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Geometry of Look-Back -- lensing



 
 
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  #1  
Old September 12th 14, 09:44 PM posted to sci.astro.research
Eric Flesch
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Posts: 321
Default Geometry of Look-Back -- lensing

Pursuant to an old thread, I wonder who can answer this:

Let's pretend for a moment that there is no dark matter, but that the
gravitational lensing that we see happening out there is affected as
follows:

(1) The lens / target are at different distances than we suppose, and

(2) There is a migrating universal "constant" such that in earlier
epochs matter bent light more per kg than it does today. In other
words, lensing effectiveness is proportional to 1+z, or maybe the
square root of 1+z.

For (1), my question is, if we alter the distances to lens and target,
even if very unreasonably so, can we recover the lensing that we see?
Or is the only working solution to make the lens much larger & further
away? A smaller closer lens can't bend the light that much, is that
right?

For (2), my question is, is there broadly a redshift dependency in
lens power, that is, lens mass? Are high-z lenses seen to be more
powerful than low-z lenses, or is that susceptible to a Malmquist
bias?

A while ago I speculated that time dilation might go as the square
root of 1+z instead of the standard 1+z. This is because if there is
a migrating universal constant which operated on the space-time
manifold, then redshift would be half time dilation and half spatial
lengthening. In other words, the past would look bigger but this
self-corrects via Riemannian geometry. I'm wondering how this would
affect the lensing that we see, thus these questions. Appreciate any
help.

cheers, Eric
  #2  
Old September 13th 14, 08:31 PM posted to sci.astro.research
Phillip Helbig---undress to reply
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Posts: 629
Default Geometry of Look-Back -- lensing

In article , Eric Flesch
writes:

Let's pretend for a moment that there is no dark matter, but that the
gravitational lensing that we see happening out there is affected as
follows:

(1) The lens / target are at different distances than we suppose, and

(2) There is a migrating universal "constant" such that in earlier
epochs matter bent light more per kg than it does today. In other
words, lensing effectiveness is proportional to 1+z, or maybe the
square root of 1+z.


Either you are assuming a change in the gravitational constant (which is
ruled out---at interesting levels, at least) or proposing some method
other than that described by GR to bend light. However, if GR is not
valid, then it probably makes little sense to leave the cosmological
model otherwise alone and just change the lensing effectiveness.

For (1), my question is, if we alter the distances to lens and target,
even if very unreasonably so, can we recover the lensing that we see?
Or is the only working solution to make the lens much larger & further
away? A smaller closer lens can't bend the light that much, is that
right?


I don't follow you here. If the lensing strength is higher at high
redshift, why do you want to make the lens larger and further away?

For (2), my question is, is there broadly a redshift dependency in
lens power, that is, lens mass? Are high-z lenses seen to be more
powerful than low-z lenses, or is that susceptible to a Malmquist
bias?


Check out the classic Turner, Ostriker & Gott ApJ paper for a plot of
lensing effectiveness as a function of redshift or, for a non-zero
cosmological constant, Fukugita, Futamase, Kasai and Turner, also in
ApJ. Lensing effectiveness is roughly Gaussian when plotted against
redshift, but is down to almost zero well before the redshift of the
source is reached. It depends on the redshift of the source and on the
cosmological model.

A while ago I speculated that time dilation might go as the square
root of 1+z instead of the standard 1+z.


This seems rather ad-hoc.

This is because if there is
a migrating universal constant which operated on the space-time
manifold, then redshift would be half time dilation and half spatial
lengthening.


Why? Unless you have an underlying theory, I don't see how you arrive
at this.

In other words, the past would look bigger but this
self-corrects via Riemannian geometry. I'm wondering how this would
affect the lensing that we see, thus these questions. Appreciate any
help.


You need to have a more concrete model in order to make concrete
predictions. People have looked for signals of unorthodox models in
lensing and found none.
  #3  
Old September 14th 14, 10:49 AM posted to sci.astro.research
Eric Flesch
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Posts: 321
Default Geometry of Look-Back -- lensing

On Sat, 13 Sep 14, Phillip Helbig wrote:
writes:
... lensing effectiveness proportional to 1+z, or ... sqrt 1+z.

you are ... proposing some method other than ... GR to bend light.


Heavens no, all I'm saying is that just as GR generalized SR, so can
GR be generalized into a larger frame where G (or some such) migrates.

I don't follow you here. If the lensing strength is higher at high
redshift, why do you want to make the lens larger and further away?


I am looking at (1) and (2) separately.

For (2), my question is, is there broadly a redshift dependency in
lens power, that is, lens mass? Are high-z lenses seen to be more
powerful than low-z lenses, or is that susceptible to a Malmquist
bias?


