I was wondering whether it has proven whether the universe is expanding and if so, what is its ratio, one more question that whether it has an effect for the centrifugal forces, i.e gravity and the weight of the planets and far- away stars. Many thanks.

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    $\begingroup$ I don't understand much of this. What do you mean by "ratio"? What do you mean by "centrifugal forces ie gravity"? Gravity isn't a centrifugal force. By "weight" do you mean "mass"? How do you think expansion might affect mass? You have tagged "star" and "milky-way" How is this question about stars or the milky-way? Don't comment; please edit. $\endgroup$ – James K Jan 13 '18 at 21:04
  • $\begingroup$ Do you mean "what is its rate", perhaps? $\endgroup$ – pela Jan 13 '18 at 22:24
  • $\begingroup$ If I understand your question, the answer is that expansion doesn't affect gravity or other factors, because expansion of space doesn't apply over short distances. Space expands, objects in space aren't affected, it's the space between objects that expands - and, again, only over very large distances. Space doesn't expand between the Earth and the Sun, for example at least, not in any meaningful way. It certainly doesn't expand within the Earth. $\endgroup$ – userLTK Jan 13 '18 at 23:06
  • $\begingroup$ The speed of the expension is the Universe is measured by the so-named Hubble-constant. Its value is around $70 \frac{km}{s}$ for every 100 Megaparsec. Thus, anything 100 MPc away, is receding with $70 \frac{km}{s}$ from us. Currently there is no evidence that gravity would change in time. Of course things more far away attracts eachother lesser. $\endgroup$ – peterh - Reinstate Monica Jan 14 '18 at 1:09
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    $\begingroup$ It's accepted now that the universe is expanding and this Wikipedia page discusses the reason and evidence. $\endgroup$ – StephenG Jan 14 '18 at 1:36