Answer:
The Big Bang is the leading theory for the origin of the universe and has excellent evidence in its favor. The Steady State theory was an alternative theory to the Big Bang and was ruled out in the 1960’s by the discovery of the cosmic background radiation. Cosmic inflation is an event early in the history of the universe; it could very well be the bang of the big bang.
The Big Bang proposes that the universe began in a hot, dense state and has been expanding and cooling ever since. Observational evidence includes:
- the Hubble law — distant galaxies and clusters are flying away from us, and farther ones are flying away faster, indicating ALL of space is expanding and that at earlier times the universe was smaller/denser.
- the cosmic microwave background: the universe is emitting radiation that has a temperature of 2.7 degrees above absolute zero. Measurements of this radiation over many wavelengths indicate this radiation has a particular form, known as a black body. A blackbody is a perfect emitter and only dense objects emit black body radiation, so measurements of this radiation indicate the early universe was dense. As a black body expands it will cool, so this radiation also indicates the early universe was hot.
- the light element abundance. The Big Bang predicts particular amounts of helium and light elements like lithium, and these predictions are in excellent agreement with observations.
- cosmic ages. Stellar remnants like white dwarfs could be billions upon billions of years old, yet we find the oldest being 10–12 billion years old. Similarly star clusters could have a range of ages, but we find none older than about 13 billion years old, in agreement with the expansion age for the universe (13.7 billion years).
So, as scientific theories go, the Big Bang is solid. It does not have a good answer for why the universe is so uniform though (regions on opposite sides of the universe have the same temperature, but they weren’t in contact at early times) and this is where inflation comes in — a proposal that the universe underwent an extremely rapid expansion at very early times. Inflation predicts that spacetime will be flat, as is observed, but we’d love to detect things like gravitational waves from it to bolster the evidence.
