📚 Cosmology | 宇宙学
Cosmology is the branch of astronomy that investigates the origin, large-scale structure, evolution, and ultimate fate of the entire universe. It draws on observations of distant galaxies, the cosmic microwave background, and the laws of physics to build a coherent picture of the cosmos from the Big Bang to the present day.
宇宙学是天文学的一个分支，研究整个宇宙的起源、大尺度结构、演化和最终命运。它利用对遥远星系、宇宙微波背景的观测以及物理定律，构建出从大爆炸至今的宇宙连贯图景。
1. Our Solar System and Our Place | 我们的太阳系与我们在宇宙中的位置
Our Solar System consists of the Sun, eight planets, dwarf planets, moons, asteroids, and comets, all held together by gravity. The Sun is a medium-sized star that provides the energy necessary for life on Earth.
我们的太阳系包括太阳、八大行星、矮行星、卫星、小行星和彗星，它们都由引力维系在一起。太阳是一颗中等大小的恒星，为地球上的生命提供所需的能量。
The Solar System resides in the Milky Way galaxy, a vast collection of hundreds of billions of stars. Our Sun is located about 26,000 light-years from the galactic centre, in one of the galaxy's spiral arms.
太阳系位于银河系内，银河系是一个由数千亿颗恒星组成的庞大集合体。我们的太阳距离银河系中心约26,000光年，处于星系的一条旋臂之中。
Understanding our local neighbourhood is the first step in grasping the immense scales of cosmology. Just as Earth is one planet among many, the Sun is one star among billions in its galaxy.
了解我们所在的本地环境是理解宇宙学巨大尺度的第一步。正如地球是众多行星中的一颗，太阳也是银河系数千亿颗恒星中的一颗普通恒星。
2. Galaxies: Building Blocks of the Universe | 星系：宇宙的组成单元
A galaxy is a massive, gravitationally bound system comprising stars, stellar remnants, interstellar gas, dust, and dark matter. Galaxies are the fundamental building blocks of the visible universe and come in several distinct shapes.
星系是由恒星、恒星残骸、星际气体、尘埃和暗物质组成的巨大引力束缚系统。星系是可见宇宙的基本组成单元，具有几种不同的形态。
| Galaxy Type | Shape | Example |
|---|---|---|
| Spiral | Flat disk with spiral arms and a central bulge | Milky Way, Andromeda |
| Elliptical | Oval or spherical, little gas and dust | M87 |
| Irregular | No definite shape; often rich in gas | Large Magellanic Cloud |
Our Milky Way is a barred spiral galaxy about 100,000 light-years in diameter. Together with the Andromeda galaxy and about 80 smaller galaxies, it forms the Local Group, a typical galaxy group.
我们的银河系是一个直径约10万光年的棒旋星系。它与仙女座星系以及约80个较小的星系一起组成了本星系群，这是一个典型的星系群。
Galaxies are not isolated; they interact and merge over cosmic time. These interactions can trigger bursts of star formation and reshape galactic structures.
星系并非孤立存在；它们在宇宙时间尺度上相互作用和合并。这些相互作用会引发恒星形成的爆发，并重塑星系的结构。
3. Large-Scale Structure of the Universe | 宇宙的大尺度结构
On scales larger than individual galaxies, matter is organised into clusters and superclusters. Galaxy clusters contain hundreds to thousands of galaxies bound by gravity, along with hot gas and dark matter.
在比单个星系更大的尺度上，物质组织成星系团和超星系团。星系团包含数百至数千个由引力束缚的星系，以及热气体和暗物质。
Superclusters are even larger agglomerations spanning hundreds of millions of light-years. For instance, our Local Group is part of the Virgo Supercluster. Beyond superclusters, galaxies form enormous filaments and sheets, leaving vast empty regions called voids.
超星系团是更大的聚合体，跨越数亿光年。例如，我们的本星系群是室女座超星系团的一部分。在超星系团之外，星系形成巨大的纤维状结构和片状结构，留下被称为空洞的巨大空旷区域。
The observable universe is the spherical region centred on Earth from which light has had time to reach us since the Big Bang. Its diameter is estimated at about 93 billion light-years.
可观测宇宙是以地球为中心的球状区域，自大爆炸以来光有足够时间从那里到达我们。其直径估计约为930亿光年。
4. Evidence for an Expanding Universe: Redshift | 宇宙膨胀的证据：红移
When astronomers examine the light from distant galaxies, the dark absorption lines in their spectra are found to be shifted towards the red end of the spectrum. This phenomenon is known as redshift.
当天文学家检查来自遥远星系的光时，会发现其光谱中的暗吸收线向红端移动。这种现象被称为红移。
Redshift (z) is defined by the fractional increase in wavelength: z = (λobserved - λrest) / λrest. For galaxies outside the Local Group, z is positive, indicating that the wavelengths are stretched.
