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New to this so apologies for my ignorance, the simpler the answer the better. Here goes.

Light took 13.5 billion years to get to us from the big bang.

On an imaginary neighboring planet that is much larger, say for example, half the mass of a black hole, time moves slower than it does for us on Earth.

If their time moves slower relative to our time do they still see the big bang as occurring 13.5 billion years ago?

Qmechanic
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Nick Yiannop
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    Does this answer your question? Is the cosmological time grosso modo isochrone? – John Rennie May 21 '23 at 09:28
  • Also, a black hole can be any mass as far as we know. Nature simply can't seem to make them from stars below the Chandrasekhar limit, but that's not a general limitation of black hole physics, hence one should probably not try to characterize a situation like this by mass. The local physics of the gravitational well and the physics of "flatland" can also be separated fairly well, so it's not like the people in gravitational wells (like us) are somehow seeing the world all that differently from the "flatlanders". – FlatterMann May 21 '23 at 14:26
  • No not the answer, a waste of time asking here. – Nick Yiannop May 23 '23 at 02:04
  • @NickYiannop If we see the universe as 14 billion years old, people on a planet where time moves twice slower would see the universe as 7 billion years old by their clock. They however may be smart enough to realize that their time is dilated and calculate the true age of the universe. – safesphere May 24 '23 at 04:04
  • @safesphere so its just numbers and perception all the rest actually stays the same. Thank you. – Nick Yiannop May 25 '23 at 06:55
  • @NickYiannop Yes, but some effects are very real. For example, if you had a twin separated from you at birth to this planet while you remain on the Earth, then, when you meet again, he would be actually twice younger than you. – safesphere May 26 '23 at 04:33
  • @safesphere yes you are right, that I'd did know, just like the men who went to the moon came back a fraction of a second younger due to the speed they were traveling. That affect is real. But darn it's till tricky. Let's say the other larger planet uses the same time system as us, same clocks we will see theirs as moving slower but will they still see the big bang as us? Is it that for them light will have a lower speed value there fore it will be the same 13.8 billion light years away/ back. – Nick Yiannop May 27 '23 at 10:26
  • @NickYiannop Time dilation works differently in Special Relativity and gravity. If your twin is in a fast spaceship, you see his time moving slower and he also sees your time moving slower. However, if he is on that big planet, you se his time moving slower, but he sees your time moving faster. So he will see you and your light moving twice faster and the whole universe developing twice faster over 7 billion years instead if 14 by his clock. He will see that your clock is at the 14 billion years mark when his is at 7. – safesphere May 27 '23 at 20:28
  • @safespere yeap I follow you there, but to be 100% clear, would you say that despite how we both see the time since the big bang the time it's actually taken is the same. – Nick Yiannop May 29 '23 at 05:47
  • @NickYiannop Most of the universe exists in a negligible time dilation, so yes, it has taken 14 billion years for the most of the universe to develop. However, hypothetically, if this big planet was created very early and time is twice dilate there, then the development of things on this planet has taken 7 billion years until cosmological "now". – safesphere May 30 '23 at 05:43

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