If every part of the universe has aged differently owing to time running differently for each part, why do we say the universe is 13.8 billion years old?

Question

For some parts relative to us, only a billion years would have passed, for others maybe 20?

In: Physics

Answer 1

Physicist here. Here’s a copy/paste from [my answer to an old /r/askscience thread on the topic that included lots of good discussion.](https://www.reddit.com/r/askscience/comments/1m3yql/since_time_is_relative_how_do_we_define_it_when/cc5kh6z)

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It depends on how we measure it, but all reasonable reference frames give about the same value.

The most precise measurements are based on the [Cosmic Microwave Background (CMB)](https://en.wikipedia.org/wiki/Cosmic_microwave_background). There is a convenient reference frame called the [comoving frame](https://en.wikipedia.org/wiki/Comoving_coordinates#Comoving_coordinates), in which the CMB light coming from all directions is equally redshifted. This is also the reference frame in which the universe is the oldest, and is the reference frame we use when doing most cosmology.

Our solar system is moving at about 371 km/s relative to the comoving frame, which gives a time dilation factor of only 1.0000008, which is why it doesn’t matter much what (reasonable) reference frame we pick. In this frame the universe is only about 10,000 years younger, out of 13.8 billion years.