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Chris Boar
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Red/Blue shift due to Doppler effect = assumption?
      #5470861 - 10/15/12 12:40 AM

A real basic question on this (my Dad asked me this today actually), just wanted to check on this.

Our estimation on distance to other stars/galaxies based on red shift measurement relies on the premise that red/blue shift is caused by the Doppler effect. But we cannot verify this, we just take it as a given. Is that correct?

If our assumption on the relationship between red shift/doppler effect is wrong then all our distance calculations are incorrect as well. Hubble's law would be wrong, We would in fact have no idea how far away these objects are?

Is my understanding on this correct?


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russell23
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Re: Red/Blue shift due to Doppler effect = assumption? new [Re: Chris Boar]
      #5471136 - 10/15/12 09:24 AM

Quote:

A real basic question on this (my Dad asked me this today actually), just wanted to check on this.

Our estimation on distance to other stars/galaxies based on red shift measurement relies on the premise that red/blue shift is caused by the Doppler effect. But we cannot verify this, we just take it as a given. Is that correct?

If our assumption on the relationship between red shift/doppler effect is wrong then all our distance calculations are incorrect as well. Hubble's law would be wrong, We would in fact have no idea how far away these objects are?

Is my understanding on this correct?




Let me help clarify a couple of things here. Almost all galaxies are observed to have redshifted spectral lines. A few nearby galaxies and a few galaxies in the Virgo cluster have blueshifts, but almost all galaxies have redshifts.

For large samples of galaxies with distances estimated from distances indicators such as the Tully-Fisher relation, surface brightness fluctuation method, or Type Ia SN (all of which are calibrated with Cepheid variables), there is a trend of the observed redshift increasing with the distance.

The standard interpretation of this observational result is that the universe is expanding. If this interpretation is incorrect, there is still a trend of redshift increasing with distance, so any alternate theory would need to be able to explain redshift increasing with distance.

There are also doppler shifts interpreted to be superposed upon the expansion redshift. These are due to peculiar motions in large galaxy clusters and amount to no more than ~ 1500 km s-1 at the most for individual galaxies.

Stars within a galaxy are not participating in the expansion of the universe because that affect is small relative to the gravitational forces in a galaxy. So doppler motions are detected for stars in galaxy, but the redshifts and blueshifts are stars within a galaxy are not correlated with distance.

Hope that helps.

Dave


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GlennLeDrew
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Re: Red/Blue shift due to Doppler effect = assumption? new [Re: russell23]
      #5471173 - 10/15/12 09:53 AM

We see Doppler shifting in rotating bodies like the Sun and planets, which are congruent with the rotation rates directly observed. From this we might reasonably conclude that Doppler shifts observed in such phenomena as spectrographic binary stars and stars in clusters (particularly globulars), which seem to accord with dynamical theory and certain other observations, is a real indication of radial velocity. And for the distant galaxies in their turn, Doppler shifts can again, by extrapolation, be reasonably assumed to indicate motion along the line of sight.

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Chris Boar
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Reged: 02/23/09

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Re: Red/Blue shift due to Doppler effect = assumption? new [Re: GlennLeDrew]
      #5471997 - 10/15/12 06:01 PM

Thanks guy's, that makes sense.

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deSitter
Still in Old School


Reged: 12/09/04

Re: Red/Blue shift due to Doppler effect = assumption? new [Re: GlennLeDrew]
      #5472267 - 10/15/12 08:48 PM

Quote:

We see Doppler shifting in rotating bodies like the Sun and planets, which are congruent with the rotation rates directly observed. From this we might reasonably conclude that Doppler shifts observed in such phenomena as spectrographic binary stars and stars in clusters (particularly globulars), which seem to accord with dynamical theory and certain other observations, is a real indication of radial velocity. And for the distant galaxies in their turn, Doppler shifts can again, by extrapolation, be reasonably assumed to indicate motion along the line if sight.




Neverthess, it *is* possible to provide a purely kinematic explanation of cosmic redshifts. Whether or not it's the right one, it still is extremely interesting that this is even possible.

