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Name: Pam
Status: educator
Grade: 6-8
Location: Outside U.S.
Country: USA
Date: Fall 2013

If galaxies and stars will be moving away from each other then at one point in time would they be moving away from each other faster than the speed of light?

Dear Pam,

Good question. No they would not. Even if both are moving at the speed of light somehow, their combined velocity could not be greater than the speed of light.

Sincerely David H. Levy


First, let us look at what is meant by "stars moving away from each other". The expansion of the universe (as a result of the Big Bang) is not so much from galaxies physically moving away from each other, but rather that the intervening space in between galaxies are expanding - resulting in more space between galaxies. New theories suggest that it is a fundamental nature of space (or vacuum) to expand. Can this expansion go at such a rate as to be faster than the speed of light? We do not yet have enough data to make any kind of statement on that since we are barely beginning to understand this nature of vacuum. Observations suggest that the intervening space is expanding at an increasing rate, however, that expansion speed is very far from being at the speed of light. The rate of expansion might change, might have an upper limit, might reverse, we do not know yet.

Greg (Roberto Gregorius) Canisius College

This is perfectly feasible. In this case, what are moving faster than the speed of light (300,000 km/s) are 'reference points', not physical objects.

A good example of this is; if you shine a torch towards a point up in the sky, then sweep around fast, say 160 degrees, then there is a very distant point along that light beam (millions of light-years away) that actually travels faster than light.

However, the 'Big Crunch', the Universe collapsing in on itself, will PROBABLY occur before then.

Howard Barnes Astronomer.

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