Kinematic Origins of Cosmo Redshift

[CraigS]

Hi Folks;

I'm finding this paper, "The Kinematic Origin of the Cosmological Redshift" to be very helpful for learning how to communicate with others on this topic (for those interested). Great learning paper, reasonably easy to read, also. (Dated: April 14, 2009).

It dances around the different interpretations of redshift from several different perspectives.

Some notable quotes:

"A common belief among cosmologists is that the cosmological redshift cannot be properly viewed as a Doppler shift (that is, as evidence for a recession velocity), but must instead be viewed in terms of the stretching of space. We argue that the most natural interpretation of the redshift is in fact as a Doppler shift, or rather as the accumulation of many infinitesimal Doppler shifts."

"In the context of the expanding Universe, the kinematic interpretation corresponds to a family of comoving observers and hence seems to be the more natural one."

"The common belief that the cosmological redshift can “only” be explained in terms of the stretching of space is based on conflating the properties of a specific coordinate system with properties of space itself. This is precisely the opposite of the correct frame of mind in which to understand relativity."

Very cool .. then I read the "Acknowledgments" section:
"We thank Sean Carroll, Jim Peebles, Dave Spiegel, and Ned Wright for helpful comments."

My thanks to Carl (again) for leading me to this one.

Cheers
PS: Apologies if this is 'old hat' for some. I mean this message as a 'heads-up' for learning junkies .. like me ..

[renormalised (Carl)]

I suppose so....Doppler shift is due to motion towards or away from the observer. In this case away from. Now, technically speaking, Hubble shift is due to the expansion of spacetime itself, and not the movement of the individual galaxies, which are at rest w.r.t. spacetime in regards to the expansion. However, if you wanted to call the expansion and the subsequent redshifting of the light a "species" of Doppler shift, caused by that movement of spacetime (not the galaxies), then I suppose you could. But really, it seems to be a matter of semantics here. And the whole idea of separating the two, Hubble expansion and Doppler shift due to the actual movement of the galaxies, was done for a specific purpose. To differentiate the two types of redshift and allow the astronomers to calculate both, without getting either mixed up. They have two different mechanisms of causation...one being the stretching of spacetime and the other being the movement of the galaxies (due to gravitational forces etc) within spacetime. That's why it was important to separate the two to begin with.

[CraigS]

Originally Posted by renormalised

I suppose so....Doppler shift is due to motion towards or away from the observer. In this case away from. Now, technically speaking, Hubble shift is due to the expansion of spacetime itself, and not the movement of the individual galaxies, which are at rest w.r.t. spacetime in regards to the expansion. However, if you wanted to call the expansion and the subsequent redshifting of the light a "species" of Doppler shift, caused by that movement of spacetime (not the galaxies), then I suppose you could. But really, it seems to be a matter of semantics here. And the whole idea of separating the two, Hubble expansion and Doppler shift due to the actual movement of the galaxies, was done for a specific purpose. To differentiate the two types of redshift and allow the astronomers to calculate both, without getting either mixed up. They have two different mechanisms of causation...one being the stretching of spacetime and the other being the movement of the galaxies (due to gravitational forces etc) within spacetime. That's why it was important to separate the two to begin with.

Yep. That's cool. They say:

"WHY THIS MATTERS:

While there is no “fact of the matter” about the origin of the redshift—what one concludes is a function of the coordinate system or method of calculation—we believe it is instructive to analyze the differing interpretations of the cosmological redshift, partly to improve understanding of cosmology, but more importantly to improve understanding of general relativity. That analysis leads, in our view, to the conclusion that the most natural interpretation of the redshift is as having a kinematic origin."

The article's purpose seems to be to broaden the reader's understanding, so that one can manoeuvre around to see someone else's perspective on the topic. (I like the motivation behind this one).

Cheers & Rgds.

[renormalised (Carl)]

Both a Doppler shift and a Hubble redshift have kinematic origins, by definition. Both are caused by the movement of something...individual galaxies under gravity in the case of a Doppler shift. The other caused by expanding spacetime, which is also movement, but not of the galaxies themselves. Just like raisins in a dough, they may jostle about a bit when cooked (gravity/Doppler shift), but the dough rises and expands, carrying the raisins along with it (expansion/Hubble redshift). It's easy enough to see where they're coming from, but it's still a question of semantics. Unless, of course, they actually think that the redshift is being caused by actual movement of the galaxies through space (not with space, as is the case for expansion). That's a different matter entirely.

