辅助暗能量
Assisted dark energy
摘要：
在辐射和物质时代，宇宙尺度解产生与背景流体密度成比例的标量场密度，这对缓解暗能量的能量尺度问题是有吸引力的。在多个标量场存在的情况下，标度解可以通过所谓的辅助暴胀机制退出到宇宙加速时代。本文详细研究了具有广义拉格朗日密度p= sum_{i=1} ^n X_i g（X_i e^{lambda_i phi_i}）的多场系统的宇宙动力学，其中X_i=-（nabla phi_i）^2/2是第i个场phi_i的动能，lambda_i是常数，g是关于Y_i=X_i e^{lambda_i phi_i}的任意函数.这涵盖了文献中提出的大多数具有标度解的暗能量标量场模型。使用来自大爆炸核合成的束缚和每个场不能作为宇宙的单个组成部分驱动暴胀的条件，我们发现以下特征：（i）在标度物质时代之后，可以实现瞬时或永恒的宇宙加速，（ii）辅助标量场的“解冻”性质对于确定场状态方程w_{phi}的演化至关重要，（iii）在存在多个标量场的情况下，现今的场状态方程可以与观测界w_{phi} <-0.8相一致。
英文摘要：
Cosmological scaling solutions, which give rise to a scalar-field density proportional to a background fluid density during radiation and matter eras, are attractive to alleviate the energy scale problem of dark energy. In the presence of multiple scalar fields the scaling solution can exit to the epoch of cosmic acceleration through the so-called assisted inflation mechanism. We study cosmological dynamics of a multi-field system in details with a general Lagrangian density p=sum_{i=1}^n X_i g(X_i e^{lambda_i phi_i}), where X_i=-(nabla phi_i)^2/2 is the kinetic energy of the i-th field phi_i, lambda_i is a constant, and g is an arbitrary function in terms of Y_i=X_i e^{lambda_i phi_i}. This covers most of the scalar-field models of dark energy proposed in literature that possess scaling solutions. Using the bound coming from Big-Bang-Nucleosynthesis and the condition under which the each field cannot drive inflation as a single component of the universe, we find the following features: (i) a transient or eternal cosmic acceleration can be realized after the scaling matter era, (ii) a "thawing" property of assisting scalar fields is crucial to determine the evolution of the field equation of state w_{phi}, and (iii) the field equation of state today can be consistent with the observational bound w_{phi}<-0.8 in the presence of multiple scalar fields.
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