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dpa
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why can we believe that redshift PROOVES expansion of universe when we know that redshift could have been caused due to gravitation.
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?
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.
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.dpa said:why can we believe that redshift PROOVES expansion of universe when we know that redshift could have been caused due to gravitation.
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.
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.
PhilDSP said:Can you provide a reference to that paper or article?
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, 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).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.
Chalnoth said:... nullified... don't feel any attraction in any particular direction
Right, local inhomogeneities. A region which is sufficiently overdense compared to the expansion collapses inward on itself.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?
dpa said:why can we believe that redshift PROOVES expansion of universe when we know that redshift could have been caused due to gravitation.
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)
salvestrom said:Could you elaborate please, preferably without maths ;).
Uh, what? In what universe is this happening?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.
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.budrap said:The currently accepted model relies on a mechanism, "expanding space", which has no empirical basis.
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.
salvestrom said:
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.
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.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.
Huh? No. Not at all.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?
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.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?
Chalnoth said:Huh? No. Not at all.
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.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.
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.
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.
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)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.
Drakkith said:I don't see how that could work for reasons I stated earlier. Can you elaborate?
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.
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.
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.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.