I Light Emission by Stars: Mass, Red Shift & Hubble's Law

[gptejms]

TL;DR Summary

Light emitted by a star should be affected (red shifted) by the gravitational pull of its own meaning thereby that light from bigger/ more massive stars should be red shifted more-- is this established?

Light emitted by a star should be gravitationally red shifted by its own mass, resulting in greater red shift of light emitted by a more massive star. Is this phenomenon known? Could this be contributing in Hubble 's law in some part too-- probably more distant stars are more massive too-- just guessing!

 

[PeroK]

Summary:: Light emitted by a star should be affected (red shifted) by the gravitational pull of its own meaning thereby that light from bigger/ more massive stars should be red shifted more-- is this established?

Yes, it is well established, since at least 1954. See here, for example:

https://en.wikipedia.org/wiki/Gravitational_redshift#Astronomical_observations

 

[gptejms]

Yes, it is well established, since at least 1954. See here, for example:

https://en.wikipedia.org/wiki/Gravitational_redshift#Astronomical_observations

Hubble's law talks of only distance and not the mass of emitting star---is this effect negligible?

 

[PeroK]

Hubble's law talks of only distance and not the mass of emitting star---is this effect negligible?

The link above is about measurable gravitational redshift, not Hubble's law. It talks about measuring the gravitational redshift of light from the Sun, for example.

 

[gptejms]

The link above is about measurable gravitational redshift, not Hubble's law. It talks about measuring the gravitational redshift of light from the Sun, for example.

That's right, but my question is that some part of the redshift attributed to receding of galaxies is actually due to the gravitational pull of its own star--is this part so small that it can be neglected altogether?

 

[PeroK]

That's right, but my question is that some part of the redshift attributed to receding of galaxies is actually due to the gravitational pull of its own star--is this part so small that it can be neglected altogether?

The gravitational redshift only depends on the mass of the star. That becomes less significant the further away the star is. I'm pretty sure it's not negligible in all cases. It's another complexity in measuring the recession at a smaller redshift.

 

[Ibix]

The gravitational redshift only depends on the mass of the star.

Mass and radius, I think. I don't think one is wholely determined by the other.

Hubble's law talks of only distance and not the mass of emitting star---is this effect negligible?

Galaxies are full of stars of different masses. The effect would "blur" the cosmological red shift of a galaxy slightly, since some of its stars would have cosmological red shift plus a little gravitational red shift while others would have the same cosmological red shift plus a larger gravitational contribution. They all already have kinematic Doppler from their circulation around the galaxy.

The key point is that I wouldn't expect a pattern with distance from gravitational red shift. So it would become a less significant issue the further away you looked.

 

[anuttarasammyak]

We can observe red shift by gravity for the stars in our own galaxy where Hubble's law is negligible or does not stand due to inhomogeneous distribution of energy in that scale. As for stars in other galaxies, I wonder if we can distinguish light of a star from other stars in that same galaxy by our current technology.

 

[gptejms]

Mass and radius, I think. I don't think one is wholely determined by the other.

Galaxies are full of stars of different masses. The effect would "blur" the cosmological red shift of a galaxy slightly, since some of its stars would have cosmological red shift plus a little gravitational red shift while others would have the same cosmological red shift plus a larger gravitational contribution. They all already have kinematic Doppler from their circulation around the galaxy.

The key point is that I wouldn't expect a pattern with distance from gravitational red shift. So it would become a less significant issue the further away you looked.

Actually, I am slowly trying to inch towards alternative explanations of Hubble's law.Recession of galaxies is the accepted view, but there may be other possible explanations. For example, if space were a medium like any other medium, it would absorb radiation(more at the higher frequency end) leading to frequency attenuation. This would also explain greater redshift of farther galaxies. Even though space is not accepted to be a medium, why not look for such simpler explanations rather than jumping into expanding universe model?

 

[Ibix]

For example, if space were a medium like any other medium, it would absorb radiation(more at the higher frequency end) leading to frequency attenuation. This would also explain greater redshift of farther galaxies.

No it wouldn't. Spectral lines wouldn't appear at redshifted frequencies. And there's no dispersion.

why not look for such simpler explanations rather than jumping into expanding universe model?

Because no simpler explanation consistent with red shift, the CMB and its power spectrum, the theoretical framework of general relativity, and more abstruse concepts like the integrated Sachs-Wolfe effect has ever been found. And the FLRW solution to Einstein's field equations is one of the simplest ever found.

It's not that other solutions haven't been tried (look up "tired light" for starters). It's that you have to pile special case on special case to make them consistent with observation.