-drl


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GlennLeDrew
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Re: Red/Blue shift due to Doppler effect = assumption? new [Re: deSitter]
      #5476492 - 10/18/12 07:56 AM

And is it not a *kinematic* relationship which I'm supporting, as opposed to some 'tired light' theory or some such?

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Jarad
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Re: Red/Blue shift due to Doppler effect = assumption? new [Re: GlennLeDrew]
      #5476517 - 10/18/12 08:20 AM

I think he means purely kinematic in the sense that the various galaxies just happen to moving at those speeds, as opposed to due to the expansion of the universe.

Is that correct, Danny?

Jarad


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GlennLeDrew
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Re: Red/Blue shift due to Doppler effect = assumption? new [Re: Jarad]
      #5476528 - 10/18/12 08:38 AM

Color me naive, for I see 'kinematic' velocity and expansion of the universe as the same thing. In both cases the object is moving, as revealed by Doppler shifts. The ultimate extension of this is the redward shift into the microwave regime the echo of the big bang (again, paint me gullible.)

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Neutrino?
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Reged: 12/14/09

Loc: Wasatch Front
Re: Red/Blue shift due to Doppler effect = assumption? new [Re: GlennLeDrew]
      #5476945 - 10/18/12 01:16 PM

Quote:

Color me naive, for I see 'kinematic' velocity and expansion of the universe as the same thing. In both cases the object is moving, as revealed by Doppler shifts. The ultimate extension of this is the redward shift into the microwave regime the echo of the big bang (again, paint me gullible.)




No. (Currently, it is accepted that) Cosmological redshift is not due to kinematics. There is no conservation law for energy along geodesics in the FRW metric as it is not symmetric in time. This metric is used to describe the metric expansion of space in GR.

Edited by Neutrino? (10/18/12 01:18 PM)


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deSitter
Still in Old School


Reged: 12/09/04

Re: Red/Blue shift due to Doppler effect = assumption? new [Re: Jarad]
      #5477200 - 10/18/12 03:14 PM

Quote:

I think he means purely kinematic in the sense that the various galaxies just happen to moving at those speeds, as opposed to due to the expansion of the universe.

Is that correct, Danny?

Jarad




Actually I mean by dint of geometry itself. The trees in China are upside down relative to ours. That does not mean they are being pushed over by a mysterious dynamical tree pushing force. They grow on a sphere, not a plane. The BB adopts a dynamical explanation by assuming the trees are carried along with space that expands from nothing.

-drl


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Pess
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Loc: Toledo, Ohio
Re: Red/Blue shift due to Doppler effect = assumption? new [Re: deSitter]
      #5494771 - 10/29/12 02:46 PM

Quote:

Quote:

We see Doppler shifting in rotating bodies like the Sun and planets, which are congruent with the rotation rates directly observed. From this we might reasonably conclude that Doppler shifts observed in such phenomena as spectrographic binary stars and stars in clusters (particularly globulars), which seem to accord with dynamical theory and certain other observations, is a real indication of radial velocity. And for the distant galaxies in their turn, Doppler shifts can again, by extrapolation, be reasonably assumed to indicate motion along the line if sight.




Neverthess, it *is* possible to provide a purely kinematic explanation of cosmic redshifts. Whether or not it's the right one, it still is extremely interesting that this is even possible.

-drl




I wonder. Since redshifts are a function (supposedly) of distance. Would it be possible to observe say, several distant objects over the course of years and graph a subtle change (increase) in redshift of these observed objects as they recede from us?

I just did some back of envelope calculations and it appears we need more sensitive instruments by a factor of two and/or perhaps at least 50 to a 100 years of observing time?

I think that might settle the redshift question once and for-all.

Pesse (Just thinking out loud) Mist


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Kobayashi
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Reged: 07/10/08

Re: Red/Blue shift due to Doppler effect = assumption? new [Re: Pess]
      #5495065 - 10/29/12 05:47 PM

Quote:

Since redshifts are a function (supposedly) of distance. Would it be possible to observe say, several distant objects over the course of years and graph a subtle change (increase) in redshift of these observed objects as they recede from us?