Check out the classic Turner, Ostriker & Gott ApJ paper for a plot of
lensing effectiveness as a function of redshift or, for a non-zero
cosmological constant, Fukugita, Futamase, Kasai and Turner, also in
ApJ. Lensing effectiveness is roughly Gaussian when plotted against
redshift, but is down to almost zero well before the redshift of the
source is reached. It depends on the redshift of the source and on the
cosmological model.


Excellent, thanks.

This is because if there is a migrating universal constant which
operated on the space-time manifold, then redshift would be
half time dilation and half spatial lengthening.


Why? Unless you have an underlying theory, I don't see how you arrive
at this.


Er, that *was* the underlying theory.

You need to have a more concrete model in order to make concrete
predictions. People have looked for signals of unorthodox models in
lensing and found none.


Even concrete is wet & sloppy at the start, refer to Archimedes' "On
the Method". Thanks for your answers, Phil.

Eric
  #4  
Old September 14th 14, 10:54 AM posted to sci.astro.research
Eric Flesch
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Posts: 321
Default Geometry of Look-Back -- lensing

Pursuant to my previous post, I'd like to answer Phil's question about
the "underlying theory" in more detail (moderator allowing):

On Sat, 13 Sep 14 Phillip Helbig wrote:
writes:
This is because if there is a migrating universal constant which
operated on the space-time manifold, then redshift would be
half time dilation and half spatial lengthening.


Why? Unless you have an underlying theory, I don't see how you arrive
at this.


Here I said "that *was* the underlying theory", but to elaborate,
since you asked, Phil:

As a gedankenexperiment, let's look at two mathematical spheres, one
larger than the other. The larger sphere has a lower SA-to-V ratio
than the other. This is an intrinsic difference. Now place each
sphere into its own empty universe. The spheres haven't changed, one
still has a different intrinsic nature to the other, but we have no
metric to distinguish them. So I suggest we need a universal
parameter of "scale" to account for this -- which would be a
characteristic or dimension of the space-time manifold.

"Scale" is just a reference point and so it isn't needed in our
physical law model -- sort of Machian in that way. But if it migrates
through the epochs, then it serves as a separator between past and
future and means that our telescopes are viewing a past where the
rules are different from today-- specifically, both length (xyz-axes)
and the rate of timeflow are seen to change with time. So arriving
photons would hail from a time which both looks bigger and is seen to
run slower. The xy axes that we see with our telescopes are remapped
perforce by Riemannian geometry, but the z-axis (the direction of
arrival) is not remapped. The arriving photon shows a stretch in
length and a slower runtime in equal measure, because of the migration
of "scale".

The benefit of this is that e.g., objects at z=1 are remapped to
where, for us to equate them to physical law today, we need to shrink
them and slow them by sqrt(2). The redshift would show that dilation
already, and by spatially shrinking them, the amount of lensing that
we see comes out right! Poof, dark matter! And the Riemannian
remapping would make dark energy go poof because objects would be
expected to be seen smaller and fainter.

Mind you, my calculations are ballpark only. It needs professional
work to see if the fit is exact. If the fit is exact, it's a twofer!
And that's the underlying theory, Phil, wet concrete and all.

Apologies to the moderator,
Eric
  #5  
Old September 15th 14, 09:17 PM posted to sci.astro.research
Phillip Helbig---undress to reply
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Posts: 629
Default Geometry of Look-Back -- lensing

In article , Eric Flesch
writes:

Pursuant to my previous post, I'd like to answer Phil's question about
the "underlying theory" in more detail (moderator allowing):
As a gedankenexperiment, let's look at two mathematical spheres, one
larger than the other. The larger sphere has a lower SA-to-V ratio
than the other. This is an intrinsic difference. Now place each
sphere into its own empty universe. The spheres haven't changed, one
still has a different intrinsic nature to the other, but we have no
metric to distinguish them. So I suggest we need a universal
parameter of "scale" to account for this -- which would be a
characteristic or dimension of the space-time manifold.


Interesting concept. Julian Barbour has also been investigating scale
recently. Check up on his recent stuff.

The benefit of this is that e.g., objects at z=1 are remapped to
where, for us to equate them to physical law today, we need to shrink
them and slow them by sqrt(2). The redshift would show that dilation
already, and by spatially shrinking them, the amount of lensing that
we see comes out right!


The dependence of lensing strength on redshift is rather complicated, so
I don't see a cancellation happening here.

Poof, dark matter!


Note that much evidence for dark matter comes from z essentially 0.

And the Riemannian
remapping would make dark energy go poof because objects would be
expected to be seen smaller and fainter.


