本帖最后由 positron 于 2010-7-25 18:30 编辑
What is the Universe Made Of?
One of the key questions that needs to be answered by astrophysicists is what is really out there? And of what is it all made? Without this understanding it is impossible to come to any firm conclusions about how the universe evolved.
什么组成了我们的宇宙？
天体物理学家需要回答的重要问题之一是宇宙中到底都有些什么东西？以及这些东西由什么组成？如果缺乏对这些问题的理解，就不可能给出宇宙如何演化的确切回答。
Protons, Neutrons and Electrons: The Stuff of Life
You, this computer, the air we breathe, and the distant stars are all made up of protons, neutrons and electrons. Protons and neutrons are bound together into nuclei and atoms are nuclei surrounded by a full complement of electrons. Hydrogen is composed of one proton and one electron. Helium is composed of two protons, two neutrons and two electrons. Carbon is composed of six protons, six neutrons and six electrons. Heavier elements, such as iron, lead and uranium, contain even larger numbers of protons, neutrons and electrons. Astronomers like to call all material made up of protons, neutrons and electrons "baryonic matter".
Until about thirty years ago, astronomers thought that the universe was composed almost entirely of this "baryonic matter", ordinary atoms. However, in the past few decades, there has been ever more evidence accumulating that suggests there is something in the universe that we can not see, perhaps some new form of matter.
质子、中子和电子：组成生命的材料
你自己、你面前的电脑、我们呼吸的空气、以及遥远的恒星都是由质子、中子和电子组成的。质子和中子被束缚在一起形成原子核，而原子是原子核外围绕着电子的组合，电子的数量等于原子核电荷数。氢原子由一个质子和一个电子组成，氦原子由两个质子、两个中子以及两个电子组成。碳原子由六个质子、六个中子以及六个电子组成。（这里的说法未考虑同位素，译者注。）更重的元素，如铁、铅、铀等，包含着更多的质子、中子和电子。天文学家将由质子、中子和电子组成的物质称为“重子物质”。
直到大约30年前，天文学家仍然以为我们的宇宙差不多都是由这种“重子物质”组成的，宇宙中都是普通的原子。然而，在过去的几十年中，支持下面观点的证据越来越多：宇宙中有我们看不到的东西，也许是一些新的物质形式。
WMAP and Dark Matter / Dark energy
By making accurate measurements of the cosmic microwave background fluctuations, WMAP is able to measure the basic parameters of the Big Bang model including the density and composition of the universe. WMAP measures the relative density of baryonic and non-baryonic matter to an accuracy of better than a few percent of the overall density. It is also able to determine some of the properties of the non-baryonic matter: the interactions of the non-baryonic matter with itself, its mass and its interactions with ordinary matter all affect the details of the cosmic microwave background fluctuation spectrum.
WMAP和暗物质/暗能量
通过对宇宙微波背景波动的精确测量，WMAP卫星可以测量大爆炸宇宙模型中的各种基本参数，包括宇宙的密度和组成。WMAP测量重子物质和非重子物质的相对密度，对这一数据的测量要比整体密度精确几个百分点。WAMP还能确定非重子物质的一些性质：非重子物质之间的相互作用、非重子物质的质量以及与普通物质的相互作用，这些参数都会对宇宙微波背景波动谱产生影响。
WMAP determined that the universe is flat, from which it follows that the mean energy density in the universe is equal to the critical density (within a 1% margin of error). This is equivalent to a mass density of 9.9 x 10[sup]-30[/sup] g/cm[sup]3[/sup], which is equivalent to only 5.9 protons per cubic meter. Of this total density, we now know the breakdown to be:
WAMP数据说明宇宙是平坦的，这表明宇宙的平均密度等于临界密度（低于1%的误差幅度）。这相当于物质密度大约为 9.9 x 10[sup]-30[/sup] g/cm[sup]3[/sup]，或者相当于每立方米体积5.9个质子。根据这一总密度，我们可以获得如下的统计结果：
- 4.6% Atoms. More than 95% of the energy density in the universe is in a form that has never been directly detected in the laboratory! The actual density of atoms is equivalent to roughly 1 proton per 4 cubic meters.
- 原子（重子物质）占4.6% 。宇宙中超过95%的能量密度形式我们还从未在实验室中直接探测到过！原子的实际密度大约只有4立方米中一个质子。
- 23% Cold Dark Matter. Dark matter is likely to be composed of one or more species of sub-atomic particles that interact very weakly with ordinary matter. Particle physicists have many plausible candidates for the dark matter, and new particle accelerator experiments are likely to bring new insight in the coming years.
