Mission briefing: clues from the early universe

Scientists estimate that the universe is roughly 14 billion years old. The Big Bang theory explains that the universe began in an extremely hot, dense state and has been expanding ever since. It was not matter exploding outward from one spot into pre-existing empty space: space itself has expanded. As the universe expanded, its temperature fell. Eventually, particles formed atoms, and gravity gathered matter into stars and galaxies. Evidence for this theory includes the continuing expansion of distant galaxies and faint microwave radiation detectable across the sky. This radiation is called the cosmic microwave background; it is a remnant dating back to the universe’s earliest stages. The Big Bang theory describes the universe’s early history and development, not every detail of what may have happened at the very first instant.