结构形成(Structure formation)_维基词条翻译

来挣一下正电子版主的牧夫币,哈哈.

本来前几天就开始翻译了,因为别的事情,一直拖拖拉拉的,现在才弄完;


                                                               Structure formation
                                                                   结构形成


Structure formation refers to a fundamental problem in physical cosmology. The universe, as is now known from observations of the cosmic microwave background radiation, began in a hot, dense, nearly uniform state approximately 13.7 Gyr ago.[1] However, looking in the sky today, we see structures on all scales, from stars and planets to galaxies and, on much larger scales still, galaxy clusters, and enormous voids between galaxies. How did all of this come about from the nearly uniform early universe?[2][3][4][5]
结构形成是指物理宇宙学中的一个基础问题.现在由宇宙微波背景辐射的观测人们知道,宇宙于约137亿年以前开始于一个热,致密且几乎均匀的状态[文献1]. 然而,如果看看今天的天空的话我们会看到各种尺度的结构,从恒星和行星到星系,更大的尺度上依然有结构,如星系团和星系之间的巨大的空洞(void).这些是怎么从早期近乎均匀的宇宙中产生的?[文献2,3,4,5]

Contents
目录
1 Overview
1 概述

2 Very early universe
2 甚早期宇宙

    2.1 The horizon problem
    2.1 视界问题


3 Primordial plasma
3 原初等离子体

   3.1 Acoustic oscillations
   3.1 声学振荡

4 Linear structure
4 线性结构

5 Nonlinear structure
5 非线性结构

6 Gastrophysical evolution
6 气体天体物理演化

7 Modelling structure formation
7 结构形成的模型化

   7.1 Cosmological perturbations
   7.1 宇宙学扰动

   7.2 Inflation and initial conditions
   7.2 暴涨与初始条件

8 References
8 参考文献



Overview
概述


Under present models, the structure of the visible universe was formed in the following stages:
现有模型下,可见宇宙的结构形成经历了如下几个阶段:


The very early universe In this stage, some mechanism, such as cosmic inflation is responsible for establishing the initial conditions of the universe: homogeneity, isotropy and flatness.[3][6]
甚早期宇宙  在这一时期,一些机制,例如宇宙暴涨造就了宇宙的初始条件:均匀,各向同性以及平坦.[文献3,6]

The primordial plasma The universe is dominated by radiation for most of this stage, and due to free streaming structures cannot be amplified gravitationally. Nonetheless, important evolution takes place; big bang nucleosynthesis creates the primordial elements and the cosmic microwave background is emitted. The detailed anisotropy structure of the cosmic microwave background is also created in this epoch.[2]
原初等离子体  在这个阶段的大部分时间内,宇宙由辐射主导,而且由于自由流(free streaming) 的作用结构不能被引力放大. 不过,重要的演化还是发生了;大爆炸核合成创造了原初的元素,宇宙微波背景也发射了出来.宇宙微波背景在细节上的各向异性也在这一时期产生.[文献2]

Linear growth of structure Once matter, in particular cold dark matter, dominates the universe gravitational collapse can start to amplify the tiny inhomogeneities left by cosmic inflation, causing matter to fall towards dense regions and making rarefied regions more rarefied. In this epoch, the density inhomogeneities are described by a simple linear differential equation.[4][7]
结构的线性增长 一旦物质,特别是冷暗物质主宰了宇宙,引力坍缩开始放大宇宙暴涨留下的细小的不均匀性,使得物质掉向致密的区域并使稀有的区域更加稀有.在这一阶段,密度不均匀性可用一个简单的线性微分方程来表述.[文献4,7]


