Just bought Cynthia Brown's "Big History" today and started reading... history from the big bang to the present day. Looks like it going to be very interesting (and better than Bill Bryson's effort). There is however a mistake in the 2nd paragraph of the 1st page!
Cynthia assumes that because the universe is 13.7 billion years old, it must be 13.7 billion light years across. In fact, logic would tell you that it'd be 13.7 billion light years in radius i.e. 27.4 light years in diameter.
That would be the case, except for the fact that the universe is stretching iself out at an increasing rate that is currently measured at 74.2 kilometers per second per megaparsec of space. This makes the 'observable universe' a sphere 93 billion light years in diameter with you in the middle. In other words, the position in space that the light left from 13.7 billion years ago to finally end up in your retina or telescope now exists billions of extra light years further away.
Cynthia assumes that because the universe is 13.7 billion years old, it must be 13.7 billion light years across. In fact, logic would tell you that it'd be 13.7 billion light years in radius i.e. 27.4 light years in diameter.
That would be the case, except for the fact that the universe is stretching iself out at an increasing rate that is currently measured at 74.2 kilometers per second per megaparsec of space. This makes the 'observable universe' a sphere 93 billion light years in diameter with you in the middle. In other words, the position in space that the light left from 13.7 billion years ago to finally end up in your retina or telescope now exists billions of extra light years further away.
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