scilogs Dark Matter Crisis

This blog moves to SciLogs.com

Marcel S. Pawlowski | 08. January 2013, 13:10

Pavel and I have been too busy to blog for a while (my excuse being that I am in the final stages of my PhD studies). This is also why we did not announce this sooner: Our blog has moved from SciLogs.eu to SciLogs.com. The new site provides an improved blogging system and maybe more international visibility, as well as a pleasant neighborhood of science bloggers. The new URL for "The Dark Matter Crisis" is http://www.scilogs.com/the-dark-matter-crisis/. All future articles will be published there, but the old ones will remain accessible here on SciLogs.eu.

The first article on the 'new' blog deals with last week's Nature article by Rodrigo Ibata and collaborators: "A Vast Thin Plane of Co-rotating Dwarf Galaxies Orbiting the Andromeda Galaxy". While the media currently focusses on the 15-year old co-author of the Nature study, the scientific implications of the study are no less spectacular. The co-rotating plane of satellite galaxies around Andromeda resembles the VPOS around the Milky Way and therefore similar formation scenarios are plausible, which we discuss in our article "Andromeda’s satellites behave as expected … if they are tidal dwarf galaxies".

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

Pavel Kroupa | 05. July 2012, 10:49

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 CIIL: 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.


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13th Marcel Grossmann Meeting

Marcel S. Pawlowski | 02. July 2012, 14:28

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.


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Discussing Gravity with Erik Verlinde

Marcel S. Pawlowski | 28. June 2012, 18:22

We have just returned from a talk by the Bethe colloquium. Erik Verlinde from the university of Amsterdam spoke about “Dark Matter, Dark Energy and the Emergence of Gravity ”.

Verlinde is a dutch theoretical physicist working on string theory and gravity. He became very famous for his theory of entropic gravity and was awarded the Spinoza Prize for his work.

In his talk, he showed that his approach can not only reproduce the MONDian behavior of the different kinds of galaxies. He even gave an explanation on why the centers of galaxy clusters deviate from the baryonic Tully-Fisher relation by a factor of four. The reason, he says, lies in the distribution of matter. Very roughly, galaxies can be approximated by a point mass if we look at their outskirts only. In galaxy clusters, however, the matter is more evenly distributed. Assuming a spatially constant matter density, he can even motivate the amount of the deviation

All in all, his results look very promising. After the talk, Pavel and I discussed with him for about an hour, explaining some of the failures of the LCDM model, but mostly asking about details and implications of his approach. He explained that his intention is to understand gravity by starting from scratch. So not only change and modify the formula used so far, but basing our understanding of gravity on a more fundamental basis. To do so, he looks at the observational evidence unbiased. We agreed that this is not always easy because especially cosmological results are usually analyzed and expressed in a model-dependent form. He does not aim at reproducing MOND, which even its adherents usually describe rather as a phenomenological effect than a new fundamental law. But his model naturally contains MONDian behavior, it seems to explain/give a reason for MOND's free parameter (the acceleration a0) and he also showed that his approach can predict the ratio of baryonic to (phantom) dark matter correctly: 4% to 22.5%.

What he told was impressive and looks like it could be a major step forward in our understanding of gravity and the universe. Unfortunately, we have to wait some more until he will publish a paper on this topic. But there is good reason to look forward to it.

By Pavel Kroupa and Marcel Pawlowski  (28.06.2012): "Discussing Gravity with Erik Verlinde" on SciLogs. See the overview of topics in  The Dark Matter Crisis.


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Does filamentary accretion of dark matter sub-halos naturally produce a VPOS-like structure?

Marcel S. Pawlowski | 15. May 2012, 10:44

In the previous post we discussed the VPOS, the vast polar structure of satellite objects around the Milky Way. One of the suggested origins within the cosmological cold dark matter paradigm is that the satellites have been preferentially accreted along large, cosmic filaments. These are long, thread-like structures which arise naturally during the formation of structure in the cosmos. The movie below shows how they come about:


One work suggesting that filamentary accretion can solve the VPOS-problem is Lovell et al. (2011). Its abstract claims that:

“All [six] haloes [of the Aquarius simulations] possess a population of subhaloes that rotates in the same direction as the main halo and three of them possess, in addition, a population that rotates in the opposite direction. These configurations arise from the filamentary accretion of subhaloes. Quasi-planar distributions of coherently rotating satellites, such as those inferred in the Milky Way and other galaxies, arise naturally in simulations of a ΛCDM universe.

Note the part we marked in bold face. This statement of theirs suggests that a structure like the VPOS is a natural outcome of cosmological simulations, which arises due to the filaments around a dark matter halo. That filaments can lead to anisotropies in the direction from which sub-halos are accreted onto larger halos is obvious, and it was a good idea that this might be a way to form anisotropic distributions of subhalos. However, there are several reasons to doubt this scenario.

First of all, the filaments are way too thick. For example, in Vera-Cirro et al (2011) it is shown that the filaments in the Aquarius simulations are very wide, of the order of 0.5 – 1 Mpc (see figure below). The VPOS has a thickness of only about 50 kpc. There is no way it can have been formed out of a much bigger filament. The filaments are in fact larger than the halo of the main galaxy (virial radius 200-250 kpc). Vera-Cirro et al (2011) write:

“[...] when the surrounding filament is sufficiently wide, i.e. of comparable or larger cross-section than the virial radius of the halo, the infalling particles will appear to be more isotropically distributed on the sky [...]. We have argued  [...] that [this] case is characteristic of the late stages of mass assembly in  10^12  Msun objects.”

This statement is contrary to the Lovell et al. (2011) one. In the simulations Vera-Cirro et al. (2011) discuss, the filaments are much wider than the central halo for most of the time of the simulation (from about 5 Gyr on). Thus, for about the past 9 Gyr, the accretion must have been more isotropically. Interestingly, the Vera-Cirro et al. (2011) work is based on the Aquarius simulations, the same set of cosmological simulations as the Lovell et al. (2011) paper. And it has been accepted for publication before the Lovell et al. (2011) paper.

Illustrating filament size from Vera-Cirro et al. 2011

Caption: Part of figure 4 of Vera-Ciro et al. (2011). It illustrates the size of a cosmic filament around a Milky Way like halo. The virial radius of the central halo is shown by the white ellipse. The thickness of the VPOS is less than one 10th of the white line at the bottom giving the scale of about 700 kpc.