Nonlinear growth of structure As the dense regions become denser, the linear approximation describing density inhomogeneities begins to break down – adjacent particles may even begin to cross in caustics – and a more detailed treatment, using the full Newtonian theory of gravity, becomes necessary. (Aside from the background expansion of the universe, which is due to general relativity, evolution on these comparatively small scales is usually well approximated by the Newtonian theory.) This is where structures, such as galaxy clusters and galaxy haloes begin to form. Still, in this regime only gravitational forces are significant because dark matter, which is thought to have very weak interactions, is the dominant player.[8]
结构的非线性增长 随着致密的区域变得更加致密,描述密度非均匀性的线性近似开始失效---临近的粒子甚至开始穿越焦散线--这需要更细致的处理,也就是使用完整的牛顿引力理论(除了广义相对论性的宇宙背景膨胀以外,小尺度上的演化可以有牛顿理论很好近似).而这正是星系团及星系暗晕开始形成的尺度.跟前个阶段一样,在这一时期只有引力是重要的,因为此时暗物质是主角,而其被认为是只有很微弱的相互作用.


"Gastrophysical" evolution The final step of the evolution is when electromagnetic forces become important in the evolution of structure, where baryonic matter clusters densely, as in galaxies and stars. In some cases, such as active galactic nuclei and quasars, Newtonian theory works poorly and general relativity becomes significant. It is called "gastrophysical" because of its complexity: many different, complicated effects, including gravity, magnetohydrodynamics and nuclear reactions must be taken into account.[8]
“气体天体物理学”演化 最后一步,电磁力在结构演化中变得重要,重子物质致密成团,如在星系和恒星中一样.有些情况下,比如在活动星系核和类星体中,牛顿理论效果不佳而广义相对论则变得重要.由于其复杂性,这个过程被叫做”气体天体物理的(gastrophysical)” :许多不同的,复杂的效应,包括引力,磁流体动力学(magnetohydrodynamics)以及核反应必须考虑在内.

The last three stages of evolution occur at different times depending on the scale. The largest scales in the universe are still well-approximated by linear theory, whereas galaxy clusters and superclusters are nonlinear, and many phenomena in the local galaxy must be modelled by a more nuanced approach, accounting for all the forces. This is what is called hierarchical structure formation: the smallest gravitationally bound structures – quasars and galaxies – form first, followed by groups, clusters and superclusters of galaxies. It is thought that, because of the presence of dark energy in our universe, no larger structures will be able to form.
最后三个阶段在不同尺度上发生的时间并不相同.宇宙在最大尺度上依然可以被很好地做线性近似,而星系团及超星系团则是非线性的,星系内的现象则需要更细致的模型来表述,所有的力都要考虑进去.这个现象被称为等级的结构形成:最小的引力束缚结构--类星体与星系--最先形成,接着是星系群,星系团以及超团.人们认为,由于暗能量的存在,不可能再形成更大的结构.


Very early universe
甚早期宇宙


The very early universe is still a poorly-understood epoch, from the viewpoint of fundamental physics. The prevailing theory, cosmic inflation, does a good job explaining the observed flatness, homogeneity and isotropy of the universe, as well as the absence of exotic relic particles (such as magnetic monopoles). In addition, it has made a crucial prediction that has been borne out by observation: that the primordial universe would have tiny perturbations which seed the formation of structure in the later universe. These fluctuations, while they form the foundation for all structure in the universe, appear most clearly as tiny temperature fluctuations at one part in 100,000. (To put this in perspective, the same level of fluctuations on a topographic map of the United States would show no feature higher than a few meters high.) These fluctuations are critical, because they provide the seeds from which the largest structures within the universe can grow and eventually collapse to form galaxies and stars. COBE (Cosmic Background Explorer) provided the first detection of the intrinsic fluctuations in the cosmic microwave background radiation in the 1990s.
从基础物理的角度来说,甚早期宇宙依然是一个很少被人们了解的时期.目前盛行的理论,宇宙暴涨学说很好地解释了宇宙的平坦性,均匀及各向同性,以及为什么存在奇异的残余粒子(relic particles)(例如磁单极子(magnetic monopoles)).此外,该理论还做了一个已被观测证实的关键预言:原初宇宙具有微小的扰动,这些扰动是日后结构形成的种子.这些构成宇宙中所有结构形成基础的扰动,非常清晰地表现为十万分之一的微小的温度扰动.(直观一些,同样量级的扰动在美国地形图上不会超过几米高.)这些扰动非常关键,因为它们提供了种子,宇宙中最大的结构就由这样的种子生长而来,并最终坍缩形成星系及恒星.COBE(cosmic Background Explorer)卫星首次在20世纪90年代探测到了宇宙微波背景上的这些内秉扰动.

