The universe physicist Jean-Pierre Petit said,"In the scientists' opinion, Cosmology consists of two parts, one is to investigate the universe which we can observe, and the other is to speculate the universe which we cannot observe by a series of theoretical reasoning and bold assumption. The two parts are closely correlated and difficult to prioritize. I think for a teenager who is interested in science and starts to learn the cosmology, he has to understand the difference and learn the way scientists research the universe."