In addition to this, the orientation of the preferred direction of orbits of subhalos is at odds with the expectations. Lovell et al. (2011) show that there is a slight over-abundance of subhalos orbiting in the same direction as the main halo (and in some cases also in the opposite direction). However, the galaxies forming in the main halos preferentially spin in the same direction as the main halo, so the sub-halo over abundance lies in the same plane as the galactic disc. In the case of the VPOS around the MW, the orientation is perpendicular.

Finally, Lovell et al. (2011) did not test the rather strongly worded statement of their abstract quantitatively. From their figures, it is already obvious that the before mentioned over-abundance of co-orbiting subhalos is small, only a factor of about 2 compared to the isotropic case in the bin closest to the main halo spin. The majority of subhalo orbital directions is distributed more evenly around the main halo.

Aquarius Orbital Poles

MW satellite orbital poles

Caption: The directions of angular momentum vectors, called orbital poles, of sub-halos coming from a cosmological cold-dark matter simulation (upper) and satellite galaxies of the Milky Way (lower). The question we addressed in our recent paper was: how likely is it that a distribution like the observed (lower) one can arise when drawing from the modeled (upper) one.


To allow a fair comparison, we have developed a method to test this claim. It is described in our recent paper “Can filamentary accretion explain the orbital poles of the Milky Way satellites?” (by Marcel S. Pawlowski, Pavel Kroupa, Garry Angus, Klaas S. de Boer, Benoit Famaey and Gerhard Hensler). In it, we determine how likely it is to find sets of angular momenta in model data (e.g. upper plot in the figure above) which are as concentrated and as close to a polar orientation as is observed for the MW satellite orbital poles (lower plot in the figure above). We have applied the method to both cosmological simulation data as well as models of galaxy collisions resulting in polar distributions of tidal debris.

The results are clear. They unambiguously disfavor the cold dark matter models.

Part of Fig. 3 from Pawlowski et al. 2012b

Caption: A part of Fig. 3 of our paper, illustrating the results of one of our criteria. The plot shows how likely it is that orbital poles derived from  models can be at least as concentrated as the observed value of 35.4 degree. The integral below the curves within the shaded region give the probability that randomly drawn orbital poles from the model are as concentrated as is observed. The two cosmological simulations (Aquarius D2 and Via Lactea 1, upper panels) show curves which are very similar to that for an isotropic distribution of satellite galaxies (thin line), it is unlikely that they fall into the shaded area. The lower panel shows the results for tidal debris of a galaxy collision, which is much more concentrated towards the left. In this latter case, it is most likely to draw orbital poles as concentrated as observed.

Using data from high-resolution cosmological simulations of halos that should host Milky-Way-like galaxies, we were able to show that the sub-halo orbits do not naturally produce the observed properties. In contrast, models in which the satellite galaxies are formed as tidal dwarfs from the debris of a galaxy-collision can easily reproduce the observed distribution of orbital poles. The claim that cosmological ΛCDM simulations naturally produce satellite distributions as inferred in the Milky Way has therefore been falsified. At the same time, this shows that the tidal scenario passes the test.

For more details, please read our paper (accepted by MNRAS). It is available as a preprint.

By Pavel Kroupa and Marcel Pawlowski  (15.05.2012): "Does filamentary accretion of dark matter sub-halos naturally produce a VPOS-like structure?" on SciLogs. See the overview of topics in  The Dark Matter Crisis.


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The vast polar structure - VPOS - of satellite objects around the Milky Way

Marcel S. Pawlowski | 28. April 2012, 23:54

After the worrisome news for dark matter in the last weeks, we have to add another today, based on our research (and there is more to come very soon). We show that the disc of satellite galaxies is only a part of a bigger structure: a vast polar structure (VPOS) of diverse satellite objects surrounds the Milky Way, unexpected from cosmological models. The work was done at the University of Bonn, largely through the support of the German Research Foundation (DFG) via its priority programme 1177 "Witnesses of Cosmic History: Formation and Evolution of Black Holes, Galaxies and their Environment" and partially with support from the Bonn-Cologne Graduate School of Physics and Astronomy (BCGS).

With the increasing resolution of cosmological simulations of structure and galaxy formation, it became possible to make predictions on smaller scales. In particular, it became apparent that the dark matter subhalos, typically identified as the sites of luminous satellite galaxies around a host galaxy, are more abundant than observed galaxies. This has been termed the Missing Satellites Problem. (The name, by the way, is an interesting contortion of perspective: taking the model for granted, it blames the observed number of satellites to be too low. One could have termed it the “Over-Abundance of Subhalos Problem”, or something similar.)

But the Missing satellites problem, its suggested solutions and the problems appearing with them are not the topic of today. Instead, let's focus on a different, more roboust test for cosmological models: the spatial distribution of subhalos / satellite galaxies. More roboust, because baryonic physics should not have an effect on scales of tens to hundreds of kpc, which are the typical observed distances between the satellites of the Milky Way.

The characteristic prediction of cold-dark-matter models is well illustrated in the following video, showing a dark matter halo similar to that assumed for the Milky Way. The 'camera' zooms in to the center of the halo, the size of the area shown is given in kpc the upper left corner.

 

 



It visualizes thedark matter density resulting from the Via Lactea-2 simulation (via lactea project, Jürg Diemand). The bright spots show the positions of dark matter subhalos, which might host luminous satellite galaxies. Not only does the number of predicted subhalos (> 1000) not match the observed number (currently 24, probably a few more), they are distributed rather evenly around the center, where the Milky Way would be situated. Simply put, there are subhalos in every direction.

 

The distribution of satellite galaxies

The MW satellites are distributed differently: they trace a disk of satellites (DoS), a planar distribution that is perpendicular to the Milky Way disc. With a radius of up to 250 kpc, this planar structure has a thickness of only 50-60 kpc.  That it is incompatible with the expectations from the standard cosmological model has been pointed out for the first time by Kroupa, Theis & Boily (2005).

It has been verified for the 11 'classical' satellites (Metz et al. 2007) that they are distributed in a thin (40 kpc) planar structure that it oriented perpendicular to the Milky Way disc. This study was complemented by Metz et al (2009) with the inclusion of several fainter Galactic satellites, which led to the same orientation of the 'disc of satellites'. Including a few additional faint satellite galaxies, we have shown in our 2010 paper (Kroupa et al. 2010) that, if you only look at the 13 faint satellite galaxies detected in the SDSS, they even independently describe the same planar orientation as the 11 classical ones.