红移 (z) 由波长的相对增量定义：z = (λ观测 - λ静止) / λ静止。对于本星系群以外的星系，z 为正值，表明波长被拉长。
This stretching is interpreted as a Doppler-like effect caused by galaxies receding from us. The fact that almost all distant galaxies show a redshift tells us that the universe is expanding in all directions.
这种拉伸被解释为由星系远离我们引起的类多普勒效应。几乎所有遥远星系都呈现红移这一事实告诉我们，宇宙正在向四面八方膨胀。
It is space itself that expands, carrying galaxies with it. The raisin bread analogy is often used: as dough rises, raisins move apart from one another, not because they are travelling through the dough, but because the dough itself expands.
膨胀的是空间本身，它携带着星系运动。葡萄干面包常被用作类比：面团发酵膨胀时，葡萄干彼此远离，这并不是因为葡萄干在面团中移动，而是因为面团本身在膨胀。
5. Hubble's Law and the Age of the Universe | 哈勃定律与宇宙年龄
In 1929, Edwin Hubble discovered a linear relationship between the recessional velocity of a galaxy and its distance from Earth. This is summarised as v = H0 d, where H0 is the Hubble constant.
1929年，埃德温·哈勃发现星系的退行速度与其到地球的距离呈线性关系。这总结为 v = H0 d，其中 H0 是哈勃常数。
The value of the Hubble constant is approximately 70 km s-1 Mpc-1. This means that a galaxy 1 megaparsec (about 3.26 million light-years) away recedes at about 70 km/s.
哈勃常数的值约为 70 km s-1 Mpc-1。这意味着一个距离1百万秒差距（约326万光年）的星系以大约70 km/s的速度退行。
Hubble's Law implies that the universe is expanding uniformly. By imagining the expansion running backward, cosmologists can estimate the time when all matter was concentrated in an infinitely dense point—the Big Bang. This gives an age of roughly 13.8 billion years.
哈勃定律意味着宇宙正在均匀膨胀。通过设想膨胀倒放，宇宙学家可以估算出所有物质聚集在一个无穷密度的点——大爆炸——的时间。由此得出宇宙年龄大约为138亿年。
6. The Big Bang Theory | 大爆炸理论
The Big Bang theory proposes that the universe began about 13.8 billion years ago from an extremely hot, dense singularity. Space, time, energy, and matter all came into existence at that moment, and the universe has been expanding and cooling ever since.
大爆炸理论提出，宇宙约在138亿年前从一个极热、极密的奇点开始。空间、时间、能量和物质都在那一刻诞生，此后宇宙便不断膨胀和冷却。
It is crucial to understand that the Big Bang was not an explosion into pre-existing space; it was the rapid expansion of space itself. In the earliest moments, the universe was a searing soup of fundamental particles.
关键是要理解，大爆炸并不是在已有空间中的爆炸，而是空间本身的急速膨胀。在最初时刻，宇宙是一锅由基本粒子组成的高温浓汤。
As the universe expanded, it cooled enough for protons and neutrons to form, then light nuclei like hydrogen, helium, and small amounts of lithium. This process, called Big Bang nucleosynthesis, successfully predicts the observed cosmic abundances of these light elements.
随着宇宙膨胀，它冷却到足以形成质子和中子，然后形成氢、氦和少量锂等轻原子核。这一过程称为大爆炸核合成，成功预言了观测到的这些轻元素的宇宙丰度。
- Galaxy redshifts show universal expansion
- Cosmic microwave background radiation is the relic heat
- Abundance of light elements matches nucleosynthesis predictions
- 星系红移表明宇宙正在膨胀
- 宇宙微波背景辐射是残余的热量
- 轻元素丰度与核合成预测相符
7. Cosmic Microwave Background (CMB) | 宇宙微波背景辐射
The cosmic microwave background is a faint glow of microwave radiation that fills all of space. It was discovered accidentally by Penzias and Wilson in 1965 and is considered the strongest piece of evidence for the Big Bang.
宇宙微波背景是充满整个空间的微弱微波辐射。它于1965年被彭齐亚斯和威尔逊意外发现，被认为是大爆炸最有力的证据。
The CMB is the cooled remnant of the intense radiation that filled the early universe. As the universe expanded and cooled, the radiation was stretched to longer wavelengths, now peaking in the microwave region at a temperature of approximately 2.73 K.
宇宙微波背景是早期宇宙充满的强辐射经冷却后的残余。随着宇宙膨胀和冷却，辐射被拉伸到更长的波长，现在在微波波段达到峰值，温度约为2.73 K。
The CMB is remarkably uniform across the sky, with tiny temperature variations of only about 1 part in 100,000. These anisotropies reflect slight density differences in the early universe that eventually grew into galaxies and galaxy clusters.
宇宙微波背景在全天空极其均匀，温度涨落仅有约十万分之一。这些各向异性反映了早期宇宙中微小的密度差异，这些差异最终成长为星系和星系团。
Mapping these fluctuations with satellites such as COBE, WMAP, and Planck has provided a precise 'baby picture' of the universe when it was only 380,000 years old.
