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I learned that space is always expanding. Even between sun and earth, it's just gravity is stronger.

Imagine space as graph paper and there is a square object that fills it. Space expands, 1x1 cell becomes 2x.

The space expanded not the objects in it. The size of an object remains the same.

So if N amount of matter was present, it's still N. Just that before it was occupying 1 cell now 1/2 cell.

Did the density of object increase? something changed definitely. If it wasn't density then what changed.

Qmechanic
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Muhammad Umer
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1 Answers1

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When space expands, the density of matter neither remains the same nor increases. It decreases, because there is the same amount of matter in a larger volume of space.

G. Smith
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  • then matter has expanded as well? – Muhammad Umer Mar 21 '19 at 16:35
  • I don’t know what you mean by that. The matter has the same mass. I would not describe the matter as having expanded. – G. Smith Mar 21 '19 at 16:41
  • relative to space it hasn't, but space has expanded so it has too. density relative to space is same but as you say it has decreased – Muhammad Umer Mar 22 '19 at 19:35
  • Physicists do not ever talk about “density relative to space”. Mass density is mass per volume. Since mass stays the same as the universe expands but volume increases, the mass density decreases. Saying that the density stays the same or increases is incorrect. – G. Smith Mar 22 '19 at 21:04
  • BTW, gravity is not “getting stronger”. There is no evidence that the gravitational constant $G$ is increasing. I think this misconception may be what is confusing you. – G. Smith Mar 22 '19 at 21:10
  • not negating statement about G but https://www.youtube.com/watch?v=YJzoelANL_Y – Muhammad Umer Mar 23 '19 at 04:54
  • my confusion is that why there is no independent unit to measure distance, independent of space. which will lead to distance -> volume -> density. Density independent of what is a meter, meter is fixed to space, space expands, it expands with it. If we had that independent unit for density then it'd show that it's decreasing as i.e. 1m:1im (independent meter) . would slowly become 2m:1im. But I guess without space there is no distance. Thanks for going with it – Muhammad Umer Mar 23 '19 at 05:01