- 冷暗物质占23%。暗物质可能由一种或多种和普通物质相互作用很弱的亚原子粒子组成。粒子物理学家给出了暗物质的多种候选者，并且新的粒子加速器可能在未来数年内带来新的发现。
- 72% Dark Energy. The first observational hints of dark energy in the universe date back to the 1980's when astronomers were trying to understand how clusters of galaxies were formed. Their attempts to explain the observed distribution of galaxies were improved if dark energy was present, but the evidence was highly uncertain. In the 1990's, observations of supernova were used to trace the expansion history of the universe (over relatively recent times) and the big surprise was that the expansion appeared to be speeding up, rather than slowing down! There was some concern that the supernova data were being misinterpreted, but the result has held up to this day. In 2003, the first WMAP results came out indicating that the universe was flat (see above) and that the dark matter made up only ~23% of the density required to produce a flat universe. If 72% of the energy density in the universe is in the form of dark energy, which has a gravitationally repulsive effect, it is just the right amount to explain both the flatness of the universe and the observed accelerated expansion. Thus dark energy explains many cosmological observations at once.
- 暗能量占72%。
- Fast moving neutrinos do not play a major role in the evolution of structure in the universe. They would have prevented the early clumping of gas in the universe, delaying the emergence of the first stars, in conflict with the WMAP data. However, with 5 years of data, WMAP is able to see evidence that a sea of cosmic neutrinos do exist in numbers that are expected from other lines of reasoning. This is the first time that such evidence has come from the cosmic microwave background.
- 快速运动的中微子在宇宙的结构演化中起的作用不大。
Another Probe of Dark Matter
By measuring the motions of stars and gas, astronomers can "weigh" galaxies. In our own solar system, we can use the velocity of the Earth around the Sun to measure the Sun's mass. The Earth moves around the Sun at 30 kilometers per second (roughly sixty thousand miles per hour). If the Sun were four times more massive, then the Earth would need to move around the Sun at 60 kilometers per second in order for it to stay on its orbit. The Sun moves around the Milky Way at 225 kilometers per second. We can use this velocity (and the velocity of other stars) to measure the mass of our Galaxy. Similarly, radio and optical observations of gas and stars in distant galaxies enable astronomers to determine the distribution of mass in these systems.
The mass that astronomers infer for galaxies including our own is roughly ten times larger than the mass that can be associated with stars, gas and dust in a Galaxy. This mass discrepancy has been confirmed by observations of gravitational lensing, the bending of light predicted by Einstein's theory of general relativity.
暗物质的其他探索
通过测量恒星和星际气体的运动，天文学家可以“称”星系的质量。以我们的太阳系为例，我们可以利用地球围绕太阳运动的速度算出太阳的质量。地球大约以30km/s（大约60k英里每小时）围绕太阳转动。如果太阳质量是现在的4倍，那么地球就需要以60km/s的速度绕太阳运动，以便保持在现有的轨道上。而太阳以225km/s的速度绕银心转动。我们可以利用这一速度（以及其他恒星的转动速度）计算银河系的质量。同样，对遥远星系中恒星和星际气体的射电和可见光观测结果可以帮助天文学家确定这些星系中的物质分布。
天文学家通过推算得到的星系（包括我们所在的银河系）质量，大约比星系中通过电磁手段观测到的恒星、星际气体及尘埃的质量总和还要多10倍。这种质量差已经被对引力透镜的观测所证实，引力透镜效应是爱因斯坦的广义相对论预言的光线弯曲。
HST Image of a gravitational lens
Text Link for an HST press release describing this image.
By measuring how the background galaxies are distorted by the foreground cluster, astronomers can measure the mass in the cluster. The mass in the cluster is more than five times larger than the inferred mass in visible stars, gas and dust.
通过测量背景星系的光线被前景星系团的扭曲程度，天文学家可以计算星系图的质量。这样得到的星系团质量比由可见恒星、星际气体及尘埃计算得到的数据大5倍。
Candidates for the Dark Matter
What is the nature of the "dark matter", this mysterious material that exerts a gravitational pull, but does not emit nor absorb light? Astronomers do not know.
暗物质的候选者
暗物质的本质是什么？这种神秘的物质可以提供万有引力效应，但不发出、也不吸收光？天文学家不知道有什么可以做到这一点。
There are a number of plausible speculations on the nature of the dark matter:
对暗物质的本质有很多可能的推测：
- Brown Dwarfs: if a star's mass is less than one twentieth of our Sun, its core is not hot enough to burn either hydrogen or deuterium, so it shines only by virtue of its gravitational contraction. These dim objects, intermediate between stars and planets, are not luminous enough to be directly detectable by our telescopes. Brown Dwarfs and similar objects have been nicknamed MACHOs (MAssive Compact Halo Objects) by astronomers. These MACHOs are potentially detectable by gravitational lensing experiments. If the dark matter is made mostly of MACHOs, then it is likely that baryonic matter does make up most of the mass of the universe.
