The idea for the Big Bang came in the early parts of the twentieth century when scientists discovered that all the galaxies we could see were moving away from us, which meant that the universe was expanding. Cosmologists tried to make sense of this observation by playing with Einstein’s new equations for general relativity, which describe how space and time and gravity work, and found that these equations could easily describe an expanding universe. But they also found something odd. If you project that expansion backward in time as far back as possible, then the equations predict something
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