If you're talking about redshift of galaxies, the redshift is smeared by all the internal motion within the galaxy (i.e. movement of individual stars within the galaxy), so I don't think you'll get a very good precision. Also, since galaxies are tens or thousands of light-years across, you won't see any fluctuation at a timescale shorter than tens of thousands of years.


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Jarad
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Re: Red/Blue shift due to Doppler effect = assumption? new [Re: Kobayashi]
      #5495219 - 10/29/12 07:54 PM

You would have to use something like a quasar. Those are presumed to be physically small, and giving off large amounts of light.

Still not trivial, but if it can be measured with sufficient precision, we could potentially measure a shift in a high-Z quasar over a few hundred years or so.

Sounds perfect for a new grad student to take on...

Jarad


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Rick Woods
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Re: Red/Blue shift due to Doppler effect = assumption? new [Re: Kobayashi]
      #5495421 - 10/29/12 10:15 PM

Quote:

[If you're talking about redshift of galaxies, the redshift is smeared by all the internal motion within the galaxy (i.e. movement of individual stars within the galaxy), so I don't think you'll get a very good precision.




Is that true? I would think the redshift of a distant galaxy indicative of relativistic speeds, while any local motion within the galaxy itself should be almost indetectable.


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deSitter
Still in Old School


Reged: 12/09/04

Re: Red/Blue shift due to Doppler effect = assumption? new [Re: Jarad]
      #5495729 - 10/30/12 03:28 AM

Quote:

You would have to use something like a quasar. Those are presumed to be physically small, and giving off large amounts of light.

Still not trivial, but if it can be measured with sufficient precision, we could potentially measure a shift in a high-Z quasar over a few hundred years or so.

Sounds perfect for a new grad student to take on...

Jarad




Yes or an AGN presumably embedded in thick material. The first step is to start looking.

-drl


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russell23
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Loc: Upstate NY
Re: Red/Blue shift due to Doppler effect = assumption? new [Re: Rick Woods]
      #5495900 - 10/30/12 09:08 AM

Quote:

Quote:

[If you're talking about redshift of galaxies, the redshift is smeared by all the internal motion within the galaxy (i.e. movement of individual stars within the galaxy), so I don't think you'll get a very good precision.




Is that true? I would think the redshift of a distant galaxy indicative of relativistic speeds, while any local motion within the galaxy itself should be almost indetectable.




Rotation curves are measured by measuring the redshift of a galaxy across its disk. See for example figure 5-8 of this paper:

http://adsabs.harvard.edu/abs/1997AJ....114.2402C

(click on "Full refereed article (pdf/postscript)

So redshift varies across the disk in a systematic way that is consistent with rotation. The figures put the redshift of the nucleus at zero. The redshift relative to that value is mapped across the disk and results from doppler effects due to one side of the galaxy rotating toward us in the line of sight and the other side of the galaxy rotating away from us in the line of sight.

The problem is really the timescales involved in detecting a change in redshift. As part of a paper I just submitted I listed the observed redshift values of the galaxy being discussed and the measured redshift values had a range of 110 km s-1. With a Hubble constant of 70 km s-1 Mpc-1 that is equivalent to a distance of ~1.5 Megaparsecs. Note the galaxy has not actually moved 1.5 Mpc in the 20 years of measurements the data covers. That is just the range of redshift values provided by different instruments. So the uncertainty in our redshift measurements will swamp out any changes in expansion redshifts over any timescales realistic for us to conduct measurements.

Dave


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russell23
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Re: Red/Blue shift due to Doppler effect = assumption? new [Re: Chris Boar]
      #5495961 - 10/30/12 10:09 AM

Quote:

A real basic question on this (my Dad asked me this today actually), just wanted to check on this.

Our estimation on distance to other stars/galaxies based on red shift measurement relies on the premise that red/blue shift is caused by the Doppler effect. But we cannot verify this, we just take it as a given. Is that correct?