These perturbations are thought to have a very specific character: they form a Gaussian random field whose covariance function is diagonal and nearly scale-invariant. The observed fluctuations appear to have exactly this form, and in addition the spectral index measured by WMAP – the spectral index measures the deviation from a scale-invariant (or Harrison-Zel'dovich) spectrum – is very nearly the value predicted by the simplest and most robust models of inflation. Another important property of the primordial perturbations, that they are adiabatic (or isentropic between the various kinds of matter that compose the universe), is predicted by cosmic inflation and has been confirmed by observations.
人们认为这些扰动有一个很特殊的特性:其协方差函数是对角化并且近乎尺度无关的.人们观测到的扰动表现出严格的这种形式,此外WMAP测到的谱指数--谱指数表征跟尺度无关谱(或者叫 Harrison-Zel’dovich谱)的偏离--非常接近最简单有效的暴涨模型的预言.原初扰动的另一个重要的性质是它们是绝热的(或者说在构成宇宙的不同的物质之间是等熵的),这也是宇宙暴涨学说预言出来的,并已被观测证实.

Other theories of the very early universe, which are claimed to make very similar predictions, have been proposed, such as the brane gas cosmology, cyclic model, pre-big bang model and holographic universe, but they remain in their nascency and are not as widely accepted. Some theories, such as cosmic strings have largely been refuted by increasingly precise data.
人们也提出过其它的甚早期宇宙理论,例如膜气体宇宙学(brane gas cosmology),循环模型(cyclic model),前爆炸模型(pre-big bang model)以及全息宇宙(holographic universe),这些理论声称也能作出类似的预言,但是它们依然处在被创建阶段并且没有像暴涨那样广为接受.有些理论,如宇宙弦理论(cosmic strings),已被日益增长的精确数据给排除掉.

The horizon problem
视界问题


An extremely important concept in the theory of structure formation is the notion of the Hubble radius, often called simply the horizon as it is closely related to the particle horizon. The Hubble radius, which is related to the Hubble parameter H as R = c / H, where c is the speed of light, defines, roughly speaking, the volume of the nearby universe that has recently (in the last expansion time) been in causal contact with an observer. Since the universe is continually expanding, its energy density is continually decreasing (in the absence of truly exotic matter such as phantom energy). The Friedmann equation relates the energy density of the universe to the Hubble parameter, and shows that the Hubble radius is continually increasing.
结构形成理论中一个极其重要的概念是哈勃半径(Hubble radius)的定义,这一半径经常被简称为视界,因为其与粒子视界(particle horizon)紧密相关.哈勃半径跟哈勃参数的关系是R=c/H,其中c是光速.大致来说,这个半径定义了跟一个观测者最近(最近的膨胀时刻)有因果联系的宇宙的邻近部分的体积.由于宇宙在持续膨胀,其能量密度在持续减小(如果不考虑那些奇异的物质的存在性的话,如精灵暗能量(phantom energy)).弗里德曼方程将宇宙的能量密度跟哈勃参数联系了起来,表明哈勃半径在持续增长.

