Eli5 if time is relative and gets effected by gravity how could we possibly determine the age of our universe?
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By using other constants, mainly the speed of light.
Using calculations involving light and gravity, we can calculate the distances of stuff like galaxies. The speed of light is a constant. If a galaxy is calculated to be 14 billion light years away, and we’re able to see the light it emits, that galaxy has to be at least 14 billion years old. If the galaxy is 14 billion years old, the universe has to be at least that old as well.
We aren’t sure if the universe is is fact ~14 billion years old, but we know the oldest things we’ve found are around that age.
Another theory that confirms the first is that we have measured the expansion of the universe as it is now. If we run those numbers back until the universe is miniscule (which is when the Big Bang would have occurred), we find that it would have taken roughly 14 billion years for the universe to expand to its current size.
It’s worth noting that all of these are theories, and not indisputable fact. Though there is a lot of evidence supporting the age of the universe is 14 billion years, not everyone agrees. A relatively new study from the University of Ottowa claims the universe is really ~27 billion years old.
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