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Afaik. mass slows down time, while expansion means that the universe will be less dense in mass. Does that mean that time will go faster everywhere?

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    $\begingroup$ go faster in comparison to what? $\endgroup$ – robert bristow-johnson May 2 at 2:15
  • $\begingroup$ @robertbristow-johnson Isn't it obvious? $\endgroup$ – inf3rno May 2 at 3:18
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    $\begingroup$ @inf3rno it's not obvious $\endgroup$ – Yashas May 2 at 7:19
  • $\begingroup$ You need to define your answer better. Time is a relative and also local concept. Say, you are right and today time runs twice faster than yesterday, but what difference does it make? Your heart rate would still be the same as measured by your wristwatch. All light would still have the same color (frequency), the atomic clock would still measure 24 hours per day. What measurable difference specifically are you looking for? $\endgroup$ – safesphere May 4 at 21:45
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    $\begingroup$ "if we manage to communicate with quantum entanglement between 2 places where clocks tick differently, then will it change the frequency of the signals?" - If your communication is at the speed of light or slower, then yes, the frequency shift will be defined by the relative speed of time. If your communication is instantaneous, then it is prohibited in relativity and would create various paradoxes. If we manage to create instant communication, we also would need a different theory of relativity, because the one we have would break and with no theory there is no way to say what would happen. $\endgroup$ – safesphere May 5 at 19:26
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Your question assumes that there would be a absolute time or clock that we could compare to.

There is no such absolute time.

It is a misconception that mass causes the effects of gravity and time dilation. In reality it is stress-energy.

Even if you are inside the event horizon of a black hole (where stress energy is very strong), you will see your own clock to tick normally for you. You have to compare your clock to a clock outside the black hole to see that your clock ticks slower then the clocks outside.

Now in your question you are saying that with the expansion of space, the universe will be less dense in mass. This would only be true on average.

Space is only expanding in the intergalactic voids. Space is not expanding inside galaxies, because inside galaxies, gravity dominates.

So if you look at the mass density inside the galaxies, you will see it is not becoming less dense.

It is just that galaxies are becoming more isolated.

Now in your case, if you would compare your clock here in the Milky Way to the clock at another galaxy, you would see that the clocks tick differently, but the difference (time dilation) would stay the same despite the expansion of space.

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  • $\begingroup$ My question does not assume there is an absolute clock. You can compare it to the clock here on Earth or to an average clock of the solar system, of the galaxy or the whole universe. $\endgroup$ – inf3rno May 2 at 15:04
  • $\begingroup$ It is hard to come up with a method to count average though. $\endgroup$ – inf3rno May 2 at 15:10
  • $\begingroup$ Your answer assumes that the gravity of other galaxies do not have any effect on our clock, which is not true. They have a slight effect here. $\endgroup$ – inf3rno May 2 at 15:15

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