Cosmology Exam Guide | 宇宙学考点精讲

📚 Cosmology Exam Guide | 宇宙学考点精讲

Cosmology is the branch of physics that studies the origin, structure and evolution of the Universe as a whole. It addresses questions such as: How did the Universe begin? Why is it expanding? What will its final fate be? In this IGCSE Edexcel revision guide, we cover the essential concepts, observations and equations you need for your exams.

宇宙学是物理学中研究宇宙整体起源、结构和演化的分支。它回答的问题包括：宇宙是如何开始的？为什么它在膨胀？它的最终命运是什么？在本文中，我们将系统梳理IGCSE Edexcel考试中你需要掌握的宇宙学核心概念、观测证据和关键公式。

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1. The Structure of the Universe | 宇宙的结构

The Universe is defined as everything that exists: all matter, energy, space and time. It is estimated to contain over two trillion galaxies, each holding hundreds of billions of stars. Our Sun is just one star among roughly 200 billion stars in our galaxy alone.

宇宙的定义是存在的一切：所有物质、能量、空间和时间。据估计，宇宙包含超过两万亿个星系，每个星系拥有数千亿颗恒星。仅我们的银河系就约有2000亿颗恒星，我们的太阳只是其中之一。

Our galaxy is called the Milky Way. It is a large spiral galaxy located within a cluster of galaxies known as the Local Group. Astronomers use the following hierarchy to describe the structure of the Universe:

我们的星系被称为银河系。它是一个巨大的旋涡星系，属于一个叫做本星系群的星系团。天文学家使用以下层级来描述宇宙的结构：

Level | 层级	Contains | 包含	Example | 例子
Planet | 行星	Rock, gas, ice | 岩石、气体、冰	Earth | 地球
Star | 恒星	Gas, emits light | 气体，发光	The Sun | 太阳
Galaxy | 星系	Billions of stars | 数十亿颗恒星	Milky Way | 银河系
Universe | 宇宙	All galaxies and everything | 所有星系及万物	The Universe | 宇宙本身

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2. The Solar System | 太阳系

The Solar System consists of the Sun at its centre, eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune), plus dwarf planets, moons, asteroids and comets. All of these bodies orbit the Sun because of the gravitational attraction between them and the Sun.

太阳系由位于中心的太阳和八大行星（水星、金星、地球、火星、木星、土星、天王星和海王星）组成，此外还有矮行星、卫星、小行星和彗星。所有这些天体都因为与太阳之间的万有引力而绕太阳运行。

The Sun contains over 99% of the total mass of the Solar System, so its gravity dominates the motions of all other objects. The planets are arranged roughly in a flat plane, and they orbit in the same direction.

太阳占据了太阳系总质量的99%以上，因此它的引力主宰着所有其他天体的运动。行星大致分布在同一平面内，并且沿着同一方向绕太阳公转。

For the exam, you must be able to recall that the order of the planets from the Sun is: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. A useful mnemonic is ‘My Very Educated Mother Just Served Us Noodles’.

考试中你需要记住行星距太阳由近及远的顺序：水星、金星、地球、火星、木星、土星、天王星、海王星。助记口诀是：水金地火木土天海。

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3. Gravity and Orbital Motion | 引力与轨道运动

Orbital motion arises from a balance between two factors: gravity, which pulls the orbiting body towards the central object, and the body’s inertia, which would otherwise send it moving in a straight line. The result is a curved path around the central object.

轨道运动源于两种因素的平衡：引力将天体拉向中心物体，而惯性则使天体沿直线运动。二者共同作用的结果，就是围绕中心物体的弯曲运动路径。

For a planet orbiting the Sun, or a satellite orbiting Earth, the gravitational force provides the centripetal force required for circular motion. The key relationship is:

对于绕太阳运行的行星或绕地球运行的卫星，引力提供了圆周运动所需的向心力。关键关系式为：

F = mv²/r

where F is the gravitational force, m is the mass of the orbiting body, v is its orbital speed, and r is the orbital radius.

其中F是引力，m是轨道天体的质量，v是它的轨道速度，r是轨道半径。

Artificial satellites, such as communications satellites, may be placed in geostationary orbits. A geostationary satellite orbits above the equator, moving in the same direction as Earth’s rotation, with a period of 24 hours, so it remains above the same point on Earth’s surface.

