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Testing Newtonian Gravity

by inflector
Tags: gravity, newtonian, testing
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inflector
#1
Oct8-10, 04:12 PM
inflector's Avatar
P: 337
Has anyone here looked at this paper?

Testing Newtonian gravity with distant globular clusters: NGC1851 and NGC1904
R. Scarpa, G. Marconi, G. Carraro, R. Falomo, S. Villanova

http://arxiv.org/pdf/1008.3526v1

(Submitted on 20 Aug 2010)
Globular clusters are useful to test the validity of Newtonian dynamics in the low acceleration regime typical of galaxies, without the complications of non-baryonic dark matter. Specifically, in absence of disturbing effects, e.g. tidal heating, their velocity dispersion is expected to vanish at large radii. If such behaviour is not observed, and in particular if, as observed in elliptical galaxies, the dispersion is found constant at large radii below a certain threshold acceleration, this might indicate a break down of Newtonian dynamics.

To minimise the effects of tidal heating in this paper we study the velocity dispersion profile of two distant globular clusters, NGC 1851 and NGC 1904.

The velocity dispersion profile is derived from accurate radial velocities measurements, obtained at the ESO 8m VLT telescope. Reliable data for 184 and 146 bona fide cluster star members, respectively for NGC 1851 and NGC 1904, were obtained.

These data allow to trace the velocity dispersion profile up to ~2r0, where r0 is the radius at which the cluster internal acceleration of gravity is a0 = 10e-8 cm/s/s. It is found that in both clusters the velocity dispersion becomes constant beyond ~r0. These new results are fully in agreement with those found for other five globular clusters previously investigated as part of this project. Taken all together, these 7 clusters support the claim that the velocity dispersion is constant beyond r0, irrespectively of the specific physical properties of the clusters: mass, size, dynamical history, and distance from the Milky Way. The strong similarly with the constant velocity dispersion observed in elliptical galaxies beyond r0 is suggestive of a common origin for this phenomenon in the two class of objects, and might indicate a breakdown of Newtonian dynamics below a0.

Comments: Accepted for publication by A&A main journal. 12 pages, 12 figures
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Upisoft
#2
Oct8-10, 05:18 PM
P: 349
Nice. So called dark matter finally started to disappear. I can live more easily with yet another breakdown (space, velocity and now acceleration). At least there is some kind of pattern. Do any quantum gravity theory predict such thing?
Phyisab****
#3
Oct8-10, 06:13 PM
P: 592
Why are globular clusters known to contain negligible amounts of dark matter(last sentence of page 1)? Is this considered to be common knowledge these days?

I am not well versed in recent literature on this topic. The inclusion of this single sentence completely changes the nature of their argument to me. The observation of any flattening of the VDP then seems to be strong evidence against dark matter. But my common sense says that the confidence in this statement must not be that high.

The discussion of a0 is interesting, but is that really the main point of the paper? It seems to me that the more important point may be that, before this paper, there had been little measurement of the VDP for clusters for which tidal heating can be considered negligible. Of course I don't know this without surveying the literature.

This is convincing evidence that the flattening of the VDP in globular clusters is not predominantly due to tidal heating. The fact that the flattening occurs at a0 seems like a secondary result to me.

Can anyone help me here?

inflector
#4
Oct8-10, 06:27 PM
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P: 337
Testing Newtonian Gravity

Quote Quote by Phyisab**** View Post
Why are globular clusters known to contain negligible amounts of dark matter(last sentence of page 1)? Is this considered to be common knowledge these days?
Wikipedia at Dark Matter says:

There are places where dark matter seems to be a small component or totally absent. Globular clusters show little evidence that they contain dark matter,[17] though their orbital interactions with galaxies do show evidence for galactic dark matter.
The reference at [17] is:

http://adsabs.harvard.edu/abs/2008IAUS..246..418R

but unfortunately, its behind a paywall and I couldn't find a free version anywhere with a quick search.
Phyisab****
#5
Oct8-10, 06:39 PM
P: 592
Hmm I can't find that paper in any of the databases available to me. Judging from the abstract however, it does not seem to be a satisfactory reference for a statement of that magnitude.
Phyisab****
#6
Oct8-10, 06:59 PM
P: 592
Well after some searching I am still very confused by this. I haven't found any papers which support that statement.
inflector
#7
Oct8-10, 07:30 PM
inflector's Avatar
P: 337
We've been here before:

http://www.physicsforums.com/showthread.php?t=181483
Phyisab****
#8
Oct8-10, 07:43 PM
P: 592
Quote Quote by Phyisab**** View Post
The discussion of a0 is interesting, but is that really the main point of the paper? It seems to me that the more important point may be that, before this paper, there had been little measurement of the VDP for clusters for which tidal heating can be considered negligible. Of course I don't know this without surveying the literature.

