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.


Author: Prof. Dr. Pavel Kroupa

I am a Czech-Australian teaching and researching at the University of Bonn on dynamics and stellar populations. After studying physics at The University of Western Australia, Perth, I obtained my PhD from Cambridge University, UK, as an Isaac Newton Scholar at Trinity College. After spending eight years in Heidelberg I habilitated at the University of Kiel, Germany. I then took up a Heisenberg Fellowship and later accepted the position as a professor at Bonn University in 2004. I was awarded a Leverhulme Trust Visiting Professorship (2007, Sheffield, UK) and a Swinburne Visiting Professorship (2007, Melbourne, Australia). In 2013 I received the Silver Commemorative Medal of the Senate of the Czech Republic, and I took-up an affiliation with the Charles University in Prague in 2016. Pure innovative science can only truly thrive in non-hierarchical societies in which competition for resources is not extreme. Therefore I see the need for the German academic system to modernise (away from its hierarchies) and warn of academic systems that are based on an extreme competition for resources (USA), as these stifle the experimentation with new ideas.

One thought on “A filament of dark matter between two clusters of galaxies: dark matter detected”

  1. Well saidGlad to see a call for a realistic assessment of these results. The hyperbolic news reports, based on the misleading aspects of this paper, are quite frustrating.

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