When two Bangs meet Sheldon Cooper is a scientific genius on the popular American TV show, The Big Bang Theory (<<>> ). He finally met his match last year: Stephen Hawking. This is not the first time that the scientist has appeared on TV. He has also been on Star Trek (in 1987) and The Simpsons (in 1989). Each time, he played himself. Hawking, 71, is perhaps the world's most famous scientist after Albert Einstein. He has spent his whole life studying the beginning and the end of the universe, including the Big Bang   theory. The Big Bang theory explains the early development of the universe. According to the theory, about 13.7 billion years ago everything was all squeezed  together in a tiny, tight little ball, and then the ball exploded. The results of that explosion are what we call the universe. Hawking has always tried to make science more popular with people. His book: A Brief History of Time was published in 1988. In the book he shares his understanding of the universe in simple language. The book tries to explain many subjects about the universe to common readers, including the Big Bang, black holes and light cones  . Hawking's achievements are even greater if you think about his disability. When he was 21, Hawking caught a bad illness that slowly stopped him from moving or talking. Now he sits on a wheelchair with a computer by his side. To communicate, he moves two fingers to control the computer's mouse. He chooses his words from the screen, which are then spoken by a voice synthesizer  . Hawking also believes that there might be aliens in space. However, he believes they are probably very dangerous, so we should not look for them. "I imagine they might exist in very big ships ... having used up all the resources from their home planet," Hawking said in a British documentary  named Into the Universe with Stephen Hawking. The Big Bang theory mainly explains   _  .
how the universe started what the universe is like how old the universe is how the universe exploded