I think Big Bang theory is valid because of Inflation theory, it states that universe expanded faster than a light(right?) and that theory can solve the problems that Big Bang had: the Flatness and the Horizon problem. Another reason is Cosmic Microwave Background Radiation, which is first predicted to exist by scientists who support the Big Bang theory, and later was discovered by Hubble. Are my arguments correct? Is there any more evidence that support the Big Bang theory??