Redshift as evidence of expansion

In summary: Therefore, in summary, the currently accepted model for the expansion of the universe is based on the concept of "expanding space" and is supported by empirical evidence such as redshift, while the alternative theory of gravitational redshift lacks empirical evidence and does not fully explain the observed redshift patterns. Additionally, the idea that our local area of space is shifting in a flow of space, causing apparent redshift, has not been widely accepted and is still being studied. Finally, the role of gravitational forces in the formation of galaxies and the effect of gravitational lensing on redshift is still an area of ongoing research.
  • #1
dpa
147
0
why can we believe that redshift PROOVES expansion of universe when we know that redshift could have been caused due to gravitation.
 
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  • #2
Are you suggesting that things farther away are progressively more massive so as to create a larger redshift in exactly the pattern that we would expect if these objects were simply moving away from us due to a homogeneous expansion of space?
 
  • #3
well. I guess i wanted to ask that.

So?
 
  • #4
I don't think belief comes into it. Its the best explanation for what we can observe.
 
  • #5
Matterwave said:
Are you suggesting that things farther away are progressively more massive so as to create a larger redshift in exactly the pattern that we would expect if these objects were simply moving away from us due to a homogeneous expansion of space?

The more realistic gravitational redshift alternative would be to consider a spherical wavefront expanding outward from a source. As the volume of the sphere increases it encompasses an ever increasing quantity of mass. Calculating an expected redshift at the sphere's surface for increasing radii will yield a correlated increasing redshift.

The currently accepted model relies on a mechanism, "expanding space", which has no empirical basis. Gravitational redshifting, on the other hand has been empirically verified.
 
  • #6
budrap said:
The more realistic gravitational redshift alternative would be to consider a spherical wavefront expanding outward from a source. As the volume of the sphere increases it encompasses an ever increasing quantity of mass. Calculating an expected redshift at the sphere's surface for increasing radii will yield a correlated increasing redshift.

The currently accepted model relies on a mechanism, "expanding space", which has no empirical basis. Gravitational redshifting, on the other hand has been empirically verified.

Interesting. I've not heard of this. Then again, I read an article yesterday that suggested our local area of space (cluster scale) is shifting about in a flow of space causing apparent redshift, while in reality the universe isn't moving. Or at least accelerating.
 
  • #7
dpa said:
why can we believe that redshift PROOVES expansion of universe when we know that redshift could have been caused due to gravitation.
That's not really possible. First, you can't get redshifts close to the redshift 5-10 we see for some of the most distant objects without being right outside the event horizon of a black hole. It takes extreme space-time curvature for that to be due to a gravitational redshift at the source. Secondly, we can actually measure the masses of intermediate-distance objects via gravitational lensing, and their masses are nowhere near the amount required to generate noticeable gravitational redshifts.
 
  • #8
salvestrom said:
Interesting. I've not heard of this. Then again, I read an article yesterday that suggested our local area of space (cluster scale) is shifting about in a flow of space causing apparent redshift, while in reality the universe isn't moving. Or at least accelerating.

Can you provide a reference to that paper or article?
 
  • #9
budrap said:
The more realistic gravitational redshift alternative would be to consider a spherical wavefront expanding outward from a source. As the volume of the sphere increases it encompasses an ever increasing quantity of mass. Calculating an expected redshift at the sphere's surface for increasing radii will yield a correlated increasing redshift.

The currently accepted model relies on a mechanism, "expanding space", which has no empirical basis. Gravitational redshifting, on the other hand has been empirically verified.

This only works if all the mass is on the inside of the sphere. If the universe is homogenous and isotropic, as our current view suggets, then there is approximately equal mass in every direction on a large scale.
 
  • #11
Drakkith said:
This only works if all the mass is on the inside of the sphere. If the universe is homogenous and isotropic, as our current view suggets, then there is approximately equal mass in every direction on a large scale.

So Gauss's Law of gravity is nullified on a universal scale? I'm assuming in a finite, unbound universe every source of gravity is pulling on every other source from every possible direction owing to the wrap around, while in a infinite universe there's equal, but unwrapped gravity in all directions.

Wouldn't this make gravitational attraction impossible? It would be like being in the center of the Earth, all the mass pulling equally each way.
 
  • #12
salvestrom said:
So Gauss's Law of gravity is nullified on a universal scale? I'm assuming in a finite, unbound universe every source of gravity is pulling on every other source from every possible direction owing to the wrap around, while in a infinite universe there's equal, but unwrapped gravity in all directions.
No, it isn't nullified. It just isn't useful in this situation. Objects within an expanding universe don't feel any attraction in any particular direction. Looking at it simply as the force on a single object, however, doesn't get you anywhere. Instead the easiest way to look at this is to consider the universe to be a uniform, compressible fluid with attractive forces between the different bits of the fluid. These internal forces will act in a similar way to pressure, causing the fluid as a whole to change how it expands (or contracts).
 