The horizon problem of the big bang cosmology says that, without inflation, perturbations were never in causal contact before they entered the horizon and thus the homogeneity and isotropy of, for example, the large scale galaxy distributions cannot be explained. This is because, in an ordinary Friedmann-Lemaitre-Robertson-Walker cosmology, the Hubble radius increases more rapidly than space expands, so perturbations are only ever entering the Hubble radius, and they are not being pushed out by the expansion of space. This paradox is resolved by cosmic inflation, which suggests that there was a phase of very rapid expansion in the early universe in which the Hubble radius was very nearly constant. Thus, the large scale isotropy that we see today is due to quantum fluctuations produced during cosmic inflation being pushed outside the horizon.
大爆炸的视界问题是说,在没有暴涨的情况下,扰动在进入视界之前不可能有因果联系,因此例如星系的大尺度分布的均匀及各向同性无法解释.这是因为,在通常的弗里德曼-勒梅特-罗伯逊-沃克宇宙学 (Friedmann-Lemaitre-Robertson-Walker cosmology)中,哈勃半径的增长比空间膨胀要快得多,所以扰动从不曾完全扩展至哈勃半径,也不会被空间膨胀给推出去.宇宙暴胀解决了这一悖论.它认为在早期宇宙中有一个非常快速膨胀的时期,期间哈勃半径则几乎是个常数.因此现今我们所见到的大尺度的各向同性其实是由于宇宙暴涨时期产生的量子扰动被推到了视界之外而造成的.


Primordial plasma
原初等离子体


The end of inflation is called reheating, when the inflation particles decay into a hot, thermal plasma of other particles. In this epoch, the energy content of the universe is entirely radiation, with standard model particles having relativistic velocities. As the plasma cools, baryogenesis and leptogenesis are thought to occur, as the quark-gluon plasma cools, electroweak symmetry breaking occurs and the universe becomes principally composed of ordinary protons, neutrons and electrons. As the universe cools further, big bang nucleosynthesis occurs and small quantities of deuterium, helium and lithium nuclei are created. As the universe cools and expands, the energy in photons begins to redshift away, particles become non-relativistic and ordinary matter begins to dominate the universe. Eventually, atoms begin to form as free electrons bind to nuclei. This suppresses Thompson scattering of photons. Combined with the rarefaction of the universe (and consequent increase in the mean free path of photons), this makes the universe transparent and the cosmic microwave background is emitted at recombination (the surface of last scattering).
暴涨的终结被称为再加热(reheating),此时暴涨粒子衰变为一种由其它粒子构成的热等离子体.在此时期,宇宙的能量全是辐射,标准模型粒子具有相对论性的速度.随着等离子体的冷却,重子合成(baryogenesis)与轻子合成(leptogenesis)发生了,而随着夸克-胶子等离子体 (quark-gluon plasma)的冷却,弱电对称破坏发生,宇宙变成了主要由通常的质子,中子与电子构成.随着宇宙的进一步冷却,大爆炸核合成发生,少量的氘,氦以及锂被制造出来.再随着宇宙膨胀,光子的能量开始被红移,粒子变成非相对论性的,普通物质开始主宰宇宙.最终,自由电子跟原子核束缚到一起,原子开始形成.因此汤姆逊散射也受到抑制.考虑到宇宙中自由电子的极低的比率(以及相应的光子的平均自由程的增长),这使得宇宙成为透明,宇宙微波背景则在再复合(最后散射面)的时候被发射了出来.


Acoustic oscillations
声学振荡
Main article: baryon acoustic oscillations
主条目:重子声学振荡

The amplitude of structures does not grow substantially during this epoch. For dark matter the expansion of space (which is caused by the large radiation component) is so rapid that growth is highly suppressed for the non-relativistic dark matter particles. Moreover, because dark matter is pressureless, free-streaming prevents the growth of small structures. In the relativistic fluid, on the other hand, the very large pressure prevents the growth of structures larger than the Jeans length, which is very nearly equal to the Hubble radius for radiation. This causes perturbations to be damped.
结构的强度在这一时期增长不大.对于暗物质来说,空间的膨胀(由大量的辐射组分造成)速率太快以致非相对论性的暗物质粒子的结构增长受到极大的抑制.除此之外,由于暗物质是没有压强的,自由流也阻止了小尺度结构的增长.另一方面,对相对论流体来说,巨大的压强阻止了大于金斯尺度的结构的增长(译注:此处似乎有问题,应该是小于金斯尺度?),而此时辐射的金斯尺度几乎与哈勃半径相等.扰动因此而被减弱.

