📚 The Big Bang and the Expanding Universe | 大爆炸与膨胀的宇宙
This article explains the key ideas of modern cosmology that are covered in the Edexcel IGCSE Science (Physics) specification, focusing on redshift, Hubble’s law and the Big Bang theory. You will also learn the observational evidence that supports our current understanding of an expanding universe.
本文讲解 Edexcel IGCSE 科学（物理部分）考纲中关于现代宇宙学的核心概念，重点介绍红移、哈勃定律和大爆炸理论，并说明支持“宇宙正在膨胀”这一观点的主要观测证据。
1. The Universe and Galaxies | 宇宙与星系
The Universe is everything that exists — all matter, energy, space and time. It contains enormous numbers of galaxies, and each galaxy contains billions of stars. Our own galaxy is called the Milky Way, and our Solar System lies within it.
宇宙是存在的一切——所有物质、能量、空间和时间。宇宙包含数量巨大的星系，而每个星系又包含数十亿颗恒星。我们所在的星系称为银河系，太阳系就位于银河系之中。
Because the Universe is so large, everyday distance units such as kilometres are impractical for describing distances between stars and galaxies. Scientists use the light year instead.
由于宇宙极其辽阔，用公里这样的日常距离单位来描述恒星或星系之间的距离非常不方便，因此科学家改用光年这个单位。
2. Light Year and Astronomical Distances | 光年与天文距离
A light year is the distance that light travels in one year. Light moves at a speed of approximately 3 × 10⁸ m/s, so in one year it covers about 9.46 × 10¹⁵ metres.
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