  • #13
Chalnoth said:
... nullified... don't feel any attraction in any particular direction

The two things mean the same to me. Cancelled out by each other. Gauss's Law on the universe scale essentially means in every direction is an equally sized sphere of equal mass. I wasn't meaning to imply the law ceases to function.

Having said that, how does a galaxy form if all gravitational effects are equal in all directions? Local inhomogenity? (I think that's a word). Can't the same effect redshift light?
 
  • #14
The gravity of intervening masses along the route a photon follows across the universe to reach our instruments has zero effect on redshift. They bend the path [ie, gravitational lensing], but, have no effect of frequency. As photons approach a gravitational field, they are blueshifted, as they depart, they are redshifted by exactly the same amount.
 
  • #15
salvestrom said:
Having said that, how does a galaxy form if all gravitational effects are equal in all directions? Local inhomogenity? (I think that's a word). Can't the same effect redshift light?
Right, local inhomogeneities. A region which is sufficiently overdense compared to the expansion collapses inward on itself.
 
  • #16
dpa said:
why can we believe that redshift PROOVES expansion of universe when we know that redshift could have been caused due to gravitation.

I do not know exactly what you mean by gravitation. But if you mean gravitational redshift, it is not compatible with Hubble linear law (unless you assume a fractal universe with D ≈ 2)
 
  • #17
juanrga said:
I do not know exactly what you mean by gravitation. But if you mean gravitational redshift, it is not compatible with Hubble linear law (unless you assume a fractal universe with D ≈ 2)

Could you elaborate please, preferably without maths ;).
 
  • #18
salvestrom said:
Could you elaborate please, preferably without maths ;).

Due to difficulties of the Big bang model to explain the observed linear relationship between redshift and distance (Hubble law), some theoreticians are seeking for alternative interpretations.

One of these interpretations assumes that the nature of redshift is not due to space expansion but to gravitational field and obtains a linear law for an universe with a fractal distribution of matter with fractal dimension 2.

It seems some recent surveys support a fractal distribution with D ≈ 2, but this is still open.
 
  • #19
juanrga said:
Due to difficulties of the Big bang model to explain the observed linear relationship between redshift and distance (Hubble law), some theoreticians are seeking for alternative interpretations.
Uh, what? In what universe is this happening?
 
  • #20
Gauss's law is not nullified, and the attraction between distant galaxies does not cancel out. That is why the density of matter in the universe causes a deceleration of the expansion.
budrap said:
The currently accepted model relies on a mechanism, "expanding space", which has no empirical basis.
No it does not rely on "expanding space". Expanding space is just used to try to give an intuitive picture of what the equations of general relativity are saying. Goodness knows why, as it is a source of endless confusion.
 
  • #21
  • #22
Here is another:

http://arxiv.org/abs/1109.0941
Direction Dependence of the Deceleration Parameter
"... For the wCDM model, the preferred direction is
(l; b) = 314 [-13, +20] ; 28 [-33, +11) . . . While in the case of CPL model, the direction of preferred axis is (l; b) = 309 (-23+30) ; 21 (-26+35) ..."

as compared to

http://arxiv.org/abs/1109.3856
3D Velocity and Density Reconstructions of the Local Universe with Cosmicflows-1
"The bulk velocity of the full velocity field is very robustly determined to
be Vbulk = 401  19 km/s in the direction of [galactic coordinates l:b] lgal = 299 +/-6 and bgal = 24 +/-6."

I find it fascinating the alleged axis of acceleration dependency happens to be so well aligned with the direction of travel of the local group.
 
  • #23
budrap said:
The more realistic gravitational redshift alternative would be to consider a spherical wavefront expanding outward from a source. As the volume of the sphere increases it encompasses an ever increasing quantity of mass. Calculating an expected redshift at the sphere's surface for increasing radii will yield a correlated increasing redshift.

Drakkith said:
This only works if all the mass is on the inside of the sphere. If the universe is homogenous and isotropic, as our current view suggets, then there is approximately equal mass in every direction on a large scale.

No, that's not correct. I'm simply treating an expanding spherical wavefront as an object at the moment of observation and calculating an expected redshift at the surface of that object using any accepted value for the average mass density. The aggregate gravitational pull of the external cosmological matter is of no more consequence in this case than for any other object one might consider.
 
