# the *answer*

Don Page answered on May 23, 2011,
A:

The age is taken to be the proper time (that is, the time measured by a good clock) along the worldline of moving with the matter of the universe. The proper time is given by the metric of spacetime, which is chosen to obey Einstein's equations and with parameters corresponding to the present expansion rate, the acceleration, etc., that are chosen to fit best the observations. The observations have become sufficiently precise in recent years that the parameters and current age are now known quite accurately (just a few percent uncertainty).

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