上一片说有只推手叫暗能量。这片🌌星空向导拉响救护车：证据藏在远去的光里。Last leaf met dark energy, the pusher. Now the 🌌 star guide sounds an ambulance: the evidence hides in leaving light.
第 1 步：救护车远去，声音变低Step 1: The siren drops as the ambulance leaves
先听救护车——光玩的是同一个把戏。First hear the ambulance — light plays the same trick.
救护车开远，声波被拉稀疏，音调变低。这叫多普勒效应。光也是波：星系远去，光波被拉长、颜色变红，就叫红移。An ambulance drives off, its sound waves stretch thin, the pitch drops. That's the Doppler effect. Light is a wave too: a leaving galaxy stretches its light redder — that's redshift.
红一点点能说明什么？看谱线跑多远。What does a little redder prove? See how far the lines run.
第 2 步：谱线跑多远，退得有多快Step 2: How far the lines run, how fast it leaves
对比两条光谱：纹路相同，位置整体往红端偏。偏得越多，退行越快。距离翻倍速度翻倍，这就是哈勃定律。Compare lab lines with galaxy lines: same pattern, shifted redward as a whole. Bigger shift, faster retreat. Double the distance, double the speed — that's Hubble's law.
满天星系都在红移，说明什么大事？Galaxies everywhere are redshifted — what big story is that?
第 3 步：满天红移，宇宙在胀大Step 3: An all-red sky, a growing universe
远星系几乎全在远离：宇宙在膨胀，不静止。倒带回去，就能算出开端：约 138 亿年前。数百万星系的红移巡天，就是给宇宙量年龄。Distant galaxies are nearly all leaving: the universe expands, never still. Rewind the tape to find the start: about 13.8 billion years back. Redshift surveys of millions of galaxies date the universe.
🎮 红移实验室（30 秒）🎮 Redshift lab (30 seconds)
道理讲完了。拖动速度看谱线跑多远，再闯 4 关：量退行速度，认膨胀证据。Theory done. Drag the speed, watch the lines run — then clear 4 levels: measure speeds, spot the evidence.
一句话记住它：退得越快越红，越红退得越快；满天红移就是宇宙膨胀的证据。Remember it in one line: faster means redder, redder means faster — an all-red sky is the evidence for expansion.
救护车类比：远去拉长波，声音变低、光变红（多普勒效应）Ambulance analogy: leaving stretches waves — lower sound, redder light (Doppler effect)红移量 z：谱线偏多远，退行有多快；距离翻倍速度翻倍Redshift z: how far lines shift, how fast it leaves; double distance, double speed满天红移 = 宇宙膨胀；红移巡天量出 138 亿年An all-red sky = expansion; surveys date it 13.8 billion years
内容参考 OpenStax 等公开教材，多来源核对 · AI 生成、人工审核 · 发现错误欢迎指正，帮这片叶子长得更好。Based on OpenStax and other open textbooks, cross-checked across sources · AI-generated, human-reviewed · Spotted a mistake? Tell us — help this leaf grow.
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