  • #24
Chronos said:
I find it fascinating the alleged axis of acceleration dependency happens to be so well aligned with the direction of travel of the local group.
Interesting. This may require some correction to our estimates of dark energy. But I have a hard time believing it will be anything but a small correction, and there is a good chance that it will cancel out entirely.
 
  • #25
So here's me. Confused again. So Gauss's Law does work the same and so gravitational redshift is back on the table as a possible alternate explanation that does away with accelerating expansion?

Also, I take it from those links which were pretty maths heavy from the outset that the link I was asked to provide is actually a genuine published paper and the effect they describe is taken seriously?
 
  • #26
salvestrom said:
So here's me. Confused again. So Gauss's Law does work the same and so gravitational redshift is back on the table as a possible alternate explanation that does away with accelerating expansion?
Huh? No. Not at all.

salvestrom said:
Also, I take it from those links which were pretty maths heavy from the outset that the link I was asked to provide is actually a genuine published paper and the effect they describe is taken seriously?
The basic claim of those links is that some of the accelerated expansion might be due to a local observational effect, due to our own motion. I really, really doubt that this can possibly explain the lion's share of the effect, though.
 
  • #27
Chalnoth said:
Huh? No. Not at all.

Budrap apparently does not agree with you. Logic dictates at least one of you is wrong. Chronon also states that Gauss's Law isn't nuliffied, along with a statement that distant galaxies don't cancel each other out, but I can't tell which way he's arguing in the issue.
 
  • #28
salvestrom said:
Budrap apparently does not agree with you. Logic dictates at least one of you is wrong. Chronon also states that Gauss's Law isn't nuliffied, along with a statement that distant galaxies don't cancel each other out, but I can't tell which way he's arguing in the issue.
Gauss's Law works. It's just not very relevant to the discussion at hand, because the Gauss's Law forces all cancel in a homogeneous universe. There are some differences due to local overdensities/underdensities and bulk flows, but that's a somewhat separate issue. In general, you just can't understand the expansion through Gauss's Law. Instead, the easiest way is to model the universe as a compressible fluid with some amount of pressure, as I mentioned earlier in the thread.
 
  • #29
budrap said:
No, that's not correct. I'm simply treating an expanding spherical wavefront as an object at the moment of observation and calculating an expected redshift at the surface of that object using any accepted value for the average mass density. The aggregate gravitational pull of the external cosmological matter is of no more consequence in this case than for any other object one might consider.

I don't see how that could work for reasons I stated earlier. Can you elaborate?
 
  • #30
budrap said:
The currently accepted model relies on a mechanism, "expanding space", which has no empirical basis. Gravitational redshifting, on the other hand has been empirically verified.

Right! Expansion redshift has not laboratory basis, because the expansion paradigm of the Big Bang model cannot be tested at laboratory (unlike gravitational redshift which is well-tested).

However, gravitational redshift could explain Hubble empirical law only for the case of a fractal universe with fractal dimension ≈ 2, and I think that this last requirement has not been empirically fulfilled, although some astronomers argue that some recent surveys could support the fractal structure.
 
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  • #31
salvestrom said:
Budrap apparently does not agree with you. Logic dictates at least one of you is wrong. Chronon also states that Gauss's Law isn't nuliffied, along with a statement that distant galaxies don't cancel each other out, but I can't tell which way he's arguing in the issue.
I'm arguing that general relativity says that the redshift is due to galaxies getting further away, and can't be explained by gravitational redshift in a static universe. (Note that the OP was talking about expansion rather than acceleration of expansion; there seems to be a separate conversation going on about whether dark flow can explain the apparent acceleration of the expansion)

If Gauss's law applies then it's nonsense to think that you can explain the redshift by gravity in a static universe, since Gauss's law means that you can't have a static universe.

budrap's claim is that we can see ourselves at the edge of a sphere, with a source of light in the centre of the sphere. The source is at the bottom of a potential well, and so it's light is redshifted. This would be true if we were stationary with respect to the source in an otherwise expanding universe, but we're not.
 
  • #32
Drakkith said:
I don't see how that could work for reasons I stated earlier. Can you elaborate?

I am not implying that the mass contained within the spherical wavefront constitutes a coherent body in and of itself, only that it can be treated as such in the reference frame of the wavefont because the wavefront does constitute a kind of coherent body.

But I agree with your analysis with regard to the matter content of the universe - that in the case of homogeneous and isotropic mass distribution of sufficient extent (say with a radius considerably larger than Schwarzschild radius implied by the mass density, the universe would not have a tendency to collapse gravitationally.)
 