If our assumption on the relationship between red shift/doppler effect is wrong then all our distance calculations are incorrect as well. Hubble's law would be wrong, We would in fact have no idea how far away these objects are?

Is my understanding on this correct?




Maybe it is worth coming back to the original questions. When Hubble published the first velocity-distance result in 1929 he did not assume that there was a law in which redshift increases with distance. What he noticed is that there is a correlation between the observed redshifts of the galaxies in his sample and the distances to those derived from the magnitudes of stars within the individual galaxies. So as the distance to the galaxy increases, the observed redshift velocity increases.

Hubble did interpret scatter in the velocity-distance relationship as coming from two primary sources:

1. Errors in his distances derived from magnitude data
2. Doppler motions of individual galaxies (peculiar motions)

With Hubble's original data he inferred peculiar motions should be small - typically less than about 200 km s-1. Hubble's original distances were severely in error due to less precision of magnitude measures and an incorrect calibration of the Cepheid distance scale. So his original value of what has become known as the "Hubble Constant" was over 500 km s-1 Mpc-1.

Hubble himself was very cautious about interpreting the velocity-distance relationship. While the research community preferred an expanding universe interpretation Hubble was not really committed to that or any other interpretation.

Jump ahead to modern data. Modern calibrations of the Cepheid distance scale applied to secondary distance indicators methods such as the Surface Brightness fluctuation method, Tully-Fisher Relation, Type Ia SN, Type II SN, Tip of the red Giant branch ... and a few other methods have led to a very clear picture that redshift increases with distance ... for the normal galaxies to which these methods can be applied.

This is a very important realty that people need to understand. Even in Halton Arp's "intrinsic redshift" models there is an underlying redshift-distance relationship upon which his "intrinsic" redshifts would be superposed.

What mainstream astrophysical research community sees in all their data is that there are no significant deviations from the Hubble law. When evidence appears that suggests the possibility that some very high redshift objects could actually be much closer than their redshifts imply the mainstream response is that the evidence results from purely chance alignments of lower redshift foreground objects with higher redshift background objects.

For example one of the most famous cases is NGC 7603:

http://adsabs.harvard.edu/abs/2002A%26A...390L..15L

The spiral galaxy NGC 7603 is apparently connected to the companion NGC 7603B by a bridge of material. These sorts of bridges are always interpreted as evidence for interaction between two objects. But in this case NGC 7603 has a redshift of ~8700 km s-1 wherease the companion NGC 7603b has a redshift of 16700 km s-1. Since interaction models cannot generate a redshift difference this large the mainstream concludes that NGC 7603b is a background object. However, Arp and supporters point out correctly that NGC 7603B is the most obvious candidate to explain the existence of the luminous filament that appears to connect the two objects.

Another example that has gotten no attention is the radio galaxy B21637+29. In 1988 de Ruiter et al observed this galaxy and found that the interacting companion had a redshift ~4300 km s-1 greater than the main galaxy. They went on to develop an interaction scenario that was capable of explaining the redshift differential as resulting from a strong interaction. Then in 1998 they measured additional redshifts in the cluster and found out that additional galaxies in the cluster also had the 4000 km s-1 greater redshift even though they were not involved in interactions. This created a dilema for mainstream views that de Ruiter et al correctly characterized. Either the 4000 km s-1 higher redshift group is a background group which means there is no explanation for the strong interaction evidence for B2 1637+29 or... the higher redshift group is at the same distance as B2 1637+29 in which case we would have to accept entire groups of galaxies moving at 4000 km s-1 relative to the Hubble flow. Here is the 1998 paper:

http://adsabs.harvard.edu/abs/1998A%26A...337..711D

Note that this important paper has been cited exactly twice in 14 years - both times by Lopez-Corredoira whom published the NGC 7603 paper. How can this be? Nobody in the astrophysics research community wants to explore the implications of this? Only one researcher in 14 years has cited this important paper that identifies a 4000 km s-1 deviation from Hubble's law! And 4000 km s-1 is halfway to the difference between NGC 7603 and NGC 7603B. If a 4000 km s-1 diffeence is possible, then why not an 8000 km s-1 difference?

