Which evidence supports the Big Bang Theory?

• A. Cosmic microwave background radiation
• B. Fossil records
• C. Plate tectonics data
• D. Hydrological cycles

Answer: (A)

The key idea is that the universe began in a hot, dense state and has been expanding and cooling since. A strong piece of evidence for that model is the cosmic microwave background—the faint microwave glow found in all directions. This afterglow comes from when the early universe cooled enough for photons to travel freely, about 380,000 years after the Big Bang. The radiation has a nearly uniform blackbody spectrum at about 2.7 Kelvin, with only tiny fluctuations that reflect the primordial density variations. This uniform, all-sky radiation is exactly what the hot, dense early universe would leave behind as it expanded, making it strong support for the Big Bang model.

The other options relate to processes on Earth—fossil records document the history of life, plate tectonics explains how Earth's crust moves, and hydrological cycles describe how water moves through the environment. They don’t address the origin or early evolution of the universe.