But not only are the satellite galaxies distributed in this plane, they also move within it, as Metz et al. (2008) have shown. They looked at the 8 satellite galaxies for which the proper motion is known (that is the direction of motion on the sky, in addition to the radial velocity). Out of these 8, 7 are in agreement with moving within the plane described by all the satellites' positions.

All of this work was done exclusively by members of the Stellar Populations and Dynamics Research (SPODYR) group at the University of Bonn.

 

A new idea, adding more orbit-information: streams of stars and gas

In our most recent work, we now added more different objects to this polar structure. The idea came to my mind when I (Marcel) was reading a paper about several newly discovered stellar streams (Grillmair 2009). When an object (a star cluster or dwarf galaxy) orbits around the Milky Way it looses stars. These stars will either be slightly faster or slightly slower than the object, and therefore take over or fall back along the orbit of the object. Some streams can be clearly assigned to an object, in other cases the object has been completely torn apart and only the stream is left. In both cases, the stars in the stream move more or less into the same direction as the object they came from. The streams, therefore, are situated in the same orbital plane at the progenitor object. They tell us about the object's path around the Milky Way.

If the distribution of satellite objects around the Milky Way is stable, then the objects should move within this plane. So the orbital planes, traced by the streams of stars (or gas in some cases), should preferentially align with the satellite galaxies distributed in the DoS.

After the idea was born, we developed a method for determining the orientation of a stream. The results of the first few streams looked very promising, and so I searched the literature in order to collect data for as many streams around the Milky Way as possible. In the end, we were able to use 14 streams.

Half of them turned out to be well aligned with the DoS. If they would have been drawn from an isotropic distribution (a zeroth-order approximation of the distribution expected from cosmological models) the likelihood to find that many streams this close to the DoS is only 0.3 per cent.

Since now we know that the satellite galaxies and half of all streams align in the same structure, we began referring to this structure as the 'vast polar structure' (VPOS) of the Milky Way.

 

A different class of objects in the VPOS

The streams, as mentioned before, not only originate from dwarf galaxies, but also from globular clusters. When streams stemming from globular clusters are also located in the VPOS, shouldn't these dense stellar systems be distributed within it, too? We checked this idea in the paper.

To do this, one has to know that globular clusters (GCs) can be classified in different groups, which are thought to have different origins (e.g. Mackey & van den Bergh 2005). There are GCs that lie in the disc of the Milky Way, so their distribution must be in the Galactic plane and not perpendicular to it like the VPOS. There are so-called old halo GCs. Their very old ages tall us that they have formed together with the Milky Way. They should not show signs of the VPOS. And there are 'young' halo GCs, which exhibit similarities to GCs associated with satellite galaxies, and must have a different origin than the other two groups.

When analysing the distributions of these different groups, we were stunned how well our expectations were met. The first two groups of GCs turned out to be completely unrelated to the VPOS. But the young halo globular clusters in fact define the same vast polar structure as the satellite galaxies, their motions and the streams. Even if you look only at the near young GCs (within 20 kpc) and the far ones (beyond 20 kpc) individually, they follow virtually the same structure. The likelihood of this happening in a random distribution is only 0.1 per cent.

The video below illustrated the distribution of all the different objects forming the VPOS, extending from 10 out to 250 kpc around the Milky Way. Note that the streams are magnified by a factor of three for better visibility.

 

 

 


The vast polar structure - VPOS - about the MW in Cartesian coordinates. The movie rotates the view over 360 degree, adding different objects around the Milky Way galaxy. The y-axis points towards the Galactic north pole. The 11 classical satellites are shown as yellow dots, the 13 new satellites are represented by the smaller green dots, young halo globular clusters are plotted as blue squares. The red curves connect the anchor points of streams of stars and gas, the (light-red) shaded regions illustrating the planes defined by these and the Galactic centre. Note that the stream coordinates are magnified by a factor of 3 to ease the comparison. The obscuration-region of 10 degree around the Milky Way disc is given by the horizontal grey areas. In the centre, the Milky Way disc orientation (edge-on) is shown by a short horizontal cyan line. One can clearly see when the view is edge-on to the VPOS: The extend of all types of objects becomes minimal, also the streams align preferentially with this structure. From standard dark matter cosmology, a much more spheroidal distribution of objects around the Milky Way is expected. We therefore propose the satellite galaxies of the Milky Way to be Tidal Dwarf Galaxies. Feel free to download the movie to be used in talks.

 

Suggested solutions (and why they do not work)

Within the standard cold dark matter cosmological model, such a strongly correlated structure was not predicted. This is why there have been several attempts to explain planar distributions of satellites after it became known. The list below motivates why none of these are satisfying:

  • Chance alignment: This is one of the initial and most simple ideas. If the structure is made up of only a few objects, it might just be bad luck that they all fall into a planar distribution right now. But the addition of more and more objects has reduced the likelihood of a chance alignment. A chance-alignment of the positions of the 11 classical satellites alone can be excluded at a 99.5 confidence level (Metz et al. 2007). Including the correlated motions of these or adding more objects makes this statement even more stringent, such that a chance alignment must be excluded. In addition, the preferred motion of the objects within the plane (from proper motions and stream orientations) shows that the structure is stable over time.
  • Group infall: Maybe a number of satellite galaxies were accreted by the Milky Way together in a group. It is known that associations of dwarf galaxies exist, so this was a good idea put forward by Li & Helmi (2008) and D'Onghia & Lake (2008). But as Metz et al. (2009) have shown, the observed associations are much wider than the VPOS. A structure as thin as observed can not have formed this way. In addition, the increased number of satellite objects within the VPOS speaks against this scenario, because in addition to the infall of a group of a few subhalos, a more evenly distributed population of subhalos has to be around.
  • Filamentary accretion: As seen from simulations of structure formation in the universe, there is a giant 'cosmic web' of material connecting galaxies, which are formed preferentially within such filaments. Maybe small dark matter halos / dwarf galaxies are accreted preferentially along such filaments, resulting in a preferred spacial distribution? This seems not to work out because the filaments, like the groups before, are too thick and not the only source of subhalos. While there are overdensities of infall-directions at large distances (see for example Libeskind et al. 2011), no structure as well defined as the VPOS is produced. Nevertheless, the abstract of Lovell et al. (2011) claims that “Quasi-planar distributions of coherently rotating satellites, such as those inferred in the Milky Way and other galaxies, arise naturally in simulations of a ΛCDM universe”. In a few days we will show in detail why this claim is unjustified. UPDATE: The preprint of our paper "Can filamentary accretion explain the orbital poles of the Milky Way satellites?" is now available on the arXiv. We will blog about it soon. UPDATE: Here is our blog post on this paper.

