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I have recently noticed this answer by wetsavannaanimal to a Physics.SE question,

if a star is not a black hole, light shone upwards will escape the star's gravitational field, although light is red-shifted in doing so, heavily so if the star is massive

I'm particularly interested in the red-shifting part of the answer.

Although it's not the first time I'm seeing something similar, I wasn't sure whether this is something universally accepted.

Because if it is, I'm wondering whether the whole theory of the accelerated expansion still stands. As I understand the expansion the red-shift is quite crucial in the conclusion.

Community
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adsp42
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  • Nobel prizes aren't awarded for bunk research.... – Kyle Kanos Oct 17 '14 at 20:21
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    @KyleKanos They are occasionally awarded for research that eventually proves wrong. See 1922. Science rocks because when it's wrong, we aren't stuck with it. – user121330 Oct 17 '14 at 20:44
  • We see a correlation between distance and red-shift. Are you asking whether the stars we see that are further away are all more massive? Both the universe's expansion and the red-shift of massive stars are well accepted experimentally and theoretically. – user121330 Oct 17 '14 at 20:48
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    @user121330: While Bohr's model was not correct, it's still taught in every Modern Physics & QM book I've seen. However, your comment is duly noted; I will try to use "Nobel prizes aren't (often) awarded for bunk research..." in the future ;) – Kyle Kanos Oct 17 '14 at 21:00
  • @KyleKanos I think the Bohr model was remarkably innovative even if it proved wrong - qualitatively and quantitatively quite different from homeopathy or astrology in my book. Anyhow, no problem. :D – user121330 Oct 17 '14 at 21:45

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Gravitational redshift is extremely small for stars, it has no significant impact on cosmic redshift measurements of galaxies. The gravitational redshift of light emitted by a star is of the form $$ z = \frac{1}{\sqrt{1 - \frac{2GM}{Rc^2}}} - 1 \approx \frac{GM}{Rc^2}, $$ where $R$ is (approximately) the radius of the star. For the Sun, this is of the order $10^{-6}$. While the mass of giant stars is obviously larger, their radius is also larger, so the redshift for those stars is of the same order. These values are much smaller than cosmic redshift due to the expansion of space.

Gravitational redshift does play a subtle role in CMB measurements. Cosmic microwave background radiation is affected when it passes through a supercluster of galaxies or a supervoid, causing very small but detectable changes. This is called the (integrated) Sachs-Wolfe effect.

benrg
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Pulsar
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  • Should I take it then that exactly because the gravitational redshift is "much" smaller it was not taken into account when the theory was developed? – adsp42 Oct 18 '14 at 20:04
  • So "heavily so if the star massive" is a bit abusive in my question? (comes from http://physics.stackexchange.com/users/26076/wetsavannaanimal-aka-rod-vance who answered the original question). – adsp42 Oct 18 '14 at 20:13

  • While the mass of giant stars is obviously larger, their radius is also larger -- isn't it possible that the mass is much-much larger and the radius not so much :-) I'm thinking of collapsing stars (supernovae?), isn't that how black holes come into existence? And actually the high-redshift objects that we see (and count for the expansion theory) are not stars are galaxies are they not?

     – adsp42 Oct 18 '14 at 20:20