通过COBE、WMAP和Planck等卫星绘制这些涨落，为我们提供了宇宙仅38万年时的精确“婴儿照”。
8. Stellar Life Cycles and Cosmic Chemical Evolution | 恒星生命周期与宇宙化学演化
Stars are born within giant molecular clouds of gas and dust. Under self-gravity, a protostar forms and, once core temperatures reach millions of kelvin, nuclear fusion begins, joining hydrogen into helium in the main sequence phase.
恒星诞生于巨大的气体尘埃分子云中。在自身引力作用下，原恒星形成，一旦核心温度达到数百万开尔文，核聚变开始，在主序星阶段将氢聚变为氦。
The subsequent evolution of a star depends on its mass. A star like the Sun will expand into a red giant and eventually shed its outer layers, leaving a white dwarf. Stars much more massive than the Sun end in violent supernova explosions.
恒星后续的演化取决于其质量。类似太阳的恒星会膨胀成红巨星，最终抛射外层，留下白矮星。质量远大于太阳的恒星则以剧烈的超新星爆发告终。
In the cores of stars and during supernova explosions, elements heavier than helium — carbon, oxygen, silicon, iron, and even gold — are synthesised. These elements are scattered into space, seeding future generations of stars and planets.
在恒星核心和超新星爆发过程中，比氦更重的元素——碳、氧、硅、铁甚至金——得以合成。这些元素被散布到太空，为后代的恒星和行星播下种子。
Every atom in our bodies heavier than hydrogen was forged in the hearts of stars that lived and died long before our Solar System formed. Cosmology thus connects the grandest scales to our own existence.
我们身体中每一个比氢重的原子，都是在太阳系形成之前早已诞生与死亡的恒星核心中锻造出来的。宇宙学由此将最宏大的尺度和我们自身的存在联系起来。
9. Methods of Exploring the Cosmos | 探索宇宙的方法
Astronomers exploit the entire electromagnetic spectrum to gather information about the universe. Radio telescopes reveal cold hydrogen clouds and the CMB; infrared observatories pierce dust clouds; optical telescopes capture starlight; X-ray and gamma-ray telescopes detect high-energy phenomena.
天文学家利用整个电磁波谱来收集宇宙的信息。射电望远镜揭示冷氢云和CMB；红外天文台穿透尘埃云；光学望远镜捕捉星光；X射线和伽马射线望远镜探测高能现象。
Distance measurement is fundamental. For relatively nearby stars, parallax provides direct distances. For galaxies, astronomers use standard candles such as Cepheid variable stars and Type Ia supernovae, whose intrinsic brightness is known, to gauge vast cosmological distances.
距离测量是基础。对于较近的恒星，视差法可直接提供距离。对于星系，天文学家使用标准烛光，如造父变星和Ia型超新星，它们的固有亮度已知，用以测量巨大的宇宙学距离。
Spectroscopy uses diffraction gratings to spread light into its component wavelengths. The spectra not only reveal redshift and hence recessional velocity, but also the chemical composition and temperature of celestial objects.
光谱学利用衍射光栅将光分解为不同波长。光谱不仅能揭示红移进而得出退行速度，还能展示天体的化学成分和温度。
Space telescopes like Hubble and the James Webb Space Telescope observe above Earth's atmosphere, avoiding blurring and absorption, and have revolutionised our understanding of galaxy formation and the early universe.
哈勃和詹姆斯·韦伯等太空望远镜在地球大气层上方观测，避免了模糊和吸收效应，彻底改变了我们对星系形成和早期宇宙的认识。
10. Future of Cosmology and Open Questions | 宇宙学的未来与开放问题
Despite the remarkable success of the Big Bang model, profound mysteries remain. Observations of galaxy rotation curves and gravitational lensing suggest the existence of dark matter, an unseen form of matter that exerts gravitational pull but does not emit light.
尽管大爆炸模型取得了非凡的成功，仍然存在深层的奥秘。对星系旋转曲线和引力透镜的观测表明暗物质的存在，这是一种不可见、产生引力但不发光的物质形式。
Even more puzzling, measurements of distant supernovae show that the universe's expansion is accelerating. This acceleration is attributed to dark energy, a mysterious repulsive force that makes up about 68% of the total energy content of the cosmos.
更令人困惑的是，对遥远超新星的测量显示宇宙膨胀正在加速。这种加速归因于暗能量，一种神秘的排斥力，约占宇宙总能量含量的68%。
The ultimate fate of the universe depends on the balance between gravity and dark energy. Possibilities include an eternal expansion leading to a 'Big Freeze', a reversal into a 'Big Crunch', or a 'Big Rip' in which dark energy tears everything apart.
宇宙的最终命运取决于引力和暗能量之间的平衡。可能的结果包括导致“大冻结”的永恒膨胀、反转为“大挤压”，或者暗能量撕裂一切的“大
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