  • #33
chronon said:
If Gauss's law applies then it's nonsense to think that you can explain the redshift by gravity in a static universe, since Gauss's law means that you can't have a static universe.

budrap's claim is that we can see ourselves at the edge of a sphere, with a source of light in the centre of the sphere. The source is at the bottom of a potential well, and so it's light is redshifted. This would be true if we were stationary with respect to the source in an otherwise expanding universe, but we're not.

I don't think that Gauss's law would apply, however, in a universe that is static yet unbounded. It seems from your correct interpretation of the wavefront analysis that you agree with me that in such a universe there would still be a redshift-distance relationship. And that is the point of the wavefront analysis, to suggest that there is an alternative cosmological model that does not require a universal expansion to account for the observed cosmological redshift. To the best of my knowledge that approach has never been considered.
 
  • #34
budrap said:
I am not implying that the mass contained within the spherical wavefront constitutes a coherent body in and of itself, only that it can be treated as such in the reference frame of the wavefont because the wavefront does constitute a kind of coherent body.

How so?
 
  • #35
budrap said:
I don't think that Gauss's law would apply, however, in a universe that is static yet unbounded. It seems from your correct interpretation of the wavefront analysis that you agree with me that in such a universe there would still be a redshift-distance relationship. And that is the point of the wavefront analysis, to suggest that there is an alternative cosmological model that does not require a universal expansion to account for the observed cosmological redshift. To the best of my knowledge that approach has never been considered.
You seem to want to have your cake and eat it too. Isaac Newton supposed the universe was unbounded, but static because gravitational forces canceled out, (so Gauss's law didn't apply). You seem to want to take this universe, but then not have gravitation cancelling out when it affects light.
 
<h2>1. What is redshift?</h2><p>Redshift is a phenomenon in which light from an object appears to be shifted towards the red end of the electromagnetic spectrum. This is caused by the expansion of the universe, which stretches the wavelengths of light as it travels through space.</p><h2>2. How does redshift provide evidence of expansion?</h2><p>Redshift is evidence of expansion because it is a direct result of the stretching of light as the universe expands. The further away an object is from us, the faster it is moving away from us and the greater its redshift will be. This indicates that the universe is expanding uniformly in all directions.</p><h2>3. Can redshift be caused by anything other than expansion?</h2><p>Yes, redshift can also be caused by the Doppler effect, in which the motion of an object can shift the wavelengths of light. However, this effect is only significant for objects that are relatively close to us. For objects that are very far away, the redshift is primarily due to the expansion of the universe.</p><h2>4. How is redshift measured?</h2><p>Redshift is measured using a device called a spectroscope, which splits light into its component wavelengths. By comparing the observed wavelengths of light from an object to the expected wavelengths, scientists can determine the amount of redshift and therefore the object's distance and speed of recession.</p><h2>5. Does redshift provide proof of the Big Bang theory?</h2><p>Redshift is one of the key pieces of evidence for the Big Bang theory. The observed redshift of galaxies and the cosmic microwave background radiation support the idea that the universe is expanding from a single, highly dense and hot point in the past. However, redshift alone does not prove the Big Bang theory, as there may be other explanations for the expansion of the universe.</p>

1. What is redshift?

Redshift is a phenomenon in which light from an object appears to be shifted towards the red end of the electromagnetic spectrum. This is caused by the expansion of the universe, which stretches the wavelengths of light as it travels through space.

2. How does redshift provide evidence of expansion?

Redshift is evidence of expansion because it is a direct result of the stretching of light as the universe expands. The further away an object is from us, the faster it is moving away from us and the greater its redshift will be. This indicates that the universe is expanding uniformly in all directions.

3. Can redshift be caused by anything other than expansion?

Yes, redshift can also be caused by the Doppler effect, in which the motion of an object can shift the wavelengths of light. However, this effect is only significant for objects that are relatively close to us. For objects that are very far away, the redshift is primarily due to the expansion of the universe.

4. How is redshift measured?

Redshift is measured using a device called a spectroscope, which splits light into its component wavelengths. By comparing the observed wavelengths of light from an object to the expected wavelengths, scientists can determine the amount of redshift and therefore the object's distance and speed of recession.

5. Does redshift provide proof of the Big Bang theory?

Redshift is one of the key pieces of evidence for the Big Bang theory. The observed redshift of galaxies and the cosmic microwave background radiation support the idea that the universe is expanding from a single, highly dense and hot point in the past. However, redshift alone does not prove the Big Bang theory, as there may be other explanations for the expansion of the universe.

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