Since 1998, astronomers have narrowed down our uncertainty for this important number so that a value beteen 13 and 15 billion years was the most likely age. Astronomers are accustomed to quoting numbers in terms of ranges, but it has always been rather awkward to explain why we can't seem to know this number with, say, the same accuracy that we know the mass of the Sun, or the distance from the Sun to the center of the Milky Way.

This all changed on February 11, 2003 when the Wilkinson Microwave Anisotropy Probe (WMAP) reported the results from their first-years measurements of the cosmic microwave background. This feeble light, hiding out in the microwave portion of the electromagnetic spectrum, is all that remains of the fireball that was the Big Bang. Through a detailed study of the temperature irregularities in this light, NASA astronomer Charles Bennett and his colleagues delivered as they had promised, a high-precision answer to this question: 13.7 billion years with an uncertainty of only 100 million years.

The WMAP results are completely consistent with ages determined from the oldest stars since the actual formation of stars and galaxies is now known from the WMAP analysis to have started no earlier than 200 million years after the Big Bang, or 13.7 billion years ago. This signifies the end of the so-called Dark Ages. Our Milky Way was probably among the first galaxies that formed as the Dark Ages came to an end.

This answer was updated in 2011. See my books: The Astronomy Cafe (1998) and Back to the Astronomy Cafe (2003) for more FAQs in printed form. Author: Dr. Sten Odenwald, Copyright 2011 Return to Ask the Astronomer