<p>Ultimately this is an application of Occam's razor</p>

<p>In order for the light to be gravitationally redshifted, it would have to be coming out of a <strong>deep</strong> gravitational well. For the red shift observed in galaxies to be gravitational, you would have to suppose several things. </p>

<p>First, that the stars in distant galaxies are somehow much denser: more than neutron star dense. (or possibly that entire galaxies are as dense as neutron stars.) <a href="http://www.nature.com/nature/journal/v420/n6911/abs/nature01159.html" rel="noreferrer">A neutron star has a redshift of about z=0.35</a> distant galaxies have redshift of more than 7. No known object has a gravitational redshift like that.</p>

<p>Secondly that the density is proportional to distance from us. Placing us at a special position in the universe. </p>

<p>It is much simpler to interpret the redshift as a doppler redshift, and therefore an expanding universe</p>

CC BY-SA 4.0
2019-11-15T19:24:25.820Z
Isn't this assuming that the local gravity well at source of emission is the only possible gravitational redshift factor though? Integrated Sachs-Wolfe effect is mainly concerned with CMB, but it confirms that photons suffer a net energy loss every time they pass through a void. If the proportion of inter-stellar and intergalactic void space is greater than the proportion of high-density spaces that result in net blueshift as photons pass through, it would be reasonable to expect an average net redshift relative to distance traveled for photons coming from most stars.
MacThule
CC BY-SA 4.0
2022-02-09T23:36:20.530Z
@MacThule and also there's Rayleigh Scattering which may contribute something towards it.
Owl
CC BY-SA 4.0
2022-02-09T23:45:22.443Z
Regarding ockham's razor, if we are going down that route, maybe the simplest solution is not a theory requires the invention of dark matter, an inexplicable expansion phase, the creation of a universe, and multiverses colliding to explain something. When you have to go to that length to explain a phenominon, maybe it's time to return to the basics and check everything again before we spend trillions $ and waste decades down a rabbit hole... The simplest solution is that the universe has always existed in more or less this form, and the red shift is caused by something poorly understood.
Owl
CC BY-SA 3.0
2015-11-07T22:36:51.933Z
The basic reason is Hubble shift is not what you'd see if the osberved red shift was down to the local gravitational field around the source. However I would just point out that Hubble shift is not Doppler shift. Doppler shift depends on the direction of motion and  velocity of the source relative at emission relative to the observer when received. Hubble shift depends on the cosmological scale factor at emission relative to scale factor when received.
John Davis
CC BY-SA 3.0
2015-11-07T21:46:43.373Z
2015-11-07T21:46:43.373Z
James K
<p>I would hazard a guess at this point, but effects of gravitational red-shift would perhaps average out, so approximately 50% of light would orginate from objects with stronger gravitational fields and 50% from weaker gravitational fields. Hence, we would see blue-shifted and red-shifted objects if gravitational redshift is an prominent as assumed.</p>

<p>However, with the exception of the local group (where due to being gravitationally bound M33, Andromeda and ourselves are all moving towards each other), most of the extragalactic objects we observe have a redshift. Hence, the Doppler effect must be dominating. </p>

CC BY-SA 4.0
2022-02-09T23:37:14.007Z
@user1991 as opposed to the dark matter that's needed to explain the gravitational expansion?
Owl
CC BY-SA 3.0
2015-11-08T11:02:59.663Z
The shear magnitude of the redshift of (especially) high redshift galaxies couldn't possibly be explained by a gravitational redshift, because that would imply exotic, much denser objects at high redshift that is not only contrived, but also completely incompatible with observations.

In this scenario, the redshift itself (as opposed to a blueshift as you say) is not the biggest problem. The increased redshift with distance is.
user1991
CC BY-SA 3.0
2015-11-07T13:21:02.773Z
2015-11-07T13:21:02.773Z
MichaelJRoberts
<p>Is it possible that the redshift observed by Edwin Hubble is really from a gravitational redshift and the universe isn't expanding as he has predicted?</p>

<p>What I think I know thus far is this:</p>

<ul>
<li>Redshift due to the Doppler effect of a moving object is happening due to universe expansion. Galaxies further away are speeding away from us faster than those close to us, hence having a stronger redshift effect. </li>
<li>Gravitational redshift is happening due to light "escaping" the gravitational pull of the source (star) and it has nothing to do with the movement of such source.</li>
</ul>

CC BY-SA 4.0
2022-02-09T23:42:56.540Z
@ProfRob, have there been any experiments done to validate redshift at large distances, say sending a strong laser pulse to reflect off a probe, measuring the round trip time, calculating the distance and the doppler shift and checking that this all matches up?
Owl
CC BY-SA 3.0
2015-11-07T14:54:26.277Z
Perhaps you could indicate what you have already found out about the [Hubble Flow](https://en.wikipedia.org/wiki/Hubble%27s_law) or [gravitational red shift](https://en.wikipedia.org/wiki/Gravitational_redshift).
James K
CC BY-SA 3.0
2015-11-07T16:22:35.463Z
Added to original question.
Ska
CC BY-SA 3.0
2015-11-07T22:20:02.367Z
You haven't thought this through. In your model, why should there be any relationship between redshift and distance? Or to put it another way, why do objects at similar distances, but in opposite directions, have similar redshifts?
ProfRob
CC BY-SA 3.0
2015-11-07T22:22:14.570Z
BTW Better to think about cosmological redshift as due to the expansion of the universe not the movement of sources. Wavelengths are stretched in transit.
ProfRob
<gravity><expansion><redshift><doppler-effect>
Gravitational red shift vs Doppler redshift: Is the universe really expanding?
Ska
# Gravitational red shift vs Doppler redshift: Is the universe really expanding?

