关于宇宙中的暗能量

近年中，WMAP卫星通过对微波背景辐射的各向异性的详尽测量，推断出了宇宙物质各种主要组分的密度．它引起了人们广泛的兴趣和关注．其中两点倍受注意：一是真空对宇宙总密度的贡献最大，约占70％；二是实物只约占30％，而其5/6是非常规的暗物质．这些事实向物理学提出了两个很深刻的问题：什么是真空能？以及什么是非重子暗物质？我在上一篇文章中已讨论了后一个问题，即暗物质问题．本文要讨论的是前一个问题，它原叫真空能问题，现在人们常叫它暗能量问题．这问题的物理渊源较复杂．本文的意图是对它的来龙和去脉做些初步的说明．     

第三讲宇宙中的暗物质和暗能量

文章对暗物质粒子的候选者和宇宙中暗能量的研究现状作一简单介绍．     

“暗宇宙”之谜

 夜幕降临,星光灿烂,银河、星斗与皎洁的月光交辉相映,编织出美丽的夜空,这就是我们从小获得的知识:恒星、星云、星系……构成的浩瀚宇宙。然而,这只是个错觉,我们的宇宙实际上是“暗无天日”的,这就是2003年过去一年最新的科学成就:宇宙是黑暗的,或者说是个“暗宇宙”(请参阅封二精美彩图)。什么是暗宇宙?那是指组成宇宙的绝大多数物质和能量是不可见的,发光的物质和辐射只占宇宙物质的极小部分,这种不可见的物质和能量称为暗物质和暗能量。根据最新的测量,暗能量和暗物质合计占宇宙质量密度的96%,而我们熟知的恒星、星系等发光物质或重子物质仅仅只占4%。     

反物质和暗能量

 上个世纪,人类在探索宇宙奥秘和物质基本结构及其相互作用的道路上取得了辉煌的成就,建立了描述微观世界的粒子物理标准模型和描述宇观世界的大爆炸宇宙学标准模型。粒子宇宙学将微观世界和宇观世界、粒子物理学和天文学结合起来,研究早期宇宙这一极端条件下的物理规律,探讨基本粒子的相互作用的统一。近代宇宙学研究表明,在宇宙演化过程中经历了暴涨(Inflation)阶段。基于粒子物理的标量场理论,暴涨宇宙学不仅为经典大爆炸宇宙模型中的初始条件和疑难给予了答案,而且提供了一个描述宇宙大尺度结构成因的合理理论,并为近年观测所支持。     

哈勃定律和哈勃常数测定的进展

哈勃常数测定是２０世纪天文学中最重大的发现。哈勃定律的发现架起了天文学和近代物理学之间的桥梁。近年来;由两者结合而形成的一门新的学科－近代天体物理学一直处于自然科学的前沿领域。本文系统地介绍了哈勃定律的发现过程及其重要意义。哈勃常数是哈勃定律的发现过程及其重要意义。哈勃常数是哈勃定律中的关键物理量。虽然哈勃常数的测量已经超过了半个世纪;但至今仍存在一些不确定的因素。文中叙述了哈勃常数的测量原则和最     

暗能密度测量的新结果

宇宙膨胀的加速是由一种排斥力引起的 .虽然这种所谓的“暗能”被认为占宇宙的 2 / 3左右 ,但是一开始谁也不知道暗能是由什么组成的 .爱因斯坦在 1917年所预言的不随时间变化的“宇宙常数”是对暗能的一种可能的解释 .对暗能还有一些更奇特的解释 ,例如quintessence理论、包括     

第五讲暗能量和德西特时空

最近的天文观测表明，宇宙是在加速膨胀，而不是原来认为的减速膨胀．为解释加速膨胀，必须在宇宙的物质能量中引入暗能量这一成分，文章讨论了暗能量的可能侯选者，特别强调了宇宙常数问题、德西特时空问题以及和德西特时空相关的一些基本问题．     

第三讲 宇宙中的暗物质和暗能量

文章对暗物质粒子的候选者和宇宙中暗能量的研究现状作一简单介绍.     

幻能与宇宙末日

大破裂(big rip)——一种新的宇宙末日图景使当今的宇宙加速膨胀的理论发展到一个新的极端，Dartmouth学院的物理学家Robert Caldwell和他在Caltech的同事Marc Kamionkowski及Nevin Weinberg提出：如果假设中的使宇宙加速膨胀的暗能足够大，它将克服把物体束缚在一起的万有引力、电磁力与强力,不仅星系问的空间继续增加，星系本身也     

最新测量证实“哈勃常数”的实用性

据美国《每日科学》报道,目前研究人员最新使用一种精确方法测量了宇宙的体积大小和年龄,以及它如何快速膨胀．这项测量证实了“哈勃常数”的实用性,它指示出了宇宙的体积大小,证实宇宙的年龄为137．5亿年．同时,该结果证实了宇宙暗能量的强度,暗能量对宇宙的膨胀起到了加速作用．     

大爆炸和宇宙学红移中常被误解的几个观念

列举了大爆炸和宇宙学中几个常被误解的问题,并给出了正确的解释.     