[CraigS]

Originally Posted by renormalised

Both a Doppler shift and a Hubble redshift have kinematic origins, by definition. Both are caused by the movement of something...individual galaxies under gravity in the case of a Doppler shift. The other caused by expanding spacetime, which is also movement, but not of the galaxies themselves. Just like raisins in a dough, they may jostle about a bit when cooked (gravity/Doppler shift), but the dough rises and expands, carrying the raisins along with it (expansion/Hubble redshift). It's easy enough to see where they're coming from, but it's still a question of semantics. Unless, of course, they actually think that the redshift is being caused by actual movement of the galaxies through space (not with space, as is the case for expansion). That's a different matter entirely.

Ah ... but what happens if the spacetime dough disappears ? Would two raisins stuck together by a bit of stringy, gooey dough come together or would they fly apart ?
(Page 13 contains the answer .. for the cheats, that is).



Cheers
PS: Yep I am a bit of a stirrer at times .. after all .. what can ya expect from a hamster (Witten's, that is) !!

[renormalised (Carl)]

If the spacetime dough disappears, so do the raisins

In their answer, the galaxies will begin to fall towards one another, yes, but only if they're both still under the influence of their mutual gravities. If that mutual attraction is less than the Hubble flow for the distance between them, both will eventually get caught up in the flow and start to recede, despite of initially moving together (or not at all, if not under their mutual gravitational influence i.e. too far away). It's only when two or more galaxies are under the influence of their mutual gravitational fields, like the Milky Way and Andromeda, that the local Hubble flow becomes negligible in comparison to their gravitational connection and as far as the flow is concerned they're considered to be one object.

[CraigS]

Originally Posted by renormalised

If the spacetime dough disappears, so do the raisins

In their answer, the galaxies will begin to fall towards one another, yes, but only if they're both still under the influence of their mutual gravities. If that mutual attraction is less than the Hubble flow for the distance between them, both will eventually get caught up in the flow and start to recede, despite of initially moving together (or not at all, if not under their mutual gravitational influence i.e. too far away). It's only when two or more galaxies are under the influence of their mutual gravitational fields, like the Milky Way and Andromeda, that the local Hubble flow becomes negligible in comparison to their gravitational connection and as far as the flow is concerned they're considered to be one object.

Its off topic a bit, but seeing as you brought it up, what/where is the evidence that Andromeda and the Milky Way are actually within range to be significantly gravitationally attracted ? (I know it smells a bit like GIEC (Giant Intergalactic Eddy Currents) but what the hell ? ... its been that kind of week !).

Of course other than GIEC, there's another possibility ... (early universe inertia although, perhaps this would imply co-incident paths which couldn't have happened with Inflation ?).

Wondering ... Cheers

[renormalised (Carl)]

Originally Posted by CraigS

Its off topic a bit, but seeing as you brought it up, what/where is the evidence that Andromeda and the Milky Way are actually within range to be significantly gravitationally attracted ? (I know it smells a bit like GIEC (Giant Intergalactic Eddy Currents) but what the hell ? ... its been that kind of week !).

Of course other than GIEC, there's another possibility ... (early universe inertia although, perhaps this would imply co-incident paths which couldn't have happened with Inflation ?).

Wondering ... Cheers

Simple fact that they're approaching one another at about 140kms on a highly elliptical trajectory...they will collide or come very close to it in about 3-4 billion years. Also, in the 13.7 billion years of their existence, if they were being carried along by the Hubble flow, they'd be a lot further apart than what they are now. This will be their first, possibly second pass by of one another since their formation. At their furthest apart, they believe it was about 4 million light years.

Here's some more reading for you to do

http://en.wikipedia.org/wiki/Milky_Way

http://en.wikipedia.org/wiki/Androme..._Way_collision

[Insane Climber (Jason)]

hello
I'm new to these ideas so please just shoot me down if im on the wrong track. Doesn't this idea get even more complicated when you consider the rather large redshift cause by objects with a massive gravity, such as light from near a black hole? and as light travels through the universe to us how can we be sure of an objects distance or location or velocity considering all the factors that can warp it on its way. It seems to me your sugesting there might be something wrong with the cosmological model ,but i thought we already knew that?

