Passage: NASA's Wilkinson Microwave Anisotropy Probe (WMAP) project's nine-year data release in 2012 estimated the age of the universe to be (13.772±0.059)×109 years (13.772 billion years, with an uncertainty of plus or minus 59 million years).However, this age is based on the assumption that the project's underlying model is correct; other methods of estimating the age of the universe could give different ages. Assuming an extra background of relativistic particles, for example, can enlarge the error bars of the WMAP constraint by one order of magnitude.This measurement is made by using the location of the first acoustic peak in the microwave background power spectrum to determine the size of the decoupling surface (size of the universe at the time of recombination).  The light travel time to this surface (depending on the geometry used) yields a reliable age for the universe.  Assuming the validity of the models used to determine this age, the residual accuracy yields a margin of error near one percent.

Question: What factor can influence the travel time to the decoupling surface?
the geometry used