(Asked by: username_0 on 2015-11-07)

Is it possible that the redshift observed by Edwin Hubble is really from a gravitational redshift and the universe isn't expanding as he has predicted?

What I think I know thus far is this:

* Redshift due to the Doppler effect of a moving object is happening due to universe expansion. Galaxies further away are speeding away from us faster than those close to us, hence having a stronger redshift effect.
* Gravitational redshift is happening due to light "escaping" the gravitational pull of the source (star) and it has nothing to do with the movement of such source.

**Tags:** gravity, expansion, redshift, doppler-effect
> **Comments:**
> **username_2**: Perhaps you could indicate what you have already found out about the [Hubble Flow](https://en.wikipedia.org/wiki/Hubble%27s_law) or [gravitational red shift](https://en.wikipedia.org/wiki/Gravitational_redshift).  
> **username_0**: Added to original question.  
> **username_3**: You haven't thought this through. In your model, why should there be any relationship between redshift and distance? Or to put it another way, why do objects at similar distances, but in opposite directions, have similar redshifts?  
> **username_3**: BTW Better to think about cosmological redshift as due to the expansion of the universe not the movement of sources. Wavelengths are stretched in transit.  
> **username_1**: @ProfRob, have there been any experiments done to validate redshift at large distances, say sending a strong laser pulse to reflect off a probe, measuring the round trip time, calculating the distance and the doppler shift and checking that this all matches up?  

---

## 2 answers

### Answer by username_2 on 2015-11-07. Score: 5 | Accepted answer

Ultimately this is an application of Occam's razor

In order for the light to be gravitationally redshifted, it would have to be coming out of a **deep** gravitational well. For the red shift observed in galaxies to be gravitational, you would have to suppose several things.

First, that the stars in distant galaxies are somehow much denser: more than neutron star dense. (or possibly that entire galaxies are as dense as neutron stars.) [A neutron star has a redshift of about z=0.35](http://www.nature.com/nature/journal/v420/n6911/abs/nature01159.html) distant galaxies have redshift of more than 7. No known object has a gravitational redshift like that.

Secondly that the density is proportional to distance from us. Placing us at a special position in the universe.

It is much simpler to interpret the redshift as a doppler redshift, and therefore an expanding universe

> **Comments:**
> **username_5**: The basic reason is Hubble shift is not what you'd see if the osberved red shift was down to the local gravitational field around the source. However I would just point out that Hubble shift is not Doppler shift. Doppler shift depends on the direction of motion and  velocity of the source relative at emission relative to the observer when received. Hubble shift depends on the cosmological scale factor at emission relative to scale factor when received.  
> **username_4**: Isn't this assuming that the local gravity well at source of emission is the only possible gravitational redshift factor though? Integrated Sachs-Wolfe effect is mainly concerned with CMB, but it confirms that photons suffer a net energy loss every time they pass through a void. If the proportion of inter-stellar and intergalactic void space is greater than the proportion of high-density spaces that result in net blueshift as photons pass through, it would be reasonable to expect an average net redshift relative to distance traveled for photons coming from most stars.  
> **username_1**: @MacThule and also there's Rayleigh Scattering which may contribute something towards it.  
> **username_1**: Regarding ockham's razor, if we are going down that route, maybe the simplest solution is not a theory requires the invention of dark matter, an inexplicable expansion phase, the creation of a universe, and multiverses colliding to explain something. When you have to go to that length to explain a phenominon, maybe it's time to return to the basics and check everything again before we spend trillions $ and waste decades down a rabbit hole... The simplest solution is that the universe has always existed in more or less this form, and the red shift is caused by something poorly understood.  

---

### Answer by username_6 on 2015-11-07. Score: 1

I would hazard a guess at this point, but effects of gravitational red-shift would perhaps average out, so approximately 50% of light would orginate from objects with stronger gravitational fields and 50% from weaker gravitational fields. Hence, we would see blue-shifted and red-shifted objects if gravitational redshift is an prominent as assumed.

However, with the exception of the local group (where due to being gravitationally bound M33, Andromeda and ourselves are all moving towards each other), most of the extragalactic objects we observe have a redshift. Hence, the Doppler effect must be dominating.

> **Comments:**
> **username_7**: The shear magnitude of the redshift of (especially) high redshift galaxies couldn't possibly be explained by a gravitational redshift, because that would imply exotic, much denser objects at high redshift that is not only contrived, but also completely incompatible with observations.  In this scenario, the redshift itself (as opposed to a blueshift as you say) is not the biggest problem. The increased redshift with distance is.  
> **username_1**: @user1991 as opposed to the dark matter that's needed to explain the gravitational expansion?  

---