As a final example there is NGC 1275 - an active galaxy with a 3000 km s-1 higher redshift galaxy superposed in front of it on our line of sight:

http://adsabs.harvard.edu/abs/1996Ap.....39..334M

http://adsabs.harvard.edu/abs/1992ApJ...388..301C

There is a universe of possibilities out there involving devations from Hubble's law that the astrophysics research community avoids investigating. I don't understand why there is such resistance. It is well established that a Hubble relationship exists for most normal galaxies. It would not require abandoning the current cosmological model to acknowledge that our universe contains anomalous redshift objects.

Dave


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GlennLeDrew
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Re: Red/Blue shift due to Doppler effect = assumption? new [Re: russell23]
      #5496172 - 10/30/12 01:17 PM

I would be most cautious of interpreting 'connecting bridges' based on appearance alone. There must be more robust evidence to confirm that they result from more than chance superposition. One effect to be aware of is that subtle features can be made more apparent when seen against another smooth source. An example is the M110's tidal tail, being more apparent where seen against the faint, smooth glow in the outer parts of M31. Someone not acquainted with tidal effects might interpret this as a connecting bridge. Of course, we know with some confidence that M31 and M110 are effectively interacting. But suppose M110 was a very much more distant object having the same tidal tail-like extension; it might initially give the impression of a 'connection' to the foreground M31.

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deSitter
Still in Old School


Reged: 12/09/04

Re: Red/Blue shift due to Doppler effect = assumption? new [Re: russell23]
      #5496322 - 10/30/12 02:53 PM

Quote:

Quote:

A real basic question on this (my Dad asked me this today actually), just wanted to check on this.

Our estimation on distance to other stars/galaxies based on red shift measurement relies on the premise that red/blue shift is caused by the Doppler effect. But we cannot verify this, we just take it as a given. Is that correct?

If our assumption on the relationship between red shift/doppler effect is wrong then all our distance calculations are incorrect as well. Hubble's law would be wrong, We would in fact have no idea how far away these objects are?

Is my understanding on this correct?




Maybe it is worth coming back to the original questions. When Hubble published the first velocity-distance result in 1929 he did not assume that there was a law in which redshift increases with distance. What he noticed is that there is a correlation between the observed redshifts of the galaxies in his sample and the distances to those derived from the magnitudes of stars within the individual galaxies. So as the distance to the galaxy increases, the observed redshift velocity increases.

Hubble did interpret scatter in the velocity-distance relationship as coming from two primary sources:

1. Errors in his distances derived from magnitude data
2. Doppler motions of individual galaxies (peculiar motions)

With Hubble's original data he inferred peculiar motions should be small - typically less than about 200 km s-1. Hubble's original distances were severely in error due to less precision of magnitude measures and an incorrect calibration of the Cepheid distance scale. So his original value of what has become known as the "Hubble Constant" was over 500 km s-1 Mpc-1.

Hubble himself was very cautious about interpreting the velocity-distance relationship. While the research community preferred an expanding universe interpretation Hubble was not really committed to that or any other interpretation.

Jump ahead to modern data. Modern calibrations of the Cepheid distance scale applied to secondary distance indicators methods such as the Surface Brightness fluctuation method, Tully-Fisher Relation, Type Ia SN, Type II SN, Tip of the red Giant branch ... and a few other methods have led to a very clear picture that redshift increases with distance ... for the normal galaxies to which these methods can be applied.

This is a very important realty that people need to understand. Even in Halton Arp's "intrinsic redshift" models there is an underlying redshift-distance relationship upon which his "intrinsic" redshifts would be superposed.

What mainstream astrophysical research community sees in all their data is that there are no significant deviations from the Hubble law. When evidence appears that suggests the possibility that some very high redshift objects could actually be much closer than their redshifts imply the mainstream response is that the evidence results from purely chance alignments of lower redshift foreground objects with higher redshift background objects.

