A、The gradual development of the universe. B、The various research on the stars. C、The formation of Newton’s law of gravity. D、Th

　　Today most astronomers accept the notion that groups of stars that make up the universe are all moving farther and farther away from each other. But until very recently, this idea of an expanding universe was not a theory most European scholars believed in. (20) Since ancient times and up to about the 17th century, most of these scholars thought the size of the universe had remained unchanged since the moment of its creation or perhaps forever, with all the stars remaining more or less in place in relation to each other. But that was challenged in the late 17th century by Isaac Newton’s idea of gravity as a force of attraction, which contradicted the idea of a universe that is static, unchanging. (21) If gravity causes all the stars out there in space to attract each other as Newton said, then they could remain centrally motionless. Sooner or later all the stars will fall in towards each other. Well, scientists then proposed a new model, taking Newton’s theory into account. They didn’t want to abandon the idea of motionless stars, but for this model to work, so the stars wouldn’t fall in each other. They had to modify Newton’s law of gravity, so they theorized that for distance as large as those between stars, the gravitational force repels rather than attracts. As you might guess, this led to some other contradictions. But this was prettily resolved in the past centuries by the currently accepted theory, which says the universe is continuously expanding.
　　20. What theory challenged the idea about the size of the universe?
　　21. What will happen if it is gravity that causes all the stars in space to attract each other?
　　22. What is the speaker mainly talking about?