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How Do We Define Distance to a Star That is Currently 10 Billion Light Years From us in an Expanding Universe?

-- | July 21, 2018

Question: It is said that light traveling from a star that is currently 10 billion light years from us has taken 10 billion years to travel to us. But since the universe is expanding and that star was much closer to us when the light was emitted so wouldn’t the light have taken much less than 10 billion years to reach us? — Andy

Answer: My colleagues at Cornell University have provided a very detailed and informative explanation to this question, which I encourage you to read. The answer is a bit complicated as “distance” has multiple definitions in an expanding universe. The answer on the Cornell University Ask an Astronomer web site, though, should answer your question.

Jeff Mangum

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