These perturbations are still very important, however, as they are responsible for the subtle physics that result in the cosmic microwave background anisotropy. In this epoch, the amplitude of perturbations which enter the horizon oscillate sinusoidally, with dense regions becoming more rarefied and then becoming dense again, with a frequency which is related to the size of the perturbation. If the perturbation oscillates an integral or half-integral number of times between coming into the horizon and recombination, it appears as an acoustic peak of the cosmic microwave background anisotropy. (A half-oscillation, in which a dense region becomes a rarefied region or vice-versa, appears as a peak because the anisotropy is displayed as a power spectrum, so underdensities contribute to the power just as much as overdensities.) The physics which determines the detailed peak structure of the microwave background is complicated, but these oscillations provide the essence.[9][10][11][12][13]
尽管如此,这些扰动依然非常重要,因为它们跟产生宇宙微波背景各向异性的精妙的物理过程相关.在这个时期内,进入到视界的扰动的幅度做正弦振荡,致密的区域变得更加稀少更加致密.振荡的频率则跟扰动的尺度有关.如果扰动在进入视界到再复合之间振荡了整数或者半整数次,它会表现为宇宙微波背景上的一个声学峰. (在半振荡(half-oscillation)中,致密的区域变得更少更密,反之也成立,这样的振荡之所以表现为一个峰是因为各向异性是一个幂律谱,低密度区域跟高密度区域对功率谱的贡献一样多 (译注:此处不大明白,幂律谱到底说的是非均匀性还是各向异性,为什么幂律谱就表明低密度区域跟高密度区域的贡献一样?)决定微波背景细致的峰结构的物理过程非常复杂,但本质上是这些振荡在其作用.[参考文献9,10,11,12,13]


Linear structure
线性结构

One of the key realizations made by cosmologists in the 1970s and 1980s was that the majority of the matter content of the universe was composed not of atoms, but rather a mysterious form of matter known as dark matter. Dark matter interacts through the force of gravity, but it is not composed of baryons and it is known with very high accuracy that it does not emit or absorb radiation. It may be composed of particles that interact through the weak interaction, such as neutrinos, but it cannot be composed entirely of the three known kinds of neutrinos (although some have suggested it is a sterile neutrino). Recent evidence suggests that there is about five times as much dark matter as baryonic matter, and thus the dynamics of the universe in this epoch are dominated by dark matter.
20世纪7,80年代,宇宙学家的重要认识之一就是宇宙中主要的物质不是由原子构成,而是一种神秘的物质形式,即通常所说的暗物质.暗物质通过引力相互作用,但它并不由重子构成,而且在很高的精度上人们知道它既不发射也不吸收辐射.它可能由参与弱相互作用的粒子构成,例如中微子,但是不可能完全是三种已知的中微子(尽管有人提议说是惰性中微子(sterile neutrino)).近期的证据表明暗物质约为重子物质的5倍,因此在这个阶段宇宙的动力学由暗物质主导.


Dark matter plays a key role in structure formation because it feels only the force of gravity: the gravitational Jeans instability which allows compact structures to form is not opposed by any force, such as radiation pressure. As a result, dark matter begins to collapse into a complex network of dark matter halos well before ordinary matter, which is impeded by pressure forces. Without dark matter, the epoch of galaxy formation would occur substantially later in the universe than is observed.
暗物质在结构形成中扮演着一个关键的角色,因为它只能感受到引力:使得致密结构可以形成的引力金斯不稳定性不会被任何力给抵消,例如辐射压.结果,暗物质比普通物质要早得多地开始坍缩成暗物质晕的复杂的网状结构,普通物质则受到了压力的阻碍.如果没有暗物质的话,星系形成的时期将会大大晚于观测到的时间.