- 褐矮星：如果一个恒星的质量小于太阳质量的1/20，它的核心将不能达到足够的温度以点燃氢或氘，因此它仅仅靠引力收缩发光。这些暗淡的天体，居于恒星和行星之间，不够亮以至于不能被我们的望远镜直接探测到。天文学家给褐矮星和类似的天体起了一个绰号叫大孩子（大质量致密天体），这些大孩子是引力透镜实验潜在的探测目标。如果暗物质主要由大孩子组成，那么就有可能重子物质确实组成了我们宇宙的大部分物质。
- Supermassive Black Holes: these are thought to power distant k quasars. Some astronomers speculate that there may be copious numbers of black holes comprising the dark matter. These black holes are also potentially detectable through their lensing effects.
- 超大质量黑洞：它们被推测为遥远的K型类星体提供了能源。一些天文学家猜测可能存在着大量的黑洞形成了暗物质。这些黑洞也是引力透镜实验潜在的探测目标。
- New forms of matter: particle physicists, scientists who work to understand the fundamental forces of nature and the composition of matter, have speculated that there are new forces and new types of particles. One of the primary motivations for building "supercolliders" is to try to produce this matter in the laboratory. Since the universe was very dense and hot in the early moments following the Big Bang, the universe itself was a wonderful particle accelerator. Cosmologists speculate that the dark matter may be made of particles produced shortly after the Big Bang. These particles would be very different from ordinary "baryonic matter". Cosmologists call these hypothetical particles WIMPs (for Weakly Interacting Massive Particles) or "non-baryonic matter".
- 新的物质形式：粒子物理学家，即研究自然界基本力和物质基本组成的科学家，猜测存在着新的相互作用形式和新的粒子。建造“超级对撞机”的基本动机之一就是试图在实验室中产生这种新的物质。根据大爆炸理论，宇宙早期具有很高的密度并且非常热，因此宇宙自身就是一个极好的粒子加速器。宇宙学家猜测暗物质可能由大爆炸后不久产生的粒子组成，这些粒子或许和普通的“重子物质”完全不同。宇宙学家将这些假象粒子成为WIMP（弱相互作用大质量粒子）或“非重子物质”。
Dark Energy: a Cosmological Constant?
Dark Energy makes up a large majority ot the total content of the universe, but this was not always known. Einstein first proposed the cosmological constant (not to be confused with the Hubble Constant) usually symbolized by the greek letter "lambda" (Λ), as a mathematical fix to the theory of general relativity. In its simplest form, general relativity predicted that the universe must either expand or contract. Einstein thought the universe was static, so he added this new term to stop the expansion. Friedmann, a Russian mathematician, realized that this was an unstable fix, like balancing a pencil on its point, and proposed an expanding universe model, now called the Big Bang theory. When Hubble's study of nearby galaxies showed that the universe was in fact expanding, Einstein regretted modifying his elegant theory and viewed the cosmological constant term as his "greatest mistake".
Many cosmologists advocate reviving the cosmological constant term on theoretical grounds, as a way to explain the rate of expansion of the universe. Modern field theory associates this term with the energy density of the vacuum. For this energy density to be comparable to other forms of matter in the universe, it would require new physics theories. So the addition of a cosmological constant term has profound implications for particle physics and our understanding of the fundamental forces of nature.
The main attraction of the cosmological constant term is that it significantly improves the agreement between theory and observation. The most spectacular example of this is the recent effort to measure how much the expansion of the universe has changed in the last few billion years. Generically, the gravitational pull exerted by the matter in the universe slows the expansion imparted by the Big Bang. Very recently it has become practical for astronomers to observe very bright rare stars called supernova in an effort to measure how much the universal expansion has slowed over the last few billion years. Surprisingly, the results of these observations indicate that the universal expansion is speeding up, or accelerating! While these results should be considered preliminary, they raise the possibility that the universe contains a bizarre form of matter or energy that is, in effect, gravitationally repulsive. The cosmological constant is an example of this type of energy. Much work remains to elucidate this mystery!
There are a number of other observations that are suggestive of the need for a cosmological constant. For example, if the cosmological constant today comprises most of the energy density of the universe, then the extrapolated age of the universe is much larger than it would be without such a term, which helps avoid the dilemma that the extrapolated age of the universe is younger than some of the oldest stars we observe! A cosmological constant term added to the standard model Big Bang theory leads to a model that appears to be consistent with the observed large-scale distribution of galaxies and clusters, with WMAP's measurements of cosmic microwave background fluctuations, and with the observed properties of X-ray clusters.
source：http://map.gsfc.nasa.gov/universe/uni_matter.html
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