Cheers

[CraigS]

Originally Posted by Insane Climber

hello
I'm new to these ideas so please just shoot me down if im on the wrong track. Doesn't this idea get even more complicated when you consider the rather large redshift cause by objects with a massive gravity, such as light from near a black hole? and as light travels through the universe to us how can we be sure of an objects distance or location or velocity considering all the factors that can warp it on its way. It seems to me your sugesting there might be something wrong with the cosmological model ,but i thought we already knew that?

Cheers

Yes, it does get more complicated to take 'gravitational lensing' into account. This is what happens when light has to pass close to a large gravitational object before we see it. Among other effects, the light intensity patterns can shift in time and we can see the same signal (object image) repeated multiple times in the same image, for example.
This phenomenon can actually tell us lots about the physical properties of the intervening object and thus can reveal more information, had it not occurred. Some of this information is being used in the chase to ascertain the nature of some of the more mysterious stuff in the Universe.
The scientists doing research are very meticulous about which objects they choose to base their cosmological models on. Whilst the papers I am adding to this board, seemingly challenge some of the tenets of cosmology, they are part of the normal scientific process and really serve as a double check to ensure that cosmological models are 'on track'.
I, for one, am glad that this process occurs. It re-assures me that the basis of the cosmological models we hear about aren't just based on 'wobbly science'.

Cheers & Rgds.

[sjastro]

Originally Posted by Insane Climber

hello
I'm new to these ideas so please just shoot me down if im on the wrong track. Doesn't this idea get even more complicated when you consider the rather large redshift cause by objects with a massive gravity, such as light from near a black hole? and as light travels through the universe to us how can we be sure of an objects distance or location or velocity considering all the factors that can warp it on its way. It seems to me your sugesting there might be something wrong with the cosmological model ,but i thought we already knew that?

Cheers

You are correct. There is a third type of redshift called gravitational redshift associated with extremely dense objects such as black holes.

There is a subtle manifestation of gravitational redshift occurring on Earth. It's the necessity for time corrections of GPS satellites (apart from corrections due to SR effects and the Sagnac effect.)

Gravitational redshift might be the wildcard to explain the behaviour of Quasars if they are in fact black holes surrounded by an accretion disk.

Cosmologists really hate Quasars. Apart from the lack of time dilation as discussed in previous threads, their Z values are not randomly distributed.
They cluster in two main ranges 2.2>Z>1.95 and 0.5>Z>0.

Regards

Steven

[renormalised (Carl)]

Originally Posted by sjastro

You are correct. There is a third type of redshift called gravitational redshift associated with extremely dense objects such as black holes.

There is a subtle manifestation of gravitational redshift occurring on Earth. It's the necessity for time corrections of GPS satellites (apart from corrections due to SR effects and the Sagnac effect.)

Originally Posted by sjastro

Gravitational redshift might be the wildcard to explain the behaviour of Quasars if they are in fact black holes surrounded by an accretion disk.

The scientific consensus is (apart from those mavericks) that's exactly what they are...supermassive black holes surrounded by accretion disks. All the models for accretion around these objects fits observations fairly nicely.

Originally Posted by sjastro

Cosmologists really hate Quasars. Apart from the lack of time dilation as discussed in previous threads, their Z values are not randomly distributed.
They cluster in two main ranges 2.2>Z>1.95 and 0.5>Z>0.

Regards

Steven

That depends on the size of the study and how random the selection processes were in observing the quasars. However, there are explanations for any clustering that may actually be there....one of them being the formation of the quasar mechanism itself and the nature of the objects the quasars are forming in. There's a time dependency factor that comes into play. Those distances quoted (in z) equate to 11.29 and 10.8 billion light years and 5.37 billion to 798.7 million light years, respectively, so putting them roughly 2.4 to 2.9 billion, and 8.33 to 12.9 billion years after the BB (closest quasar being at z=0.06). Given that not all quasars will form at the same time or be equally as massive/luminous etc, but that on average many quasars will be within a standard deviation or two of the average size, many of them may turn on or off and become fully fledged or defunct at around the same time, or thereabouts. This may give the apparent effect of being clustered around certain values of z.

However, that in itself isn't a given either, as the quasar mechanism and the timing of their formation isn't tied down as well as they would like.