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Astronomy - expansion of the universe
Expert: Courtney Seligman - 10/26/2009
Question
Hi Courtney! Thank you for volunteering!!! Here is my question:
Given that the expansion rate of the universe is 74.2 ±3.6 kilometers/second/megaparsec, and that the radius of the universe is 14 Gpc (given my very simplistic math abilities), I estimate that the expansion rate of the outer edge of the observable universe would be 10,388,000 km/sec. Isn't faster than the speed of light (at 300,000km/sec)? How can this be?
Bruce
Answer
The problem is your units (and a probable decimal place error). The "observable" Universe is 13.7 billion light years in radius, not 13.7 billion parsecs. Dividing by 3.26 light years per parsec yields 4 Gpc for the radius, not 13.7. Multiplying by 74.2 km/sec/Mpc yields 297000 km/sec, which is essentially the same as the speed of light. So the numbers come out "right".
This is of course not an accident. The size of the "observable" Universe is defined as the distance at which the expansion velocity would be equal to the speed of light; so if the arithmetic is done correctly, the radius (in Mpc) multiplied by the Hubble constant (in km/sec/Mpc) always comes out equal to the speed of light.
The actual size of the Universe, however, is many times larger than that. It's just that regions which lie beyond the "observable" limit can't be observed, because as light travels from there to here, the expansion of the space through which it is traveling carries the light away from us, despite its velocity in our direction.
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