The physics of structure formation in this epoch is particularly simple, as dark matter perturbations with different wavelengths evolve independently. As the Hubble radius grows in the expanding universe, it encompasses larger and larger perturbations. During matter domination, all causal dark matter perturbations grow through gravitational clustering. However, the shorter-wavelength perturbations that are encompassed during radiation domination have their growth retarded until matter domination. At this stage, luminous, baryonic matter is expected to simply mirror the evolution of the dark matter, and their distributions should closely trace one another.
这一阶段的结构形成的物理非常简单,因为不同波长的暗物质扰动是独立演化的.随着哈勃半径在膨胀宇宙中的增长,它遇到越来越大的扰动.在物质主导时期,所有有因果联系的暗物质扰动都通过引力成团性增长.然而,在辐射为主的时期,其中的短波长扰动的增长受到了阻滞,直至物质主导.在此阶段,发光的,重子的物质只是简单地镜像暗物质演化,它们的分布应当互相紧密跟随.


It is a simple matter to calculate this "linear power spectrum" and, as a tool for cosmology, it is of comparable importance to the cosmic microwave background. The power spectrum has been measured by galaxy surveys, such as the Sloan Digital Sky Survey, and by surveys of the Lyman-α forest. Since these surveys observe radiation emitted from galaxies and quasars, they do not directly measure the dark matter, but the large scale distribution of galaxies (and of absorption lines in the Lyman-α forest) is expected to closely mirror the distribution of dark matter. This depends on the fact that galaxies will be larger and more numerous in denser parts of the universe, whereas they will be comparatively scarce in rarefied regions.
计算这样的”线性功率谱”是比较简单的,做为一个宇宙学工具,它的重要性可以跟宇宙微波背景相提并论.人们已经通过星系巡天,如斯隆数字巡天,以及赖曼阿尔法森林巡天测到了功率谱.由于这些巡天测的是来自星系跟类星体的辐射,它们并不是直接测量暗物质,但是人们期望星系的大尺度分布(以及赖曼阿尔法的吸收线的分布)是紧密地跟随暗物质的分布的.这依赖于这样一个事实,在宇宙密度大的区域,星系比较大且比较多,尽管这本身也跟稀有场一样稀少.


Nonlinear structure
非线性结构

When the perturbations have grown sufficiently, a small region might become substantially denser than the mean density of the universe. At this point, the physics involved becomes substantially more complicated. When the deviations from homogeneity are small, the dark matter may be treated as a pressureless fluid and evolves by very simple equations. In regions which are significantly denser than the background, the full Newtonian theory of gravity must be included. (The Newtonian theory is appropriate because the masses involved are much less than those required to form a black hole, and the speed of gravity may be ignored as the light-crossing time for the structure is still smaller than the characteristic dynamical time.) One sign that the linear and fluid approximations become invalid are that dark matter starts to form caustics in which the trajectories of adjacent particles cross, or particles start to form orbits. These dynamics are generally best understood using N-body simulations (although a variety of semi-analytic schemes, such as the Press-Schechter formalism, can be used in some cases). While in principle these simulations are quite simple, in practice they are very difficult to implement, as they require simulating millions or even billions of particles. Moreover, despite the large number of particles, each particle typically weighs 109 solar masses and discretization effects may become significant. The largest such simulation as of 2005 is the Millennium simulation.[14]
当扰动显著增长时,一小块区域可能会变的比宇宙的平均密度要密的多.此时,涉及到的物理过程变得更加复杂.当暗物质偏离均匀分布很小时,它可以被视为没有压强的流体并遵从简单的方程演化.在比背景要密得多的区域内,完整的牛顿引力理论必须被考虑进来.(牛顿理论之所以适用是因为涉及到的质量比形成黑洞的质量要小得多,引力的速度效应可以被忽略,因为对这样的结构来说光的穿越时间依然小于特征的动力学时间.)线性和流体近似失效的一个标志是暗物质开始形成焦散线 (caustics),其中临近粒子的轨迹开始交叉,或者粒子开始沿环绕轨道运行.通过N体的数值模拟人们对这些动力学过程一般已经很好地了解(尽管一些半解析的方法,如Press-Schechter 公式,在有些情况下也可以使用).尽管从理论上来说这些数值模拟相当简单,实际上却是很难付实行,因为这需要模拟上百万甚至上十亿的粒子.此外,即使不考虑这样大的数目,每个粒子通常的质量为10^9太阳质量,离散化效应可能会变显著.最新的这种模拟是2005年的千禧模拟(Millennium simulation).[文献14]