 

A radically different scenario

In our paper, we propose a radically different alternative: the VPOS has been formed from the debris of a collision of two galaxies. The satellite galaxies would then not be dark-matter dominated objects, but tidal dwarf galaxies that formed within the tidal debris stripped from another galaxy. It is noteworthy that this had already been hinted at by the early stellar-dynamical work by Kroupa (1997)  who showed that the satellite galaxies may not require dark matter but that they may appear as if they had dark matter. Tidal dwarf galaxies are observed to form and naturally align in the plane of the interaction.

As we have shown in our paper “Making counter-orbiting tidal debris. The origin of the Milky Way disc of satellites?”, Pawlowski et al. (2011), a number of features of the satellite galaxy population of the Milky Way are consistently explained if they stem from tidal debris. In addition, Pavel has laid out more reasons in favor of a tidal origin in his recent paper (Kroupa 2012).

 

More Information

The papers reporting problems for dark matter keep coming in more frequently lately. Be prepared for another one from our side next week, discussing filamentary accretion as a possible origin of the VPOS. UPDATE: preprint available here, discussed in the blog here.

The paper this post is based on: “The VPOS: a vast polar structure of satellite galaxies, globular clusters and streams around the Milky Way”, by Marcel S. Pawlowski, J. Pflamm-Altenburg and Pavel Kroupa. It has been accepted for publication in MNRAS and a preprint can be found on the arXiv.

The Royal Astronomical Society has published a press release on this topic, too: “Do the Milky Way’s companions spell trouble for dark matter?

It was picked up by a number of news sites, a selection of which we list below:

Blog posts:

 

By Pavel Kroupa and Marcel Pawlowski  (28.04.2012): "The vast polar structure - VPOS - of satellite objects around the Milky Way" on SciLogs. See the overview of topics in  The Dark Matter Crisis.


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Dark Matter gone missing in many places: a crisis of modern physics?

Marcel S. Pawlowski | 19. April 2012, 21:41

On The Dark Matter Crisis, we have already presented numerous problems that appear within the LCDM model of cosmology. Some of these have been given names, like the “Missing Satellites Problem”, where LCDM predicts more dark matter subhaloes around the Milky Way than there are observed satellite galaxies, which are expected to trace them. Or the “Missing Baryons Problem”: from cosmological predictions we expect a certain density in the baryonic, luminous and thus in principle observable matter. But when you add up all the visible matter you observed, you only get 10-40 per cent of what you expect. The larger fraction is missing.

Even the ongoing non-detection of the DM particle in direct-detection experiments might be seen by some as another of these problems. So, there are several cases in which the model predicts something which then is not observed, thus leading to the 'missing' of that particular entity or observation thereof.

This week, two additional studies claim that even more seems to be missing (when your expectations are based on what LCDM predicts, that is). They both suggest a serious lack in the amount of expected dark matter on two very different size-scales: the local universe and our immediate neighborhood within the Milky Way.

 

Dark Matter missing in ... the Local Universe

In the work titled “Missing Dark Matter in the Local Universe”, Igor D. Karachentsev has looked at a sample of 11,000 galaxies in the local Universe around the MW. He has summed up the masses of individual galaxies and galaxy-groups and used this to test a very fundamental prediction of LCDM.

The idea is as simple as it is brilliant: cosmology has precise predictions as to what is the content of our universe. In particular, it predicts the density of matter to be Ωm,glob = 0.28 +- 0.03 (83 per cent of this in dark, 17 per cent in luminous matter). Now, to test this, all you have to do is to sum up all the mass within a certain volume of space, and you can estimate the actual density of mass within that volume. To be sure that your volume is representative, it needs to be large. If you only sum over, say, a sphere of 100 kpc in diameter, the density strongly depends on whether you have a galaxy in this volume or not. Karachentsev chose to use a volume with a radius of 50 Mpc around the MW. On this size-scale, the density is expected to fluctuate by only 10 percent, a reasonably low value in astronomy. The scale can thus be assumed to be representative and you should observe the mass density predicted by LCDM.

Except that you do not.

Karachentsev reports that the average mass density is only Ωm,loc = 0.08 +- 0.02, a factor of 3-4 lower than predicted and can not be explained by the uncertainties in the data or prediction. As most of the mass-content in the Universe is supposed to be dark matter, this means that most dark matter is missing in this volume.

It is not straight-forward to interpret this result, except that it might be a serious problem for LCDM. In the paper three solutions within the framework of standard dark matter cosmology are suggested. First of all, we might resort to the unsatisfying claim that the local Universe is exceptionally non-representative of the Universe as a whole. We would then sit in a local void, a very large under-dense region of the Universe. Unfortunately, as Karachentsev states in his paper, this is in contradiction to observations. The other two suggested solutions are based on the idea that maybe not all mass is counted. Dark matter is defined to be an elusive thing, after all. Dark halos might be more extended than predicted in the models, pushing it outside the virial radius of a halo, the region in which observations can indirectly 'measure' it from the dynamics. However, taking this as a solution to the observed mass-deficit “clearly contradicts the existing observational data”, as Karachentsev states in his work. But maybe much of the dark matter is hiding somewhere else? Karachentsev suggests it to be in massive dark clumps not filled with galaxies (he calls them 'dark attractors'), and thus is invisible to us when looking for galaxies only. But how could these dark clumps, with masses of galaxy-clusters, remain dark? You would need to separate the baryonic, luminous matter from a large bunch of dark matter to make sure no galaxies from in the dark attractor.

In any case, these suggestions require modifications to the behavior of dark matter because their processes are not predicted in current models. None of these possibilities seem very attractive, leaving us with the conclusion that, assuming we live in a LCDM universe, a large fraction of the dark matter is gone missing.

 

Dark Matter missing in ... the Solar Neighborhood

The amount of dark matter in the solar neighborhood was investigated in the work “Kinematical and chemical vertical structure of the Galactic thick disk II. A lack of dark matter in the solar neighborhood” by Christian Moni Bidin and collaborators. For a short introduction, you can have a look at this proceedings paper, and yesterday, the ESO also issued a press release about this work, titled “Serious Blow to Dark Matter Theories?”.