人造卫星（如通信卫星）可被送入地球同步轨道。地球同步卫星在赤道上方运行，与地球自转方向相同，周期为24小时，因此它始终位于地球表面同一位置的上方。

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4. The Doppler Effect: Redshift and Blueshift | 多普勒效应：红移与蓝移

The Doppler effect describes how the observed wavelength of a wave changes when the source is moving relative to the observer. When a light source moves away from an observer, the wavelengths of the light are stretched, shifting towards the red end of the spectrum. This is called redshift.

多普勒效应描述了当波源与观察者之间存在相对运动时，观察到的波长所发生的变化。当光源远离观察者时，光的波长被拉长，向光谱的红端移动。这被称为红移。

Conversely, when a light source moves towards an observer, the wavelengths are compressed, shifting towards the blue end of the spectrum. This is called blueshift.

相反，当光源朝观察者运动时，波长被压缩，向光谱的蓝端移动。这被称为蓝移。

The redshift z of a galaxy is defined as the fractional change in wavelength:

星系的红移量z定义为波长的相对变化量：

z = Δλ/λ₀ ≈ v/c

where λ₀ is the rest wavelength, Δλ is the change in wavelength, v is the recessional velocity of the galaxy, and c is the speed of light.

其中λ₀是静止波长，Δλ是波长的变化量，v是星系的退行速度，c是光速。

In 1929, Edwin Hubble observed that the light from nearly all distant galaxies is redshifted. This means that essentially all galaxies are moving away from us, which is powerful evidence that the Universe is expanding.

1929年，埃德温·哈勃发现几乎所有遥远星系的光都发生了红移。这意味着几乎所有星系都在远离我们，这是宇宙正在膨胀的有力证据。

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5. Hubble’s Law | 哈勃定律

Hubble discovered that the recessional velocity of a galaxy is directly proportional to its distance from Earth. This famous relationship is now known as Hubble’s law:

哈勃发现，星系的退行速度与它到地球的距离成正比。这一著名关系现在被称为哈勃定律：

v = H₀ × d

where v is the recessional velocity in metres per second, d is the distance to the galaxy in metres, and H₀ is the Hubble constant.

其中v是退行速度，单位是米每秒；d是到星系的距离，单位是米；H₀是哈勃常数。

The Hubble constant describes the current rate of expansion of the Universe. Its accepted value is approximately:

哈勃常数描述了宇宙当前的膨胀速率。其公认值约为：

H₀ ≈ 2.3 × 10⁻¹⁸ s⁻¹ ≈ 70 km/s/Mpc

Note the two common units: per second (s⁻¹) and kilometres per second per megaparsec (km/s/Mpc). One megaparsec equals about 3.1 × 10¹⁹ km. The IGCSE exam may use either, so always check the units given in the question.

注意两种常见单位：每秒（s⁻¹）和千米每秒每百万秒差距（km/s/Mpc）。1百万秒差距约等于3.1 × 10¹⁹千米。IGCSE考试可能使用其中任何一种，所以务必看清题目给出的单位。

Hubble’s law implies that galaxies that are farther away are moving away from us faster. This is exactly what we expect if the Universe is expanding uniformly: the space between galaxies is stretching everywhere.

哈勃定律表明，距离更远的星系远离我们的速度更快。这正是宇宙均匀膨胀时我们所预期的结果：星系之间的空间在各处都在被拉伸。

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6. The Big Bang Theory | 大爆炸理论

The Big Bang theory is the dominant scientific model for the origin of the Universe. It states that roughly 13.8 billion years ago, the Universe began from an extremely hot, dense single point, and has been expanding and cooling ever since.

大爆炸理论是描述宇宙起源的主流科学模型。它指出，大约138亿年前，宇宙始于一个极热、极稠密的点，并从此一直在膨胀和冷却。

The name ‘Big Bang’ can be misleading: the event was not an explosion that happened in space; rather, it was the expansion of space itself. The Universe did not expand “into” anything, because there was nothing outside the Universe.

“大爆炸”这个名称可能引起误解：它并不是发生在空间中的一次爆炸；相反，它是空间本身的膨胀。宇宙并没有”向”任何东西膨胀，因为宇宙之外空无一物。

According to this model, as the Universe expanded, it also cooled. Within the first few minutes, temperatures were so extreme that only elementary particles existed. As cooling continued, these particles combined to form hydrogen and helium nuclei, and much later, these condensed into stars and galaxies.