This is convincing evidence that the flattening of the VDP in globular clusters is not predominantly due to tidal heating. The fact that the flattening occurs at a0 seems like a secondary result to me.

Is this then very compelling evidence for something other than dark matter? I assume there are some doubts or this would have been big news.
Upisoft
#9
Oct9-10, 10:23 AM
P: 349
Quote Quote by Phyisab**** View Post
Is this then very compelling evidence for something other than dark matter? I assume there are some doubts or this would have been big news.
Unless observed flattening at a0 is a big coincidence. If it is not coincidence then the dark mater should be renamed to "dark spooky matter". The dark spooky matter apparently knows how to place itself, so it will cause flattening at a0 in each case. Well, it is possible answer, but I don't find it very plausible.
granpa
#10
Oct9-10, 10:54 AM
P: 2,258
http://arxiv.org/pdf/1008.3526v1
Although there are plenty of reasons to trust Newton’s law
in this very weak acceleration regime, it is well known that
spiral galaxies rotation curves systematically deviate from prediction
of Newtonian dynamics.

Similarly elliptical galaxies and cluster of galaxies velocity dispersion profile show a remarkable flattening at large radii
where a Keplerian falloff would be expected. These deviations,
ascribed to the existence of large amount of non-baryonic
dark matter (DM), appears to exhibit systematic (but not yet
understood) behaviors. The most remarkable being that
DM is needed to reconcile prediction
with observations when and only when the acceleration
of gravity goes below a critical value, of the order of a0 ∼ 10−8
cm s−2
.
I gather that 'velocity dispersion' means the difference between the most positive and the most negative velocities at any given radius from the center.
inflector
#11
Oct9-10, 12:40 PM
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P: 337
Quote Quote by granpa View Post
I gather that 'velocity dispersion' means the difference between the most positive and the most negative velocities at any given radius from the center.
From Wikipedia on Velocity Dispersion:
In astronomy, the velocity dispersion σ, is the range of velocities about the mean velocity for a group of objects, such as a cluster of stars about a galaxy. For example, in a galaxy, a typical value for the velocity dispersion of the objects orbiting the galactic centre is about 200km/s. By measuring the radial velocities of selected members, the velocity dispersion of the cluster can be estimated and used to derive the cluster's mass from the virial theorem.
The velocity dispersion profile relates this velocity dispersion to the radius and often fits a curve to the data. So it is the addition of the word "profile" that refers to the relation of the velocity dispersion to the radius.
inflector
#12
Oct9-10, 12:45 PM
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P: 337
Quote Quote by Phyisab**** View Post
Is this then very compelling evidence for something other than dark matter? I assume there are some doubts or this would have been big news.
That's what I'd like to know as well. I was hoping that someone here who follows dark matter and cosmological-level gravity more closely would know of this paper and any responses. Perhaps they still will respond.

I have been unable to find any non-paid primary source papers in my searches that address the issue of dark matter and globular clusters so far.
Phyisab****
#13
Oct9-10, 05:13 PM
P: 592
Quote Quote by Upisoft View Post
Unless observed flattening at a0 is a big coincidence. If it is not coincidence then the dark mater should be renamed to "dark spooky matter". The dark spooky matter apparently knows how to place itself, so it will cause flattening at a0 in each case. Well, it is possible answer, but I don't find it very plausible.
This is not the source of the question, you did not follow my argument carefully. Please read the quoted portion in my previous post.
Phyisab****
#14
Oct10-10, 09:08 AM
P: 592
Anyone?
Phyisab****
#15
Oct11-10, 09:22 AM
P: 592
Is this not an interesting question? Someone must be able to explain the gap in my understanding, or this should be a big deal.
granpa
#16
Oct11-10, 09:40 AM
P: 2,258
well bigger stars sink to the cores of globular clusters and
lighter stars rise to the outer parts.

So if dark matter is only found in the outer parts of the cluster then...
Phyisab****
#17
Oct11-10, 09:49 AM
P: 592
What are you trying to say? Can you please use complete sentences and proper punctuation? What is your conclusion?

If dark matter is known to not cause the flattening of the velocity profile in globular clusters, and the other posited cause, tidal heating, has also been ruled out, then does this not have serious implications? Does this not bring the whole idea of dark matter very seriously into question, and call for a serious consideration of other ideas such as flaws in our understanding of Newtonian gravity?
granpa
#18
Oct11-10, 09:57 AM
P: 2,258
I'm sorry.
And here I thought I was being helpful.
I'll try not to do it again.


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