Gravitational Scalar & Redshift Distortion

This is just a timely reminder the the terms "Dark Matter" and "Dark
Energy" are confusing astronomers. I keep up with ArXiv and I find it
amazing that so many astronomers seem unable to step outside of the
matter & energy paradigms to find cosmology solutions. So a simple
review:

(1) "Dark Matter" is used when we find there's more gravity than is
accounted for by the matter that we model. The real point is there is
extra gravity, or its equivalent. A far better term would be
"Gravitational Scalar", bespeaking an unmodelled gravitational effect.
Then all those poor ArXiv authors might flail in better directions.

(2) "Dark Energy" comes from our use of redshift as a smooth indicator
of cosmological distance, which paints structural distortions when
applied to observations of the physical universe. Maybe the answer is
that redshift has an unmodelled complication, and that (say) time
dilation goes as the square root of what we think it is. Maybe a
better term would be "Redshift Distortion" so that astronomers can
revisit old assumptions instead of creating large unseen paradigms.

The standard rejoinder to the "multiple universes" model is that we
prefer calculations which "conserve on universes". This commendable
restraint should be applied to "Dark Matter" and "Dark Energy" as
well. "Gravitational Scalar" and "Redshift Distortion". Think about
it. Thanks,

Eric Flesch

In article , Eric Flesch
writes:

This is just a timely reminder the the terms "Dark Matter" and "Dark
Energy" are confusing astronomers. I keep up with ArXiv and I find it
amazing that so many astronomers seem unable to step outside of the
matter & energy paradigms to find cosmology solutions. So a simple
review:

It's too late to turn back the tide, but "dark energy" is a really
stupid term. As Sean Carroll pointed out, essentially everything has
energy and many things are dark. The defining characteristics are the
negative pressure and the non-clumpiness, but his "smooth tension"
didn't catch on. :-(

(1) "Dark Matter" is used when we find there's more gravity than is
accounted for by the matter that we model. The real point is there is
extra gravity, or its equivalent. A far better term would be
"Gravitational Scalar", bespeaking an unmodelled gravitational effect.

But this goes too far in the opposite direction, suggesting that it
might be something other than dark matter. Also, "dark matter" can also
refer to KNOWN matter which is dark. There is in general a lack of
distinction in many cases between "dark matter", "missing matter" and
"non-baryonic matter".

Yes, it could be something different, but as long as a simple
configuration of dark matter fits the observations, that speaks for this
explanation, as opposed to something more bizarre. Ditto for dark
energy, where the cosmological constant fits ALL observations.

(2) "Dark Energy" comes from our use of redshift as a smooth indicator
of cosmological distance, which paints structural distortions when
applied to observations of the physical universe. Maybe the answer is
that redshift has an unmodelled complication, and that (say) time
dilation goes as the square root of what we think it is. Maybe a
better term would be "Redshift Distortion" so that astronomers can
revisit old assumptions instead of creating large unseen paradigms.

The term "redshift distortion" has a completely different meaning in the
context of cosmology. I also don't see what your point is here, since
the relations between various sorts of distance and redshift have been
known for decades. The fact that ALL observations can be fit with a
two-parameter model (three including the Hubble constant) based on 1930s
cosmology indicates to me the opposite: the universe is, on large
scales, quite simple. (WHY that should be the case is a different
question, of course.) There is also know evidence for "time dilation
goes as the square root of what we think it is" or whatever.

Le 26/01/2014 07:01, Eric Flesch a écrit :
(1) "Dark Matter" is used when we find there's more gravity than is
accounted for by the matter that we model.

Wouldn't it be more accurate to say that we observe movements (rotation
of galaxies) that are unexplained and that many astronomers
assume it is caused by gravity.

IF (and only IF) we assume that the effect that we observe is caused by
gravity THEN we need more matter to explain it (the dark matter).

If the unexplained movement is caused by a new unknown force that works
at galactic scales only there is no need for any new matter.

What we observe is that galaxies rotate as rigid bodies, i.. the
rotation doesn't slow down if we move away from the center.

Is this correct?

In article , jacob navia
writes:

Le 26/01/2014 07:01, Eric Flesch a écrit :
(1) "Dark Matter" is used when we find there's more gravity than is
accounted for by the matter that we model.

Wouldn't it be more accurate to say that we observe movements (rotation
of galaxies) that are unexplained and that many astronomers
assume it is caused by gravity.

Right. First level: certain motion observed. Second level: assume it
is cause by gravity (though it could presumably be caused by some other
force). Third level: assume dark matter (though it could presumably be
caused by a change in the law of gravity).

IF (and only IF) we assume that the effect that we observe is caused by
gravity THEN we need more matter to explain it (the dark matter).

Not necessarily; it could be caused by some sort of modified gravity
(e.g. MOND).

If the unexplained movement is caused by a new unknown force that works
at galactic scales only there is no need for any new matter.

This is what most people assume, though of course both effects could be
present.

What we observe is that galaxies rotate as rigid bodies, i.. the
rotation doesn't slow down if we move away from the center.

Is this correct?

No and yes. What is observed is that the speed of rotation is
APPROXIMATELY constant with distance from the center, while if the mass
distribution followed that of the stars then it should decrease.
However, a rigid rotation would have the speed INCREASING with the
distance from the centre (though the ANGULAR speed would then be
constant).

On Sun, 26 Jan 14, jacob navia wrote:
Le 26/01/2014 07:01, Eric Flesch a écrit :
(1) "Dark Matter" is used when we find there's more gravity than is
accounted for by the matter that we model.