For example one of the most famous cases is NGC 7603:

http://adsabs.harvard.edu/abs/2002A%26A...390L..15L

The spiral galaxy NGC 7603 is apparently connected to the companion NGC 7603B by a bridge of material. These sorts of bridges are always interpreted as evidence for interaction between two objects. But in this case NGC 7603 has a redshift of ~8700 km s-1 wherease the companion NGC 7603b has a redshift of 16700 km s-1. Since interaction models cannot generate a redshift difference this large the mainstream concludes that NGC 7603b is a background object. However, Arp and supporters point out correctly that NGC 7603B is the most obvious candidate to explain the existence of the luminous filament that appears to connect the two objects.

Another example that has gotten no attention is the radio galaxy B21637+29. In 1988 de Ruiter et al observed this galaxy and found that the interacting companion had a redshift ~4300 km s-1 greater than the main galaxy. They went on to develop an interaction scenario that was capable of explaining the redshift differential as resulting from a strong interaction. Then in 1998 they measured additional redshifts in the cluster and found out that additional galaxies in the cluster also had the 4000 km s-1 greater redshift even though they were not involved in interactions. This created a dilema for mainstream views that de Ruiter et al correctly characterized. Either the 4000 km s-1 higher redshift group is a background group which means there is no explanation for the strong interaction evidence for B2 1637+29 or... the higher redshift group is at the same distance as B2 1637+29 in which case we would have to accept entire groups of galaxies moving at 4000 km s-1 relative to the Hubble flow. Here is the 1998 paper:

http://adsabs.harvard.edu/abs/1998A%26A...337..711D

Note that this important paper has been cited exactly twice in 14 years - both times by Lopez-Corredoira whom published the NGC 7603 paper. How can this be? Nobody in the astrophysics research community wants to explore the implications of this? Only one researcher in 14 years has cited this important paper that identifies a 4000 km s-1 deviation from Hubble's law! And 4000 km s-1 is halfway to the difference between NGC 7603 and NGC 7603B. If a 4000 km s-1 diffeence is possible, then why not an 8000 km s-1 difference?

As a final example there is NGC 1275 - an active galaxy with a 3000 km s-1 higher redshift galaxy superposed in front of it on our line of sight:

http://adsabs.harvard.edu/abs/1996Ap.....39..334M

http://adsabs.harvard.edu/abs/1992ApJ...388..301C

There is a universe of possibilities out there involving devations from Hubble's law that the astrophysics research community avoids investigating. I don't understand why there is such resistance. It is well established that a Hubble relationship exists for most normal galaxies. It would not require abandoning the current cosmological model to acknowledge that our universe contains anomalous redshift objects.

Dave




Great post. It's very important to understand that the Hubble relation is real. I am convinced that some redshifts are intrinsic, but they appear, as you say, superposed on the Hubble effect. Too often "Arpists" are stridently against any redshift that has cosmic interpretations, while those who denigrate them often appear to be unwilling to even look with an open mind. Sounds like politics no?

-drl


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deSitter
Still in Old School


Reged: 12/09/04

Re: Red/Blue shift due to Doppler effect = assumption? new [Re: GlennLeDrew]
      #5496327 - 10/30/12 02:55 PM

Quote:

I would be most cautious of interpreting 'connecting bridges' based on appearance alone. There must be more robust evidence to confirm that they result from more than chance superposition. One effect to be aware of is that subtle features can be made more apparent when seen against another smooth source. An example is the M110's tidal tail, being more apparent where seen against the faint, smooth glow in the outer parts of M31. Someone not acquainted with tidal effects might interpret this as a connecting bridge. Of course, we know with some confidence that M31 and M110 are effectively interacting. But suppose M110 was a very much more distant object having the same tidal tail-like extension; it might initially give the impression of a 'connection' to the foreground M31.




There is a large class of galaxies that are morphologically similar - grand spiral with irregular companion connected by a material bridge. Why should these be any different? In fact the onus would be on the person proposing chance alignment, not the one assuming the simplest possibility.

-drl


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