"The church says the earth is flat, but I know that it is round, for I have seen the shadow on the moon, and I have more faith in a shadow than in the church" -- Ferdinand Magellan
Some of my readers believe that the Universe is only a few thousand years old. I've made the following argument informally on some of their blogs, but have never received a satisfactory answer. I would like to know the specific point at which they believe the argument fails.
This argument is not an attempt to refute the day in Genesis = era argument, just a literal, 6000 year-old Universe. Here goes:
a) The speed of light hasn't changed drastically in the last 6000 years. By this I mean that it did not suddenly decelerate by a factor of 1000 or more in the last 6000 years. (The speed of light in a vacuum is actually a constant.)
b) The Universe was not created with photons placed exactly where they would have been had the Universe been way older.
(Assuming a drastically changing speed of light or a purposefully deceptive Creator would be an ad hoc hypothesis.)
1) The Andromeda Galaxy is 2.9 million light-years away.
2) We can see the Andromeda Galaxy.
3) Therefore, the light from Andromeda has been travelling for 2.9 million years minus a little bit for the amount Andromeda has moved away since the light left.
4) Therefore, Andromeda must have existed as it now appears to us at least 2 million years ago.
5) Therefore, the Universe must be older than 2 million years.
6) Therefore, the Universe must be WAY older than 6000 years.
Please note that Andromeda is a close galaxy. I picked it because it is the farthest galaxy visible to the naked eye. The Hubble telescope has seen galaxies believed to be up to 13 billion light-years away.