 

[George Jones]

Actually, I am slowly trying to inch towards alternative explanations of Hubble's law.Recession of galaxies is the accepted view, but there may be other possible explanations. For example, if space were a medium like any other medium, it would absorb radiation(more at the higher frequency end) leading to frequency attenuation. This would also explain greater redshift of farther galaxies. Even though space is not accepted to be a medium, why not look for such simpler explanations rather than jumping into expanding universe model?

This not observed. Steven Weinberg, in his 2008 graduate-level book Cosmology, writes

"For instance, one important difference between "tired light" theories and the conventional big bang theory is that in the conventional theory all rates at the source are decreased by a factor $(1+z)^{−1}$, while in tired light theories there is no such slowing down. One rate that is slowed down at large redshifts in the conventional theory is the rate at which photons are emitted by the source. This is responsible for one of two factors of $(1+z)^{−1}$ in ... apparent luminosity, the other factor being due to the reduction of energy of individual photons. On the other hand, if the rate of photon emission is not affected by the redshift, then in a static Euclidean universe in which photons lose energy as they travel to us, the apparent luminosity of distant source ... will be given by ... only a single factor of $1+z$ in the denominator."

 

[Nugatory]

Actually, I am slowly trying to inch towards alternative explanations of Hubble's law.

Please respect the forum mission statement: "Our mission is to provide a place for people (whether students, professional scientists, or others interested in science) to learn and discuss science as it is currently generally understood and practiced by the professional scientific community. " (emphasis mine)

Discussion of the (overwhelming) evidence for recession is good, and discussion of why the alternatives fail is acceptable as long as it does not slide into personal speculation, but we will not host a discussion that "inches towards alternative explanations".

 

[gptejms]

Please respect the forum mission statement: "Our mission is to provide a place for people (whether students, professional scientists, or others interested in science) to learn and discuss science as it is currently generally understood and practiced by the professional scientific community. " (emphasis mine)

Discussion of the (overwhelming) evidence for recession is good, and discussion of why the alternatives fail is acceptable as long as it does not slide into personal speculation, but we will not host a discussion that "inches towards alternative explanations".

I was just trying to understand why alternate explanations if any haven't been accepted or looked into.

 

[gptejms]

This not observed. Steven Weinberg, in his 2008 graduate-level book Cosmology, writes

"For instance, one important difference between "tired light" theories and the conventional big bang theory is that in the conventional theory all rates at the source are decreased by a factor $(1+z)^{−1}$, while in tired light theories there is no such slowing down. One rate that is slowed down at large redshifts in the conventional theory is the rate at which photons are emitted by the source. This is responsible for one of two factors of $(1+z)^{−1}$ in ... apparent luminosity, the other factor being due to the reduction of energy of individual photons. On the other hand, if the rate of photon emission is not affected by the redshift, then in a static Euclidean universe in which photons lose energy as they travel to us, the apparent luminosity of distant source ... will be given by ... only a single factor of $1+z$ in the denominator."

Is z the distance? Why (1+z)^-1?

 

[Ibix]

$z$ is the usual way of expressing cosmological redshift. $z=0$ is right here and increases with distance, whether you are talking about the frequency shift (which scales as $(1+z)^{-1}$) or wavelength shift (which scales as $(1+z)$).

 

[gptejms]

$z$ is the usual way of expressing cosmological redshift. $z=0$ is right here and increases with distance, whether you are talking about the frequency shift (which scales as $(1+z)^{-1}$) or wavelength shift

$z$ is the usual way of expressing cosmological redshift. $z=0$ is right here and increases with distance, whether you are talking about the frequency shift (which scales as $(1+z)^{-1}$) or wavelength shift (which scales as $(1+z)$).

Simple question: what is z?

 

[George Jones]

Is z the distance? Why (1+z)^-1?

$z$ is relative change in wavelength. In terms of a mathematical definition, $z$ is given by

$$1 + z = \frac{\lambda_{obs}} {\lambda_{em}},$$

so that

$$z = \frac{\lambda_{obs}} {\lambda_{em}} - 1 = \frac{\lambda_{obs} - \lambda_{em}} {\lambda_{em}} = \frac{\Delta \lambda} {\lambda_{em}}.$$

If you want to compete, you have to know at least the most basic of notation and terminology.

 

[George Jones]

Is z the distance? Why (1+z)^-1?

$z$ is relative change in wavelength. In terms of a mathematical definition, $z$ is given by

$$1 + z = \frac{\lambda_{obs}} {\lambda_{em}},$$

so that

$$z = \frac{\lambda_{obs}} {\lambda_{em}} - 1 = \frac{\lambda_{obs} - \lambda_{em}} {\lambda_{em}} = \frac{\Delta \lambda} {\lambda_{em}}.$$

If you want to compete, you have to know at least the most basic of notation and terminology.