What we observe is that galaxies rotate as rigid bodies, i.. the
rotation doesn't slow down if we move away from the center.
Is this correct?

Well, two points:

(1) No, it's not correct, because with spiral galaxies we find that
rotational velocity is more or less constant for most R. That means
it still takes longer for further-out parts to perform 1 revolution,
so the galaxies don't rotate as a rigid body (else the rotational
velocity would linearly increase with R).

(2) I more had in mind the additional gravity needed to cause the
gravitational lensing that we see. The supposition is that "dark
matter" is providing the extra gravity, but my point is that the extra
gravity may indeed be there (and even that is a stretch, maybe it's
something else causing the extra lensing), but that just because we
know matter subtends gravity, doesn't mean that there can't be another
cause as well, like an extra dimension of who-knows-what.

Eric

Le 27/01/2014 09:55, Phillip Helbig---undress to reply wrote :
jacob navia wrote
What we observe is that galaxies rotate as rigid bodies, i.. the
rotation doesn't slow down if we move away from the center.

Is this correct?

No and yes. What is observed is that the speed of rotation is
APPROXIMATELY constant with distance from the center, while if the mass
distribution followed that of the stars then it should decrease.

Well, thanks, I had gotten that completely wrong. Are there any papers
about exactly how approximately? That could be an interesting subject of
study.

However, a rigid rotation would have the speed INCREASING with the
distance from the centre (though the ANGULAR speed would then be
constant).

Yes, I thought that the rotation curves had a constant anglular velocity
with increasing speeds...

Thanks again for your answers.

On 1/26/2014 12:01 AM, Eric Flesch wrote:
(1) "Dark Matter" is used when we find there's more gravity than is
accounted for by the matter that we model.

I'm not sure where to insert this into the discussion. This seems as
good as any other.

Is there anything that can be learned in the context of dark matter by
studying irregular galaxies? Either by studying the galaxies
individually or the distribution of those galaxies in space.

In article , jacob navia
writes:

No and yes. What is observed is that the speed of rotation is
APPROXIMATELY constant with distance from the center, while if the mass
distribution followed that of the stars then it should decrease.

Well, thanks, I had gotten that completely wrong. Are there any papers
about exactly how approximately? That could be an interesting subject of
study.

There is a HUGE literature on rotation curves of galaxies.

An easy place to start might be with a review of MOND (which is a theory
which claims that gravitation needs to be modified, which would allow
flat rotation curves without dark matter (but possibly not solve all
"dark matter problems"). There is one by McGough and a co-author in
Living Reviews which is just a couple of years old. Start there. There
are many references. Many of the observations, of course, are by people
who do not support MOND, or are non-committed.

Op woensdag 29 januari 2014 14:23:51 UTC+1 schreef Phillip Helbig
In article , jacob navia

writes:

No and yes. What is observed is that the speed of rotation is
APPROXIMATELY constant with distance from the center, while if the mass
distribution followed that of the stars then it should decrease.

while if the mass distribution followed that of the planets
around the Sun it should decrease.
(As described by Newton's Law)

Well, thanks, I had gotten that completely wrong. Are there any papers
about exactly how approximately? That could be an interesting subject of
study.

There is a HUGE literature on rotation curves of galaxies.

An easy place to start might be with a review of MOND (which is a theory
which claims that gravitation needs to be modified, which would allow
flat rotation curves without dark matter (but possibly not solve all
"dark matter problems").

The issue is that galaxy rotation curves are flat which they should
not be if you calculate these curves based on visible baryonic matter.
In short you need more matter in order to calculate a flat rotation curve.
One solution is to add more matter in the disc.
A different one is to add more matter in the halo.
This extra matter is supposed to be nonbaryonic matter.
A different solution is to modify Newton's Law such that you
do not need extra matter ie nonbaryonic matter.
(This has consequences to describe the planets around the Sun)
If this solution is correct than you do not need any nonbaryonic
matter to explain flat galaxy rotation curves.
However this raises a serious if you want to explain the universe in total.
As a result of the CMB radiation and WMAP 85% of all matter in the
universe should be nonbaryonic (15% baryonic).
The problem is: if there is only baryonic matter in galaxies where
is all this nonbaryonic matter?

See also:
http://users.telenet.be/nicvroom/fri...20age.htm#Ref1

Nicolaas Vroom

In article ,
jacob navia writes:
Wouldn't it be more accurate to say that we observe movements (rotation
of galaxies) that are unexplained

Also galaxy cluster velocity dispersions, historically the first
evidence for dark matter. Also local flows such as towards the Great
Attractor. Also velocity dispersions for elliptical and irregular
galaxies.

and that many astronomers assume it is caused by gravity.

"Many astronomers" should be "nearly all." The assumption is
consistent with light element abundances, with the CMB fluctuations,
and with baryon acoustic oscillations, so at this point it will take
quite strong evidence for any other model to take over. That doesn't
stop people from trying, of course.

If the unexplained movement is caused by a new unknown force that works
at galactic scales only there is no need for any new matter.

This is a version of MOND, though the restricted version you propose
needs an extension for galaxy clusters. So far there is (so far as I
know) no MOND that explains all the data. It's not quite as bad as
needing a special gravity law for every galaxy and every cluster, but
that's the basic nature of the problem.

What we observe is that galaxies rotate as rigid bodies, i.. the
rotation doesn't slow down if we move away from the center.

As others have written, what we observe is leveling off of velocity.
I strongly recommend the "Rotcurve Applet" and explanatory materials
at http://burro.astr.cwru.edu/JavaLab/R...eWeb/main.html

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