The universe began expanding from a high-density, high-temperature state about how long ago?

• A. 13.8 million years ago.
• B. 13.8 billion years ago.
• C. 4.5 billion years ago.
• D. 10,000 years ago.

Answer: (B)

This question tests how long the universe has been expanding since its hot, dense origin. By measuring how fast galaxies are receding (redshift) and by analyzing the cosmic microwave background as a record of the early universe, scientists model the expansion history and estimate the time elapsed since that beginning. Those lines of evidence place the age at about 13.8 billion years. The other timeframes don’t fit the observed expansion and the fossil radiation: 13.8 million years is far too short to develop the structures we see; 4.5 billion years is about the age of the solar system, not the cosmos; 10,000 years is far too recent given the cosmic background and large-scale structure. So the best answer is the one that corresponds to roughly 13.8 billion years ago.