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

There are several good answers here, but I think it’s worth mentioning how rare relativistic things are. Most things in space tend to be moving at about 0.1% the speed of light. Now that seems fast, but it turns out the relativistic effects that make clocks move differently are very small until you hit about 90% the speed of light. You can also change the clock’s speed with heavy gravity, but again you need to be near a black hole for that to matter.

Overall, you get that for pretty much all the clocks out there, the age of the universe is going to be the same, give or take a few thousand years.

Last thing worth noting is the cosmic microwave background. Basically there was a time the universe was full of gas that was so hot we can still see the glow from it today. You can tell from this glow whether you are moving with respect to that gas, so you can use it as a reference point for a standard speed, and so a standard clock, for the universe. As I describe above, it doesn’t make much difference to account for this, but it’s pretty cool.