根据这一模型，宇宙在膨胀的同时也在冷却。在最初的几分钟内，温度极高，只存在基本粒子。随着进一步冷却，这些粒子结合形成氢核和氦核，再后来，这些物质凝聚成恒星和星系。

It is important to emphasise that the Big Bang was not the explosion of matter into empty space; it was the rapid expansion of space-time itself, carrying matter and radiation with it.

需要强调的是，大爆炸并不是物质向空旷空间的爆炸；而是时空本身的迅速膨胀，物质和辐射随之被携带而去。

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7. Cosmic Microwave Background Radiation | 宇宙微波背景辐射

The cosmic microwave background radiation (CMBR) is faint microwave radiation that fills the entire Universe uniformly in all directions. It is one of the most important pieces of evidence for the Big Bang.

宇宙微波背景辐射（CMBR）是均匀充满整个宇宙、来自所有方向的微弱微波辐射。它是支持大爆炸理论最重要的证据之一。

Shortly after the Big Bang, the Universe was so hot that matter existed as a plasma, and radiation could not travel freely. About 380,000 years later, the Universe had cooled enough for atoms to form; at this point, radiation was finally able to travel freely through space. As the Universe continued to expand, this radiation stretched to longer and longer wavelengths.

大爆炸后不久，宇宙温度极高，物质以等离子体形式存在，辐射无法自由传播。大约38万年后，宇宙冷却到足以形成原子，此时辐射终于能在空间中自由传播。随着宇宙持续膨胀，这些辐射被拉伸到越来越长的波长。

The CMBR we observe today corresponds to a temperature of only about 2.7 K, well within the microwave region of the electromagnetic spectrum. Its discovery in 1965 by Penzias and Wilson confirmed a key prediction of the Big Bang theory.

我们今天观测到的宇宙微波背景辐射对应的温度仅为约2.7K，位于电磁波谱的微波波段。1965年彭齐亚斯和威尔逊的发现证实了大爆炸理论的一个关键预言。

In the exam, you may be asked to explain why the CMBR supports the Big Bang theory: the radiation is the cooled remnant of the early, hot Universe, stretched into microwaves by the expansion of space.

考试中你可能被要求解释宇宙微波背景辐射为何支持大爆炸理论：这种辐射是早期高温宇宙的冷却残余，被空间膨胀拉伸到微波波段。

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8. Summary of Evidence for the Big Bang | 大爆炸理论的证据总结

Two major lines of observational evidence support the Big Bang theory. You must be able to describe both clearly in the exam.

两条主要的观测证据支持大爆炸理论。你必须在考试中清楚地描述这两者。

Evidence | 证据	Observation | 观测事实	What it supports | 支持的观点
Redshift of galaxies | 星系红移	Light from distant galaxies is redshifted; galaxies are moving apart	The Universe is expanding
CMBR | 宇宙微波背景辐射	Uniform microwave radiation at 2.7 K fills all of space	The Universe was once hot and dense
Abundance of light elements | 轻元素丰度	The Universe is about 75% hydrogen and 25% helium	Matches predictions of nucleosynthesis in the early Universe

Study tip: to earn full marks in an exam question on the Big Bang, always mention both the redshift of galaxies (expansion) and the cosmic microwave background radiation (hot early state). Naming the discovery of the CMBR by Penzias and Wilson can also earn credit.

学习提示：要在考试中关于大爆炸的题目上拿满分，一定要同时提到星系红移（膨胀）和宇宙微波背景辐射（早期高温状态）。写出彭齐亚斯和威尔逊发现宇宙微波背景辐射也能加分。

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9. Estimating the Age of the Universe | 估算宇宙的年龄

If we assume that galaxies have been moving apart at constant speed since the Big Bang, we can estimate the age of the Universe by dividing the maximum distance by the speed, which gives a simple relation involving the Hubble constant:

如果我们假设星系自大爆炸以来一直以恒定速度分离，就可以通过距离除以速度来估算宇宙的年龄，这给出了一个涉及哈勃常数的简单关系：

t = 1/H₀

This is because v = H₀d and time = d/v, so t = d/(H₀d) = 1/H₀.

这是因为v = H₀d，而时间 = d/v，因此t = d/(H₀d) = 1/H₀。

Worked example | 计算示例:

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