Is the age of the Universe absolute or relative?

04 April 2017

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Question

The latest estimate of the age of the Universe is about 13.8b years.  But would the age of the universe be different if the observer were situated on a planet near the outer edge of the universe, traveling at speeds far greater than we are?  Shouldn't that observation show the universe is younger, and if so how much younger?  If, on the other hand, the observation is absolute from our perspective, why can relativity considerations be ignored?

Answer

Chris put Stan's question to cosmologist Andrew Pontzen...

Andrew - It is a deep question and the brief answer is no. We’re pretty sure that wherever we were in the universe we would measure the age as being the same but, as the question is saying, why should that be? We do know that the universe is kind of whizzing apart in some sense. We also know that Einstein’s special relativity tells us that if something is moving fast, it experiences time very differently and, in particular, you might well expect that it would experience much less time because it’s moving fast.

The way that you reconcile this is to apply not Einstein’s special relativity, but Einstein’s general relativity to the problem. General relativity was this kind of breakthrough in physics which combined the theory of special relativity, which came considerably earlier, with the problem of gravity. What is this mysterious force that seems to keep us glued to the Earth’s surface, and keeps the planets moving around, and the stars moving around the galaxies? The older theories of gravity, which were Newton’s theory, just couldn’t really be reconciled with Einstein’s special relativity, which had all these weird effects about clocks running at different rates for things moving at different speeds.

So general relativity was this theory that brought together gravity and special relativity and it means that once gravity becomes important, if you start thinking about situations where gravity is the most important force and that would include the expansion of the universe, then you can no longer apply the laws of special relativity. And, actually, if you correctly apply the laws of general relativity, you do get the correct answer that despite everything flying apart, everyone measures the same time having past.

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