A filament of dark matter between two clusters of galaxies: dark matter detected

We briefly comment on the paper by Dietrich, Werner, Clowe et al. on “A filament of dark matter between two clusters of galaxies ” which is now in press with Nature.

The media have it that this may be a direct detection of dark matter.  The abstract of this paper reads

“It is a firm prediction of the concordance Cold Dark Matter (CDM) cosmological model that galaxy clusters live at the intersection of large-scale structure filaments. The thread-like structure of this “cosmic web” has been traced by galaxy redshift surveys for decades. More recently the Warm-Hot Intergalactic Medium (WHIM) residing in low redshift filaments has been observed in emission and absorption. However, a reliable direct detection of the underlying Dark Matter skeleton, which should contain more than half of all matter, remained elusive, as earlier candidates for such detections were either falsified or suffered from low signal-to-noise ratios and unphysical misalignements of dark and luminous matter. Here we report the detection of a dark matter filament connecting the two main components of the Abell 222/223 supercluster system from its weak gravitational lensing signal, both in a non-parametric mass reconstruction and in parametric model fits. This filament is coincident with an overdensity of galaxies and diffuse, soft X-ray emission and contributes mass comparable to that of an additional galaxy cluster to the total mass of the supercluster. Combined with X-ray observations, we place an upper limit of 0.09 on the hot gas fraction, the mass of X-ray emitting gas divided by the total mass, in the filament.”

The first sentence of the abstract is undoubtedly correct, but the following sentence here is absolutely true as well:

“It is a firm prediction of any realistic cosmological model that galaxy clusters live at the intersection of large-scale structure filaments.”

Indeed, in any realistic theory of gravity, matter, which has a significant random velocity field, will collapse to filamentary structures. This is amply observed in simulations of molecular clouds without dark matter and is now also beautifully seen in observations of real molecular clouds with the Hershel telescope (e.g. Andre et al. 2010). Filamentary structure is thus nothing special to the  concordance Cold Dark Matter (CDM) cosmological model, and so the abstract can be seen as being somewhat misleading.

Further,  the authors of this paper have only studied the lensing signal using Einstein’s General Relativity. It is true that using Einstein’s General Relativity the signal can only be interpeted with the help of postulating the presence of additional, unseen matter.

But, in a different but nevertheless realistic theory of gravity, the lensing signal may well be explainable without dark matter (e.g. Zhao et al. 2006). It is even possible that in a better theory of gravity, if there is a matter concentration at point A and one at point B then there might be a lensing signal not related to any local matter density at point C in between A and B. Wrongly interpeting such a lensing signal with General Relativity would then lead to the false result that there is unseen matter at C. For instance, in this paper Mordehai Milgrom and Robert Sanders explain how a dark matter effect appears where there is no dark matter at all. The gravitational lensing by filaments in the framework of modified gravity has also been investigated by Feix et al. (2008).

So, the above Nature paper is misleading on this account as well, because “the detection of a dark matter filament connecting the two main components of the Abell 222/223 supercluster system” relies on assuming effective gravity to be described by Einstein’s General Relativity on all scales.

That this cannot be the case has already been shown many times (e.g. “What are the three best reasons for the failure of the LCDM model? Incompatibility with observations” and “Question CII: MOND works far too well!“).  And, an invited review on these problems and matters is available as a freely-downloadable open access CSIRO-publishing paper “The Dark Matter Crisis: Falsification of the Current Standard Model of Cosmology”.

So, while the observations and the results presented in the Nature paper are a major and beautiful feat deserving much attention, a more balanced discussion of the results would have been more appropriate.

By Pavel Kroupa and Marcel Pawlowski  (05.07.2012): “A filament of dark matter between two clusters of galaxies” on SciLogs. See the overview of topics in  The Dark Matter Crisis.

13th Marcel Grossmann Meeting

We are now on our way to the 13th Marcel Grossmann Meeting in Stockholm. The meeting of physicists and astronomers covers General Relativity, Gravity and relativistic field theories and is held every three years (since 1975) in different cities. It is named after Marcel Grossmann, who was a Swiss mathematician and a collaborator of Einstein in his work on general relativity.

Following his recent review paper “The dark matter crisis: falsification of the current standard model of cosmology”, Pavel has been invited by Davit Merrit to give a talk in the parallel session “EG4: Self-Gravitating System”.

The session will take place tomorrow afternoon (Tuesday, 3rd of July) at the AlbaNova University Center, in room FA32. We will present our work on the dwarf and satellite galaxies in the second half of the session, after the coffee break at 16:30. At first, Marcel will give a talk on the Vast Polar Structure (VPOS) and why filamentary accretion can not account for it. This is followed by Pavel (at 16:45) presenting his falsification of the standard model of cosmology. The third talk in this row (17:05) then is by Mordehai Milgrom, who first proposed Modified Newtonian Dynamics (MOND) as an alternative to dark matter. He will present “MOND laws of galactic dynamics”.

The session will continue until 18:50 with more talks on dark matter, its haloes and galaxy formation. We are looking forward to an interesting meeting with lots of discussions. If we find the time, we might even report on some aspects of the meeting here in the blog.

By Pavel Kroupa and Marcel Pawlowski  (02.07.2012): “13th Marcel Grossmann Meeting” on SciLogs. See the overview of topics in The Dark Matter Crisis.