Strongest dark matter evidence so far

[Martin]

http://space.newscientist.com/article/dn11860-ring-of-dark-matter-surrounds-cosmic-collision.html

 

==exerpt==

Colossal collision

 

...They found a curious ring-like structure around the cluster's outskirts. The researchers believe the ring is made of dark matter, since there is no concentration of visible matter at the location of the ring. Most of the cluster's ordinary matter is thought to be in the form of hot gas, which was mapped by the Chandra X-ray Observatory, and is concentrated at the cluster's core.

 

Previous studies have hinted that Cl 0024+17 is not a single galaxy cluster but a pair of clusters that have collided. Simulations show that when such a colossal collision occurs – in this case, it involved almost 200 billion Suns' worth of matter – the cluster's dark matter explodes outwards, forming a roughly spherical shell. Watch an animation of the collision and dark matter explosion.

 

Projected on the sky, such a shell would look like the ring observed in the gravitational lensing study, the researchers say. They estimate that the collision happened sometime between 1 billion and 2 billion years ago.

 

Modified gravity

 

Some scientists have previously suggested that modifications to Newton's law of gravity could account for anomalies cited as evidence for dark matter (see Equinox challenge to Newton's law). This possibility can be difficult to rule out, because normally dark matter and ordinary matter are so well mixed together that it is difficult to pin down effects due to dark matter alone.

 

But because the ring in Cl 0024+17 is separate from any concentrations of ordinary matter, it would be difficult to explain without dark matter, the researchers say. "I think it's the strongest evidence for the existence of dark matter to date," Jee told New Scientist.

==endquote==

Hubblesite's own news story about it

http://hubblesite.org/newscenter/archive/releases/2007/17/full/

(Hubblesite also has some computer animation movies of the collision, with a side view plus a head-on view)

 

blog comment with some inside history

http://scienceblogs.com/principles/2007/05/life_cycle_of_science_pr.php

 

best short news report so far IMO is this one on PhysicsWeb

http://physicsweb.org/articles/news/11/5/15/1?rss=2.0

 

technical version to be published in Astrophysical journal

http://arxiv.org/abs/0705.2171

[Martin]

this is the second piece of smoking-gun evidence

 

the other was last summer's presentation of the "bullet" cluster collision, where we see a side view of two clusters colliding

 

in these collisions the (hypothetical) cloud of dark matter gets dislocated and its lensing effect on more distant stuff in the background can be studied and mapped separate from the other stuff in the cluster

 

I am still in a wait-and-see mode. I hope and expect to see a lot more really good evidence, narrowing down the characteristics of what the stuff is (if it really is stuff and there is not some better alternative explanation of the observed effects)

 

But I confess to be less skeptical now than before seeing this most recent report.

[Royston]

Some scientists have previously suggested that modifications to Newton's law of gravity could account for anomalies cited as evidence for dark matter (see Equinox challenge to Newton's law). This possibility can be difficult to rule out, because normally dark matter and ordinary matter are so well mixed together that it is difficult to pin down effects due to dark matter alone.

 

Are there any other alternatives or developments concerning MOND, I'll try and find a link, but I swear I read that the modification was a little ad hoc i.e it's only applicable for large systems, and breaks down when applied to smaller systems (e.g our solar system.)

 

I'm wondering if the 2nd law needs a radical revision, or a whole new set of equations where the 2nd law only becomes applicable to smaller systems...if that makes sense. My physics is a little rusty by the way.

 

EDIT: sorry if this is a little off topic.

[Martin]

Are there any other alternatives or developments concerning MOND, ...

 

recently a big expansion of research in general area of modified gravity.

too much to try to follow

nearly every day some new paper gets posted on arxiv.

 

they are not called "MOND" mostly. sometimes they are called "f®" theories or just modfied gravity theories. I have stopped keeping track.