The result of N-body simulations suggest that the universe is composed largely of voids, whose densities might be as low as one tenth the cosmological mean. The matter condenses in large filaments and haloes which have an intricate web-like structure. These form galaxy groups, clusters and superclusters. While the simulations appear to agree broadly with observations, their interpretation is complicated by the understanding of how dense accumulations of dark matter spur galaxy formation. In particular, many more small haloes form than we see in astronomical observations as dwarf galaxies and globular clusters. This is known as the galaxy bias problem, and a variety of explanations have been proposed. Most account for it as an effect in the complicated physics of galaxy formation, but some have suggested that it is a problem with our model of dark matter and that some effect, such as warm dark matter, prevents the formation of the smallest haloes.
N体数值模拟的结果表明,宇宙是由巨大的空洞构成,其密度可低至宇宙平均密度的十分之一.物质聚集于巨大的纤维状结构与暗晕内,具有内秉的网状结构.它们形成了星系群,星系团以及超星系团.尽管数值模拟跟观测大体吻合,其对理解致密的暗物质聚集是如何刺激了星系形成的所做的诠释却是复杂的.特别是,形成的小暗晕比我们观测到的矮星系和球状星团要多.这被称为星系偏袒问题,人们已经提出了多种解释.大多数将其归于星系形成中复杂物理过程的一种效应,但也有模型认为这是我们暗物质模型的问题,一些效应,例如温暗物质(warm dark matter),会阻碍最小的暗晕的形成.

Gastrophysical evolution
气体天体物理演化


The final stage in evolution comes when baryons condense in the centers of galaxy haloes to form galaxies, stars and quasars. A paradoxical aspect of structure formation is that while dark matter greatly accelerates the formation of dense haloes, because dark matter does not have radiation pressure, the formation of smaller structures from dark matter is impossible because dark matter cannot dissipate angular momentum, whereas ordinary baryonic matter can collapse to form dense objects by dissipating angular momentum through radiative cooling. Understanding these processes is an enormously difficult computational problem, because they can involve the physics of gravity, magnetohydrodynamics, atomic physics, nuclear reactions, turbulence and even general relativity. In most cases, it is not yet possible to perform simulations that can be compared quantitatively with observations, and the best that can be achieved are approximate simulations that illustrate the main qualitative features of a process such as star formation.
See also: galaxy formation and evolution and stellar evolution
当重子在星系暗晕的中心收缩并形成星系,恒星与类星体的时候,演化的最后阶段到来.关于结构形成,看起来似乎矛盾的是虽然暗物质由于没有辐射压而大大加速了致密晕的形成,它却不可能形成小的结构,因为其无法损耗掉角动量,而普通的重子物质可以通过辐射冷却损失角动量来坍缩形成致密的天体.理解这一过程是一个极其困难的计算问题,因为其涉及到引力物理,磁流体动力学,原子物理,核反应,湍流甚至广义相对论.在大多数情况下,人们还不可能运行可以跟观测定量比较的数值模拟,人们所能够做到的最好的成就就是做近似的模拟以阐释一个过程的主要的定性特性,如恒星形成.
另见:星系形成与演化和恒星演化.


Modelling structure formation
结构形成的模型化

Cosmological perturbations
宇宙学扰动
Main article: cosmological perturbation theory
主条目:宇宙学扰动理论