In their work, Moni Bidin et al. have looked at a sample of 400 red giant stars close to the Sun at vertical distances of 1.5 to 4 kpc above the MW disc. In addition to the stellar 3D positions, they have derived three-dimensional kinematics for these stars. From this data, they estimate the dynamical surface mass density of the MW within this range in heights from the disc. This surface mass density should be the sum of all mass, visible and dark. But it turns out, according to their analysis, that the visible mass alone is already a perfect fit to the observed value. According to the authors, no additional mass is needed (see their plot below).

Figure 1 of Moni Bidin et al. (2012)

CAPTION: Upper panel of figure 1 of Moni Bidin et al. (2012). Observational results (black) for the surface mass density within a certain distance from the Galactic plane (x-axis). The dotted and dashed lines show the 1- and 3-sigma strip of the observations. The predictions of models (grey) containing a dark matter halo all lie significantly above the observed value, except for the model accounting for visible mass only (labelled VIS).

Their analysis is based on a number of assumptions about the structure of and kinematics in the Milky Way disc, like that the density decays exponentially in both radial and vertical direction, that there is a flat rotation curve, thar there is no bulk motion of stars in vertical or radial direction and so on. It might well be that some of their assumptions are not perfectly valid. However, they have checked that changing one of their adopted input parameters or assumptions can not solve the problem of missing DM. Very exotic hypotheses (they mention an unreasonably thin thick disc as an example) can make their data fit with the expectations from DM models, but such a solution is unsatisfying and rather improbable, according to them.

Taken together, the work suggests that, given their assumptions about the the MW disc, dark matter halos as predicted by current models do not explain the observations. It might be more informative to state it the other way around, though: according to them, the observations can be easily explained with the visible matter of the Milky Way disc alone, there is no need for more.

Note added on 21.05.2012: In a recent posting on astro-ph Bovy & Tremaine point out that the deduced amount of dark matter depends on the assumptions that go into the modelling of the stellar kinematics. They assume Newtonian dynamics to be valid (as Moni Bidin et al. have) but in contradiction to Moni Bidin et al. they show that it is not correct to assume the mean azimuthal velocity is independent of Galactocentric cylindrical radius. Instead, taking the circular velocity to be independent of the radius, Bovy & Tremaine show that the usual local matter density is arrived at. If Milgromian dynamics were correct rather than Newtonian dynamics, then it emerges that the local stellar kinematics ought to show evidence for phantom dark matter (e.g. Fig.12 in Kroupa 2012). We remind the reader that in the past it has been claimed that local stellar kinematics shows evidence for significant amounts of dark matter in the disk of the Milky Way, while more thorough later analysis has found this signal to go away (Kujiken & Gilmore 1989; Kuijken 1991Flynn & Fuchs 1994). Thus, all in all, the Newtonian analysis by Bovy & Tremaine not only "saved dark matter", but more importantly although unintentionally, Bovy & Tremaine demonstrated consistency of the data with MOND. End Note.

 

Dark Matter missing in ... well, it is simply not there at all

Indeed, a 50 page review of the observational tests of the standard model has been compiled by Pavel Kroupa in "The dark matter crisis: falsification of the current standard model of cosmology" and will appear in the Publications of the Astronomical Society of Australia (PASA-CSIRO publishing). Using a huge number of different data, Pavel Kroupa performs a strict logical falsification of the currently standard cosmological model, which is based on Einstein's theory of general relativity, concluding that cold or warm dark matter cannot exist.

Note added on 21.05.2012: The implications of the Dual Dwarf Galaxy Theorem of the Kroupa 2012 paper is that cold or warm dark matter cannot be dynamically relevant in galaxies. It then implies that non-Newtonian (e.g. Milgromian) dynamics must be valid. Ironically, when interpreting Milgromian systems with Newtonian eyes, the observes will see evidence for dark matter. However, this is phanotm dark matter and it is exactly coupled to normal matter. That is, phanton dark matter is not constituted of ballistic particles which are on individual orbits within a Newtonian potential. End Note.

 

A crisis of modern physics

If there is no dynamically relevant cold or warm dark matter then we still need to explain the flat rotation curves of galaxies. This leads to a crisis in modern physics, as our very understanding of space-time and matter are now at stake.

Other posts you might find interesting:

II. The Fritz Zwicky Paradox and its solution

Question C.II: MOND works far too well !

Question C.III: Fundamental theoretical problems

By Pavel Kroupa and Marcel Pawlowski  (19.04.2012): "Dark Matter gone missing in many places: a crisis of modern physics?" on SciLogs. See the overview of topics in  The Dark Matter Crisis.


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German TV tip: physics at the verge of collapse - science in the dark

Marcel S. Pawlowski | 17. April 2012, 12:46

A short TV-tip for our German readers: on April 17 (today) at 18:30 on 3sat there will be a "nano spezial" about fundamental problems of physics and cosmology, dark matter and dark energy. It is titled "Physik vor dem Kollaps - Die Wissenschaft steht im Dunkeln". It includes an interview with Pavel Kroupa.

For those without TV: the programme can already be found online in the 3sat Mediathek and will be available for the next seven days.


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Question D: What about the Bullet cluster? And what about the Train-Wreck cluster Abell 520?

Pavel Kroupa | 15. April 2012, 20:15

Summary:

One result is very definite by now: neither the Bullet nor the Train Wreck clusters support (nor do they prove) the existence of cold or warm dark matter. And, they certainly do not disprove MOND. Quite on the contrary, according to current knowledge, they falsify the concordance cosmological (or LCDM) model.

The Bullet cluster consists of two clusters of galaxies that have penetrated each other leaving behind a slab of gas while the now seperating clusters retain matter as revealed through gravitational lensing. Assuming General Relativity (GR) to be valid the lensing measurements tell us that collisionless dark matter must be present in the separating clusters. But, it has been shown that the relative velocity of the two clusters need to be so large that the observed constellation ought to not occur in the real universe if it were described by GR, i.e. by the concordance cosmological model. Instead, it turns out that MOND-based models can readily account for the large relative velocity and the lensing signal as long as both clusters contain some hot dark matter or, alternatively, gas in cold clouds that cannot be detected. The Train Wreck cluster shows the opposite behaviour: assuming GR to be valid, the putative cold or warm dark matter has separated from the galaxies in this other collision of galaxy-clusters. The core of dark matter is evident from gravitational lensing (assuming GR to hold). This is inexplicable within GR because there is no known physical mechanism known for separating the dark matter from the galaxies as it does not dissipate like gas. In MOND-based models, the train wreck is also a challenge, but in principle it may perhaps be possible to separate the hot-dark-matter cluster core and the galaxies, and/or to obtain spurios lensing signals suggesting matter concentrations where there are none. Thus, the train wreck may, in the end, turn out to be a case supporting MOND-based models over GR-based ones.