 

some research is aimed at figuring out how to KILL these theories by observational testing

 

one new theory stands out, for me

as different from the rest. it is developed by Kirill Krasnov and Yuri Shtanov

and it is called "non-metric gravity"

it is new---2006. it has no new fields. perhaps it seems a little less ad hoc to me. but it still must be tested! most things that theorists invent turn out to be wrong---this is the way science progresses (by shooting down 95 percent of the new ideas)

[MolotovCocktail]

How many MOND theories are there? I've heard of one, in which I think goes like F=ma^2. I know acceleration is squared in one of them at least.

[Martin]

How many MOND theories are there? I've heard of one...

 

depends on what you call MOND.

if you reserve that word for the one original theory that originally took that name, then obviously there is only ONE

 

but other theories of modified gravity often get called MOND

so it is kind of vague

 

Would you like me to try to estimate how many Modified Gravity theories there are that

 

1. act just almost like usual gravity at small to medium scale

2. try to dispense with dark matter and/or dark energy at large scale

 

I don't think anyone has counted them. Nobody I know of has counted.

I suppose there might be half a dozen famous ones and among those there would be variations that people have thought of. But it is just a guess.

Lot of papers about this. hard to keep track.

 

The original use of the word MOND was by Moti Milgrom, I think.

But his original MOND (from like the 1980s?) becane obsolete some time ago.

Then there was Bekensteins relativistic MOND (from around 2004) which maybe you don't want to call MOND, but people often have called it that, by analogy with Milgrom's. And John Moffatt proposed some variants, I think about the same time as Bekenstein did.

 

Maybe Wikipedia has an article about the various kinds of Modified Gravity.

 

Yeah, Wiki has an article but the scope is rather narrow. They mainly focus on Milgrom's original MOND and they mention a few other people who have offereed improvements like Jacob Bekenstein, and John Moffatt. But the Wiki article does not give any idea of the activity going on right now.

Another strategy would be to go to the ARXIV.ORG search engine and try the keywords "f® gravity"

 

or even just say "modified gravity"

 

A recent paper I thought was interesting was by Kirill Krasnov and Yuri Shtanov----completely different approach from all the others.

[grifter]

I happened to read the exact same article:

"Non-Metric Gravity:

Spherically Symmetric Solution,

Missing Mass and Redshifts of Quasars

on the recommendation of a friend, They certainly do take a different approach: their "scale-dependent modification of

gravity"

as oppose to traditionally modifying the Hilbert–Einstein action by

1./ extra dimensions.

2./ introduction of more fields

3./ more derivatives.

 

now comes the interesting part:

the geometric structure of the theory itself is remodeled; making it non metric with regard to the above three points. The formal general-relativistic cosmological equations are also left unaltered

(And as a free bonus...)

The current theory predicts the appearance of an additional redshift factor between regions of different space-time curvature. This effect can be used to account for the observed high redshifts of quasars.

 

This is an interesting take on modified grav' and I would suggest it as a read, it has gone a little way to convince me but I think some fine tuning is required:

 

i.e. i don't like how there theory (without metric modifications) excludes black holes below a certain size, but none-the less, very interesting; give it a read

[Martin]

I'm glad to run into someone who has happened onto the Krasnov Shtanov article independently.

that is a sign that it is getting some attention.

 

An influential senior Swedish physicist named Ingemar Bengtsson has written an article about Krasnov's non-metric gravity idea.

I hope I have his name spelled right. He is on the board of directors of a new ESF (european science foundation) department set up to fund QG research and graduate/postdoc education in QG.

 

It strikes me as very edgy but still might develop into something.

 

Shtanov has mostly been in string-related research---I think brane cosmology. He is at the Bogolyubov ITP in Kiev. It seems significant to me that he crossed over from string biz to work with Krasnov on this.

 

If that happens, it can't be so much more wacky than string, right?

[grifter]

Thats true, I'm glad to see an area of Science not related to String or M; getting some attention, as oppose to those two ludicrous fields of research gaining all funding and all significant minds in the world of science.