How old is the universe?

• A. 13.8 billion years old
• B. 4.5 billion years old
• C. 2.5 billion years old
• D. 10 billion years old

Answer: (A)

The age of the universe is determined by its expansion history since the Big Bang. By measuring how fast the universe has expanded over time and observing the cosmic microwave background—the afterglow from a very early era—we can trace back to the moment of the Big Bang and calculate how long the cosmos has been evolving. These measurements, especially from the cosmic microwave background and related expansion data, point to an age of about 13.8 billion years. That number is consistent with the standard cosmological model and widely accepted by researchers. In contrast, ages like 4.5 billion years correspond to the age of the Earth, not the entire universe, and other values are not supported by the current cosmological data. So the best answer reflects the cosmological estimate derived from expansion history and CMB observations.