Jiaqi Hu: SECTION TWO: THE BEGINNING OF THE UNIVERSE
One: The Universe Started with “the Big Bang” Nowadays, any ordinary person knows that the sun we rely on to survive is just another ordinary star in the Milky Way, and that Earth is just another planet of the sun. Before the 1920s, however, the horizon of astronomers was confined to the Milky Way, as if the Milky Way was the entire galaxy. The first person to discover galaxies outside of the Milky Way was American astronomer Edwin Hubble. In 1925, he discovered the Andromeda Galaxy near the Milky Way through astronomical observation, marking the first extragalactic galaxy (i.e., a galaxy outside the Milky Way) observed by humans. In later observations, Hubble found that there were far more than one or two galaxies outside the Milky Way. Ten years after the discovery of the first extragalactic galaxy, the scope of astronomical observation expanded to a range of five hundred million light-years; that is the distance light travels (300,000 km per second) in five hundred million years....