The cosmological constant revisited

I briefly review the observational evidence for a small cosmological constant at the present epoch. This evidence mainly comes from high redshift observations of Type 1a supernovae, which, when combined with CMB observations strongly support a flat Universe with Ω _m + Ω_A ⋍ 1. Theoretically a cosmological constant can arise from zero point vacuum fluctuations. In addition ultra-light scalar fields could also give rise to a Universe which is accelerating driven by a time dependent Λ-term induced by the scalar field potential. Finally a Λ dominated Universe also finds support from observations of galaxy clustering and the age of the Universe.     

The case for the cosmological constant

I present a short overview of current observational results and theoretical models for a cosmological constant. The main motivation for invoking a small cosmological constant (or A-term) at the present epoch has to do with observations of high redshift Type Ia supernovae which suggest an accelerating universe. A flat accelerating universe is strongly favoured by combining supernovae observations with observations of CMB anisotropies on degree scales which give the ‘best-fit’ values Θ_A ⋍ 0.7 and Θ _m ⋍ 0.3. A time dependent cosmological A-term can be generated by scalar field models with exponential and power law potentials. Some of these models can alleviate the ‘fine tuning’ problem which faces the cosmological constant.     

Elliptical Solutions to the Standard Cosmology Model with Realistic Values of Matter Density

We have examined a solution to the FRW model of the Einstein and de Sitter Universe, often termed the standard model of cosmology, using wide values for the normalized cosmic constant (Ω_∧) and spacetime curvature (Ω _k ) with proposed values of normalized matter density. These solutions were evaluated using a combination of the third type of elliptical equations and were found to display critical points for redshift z, between 1 and 3, when Ω_∧ is positive. These critical points occur at values for normalized cosmic constant higher than those currently thought important, though we find this solution interesting because the Ω_∧ term may increase in dominance as the Universe evolves bringing this discontinuity into importance. We also find positive Ω_∧tends towards attractive at values of z which are commonly observed for distant galaxies.     

An approach to dark energy problem through linear invariants

The time evolution of vacuum energy density is investigated in the coherent states of inflationary universe using a linear invariant approach. The linear invariants we derived are represented in terms of annihilation operators. On account of the fact that the coherent state is an eigenstate of an annihilation operator, the wave function.in the coherent state is easily evaluated by solving the eigenvalue equation of the linear invariants. The expectation value of the vacuum energy density is derived using this wave function.Fluctuations of the scalar field and its conjugate momentum are also investigated. Our theory based on the linear invariant shows that the vacuum energy density of the universe in a coherent state is decreased continuously with time due to nonconservative force acting on the coherent oscillations of the scalar field,which is provided by the expansion of the universe. In effect, our analysis reveals that the vacuum energy density decreases in proportion to t-β where β is 3/2 for radiation-dominated era and 2 for matter-dominated era. In the case where the duration term of radiation-dominated era is short enough to be negligible, the estimation of the relic vacuum energy density agrees well with the current observational data.     

Effects of cosmological constant on motion of UHECR particles

Recent astronomical observations manifest that about two-thirds of the whole energy in the Universe is contributed by a small positive cosmological constant A (> 0). Then, an asymptotically de Sitter spacetime is premised naturally. However, physics in the de Sitter spacetime is very different from that in the Minkowski spacetime. As the first step, a covariant formalism of the kinematics in the de Sitter spacetime is presented here. By solving exactly the equations of motion for a field, we obtain the dispersion relation of a free particle. It is noticed that the dispersion relation is dependent on the degree of freedom of angular momentum of the particle. We show the threshold anomaly of the ultra high energy cosmic ray disappears naturally in the framework of the de Sitter kinematics.     

Wormhole with varying cosmological constant

It has been suggested that the cosmological constant is a variable dynamical quantity. A class of solution has been presented for the spherically symmetric space time describing wormholes by assuming the erstwhile cosmological constant Λ to be a space variable scalar, viz., Λ = Λ (r) . It is shown that the averaged null energy condition (ANEC) violating exotic matter can be made arbitrarily small.     

Holographic cosmological constant and dark energy

A general holographic relation between UV and IR cutoff of an effective field theory is proposed. Taking the IR cutoff relevant to the dark energy as the Hubble scale, we find that the cosmological constant is highly suppressed by a numerical factor and the fine tuning problem seems alleviative. We also use different IR cutoffs to study the case in which the universe is composed of matter and dark energy.