 

 

Background:


As introduced in the previous contribution to The Dark Matter Crisis, Question A: Galaxies do not work in LCDM, sociology and majority views, PK had been contacted by a few people, and here are excerpts from some of the questions asked and the replies. These help to illustrate some of the issues at hand. The questions are

A) So the LCDM model fails on scales smaller than about 8 Mpc?

B1) What is a galaxy?

B2) What is a galaxy? (Addendum on the relaxation time)

C) What are the three best reasons for the failure of the LCDM model?

     I: Incompatibility with observations

    II: MOND works far too well ! 

   III: Fundamental theoretical problems

D) What about the Bullet cluster?  And what about the Train-Wreck cluster Abell 520?  (this contribution)

E) Why is the main stream community so reluctant to  go along with accepting the failure of LCDM?

This contribution deals with Question D, while an upcoming contribution will concentrate on the remaining question. Beyond that we will keep posting on issues of relevance for the paradigm change. 


The full question posed was "And how do you respond to the bullet cluster results which seems to point to a center of mass that does not match luminosity via weak gravitational lensing"

 

We augment the answer with a brief discussion of the Train-Wreck cluster Abell 520. Our pevious contributions on this issue are:

 

A brief background to galaxy cluster dynamics:

In the Einsteinian/Newtonian theory of gravitation, galaxy clusters require about ten times as much mass in cold (or warm) dark matter than is present in normal matter.

Assuming MOND, the observed gravitational lensing and the observed kinematics in galaxy clusters merely require about a factor of two to perhaps three in additional mass. In MOND, the problem of missing mass in galaxy clusters is therefore significantly reduced. It may be completely removed if the missing mass is normal matter which is in undetectable cold gas. Or, the missing mass may be in agreement with particle physics because neutrinos oscillate and thus must have a mass. This implies the existence of additional neutral particles such as sterile neutrinos. If sterile neutrinos have a mass near 11 eV (see below) then the dark-matter problem in MOND-galaxy-clusters and in MOND-cosmological models disappears (Angus & Diaferio 2012). Such dark matter is "hot", i.e. after the Big Bang the particles had relativistic (extremely high) velocities, and so such dark matter cannot agregate into galaxies but can be captured into galaxy-cluster-sized gravitational bodies which have sufficiently deep potential wells for the hot dark matter to not be able to escape. 

This is nicely explained by Sanders (2003)  in his research paper "Clusters of galaxies with modified Newtonian dynamic., and in his review of MOND (Sanders 2009), and in the recent 160 page "Modified Newtonian Dynamics: A Review" by Famaey & McGaugh (2012).

Another approach, Modified Gravity (MOG), can deal with lensing and galaxy cluster observations entirely without dark matter (e.g. Moffat, Rahvar & Toth 2012). [note added on 16.04.2012] 

 

Answer to the Bullet cluster:

According to Tom Shanks (private communication 2010), the data reduction to get the actual weak-lensing matter distribution map is very complex and relies on subtraction of a background. There is some freedom and it is difficult to extract a signal. See also the comment by "JR" quoted below.

But, accepting the data reduction which has been published, the Bullet cluster surprisingly turns out to be a counter-argument against the validity of the LCDM model. In LCDM the required velocities of the two clusters is too high (about 3000km/s), a velocity which does not occur. Now, in a MONDian universe, such velocities occur rather naturally, and so with some hot dark matter (in fact the same as I wrote above, 11eV particles) one gets beautiful agreement with the observations!

The Bullet Cluster (1E 0657-56) is often perceived to be a disproof of Milgromian dynamics because even in Milgromian dynamics DM is required to explain the observed separation of the weak lensing signal and the baryonic matter. In actuality, the Bullet Cluster is, if anything, a major problem for the LCDM model because the large relative cluster–cluster velocity at the mass scale of the two observed clusters required to provide the observed gas shock front cannot be attained in the LCDM model, as shown by Lee & Komatsu (2010) in their research paper "Bullet Cluster: A Challenge to ΛCDM Cosmology" and as verified and deepened by Thompson & Nagamine (2012) in their research paper "Pairwise velocities of dark matter haloes: a test for the Λ cold dark matter model using the bullet cluster". Thomposn & Nagamine  "conclude that either 1E 0657-56 is incompatible with the concordance ΛCDM universe or the initial conditions suggested by the non-cosmological simulations must be revised to give a lower value of" the relative velocity.

But the high relative velocities between the two sub-clusters in the Bullet cluster arise naturally and abundantly in a Milgromian cosmology:

Assuming the Milgromian framework to be the correct description of effective gravitational dynamics, it has been shown that the Bullet Cluster lensing signal can be accounted for in it. Ibn their researhc paper,  "Can MOND take a bullet? Analytical comparisons of three versions of MOND beyond spherical symmetry",  Angus, Famaey & Zhao (2006) state "In particular, we can generate a multicentred baryonic system with a weak lensing signal resembling that of the merging galaxy cluster 1E 0657-56 with a bullet-like light distribution."

If a Milgromian cosmology is allowed to have a hot DM component then the Bullet Cluster is indeed well explainable. In the research paper on "The collision velocity of the bullet cluster in conventional and modified dynamics", Angus & McGaugh (2008) they summarise:

"We consider the orbit of the bullet cluster 1E 0657-56 in both cold dark matter (CDM) and Modified Newtonian Dynamics (MOND) using accurate mass models appropriate to each case in order to ascertain the maximum plausible collision velocity. Impact velocities consistent with the shock velocity (~ 4700kms-1) occur naturally in MOND. CDM can generate collision velocities of at most ~3800kms-1, and is only consistent with the data, provided that the shock velocity has been substantially enhanced by hydrodynamical effects. "

Using a new cosmological N-body code for MOND, Angus & Diaferio (2011) find "As a last test, we computed the relative velocity between pairs of haloes within 10 Mpc and find that pairs with velocities larger than 3000 km s-1, like the bullet cluster, can form without difficulty.

