energy density for the geometric fluid $ρ_g$, the parameter $σ_1$ must be negative for all dimensions up to $D = 11$, $(ii)$ For a suitable choice of $σ_1$, the deceleration parameter experiences signature changes in the past and future, and in the meantime it lies within a negative range which means that the current observed accelerated expansion phase of the Universe can be driven solely by the curvature of the spacetime, $(iii)$ $q$ experiences a signature change and as the dimension $D$ of spacetime increases, this signature change happens at earlier and later times, in the past and future, respectively.

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