The Big Bang theory describes how our Universe evolved from an extraordinarily hot, dense state approximately 13.8 billion years ago into the vast cosmos we observe today. Rather than an explosion in space, it represents the rapid expansion of space itself, carrying matter and energy along as temperatures dropped from trillions of degrees to the cosmic background temperature of 2.7 Kelvin we measure today.</p><p>This groundbreaking model emerged from Edwin Hubble's 1929 discovery that distant galaxies are receding from us, with more distant ones moving faster—evidence that space itself is expanding. The theory gained crucial support from the 1965 detection of cosmic microwave background radiation, the Universe's afterglow, and observations showing the predicted abundance of light elements like hydrogen (75%) and helium (25%).</p><p>The Big Bang explains three major cosmic puzzles: why the Universe is expanding, the origin of light elements, and the existence of background radiation. Modern observations reveal that normal matter comprises only 5% of the Universe, with dark matter (27%) and dark energy (68%) dominating cosmic evolution. From quantum fluctuations lasting 10^-32 seconds during cosmic inflation to the first stars igniting 400 million years later, the Big Bang provides our most comprehensive framework for understanding cosmic history and structure formation.
Examples
**Example:** The cosmic microwave background radiation, discovered in 1964, is the afterglow of the Big Bang, now cooled to just 2.7 Kelvin above absolute zero. The universe has been expanding for approximately 13.8 billion years since the Big Bang.