Dark matter

[bascule]

Is dark matter losing support? This Physorg article seems to think so:

 

http://www.physorg.com/news160726282.html

 

They suggest that perhaps there's yet another deviation in Newtonian mechanics at play which needs a different theory to explain besides the "dark matter" explanation.

 

What do you think?

[Martin]

Delighted to see this!

 

Cosmologists love to challenge their own standard model. Constantly trying to catch it at fault some way---at odds with some data or other. Good scientists in that way.

 

Sounds like another wave of interest in MOND (modified gravity).

 

I've seen no indication that Dark Matter is losing support to any significant extent. Too much in its favor. Interest in MOND was stronger back in 2006 before the bullet cluster news. Used to see several MOND articles nearly every week. Not so much research on modified gravity during 2006-2008.

But now in 2009 there may be another flurry of interest.

 

I think it is too early to be expecting General Relativity to be overturned. But this development is certainly something to watch! I will get the links to the Metz papers, and try to keep track myself.

 

Meanwhile please let us know if you see anything more.

 

IMHO from the look of the abstracts there is a bit of tub-thumping going on. PhysOrg source was Bonn University, which has an interest in overstating the importance of the local hero's results. And they don't seem to have found much that is earthshaking. DM is based on thousands of galaxies and clusters of galaxies, including maps of DM concentration by weak-lensing. These people have some isolated observations of stuff right around our galaxy which at least for the time being they can't explain otherwise and are promoting MOND to explain it. Gotta be open to this kind of thing but not go off half cocked.

http://arxiv.org/abs/0903.0375

Did the Milky Way dwarf satellites enter the halo as a group?

Manuel Metz, Pavel Kroupa, Christian Theis, Gerhard Hensler, Helmut Jerjen

(Submitted on 2 Mar 2009)

The dwarf satellite galaxies in the Local Group are generally considered to be hosted in dark matter subhalos that survived the disruptive processes during infall onto their host halos. It has recently been argued that if the majority of satellites entered the Milky Way halo in a group rather than individually, this could explain the spatial and dynamical peculiarities of its satellite distribution. Such groups were identified as dwarf galaxy associations that are found in the nearby Universe. In this paper we address the question whether galaxies in such associations can be the progenitors of the Milky Way satellite galaxies. We find that the dwarf associations are much more extended than would be required to explain the disk-like distribution of the Milky Way and Andromeda satellite galaxies. We further identify a possible minor filamentary structure, perpendicular to the supergalactic plane, in which the dwarf associations are located, that might be related to the direction of infall of a progenitor galaxy of the Milky Way satellites, if they are of tidal origin.

 

http://arxiv.org/abs/0901.1658

Discs of Satellites: the new dwarf spheroidals

Manuel Metz, Pavel Kroupa, Helmut Jerjen

(Submitted on 12 Jan 2009)

The spatial distributions of the most recently discovered ultra faint dwarf satellites around the Milky Way and the Andromeda galaxy are compared to the previously reported discs-of-satellites (DoS) of their host galaxies. In our investigation we pay special attention to the selection bias introduced due to the limited sky coverage of SDSS. We find that the new Milky Way satellite galaxies follow closely the DoS defined by the more luminous dwarfs, thereby further emphasizing the statistical significance of this feature in the Galactic halo. We also notice a deficit of satellite galaxies with Galactocentric distances larger than 100 kpc that are away from the disc-of-satellites of the Milky Way. In the case of Andromeda, we obtain similar results, naturally complementing our previous finding and strengthening the notion that the discs-of-satellites are optical manifestations of a phase-space correlation of satellite galaxies.

Edited May 12, 2009 by Martin

[bascule]

I've seen no indication that Dark Matter is losing support to any significant extent. Too much in its favor. Interest in MOND was stronger back in 2006 before the bullet cluster news.

 

What was that?

 

I think it is too early to be expecting General Relativity to be overturned. But this development is certainly something to watch! I will get the links to the Metz papers, and try to keep track myself.

 

Cool

 

IMHO from the look of the abstracts there is a bit of tub-thumping going on. PhysOrg source was Bonn University, which has an interest in overstating the importance of the local hero's results. And they don't seem to have found much that is earthshaking.

 

I see

 

DM is based on thousands of galaxies and clusters of galaxies, including maps of DM concentration by weak-lensing. These people have some isolated observations of stuff right around our galaxy which at least for the time being they can't explain otherwise and are promoting MOND to explain it. Gotta be open to this kind of thing but not go off half cocked.

 

As I don't have anything to go on but the gut feelings of scientists, what do you feel about the accusations that DM lacks explanatory power for the types of anomalies they are describing?

[Arch2008]

“The physicists do belief that this phenomenon can only be explained if the satellites were created a long time ago through collisions between younger galaxies.”

 

Think of the dark matter ‘cloud’ around a galaxy as a motor vehicle and the galaxy itself as a passenger. When two galaxies collide, then the galaxies themselves get ejected and the dark matter cloud is separated. There was an image of such an empty area of deep space that lensed background stars by its gravity, like DM would. So Professor Kroupa and his colleagues ‘believe’ that the 11 dwarf galaxies they studied are such galaxies formed by collisions that have no DM. The contradiction is that these dwarf galaxies spin rapidly as though they did have DM. However, galaxies can also form directly from a cloud of gas and so would not lose their DM in any collision mergers. If the scenario one describes creates a contradiction, then most likely this is because it is based on a belief.

 

http://map.gsfc.nasa.gov/

http://www.nasa.gov/home/hqnews/2006/aug/HQ_06297_CHANDRA_Dark_Matter.html

http://www.uslhc.us/LHC_Science/Questions_for_the_Universe/Dark_Matter

[Mr Skeptic]

As I don't have anything to go on but the gut feelings of scientists, what do you feel about the accusations that DM lacks explanatory power for the types of anomalies they are describing?

 

My gut feeling is that dark matter does not exist. However, if a new theory of gravity is to eliminate the need for dark matter, then we still need the maps of dark matter to tell us exactly how gravity needs to be modified.

[Severian]

It seems to me that the argument in the physorg article requires us to be very sure of the mechanism for how dwarf galaxies are created. Personally I would think the evidence for the mechanism of their creation is a lot weaker than the evidence in support of dark matter.

 

The effects consistent with Dark Matter have been observed as long ago as 1933 [Zwicky], and the existence of dark matter is predicted by many models of new physics, independently of cosmology.

 

Finally, two papers suggesting alternatives to dark matter does not mean dark matter is losing support. The number of papers in support of dark matter are much much more. I don't see dark matter losing support anytime soon.