📚 Redshift and the Expanding Universe | 红移与膨胀的宇宙
In IGCSE Edexcel Science (Physics), the study of the Universe is one of the most inspiring topics. One of the key pieces of evidence for the expansion of the Universe is the phenomenon of redshift. This article explains what redshift is, how it arises from the Doppler effect, and how Hubble’s law supports the Big Bang model. Worked examples and exam tips are included to help you prepare effectively.
在 IGCSE Edexcel 科学（物理）中，宇宙研究是最引人入胜的话题之一。支持宇宙膨胀的关键证据之一就是红移现象。本文将解释什么是红移、红移如何由多普勒效应产生，以及哈勃定律如何支持大爆炸模型。文中还包含例题精讲和考试技巧，帮助你高效备考。
1. What is Redshift? | 什么是红移？
Redshift is an increase in the wavelength of light or other electromagnetic radiation. When light shifts to longer wavelengths, it moves towards the red end of the visible spectrum, because red light has the longest wavelength of visible light. In astronomy, redshift is a crucial tool for measuring how fast distant objects are moving away from us.
红移是指光或其他电磁辐射的波长增加的现象。当光向更长波长方向移动时，它会向可见光谱的红端移动，因为红光在可见光中波长最长。在天文学中，红移是测量遥远天体远离我们速度的重要工具。
Blueshift is the opposite effect: the wavelength decreases, so the light moves towards the blue end of the spectrum. Redshift implies the source is receding, while blueshift implies the source is approaching. Astronomers use the amount of redshift or blueshift to calculate the relative motion between a light source and an observer.
蓝移是相反的现象：波长变短，因此光向光谱蓝端移动。红移意味着光源正在远离，而蓝移意味着光源正在靠近。天文学家利用红移或蓝移的量来计算光源与观测者之间的相对运动。
2. The Doppler Effect in Sound | 声音中的多普勒效应
Before exploring redshift in light, it is helpful to recall the Doppler effect in sound. When a source of sound moves towards you, the sound waves are compressed, so the frequency you hear is higher and the pitch is higher. When the source moves away, the waves are stretched, so the pitch is lower. This is why an ambulance siren changes pitch as it passes you.
在探讨光的红移之前，先回顾一下声音中的多普勒效应。当声源向你移动时，声波被压缩，你听到的频率更高，音调也更高；当声源远离时，声波被拉长，音调降低。这就是救护车警笛经过你身边时音调变化的原因。
Mathematically, for a stationary observer and a source moving with speed v, the observed frequency f‘ is related to the emitted frequency f. For a source moving away, the waves are longitudinally stretched, and for a source moving closer, they are compressed. The same idea applies to light waves, although the exact formula is modified by Einstein’s theory of relativity.
用数学表达，对于静止观测者和以速度 v 运动的声源，观测频率 f‘ 与发射频率 f 有关。当声源远离时，波被纵向拉长；当声源靠近时，波被压缩。同样的原理也适用于光波，不过具体公式在爱因斯坦的相对论中有所修正。
3. Redshift and Blueshift in Light | 光的红移与蓝移
Light also behaves as a wave, so when a glowing object moves away from us, its light waves are stretched and the observed wavelength becomes longer. This is called Doppler redshift. When the object moves towards us, the light waves are compressed, producing blueshift. The amount of shift depends on the relative velocity between the source and the observer.
光也具有波动性，因此当发光天体远离我们时，它的光波被拉长，观测到的波长变长，这称为多普勒红移。当天体朝向我们运动时，光波被压缩，产生蓝移。移动量取决于光源与观测者之间的相对速度。
For non-relativistic speeds, the fractional change in wavelength is approximately:
Δλ / λ₀ = v / c
Here, Δλ is the change in wavelength, λ₀ is the rest wavelength, v is the recessional speed of the source, and c is the speed of light. If v is positive for a receding source, then Δλ is positive, so the wavelength increases. This is the key equation you need to remember for calculations in the exam.
对于非相对论速度，波长的相对变化近似为：
Δλ / λ₀ = v / c
其中 Δλ 是波长变化量，λ₀ 是静止波长，v 是光源的退行速度，c 是光速。如果光源远离，v 为正，Δλ 也为正，波长变大。这是考试中做计算题需要记住的关键公式。
For example, if a galaxy’s hydrogen line is normally observed at 656 nm, but we observe it at 680 nm, then Δλ = 24 nm. Using the formula with c ≈ 3.0 × 10⁵ km/s, we can estimate the galaxy’s recessional speed. Astronomers commonly measure these shifts in spectral lines to determine galaxy velocities.
例如，某星系的一条氢发射线静止时为 656 nm，而我们观测到它为 680 nm，则 Δλ = 24 nm。利用公式并取 c ≈ 3.0 × 10⁵ km/s，我们可以估算该星系的退行速度。天文学家通常通过测量光谱线的移动量来确定星系速度。
4. Galaxy Redshift and the Expanding Universe | 星系红移与宇宙膨胀
In the 1920s, Edwin Hubble observed that the vast majority of galaxies show redshift in their spectra. This means most galaxies are moving away from us. More importantly, he discovered that the redshift of a galaxy increases with its distance. The more distant the galaxy, the faster it is receding. This is now known as Hubble’s law.
20世纪20年代，埃德温·哈勃观察到绝大多数星系的光谱都表现为红移，这说明大多数星系正在远离我们。更重要的是，他发现星系的红移量随着距离的增加而增大——星系越远，退行速度越快。这就是现在所称的哈勃定律。
This observation strongly suggests that the Universe is expanding. Imagine dots drawn on a balloon; as the balloon inflates, every dot moves away from every other dot. The farther apart two dots are, the faster they move apart. In the same way, galaxies are not all moving through a fixed space; instead, the space itself is stretching, carrying galaxies with it.
