1. Hello all, I am new to this site, I'm not a Dr or a professor, I just have an interest in this stuff, but, I was sitting here thinking about gravity and time, it was something I watched on TV, professor Brian Cox was standing next to a pyramid and he said that time moves slower where he was standing than it does from further away because of the gravity from the pyramid, albeit very slight. This got me thinking about the age of the universe, because, the way I understand it is, the universe is expanding, which means that way back it was smaller and more condensed. If it was more condensed then surely gravity was also condensed which means that time was slower then that it passes now because of the extra gravity. Not that there WAS extra gravity, just that all objects were closer together so the pull from gravity must have been stronger and therefore time passes at a slower rate. If this is the case, is the universe really 13.7 billion yrs old?

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3. Time moves slower compared to other external frames, but in the frame you measure in, it moves at it's normal rate. And there's no external frame to compare the rate of time in the universe against.

4. Whilst what Alex said is correct, what cosmologists do is to exploit something known as the cosmological principle. This principle states that our region of the universe is, on the whole, no different from any other.

We know that gravity causes time dilation, so in order to find the age of the universe we have to measure it using a frame of reference as far away from any gravitational source as possible - the frame of reference of an idealised clock that has existed since the Big-Bang in a region with the least gravity. You can think of this as a clock floating in the voids between the clusters of galaxies.

If that "cosmological clock" has remained as far away from gravitational sources as possible, throughout the history of the universe, it would today show an elapsed time of 13.7 billion years.

If we compare the elapsed time on that cosmological clock with the time on any other clock that was closer to a gravitational source, we would find the cosmological clock shows more elapsed time. So, the cosmological clock is the "fastest" clock in the universe. All other clocks, being more influenced by gravity, will have lost time in comparison (ticked "slower").

I use the quotes around "fast" and "slow" as it is not actually correct to compare times this way - all clocks tick at the same speed, but are subject to a different amount of spacetime curvature (gravity). It is more accurate to say that all clocks tick at the same speed, but take different paths through curved spacetime, so can end up with different elapsed times when we compare them.

5. Excellent, thank you very much for that, you explained that so that it was easily understood

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