What increases in density as the universe ages?

What increases in density as the universe ages?

• A. Hydrogen
• B. Helium
• C. Carbon
• D. Oxygen

Answer: (C)

Cosmic chemical evolution is at play: the universe becomes richer in heavier elements over time because stars forge new atoms and spread them into space. Carbon is a prime example of this enrichment. As stars fuse lighter elements in their cores and later eject carbon-rich material through winds and supernovae, carbon accumulates in the interstellar medium. This process boosts the density of carbon as the universe ages, even though the overall matter density drops due to cosmic expansion. Hydrogen and helium were produced in the Big Bang and, while some are locked in stars, their total cosmic density doesn’t rise in the same way; the heavier elements, including carbon, build up over time.

Cosmic chemical evolution is at play: the universe becomes richer in heavier elements over time because stars forge new atoms and spread them into space. Carbon is a prime example of this enrichment. As stars fuse lighter elements in their cores and later eject carbon-rich material through winds and supernovae, carbon accumulates in the interstellar medium. This process boosts the density of carbon as the universe ages, even though the overall matter density drops due to cosmic expansion. Hydrogen and helium were produced in the Big Bang and, while some are locked in stars, their total cosmic density doesn’t rise in the same way; the heavier elements, including carbon, build up over time.