How old is the Universe?
Most scientists estimate the time between the Big Bang and current day somewhere between 13 and 20 billion years ago. Estimates are derived from Hubble’s Constant, a rate of expansion ratio calculated by dividing the speed at which a galaxy is moving away from the Earth by it’s distance from the Earth.
By taking the inverse of Hubble’s Constant (dividing the distance of the galaxy by its speed) we get an estimate of the age of the Universe. Of course, the distance of any galaxy and the speed at which it is moving are fairly difficult measurements to make and the uncertainty in these estimates lead to the uncertainty in the age of the Universe question.
Source and more detailed information: Wikipedia’s Age of the Universe entry.
Wikipedia states: The age of the universe is the time elapsed between the Big Bang and the present day. Current observations suggest that this is about 13.73 billion years, with an uncertainty of about 120 million years.[1]
The oldest age of the universe I’ve ever heard is 15 billion years and lately that’s been coming down to 14 and now even 13. Where did 20 billion come from?
I came across the 20 billion in a book called The Handy Science Answer Book. I just reread the Wikipedia reference you mentioned and agree that that’s probably a better and more current estimate.