We know that neutrinos oscillate, therefore they must have a mass. That mass is small. This makes them a form of hot DM that we most definitely know to exist. In order to explain the oscillations, particle physics suggests the possible existence of more massive, sterile neutrinos, which interact by gravity. If they exist they might be massive enough to account for the missing mass in galaxy clusters in MOND (and they can fit the first three acoustic peaks in the CMB). A research paper discussing the possible role of sterile neutrinos for dark matter has been published by Dodelson & Wildrow (1994).

Taking this ansatz, Angus, Famaey & Diaferio (2010) demonstrate, in their research paper "Equilibrium configurations of 11 eV sterile neutrinos in MONDian galaxy clusters"  that consistency in solving the mass-deficit in galaxy clusters and accounting for the CMB radiation power spectrum is achieved if sterile neutrinos (SN) have a mass near 11 eV. They write “we conclude that it is intriguing that the minimum mass of SN particle that can match the CMB is the same as the minimum mass found here to be consistent with equilibrium configurations of Milgromian clusters of galaxies"

 

The Train Wreck cluster: 

The Train-Wreck cluster (Abell 520) has been shown to be incompatible with the LCDM model because the putative C/WDM particles have separated from the galaxies such that a core of DM is left behind and away from the concentrations of galaxies, as Mahdavi et al. (2007) find in their research paper "A Dark Core in Abell 520". There is no known physical mechanism which can separate cold or warm dark matter from galaxies to the extend required by the Train Wreck. 

Jee et al. (2012) return to the Train Wreck with their research paper "A Study of the Dark Core in A520 with the Hubble Space Telescope: The Mystery Deepens", confirming the problem. They speculate on a possible solution such as DM possibly having a self-interaction property, and interestingly they avoid discussion of any alternative theory of gravitation.

The Train Wreck remains not understood.

As pointed out by Kroupa (2012), in a MONDian cosmological model with hot dark matter (HDM) it is conceivable, at least in principle under certain conditions, for the self-bound galaxy-cluster-sized HDM core of the whole cluster to dissociate itself from the baryonic matter in galaxies since the galaxies do not reside in HDM halos. Each individual galaxy would remain on the baryonic Tully-Fisher relation, as is observed to be the case for all disk galaxies and as is required to be the case if MOND is correct (e.g. Famaey & McGaugh 2012). And, in MOND-based models it may perhaps be possible to obtain spurios lensing signals suggesting matter concentrations where there are none.

Finally, in the comment "Scientific Polemicism" to our previous contribution  "The Train Wreck Cluster - an "anti-Bullet-Cluster": disproof of Cold or Warm Dark Matter, "JR" writes on the 18.10.2010 at 14:45:

The main reason why most scientists remain sceptical about the Abell 520 "train wreck" results is that different groups analysing the *same data* obtain different mass maps (see Okabe & Umetsu 2008). Now that's a train wreck! The same cannot be said for the bullet cluster, where - to the best of my knowledge - all authors currently agree on the lensing mass maps. This does not mean, of course, that the bullet is right and Abell 520 is wrong - we should remain open minded about both. But I am particularly sceptical of the Abell 520 results because of a well-known problem with lensing mass reconstruction: the monopole degeneracy. This was illustrated beautifully in recent work by Liesenborgs et al. (2008) who show that the monopole degeneracy can lead to phantom peaks in the mass distribution (see their Figure 3). Their work focused on strong rather than weak lensing, but weak lensing suffers from exactly the same problems.

 

Within the modified gravity (MOG) framework, Moffat & Toth (2009) and Moffat, Rahvar & Toth (2012) argue to be able to account for both the Bullet and the Train Wreck cluster.

 

A ring of dark matter?:  

Milgrom & Sanders (2008) analyse in their research paper "Rings and Shells of "Dark Matter'' as MOND Artifacts" the recent detection using weak lensing of a ring of dark matter around a galaxy cluster. They write in their abstract:

We consider the possibility that this pure MOND phenomenon is in the basis of the recent finding of such a ring in the galaxy cluster Cl 0024+17 by Jee et al. (2007). We find that the parameters of the observed ring can be naturally explained in this way; this feature may therefore turn out to be direct evidence for MOND. 

 

By Pavel Kroupa and Marcel Pawlowski  (15.04.2012): "Question D: What about the Bullet cluster? And what about the Train Wreck cluster Abell 520" on SciLogs. See the overview of topics in  The Dark Matter Crisis.


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Question C.III: Fundamental theoretical problems

Pavel Kroupa | 31. March 2012, 15:25

Rather than being posted "soon after" II: MOND works far too well ! (published on the 21.03.2011), a delay caused by internal university issues arose. We are back though, for the time being, with the originally advertised "Question C.III: Fundamental theoretical problems" (this contribution).

To re-iterate: what is the purpose of this series on SciLogs? We are aiming to document, within the time we have for such matters, the already noticeable paradigm shift away from a dark-matter dominated Einsteinian inflationary cosmology model to a different description which may, or may not, be fundamentally based on Einstein's GR theory. 

Continuing now with Qestion C.III:

Summary:

The development of the concordance cosmological model (CCM) over the past 40 years is based on the addition of at least three unknown ("dark") physical phenomena (inflation, cold dark matter, dark energy), in an attempt to make Einstein's field equation account for the distribution of matter on galactic and larger scales. None of these are understood nor experimentaly verified today. While these may constitute true discoveries of new physics, much as in the spirit of the past when for example Neptune and the neutrino were postulated to exist based on not understood observations, these dark additions also have a parallel in the Ptolomaic model which is based on a series of complex additions to circular motions in order to provide a calculation tool for the Solar System prior to the discovery of Kepler's and later Newton's laws. On close scrutiny the latter analogy appears to be the favourable one because the CCM is not able to account for the observed distribution of matter on scales of 10Mpc and less, where a massive computational effort by many groups has been able to quantify the theoretical distribution of matter. Meanwhile, new dynamical laws have been discovered which are extremely successful in accounting for the appearance and motion of matter on galactic scales and above. At the same time, it is emerging that the CCM is not unique in accounting for the large-scale matter distribution nor for Big Bang Nucleosynthesis nor for the cosmic microwave radiation. This suggests rather unambiguosly that our understanding of gravity is not complete. This conclusion, obtained purely from astronomical data, is nothing else but the statement that we do not have a good physical theory of matter, mass, space and time nor do we  know how and if they can be unified. 

 

Background:


As introduced in the previous contribution to The Dark Matter Crisis, Question A: Galaxies do not work in LCDM, sociology and majority views, PK had been contacted by a few people, and here are excerpts from some of the questions asked and the replies. These help to illustrate some of the issues at hand. The questions are

A) So the LCDM model fails on scales smaller than about 8 Mpc?

