Dmitri, a 25 year old male from Toronto asks on May 21, 2011,
How do we know that universe is 13.5b years old, if time is not constant? The idea is the closer you are to the gravity source the slower the time is for you compare to the time passed for the observer without the gravity. Therefore, earlier universe was more condense with matter, therefore higher gravity, therefore time at those moments was moving slower then it for us right now.. So let say 100 year at the universe expansion phase is like 1 year for us right now. Then there should be no expansion phase, just a linear expansion, where the time should not be linear but a similar to a log function. So let say in 10 billions years scientiest will look back at our time and will call it an expansion, because for them our time passed within their moments. Also how do we know exactly that the visible universe is a sphere, if at some points the gravity could be stronger than in other, the time needed for light to travel would be different in different points?
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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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