Depending on the cosmological model, they might continue to become
fainter with redshift, but in some, including the current "concordance
model", they become brighter again at even larger redshift. Presumably,
when this is observed it will disprove your theory.
  #6  
Old September 16th 14, 06:37 AM posted to sci.astro.research
Jonathan Thornburg [remove -animal to reply][_3_]
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Posts: 137
Default Geometry of Look-Back -- lensing

Eric Flesch wrote:
(2) There is a migrating universal "constant" such that in earlier
epochs matter bent light more per kg than it does today.


and later

Heavens no, all I'm saying is that just as GR generalized SR, so can
GR be generalized into a larger frame where G (or some such) migrates.


In other words, you're effectively hypothesizing a time variation in the
Newtonian gravitational constant "big G".

There are quite good experimental bounds on the time variation of G.
See section 4 of
Jean-Philippe Uzan
"Varying Constants, Gravitation and Cosmology",
Living Reviews in Relativity 14 (2011), 2
http://relativity.livingreviews.org/...es/lrr-2011-2/
for a review.

Notably, this quotes a very tight lunar-laser-ranging bound:
J. G. Williams, S. G. Turyshev, and D. H. Boggs,
"Progress in Lunar Laser Ranging Tests of Relativistic Gravity",
Physical Review Letters, 93, 261101, (2004)
http://journals.aps.org/prl/abstract...Lett.93.261101
preprint at arXiv:gr-qc/0411113
These researchers find
(dG/dt)/G = (4 +/- 9) e-13/year
i.e., (quoting from the PRL paper)
"The $\dot{G}/G$ uncertainty is 83 times smaller than the inverse age
of the Universe, $t_0 = 13.4$ Gyr"

One can of course imagine a theory in which G varies a lot at some past
time, but then the G(t) curve flattens out so that dG/dt is small at
present times, but this seems somewhat ad-hoc.

--
-- "Jonathan Thornburg [remove -animal to reply]"
Dept of Astronomy & IUCSS, Indiana University, Bloomington, Indiana, USA
"There was of course no way of knowing whether you were being watched
at any given moment. How often, or on what system, the Thought Police
plugged in on any individual wire was guesswork. It was even conceivable
that they watched everybody all the time." -- George Orwell, "1984"
  #7  
Old September 16th 14, 06:40 AM posted to sci.astro.research
Steve Willner
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Posts: 1,172
Default Geometry of Look-Back -- lensing

In article ,
Eric Flesch writes:
is there broadly a redshift dependency in
lens power, that is, lens mass? Are high-z lenses seen to be more
powerful than low-z lenses, or is that susceptible to a Malmquist
bias?


This seems difficult to measure, and I doubt the answer is known.
You'd need a low-z and high-z lensing sample with known masses.

One approach might be to use a sample of lensing galaxy clusters
whose masses are derived from velocity dispersions, but I'd expect
big systematic errors. I might be wrong, though; there are lots of
people cleverer than I am.

--
Help keep our newsgroup healthy; please don't feed the trolls.
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
  #8  
Old September 17th 14, 04:28 AM posted to sci.astro.research
Richard D. Saam
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Posts: 240
Default Geometry of Look-Back -- lensing

On 9/16/14, 12:40 AM, Steve Willner wrote:
In article ,
Eric Flesch writes:
is there broadly a redshift dependency in
lens power, that is, lens mass? Are high-z lenses seen to be more
powerful than low-z lenses, or is that susceptible to a Malmquist
bias?


This seems difficult to measure, and I doubt the answer is known.
You'd need a low-z and high-z lensing sample with known masses.

In as much as lensable mass varies with universe density ~H^2/G (1+z)^3
then "high-z lenses will to be more powerful than low-z lenses"
  #9  
Old September 19th 14, 09:38 AM posted to sci.astro.research
Steve Willner
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Posts: 1,172
Default Geometry of Look-Back -- lensing

In article ,
"Richard D. Saam" writes:
In as much as lensable mass varies with universe density ~H^2/G (1+z)^3
then "high-z lenses will to be more powerful than low-z lenses"


I don't understand this. Lensing depends on the contrast between
high and low density regions, and all standard models have that
contrast _increasing_ with time (i.e., larger at low redshift).

In any case, what is needed for the test the OP proposed is not an
average mass or mass density but known mass for specific lensing
objects and different redshifts.

--
Help keep our newsgroup healthy; please don't feed the trolls.
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
  #10  
Old September 21st 14, 02:16 PM posted to sci.astro.research
Phillip Helbig---undress to reply
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Posts: 629
Default Geometry of Look-Back -- lensing

In article , Steve Willner
writes:

In article ,
"Richard D. Saam" writes:
In as much as lensable mass varies with universe density ~H^2/G (1+z)^3
then "high-z lenses will to be more powerful than low-z lenses"


I don't understand this. Lensing depends on the contrast between
high and low density regions, and all standard models have that
contrast _increasing_ with time (i.e., larger at low redshift).


Yes, objects tend to grow more massive with time. But for a constant
mass, there is an optimal redshift. So, the "power" will be a
combination of these two things.
 




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