 

[gptejms]

$z$ is relative change in wavelength. In terms of a mathematical definition, $z$ is given by

$$1 + z = \frac{\lambda_{obs}} {\lambda_{em}},$$

so that

$$z = \frac{\lambda_{obs}} {\lambda_{em}} - 1 = \frac{\lambda_{obs} - \lambda_{em}} {\lambda_{em}} = \frac{\Delta \lambda} {\lambda_{em}}.$$

If you want to compete, you have to know at least the most basic of notation and terminology.

Compete? What gives you that impression?

 

[George Jones]

Compete? What gives you that impression?

This:

Actually, I am slowly trying to inch towards alternative explanations of Hubble's law.

 

[gptejms]

This:

C' mon I am no expert on GTR and just trying to understand something here-- the only way to do that is by asking questions. You guys need to be more tolerant.

 

[George Jones]

C' mon I am no expert on GTR and just trying to understand something here-- the only way to do that is by asking questions. You guys need to be more tolerant.

Then a better phrasing would be something like "I am not an expert in GR or cosmology, and I am trying to understand why alternative explanations of redshift do not work."

 

[pervect]

GR does already, in some sense, incorporate gravitational redshift from large stars/masses. It does not usually separate out the effects of "gravitational redshift" from the effects of "doppler shift due to cosmological expansion". The methodolody doesn't actually care, because the methodology is coordinate independent. Conceptually differentiating these effects is only possible if one takes a coordinate dependent approach to the problem. And the results one gets will depend on the details of one's coordinate choice.

Usually, professionals adopt the cosmological coordinates to do the analysis, which are defined in such a way that expansion of the coordinates is non-zero (as measured by something called the expansion scalar). Roughly speaking, the usual cosmological coordinates expand along with what are called "co-moving" observers. This coordinate choice is most likely non-intuitive to the average reader. It is possible to adopt other coordinates that do not expand in this manner, such as Fermi-Normal coordinates.

At a basic level, we can roughly say that observers with constant cosmological coordinates move with the cosmological expansion. The CMB as measured by such an observer, called a co-moving observer, will be the same in all directions (isotropic). A signal emitted from one co-moving observer to another will experience redshift.

Observers in Fermi-Normal coordinates can be thought of as being at rest relative to each other (in some sense, at least. Technically it might be better to say that they expansion scalar of these coordinates is zero). Furthermore, observers who are at rest in cosmological coordinantes can be regarded as moving relative to observers in Fermi-Normal coordinates, with the velocity between the observers increasing as one moves away from the origin of the Fermi-Normal coordinates.

However, textbooks will generally not treat the cosmological problem in Fermi-Normal coordinates.

I have some intuitions as to what happens in Fermi-Normal coordinates, though I haven't done or seen a thourogh analysis. But basically, what I think should happen is that if it were not for the cosmological constant, there would indeed be be a gravitational redshift associated with objects at rest in Fermi-normal coordinates. However, I believe this changes with the addition of the cosmological constant in the standard $\Lambda CDM$ cosmological model. Uniituitively, the gravitational redshift becomes a gravitational blueshift, and this effect is what makes the expansion of the universe accelerate. Without the cosmological constant, the expansion of the universe would deacclerate.

 

[gptejms]

This not observed. Steven Weinberg, in his 2008 graduate-level book Cosmology, writes

"For instance, one important difference between "tired light" theories and the conventional big bang theory is that in the conventional theory all rates at the source are decreased by a factor $(1+z)^{−1}$, while in tired light theories there is no such slowing down. One rate that is slowed down at large redshifts in the conventional theory is the rate at which photons are emitted by the source. This is responsible for one of two factors of $(1+z)^{−1}$ in ... apparent luminosity, the other factor being due to the reduction of energy of individual photons. On the other hand, if the rate of photon emission is not affected by the redshift, then in a static Euclidean universe in which photons lose energy as they travel to us, the apparent luminosity of distant source ... will be given by ... only a single factor of $1+z$ in the denominator."

Tired light idea is different from what I was suggesting(read my post that you replied to above).

 

[jbriggs444]

Tired light idea is different from what I was suggesting(read my post that you replied to above).

Pulling that quote forward...

For example, if space were a medium like any other medium, it would absorb radiation(more at the higher frequency end) leading to frequency attenuation.

That theory would not explain a red shift in absorption lines. You'd shift the bulk distribution by selective attenuation, but not the lines within the distribution.

If you are going to try to promote personal theories like this, you may earn yourself some mentor attention.

 

[PeterDonis]

Actually, I am slowly trying to inch towards alternative explanations of Hubble's law.

This is personal speculation and is off limits at PF. Your valid question in the OP of this thread has been answered.

Thread closed.

 

[PeterDonis]

If you are going to try to promote personal theories like this, you may earn yourself some mentor attention.

Indeed.