The Big Bang is the hot, dense early state from which the observable universe expanded. In classical Friedmann solutions its idealized initial boundary is often a singularity where the scale factor vanishes.
The hot Big Bang model describes an initially hot expanding universe whose standard radiation- and matter-dominated eras are governed by decelerating Friedmann expansion. Without an earlier inflationary era it exhibits the Horizon problem and Flatness problem.
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The Big Bang is the leading scientific explanation for the origin of the universe. It describes the event that occurred approximately 13.8 billion years ago when the universe began to expand from an extremely hot and dense state. This event marks the beginning of time and space as we understand them. Key aspects of the Big Bang theory include: 1. **Cosmic Expansion**: The universe has been expanding since the Big Bang.