这一观测结果有力地表明宇宙在膨胀。想象气球上的几个点：气球充气时，每个点都远离其他点；两点之间距离越远，分开的速度越快。同样，星系并非都在固定的空间中运动，而是空间本身在拉伸，带动星系一起运动。
Importantly, this does not mean Earth is at the centre of the Universe. Observers in any galaxy would see the same pattern of other galaxies moving away from them. The expansion is uniform on a large scale, making our location unremarkable.
重要的是，这并不意味着地球位于宇宙的中心。任何星系中的观测者都会看到其他星系远离自己的相同模式。在宏观尺度上，膨胀是均匀的，我们所在的位置并没有特殊之处。
5. Hubble’s Law | 哈勃定律
Hubble’s law connects a galaxy’s recessional velocity v with its distance d from Earth. The equation is simple:
v = H₀ × d
Here, H₀ is the Hubble constant. Its unit is typically kilometres per second per megaparsec (km/s/Mpc). A parsec is a distance unit used in astronomy; 1 Mpc = 3.26 million light-years. Current estimates of H₀ are about 70 km/s/Mpc, though slightly different values are obtained by different measurements.
哈勃定律将星系的退行速度 v 与其距地球的距离 d 联系起来。公式很简单：
v = H₀ × d
其中 H₀ 是哈勃常数，单位通常是“千米每秒每百万秒差距”（km/s/Mpc）。秒差距是天文学中使用的距离单位，1 Mpc = 326万光年。目前 H₀ 的估计值约为 70 km/s/Mpc，不同测量方法会得到略有差异的值。
Suppose a galaxy is 100 Mpc away. Using H₀ = 70 km/s/Mpc, its recessional velocity is:
v = 70 × 100 = 7000 km/s
This shows that the galaxy is moving away from us at 7000 km/s. In the exam, you may be asked to substitute values into this equation or rearrange it to find distance: d = v / H₀.
假设一个星系距离我们 100 Mpc。取 H₀ = 70 km/s/Mpc，它的退行速度为：
v = 70 × 100 = 7000 km/s
这表明该星系正在以 7000 km/s 的速度远离我们。在考试中，你可能会被要求将数值代入公式，或通过变形 d = v / H₀ 求解距离。
6. The Big Bang and Cosmic Microwave Background | 大爆炸与宇宙微波背景辐射
If the Universe is expanding, then running this expansion backwards implies that everything was once concentrated at a single, extremely hot and dense point. This is the Big Bang theory — the idea that the Universe began about 13.8 billion years ago and has been cooling and expanding ever since.
如果宇宙在膨胀，那么把膨胀过程倒推回去，就意味着曾有某一时刻一切物质都集中在一个极其炽热致密的点上。这就是大爆炸理论——宇宙约在138亿年前诞生，此后不断冷却和膨胀。
One major piece of evidence for the Big Bang is the cosmic microwave background (CMB). In 1965, Penzias and Wilson discovered a faint, uniform microwave radiation coming from every direction in space. This radiation is the highly redshifted light from the hot early Universe. As space expanded, the original high-energy gamma rays stretched into long-wavelength microwaves, which we observe today at a temperature of about 2.7 K.
大爆炸的一个重要证据是宇宙微波背景辐射（CMB）。1965年，彭齐亚斯和威尔逊发现了来自太空各个方向的微弱均匀微波辐射。这种辐射是早期炽热宇宙光线的强烈红移遗迹。随着空间膨胀，最初的高能伽马射线被拉伸为长波微波，我们今天观测到的温度约为 2.7 K。
Other evidence includes the relative abundance of light elements such as hydrogen and helium, which matches predictions from Big Bang nucleosynthesis. The Big Bang is not an explosion that happened at a particular place in space; rather, it is the expansion of space itself.
其他证据包括氢、氦等轻元素的相对丰度与大爆炸核合成理论的预测一致。大爆炸并不是发生在空间中某个位置的爆炸，而是空间本身的膨胀。
7. Common Misconceptions and Exam Tips | 常见误解与考试技巧
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Misconception: Redshift is caused by galaxies moving through static space. Actually, for distant galaxies, the cosmological redshift is caused by the expansion of space itself stretching the light waves as they travel to us.
误解：红移是星系在静止空间中运动造成的。实际上，对于遥远星系而言，宇宙学红移是空间膨胀在光波传播过程中拉伸其波长造成的。
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Misconception: The Doppler formula Δλ/λ₀ = v/c only works for any speed. In reality, it is an approximation valid when v is much smaller than c. At very high speeds, relativistic formulas are needed, but IGCSE only requires the simple version.
误解：多普勒公式 Δλ/λ₀ = v/c 对任何速度都适用。实际上，它只是当 v 远小于 c 时的近似。速度极高时需要使用相对论公式，但 IGCSE 只要求简单形式。
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Exam tip: Always identify whether the wavelength has increased (redshift) or decreased (blueshift) before substituting values. A positive Δλ means redshift and a receding source.
考试技巧：代入数值前，先判断波长是增大（红移）还是减小（蓝移）。Δλ 为正表示红移，光源远离。
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Exam tip: Remember units. Hubble constant is often quoted in km/s/Mpc, so distance should be in Mpc and velocity in km/s when using v = H₀ × d. If you use the equation Δλ/λ₀ = v/c, ensure c and v have the same units (e.g., both km/s).
考试技巧：注意单位。哈勃常数常用 km/s/Mpc，因此 d 以 Mpc、v 以 km/s 代入 v = H₀ × d。如果使用 Δλ/λ₀ = v/c，要确保 c 和 v 单位一致（如都用 km/s）。
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Exam tip: The phrase “microwave background radiation” is often linked to the Big
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