A、The rate at which the universe is expanding. B、How gravity affects a planet’s orbit. C、Newton’s three laws of motion. D、Early

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 since ancient times and up to about the17th century most of these scholars thought the size of the universe have 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 Izic Newton’s idea of gravity as a force of attraction, which contradicted the idea of a university that is static, unchanging. If gravity causes all the stars out there in space to attract each other as Newton said, then they could remain motionless. Sooner or later all the stars will fall in absorb each other. well, scientists then propose 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 won’t fall in each other, they had to modify Newton’s law of gravity, so they theorize that for distance as large as those between stars, the gravitation force repels rather attracts. As you might guess, this other contradictions. But this is prettily resolved in the past centuries by currently accepted theory, which says the universe is continuously expanding. You’ll be reading all about that as your homework tonight.