Much of the difficulty, and many of the disputes, in understanding the large-scale structure of the universe can be resolved by understanding the choice of gauge in general relativity better. By the scalar-vector-tensor decomposition, the metric includes four scalar perturbations, two vector perturbations, and one tensor perturbation. Only the scalar perturbations are significant: the vectors are exponentially suppressed in the early universe, and the tensor mode makes only a small (but important) contribution in the form of primordial gravitational radiation and the B-modes of the cosmic microwave background polarization. Two of the four scalar modes may be removed by a physically meaningless coordinate transformation. Which modes are eliminated determine the infinite number of possible gauge fixings. The most popular gauge is Newtonian gauge (and the closely related conformal Newtonian gauge), in which the retained scalars are the Newtonian potentials Φ and Ψ, which correspond exactly to the Newtonian potential energy from Newtonian gravity. Many other gauges are used, including synchronous gauge, which can be an efficient gauge for numerical computation (it is used by CMBFAST). Each gauge still includes some unphysical degrees of freedom. There is a so-called gauge-invariant formalism, in which only gauge invariant combinations of variables are considered.
在理解宇宙大尺度结构时的许多的困难和怀疑都可以通过更好地理解广义相对论中的规范的选择而得到解决.通过标量-向量-张量分解,度规包括4个标量扰动,两个向量扰动,还有一个张量扰动.只有标量扰动是重要的:在早期宇宙向量扰动被指数压低,而张量模仅在原初的引力辐射和宇宙微波背景辐射的极化的B模的形式中做了很小(但是很重要)的贡献.通过一个没有物理含义的坐标变换,可以去掉两个标量模.去掉那些模决定了可能的规范固定的无限的数目.最流行的规法是牛顿规范(以及紧密相关的共性牛顿规范),在这一规范中剩下的是牛顿势 Φ和 Ψ,跟牛顿引力中的牛顿势能严格对应.许多其它的规范也被使用,包括同步规范(synchronous),其对数值计算来说是一个很有效的规范(被 CMBFAST采用).每个规范依然包括一些非物理的自由度. 在所谓的规范不变公式中,只有规范不变的变量的组合才被考虑.


Inflation and initial conditions
暴涨与初始条件


The initial conditions for the universe are thought to arise from the scale invariant quantum mechanical fluctuations of cosmic inflation. The perturbation of the background energy density at a given point \rho(\mathbf{x},t) in space is then given by an isotropic, homogeneous Gaussian random field of mean zero. This means that the spatial Fourier transform of ρ – \hat{\rho}(\mathbf{k},t) has the following correlation functions

    \langle\hat{\rho}(\mathbf{k},t)\hat{\rho}(\mathbf{k}',t)\rangle=f(k)\delta^{(3)}(\mathbf{k}-\mathbf{k'}),

where δ(3) is the three dimensional Dirac delta function and k=|\mathbf{k}| is the length of \mathbf{k}. Moreover, the spectrum predicted by inflation is nearly scale invariant, which means

    \langle\hat{\rho}(\mathbf{k},t)\hat{\rho}(\mathbf{k}',t)\rangle=k^{n_s-1}\delta^{(3)}(\mathbf{k}-\mathbf{k'}),

where ns − 1 is a small number. Finally, the initial conditions are adiabatic or isentropic, which means that the fractional perturbation in the entropy of each species of particle is equal.


宇宙的初始条件被认为是来自于宇宙暴涨的尺度无关的量子力学扰动.空间中某给定点的背景能量密度扰动,\rho(\mathbf{x},t),由一个各向同性,均匀的零值高斯随机场给出.这意味着空间的傅立叶变换ρ – \hat{\rho}(\mathbf{k},t)具有如下的相关函数:

\langle\hat{\rho}(\mathbf{k},t)\hat{\rho}(\mathbf{k}',t)\rangle=f(k)\delta^{(3)}(\mathbf{k}-\mathbf{k'})




其中 δ^(3)是三维的狄拉克函数,k=|\mathbf{k}|是k的长度.此外,暴涨预言的谱接近尺度无关,即:

\langle\hat{\rho}(\mathbf{k},t)\hat{\rho}(\mathbf{k}',t)\rangle=k^{n_s-1}\delta^{(3)}(\mathbf{k}-\mathbf{k'})




其中ns-1是一个小的数.最终,初始条件是绝热或者等熵的,这意味着在每种粒子的熵的扰动是相等的.



英文的原文出自:

http://en.wikipedia.org/wiki/Structure_formation