B1) What is a galaxy?

B2) What is a galaxy? (Addendum on the relaxation time)

C) What are the three best reasons for the failure of the LCDM model?

     I: Incompatibility with observations

    II: MOND works far too well ! 

   III: Fundamental theoretical problems  (this contribution)

D) What about the Bullet cluster?  And what about the Train-Wreck cluster Abell 520?

E) Why is the main stream community so reluctant to  go along with accepting the failure of LCDM?

This contribution deals with Question C III, which may be taken to be central to The Dark Matter Crisis, while upcoming contributions will concentrate on the remaining questions.


 

The three best reasons for the failure of the LCDM model: 

They can be summarised in three categories. Here is category III. Ctegories I and II can be found in seperate contributions as outlined above.

 

III) Fundamental theoretical problems

The mathematical foundation of the model is very problematical. It relies on too many completely unknown "new physics": inflation, cold dark matter and dark energy. Each of these has major problems. The reader is pointed to the review by Afshordi 2012 who addresses these issues accessibly but also in in much more depth.

Inflation is not understood from a particle physics point of view.For an introduction see this Wikipedia article, the 1999 article by Andrew Liddle and his documentation on the web. In his paper "Unconventional Cosmology" Robert Brandenberger provides a good overview of Inflation and its problems, and Starkman et al. (2012) show that the CMB fluctuations appear to be incompatible with the SMoC causing major tension with standard inflationary cosmologies.

Dark matter is very hypothetical, has not been discovered yet despite decades of search whereby all of the until now favoured dark-matter-particle properties have already been experimentally excluded (see Stacey McGaugh's compilation "Cold Dark Matter and Experimental Searches for WIMPs). And, since it is not behaving as it ought to be, as inferred from direct observed properties of galaxies, additional "dark", i.e. unknwon, forces need to be postulated to exist to arrange a model LCDM galaxy to look like a real galaxy, e.g. to "solve" the Conspiracy Problem or the MOND behaviour of galaxies (see Questions C.I and C.II).

The extensive effort world-wide to detect DM particles in terrestrial experiments has so far not been successful (e.g. Baudis & for the XENON Collaboration 2012). For example, the CRESST-II DM search has reported a possible detection of a CDM particle signal (Angloher et al. 2011), but their fig. 13 also shows this putative signal to be in the parameter region excluded by the CDMS-II (CDMS II Collaboration, Ahmed et al. 2010) and XENON100 (Aprile et al. 2011) DM-particle experiments. The search for a DM-particle-annihilation or DM-particle-decay signature from regions where high DM densities are measured assuming Newtonian dynamics to be valid has also been unsuccessful (e.g. the MW satellite galaxy Segue 1 has the highest DM density known but no DM signal has been detected, Aliu et al. 2012). Increasing loss of confidence is suffered by the experiments having to postulate ever decreasing interaction cross sections for the putative DM particles, significantly below and away from the originally favoured ones.

This is at the same time a fallacy of the adopted procedure: The existence of DM particles can never be disproven by direct experiment because ever lighter particles and/or ever smaller interaction cross sections just below the current detection threshold may be postulated for every non-detection. There exists no falsifiable prediction concerning the DM particles.

Dark energy (DE): The fluxes (i.e. brightnesses) and redshifts (i.e. distances) of observed type Ia supernovae (SNIa) do not match the cosmological models (Riess et al. 1998, Schmidt et al. 1998, Perlmutter et al. 1999) unless the universe is assumed to expand at an ever larger rate. To account for the implied accelerated expansion DE is introduced. But, as with inflation, while mathematically allowed, it remains unclear if DE constitutes physics (see e.g. the discussion in Afshordi 2012).

DE has major fine-tuning problems and is supposedly unstable to quantum corrections (e.g. Shanks 2005, "Problems with the Current Cosmological Paradigm"). Plus, a universe with DE is not energy conserving - energy appears "magically" all the time with the increasing volume of the expanding universe. This is actually well known (Kroupa et al. 2010) and would appear to be unphysical. To account for this issue one would need to postulate that the universe is not a closed system, i.e. that there is much more to it than we know (that is, yet again resort to another "dark outside" would be necessary).

Indeed, DE may not even exist, resulting from integrating a supernova (SN) Ia photon's path across the universe without correctly adding the non-linear general-relativistic sequence of time delays and spatial contractions as the photon traverses through the inhomogeneous matter distribution between the SN Ia and the observer. An observer, who does the calculation or averaging along the photon's path wrongly would indeed deduce falsly that the universe is larger than it ought to be, thus wrongly deducing the effect of an acceleration driven by DE. This has been shown to be quite possibly the case by Wiltshire (2007).

Thus, the SNIa flux--redshift data may at least partially be explained with an inhomogeneous universe(Wiltshire 2009, Smale & Wiltshire 2011, Marra & Pääkkönen 2012) rather than with DE, whereby systematics in SNIa light curve fitting remain an issue (Smale & Wiltshire 2011). Bull & Clifton (2012) find that the "appearance of acceleration in observations made over large scales does not necessarily imply or require the expansion of space to be accelerating, nor does it require local observables to indicate acceleration.'' 

It might perhaps be surprising that a homogeneous model universe should lead to a perfect agreement with the observed SNIa data. In other words, the SNIa data that stem from the real inhomogeneous universe should show some deviations from the homogeneous model. If none are seen then this may imply an over-constrained model.

 

What is gravitation?

It needs to be emphasised time and again that the failure of the CCM is not surprising as it is synonym with the well-known fact that gravitation is not understood. While Einstein paved the way for viewing space-time as a dynamic non-absolute physical object, we still do not know how mass emerges, nor whether inertial and gravitating mass are or should be the same, nor do we understand how space and time emerge or what they really are. We do not yet have a description of space, time and gravity on the quantum scale. 

In this context, the  recent break-through by Erik Verlinde who has shown that gravity may be a pseudo force which emerges "from the statistical behavior of microscopic degrees of freedom  encoded on a holographic screen" (citing from Entropic gravity). Gravity is derived by combining thermodynamics with the holographic principle.

Interestingly, in their recent paper on "Entropic corrections to Newton's law",  Modesto & Randano (2010)  suggest that Verlinde's approach leads to MONDian behaviour. Citing their abstract:

It has been known for some time that there is a deep connection between thermodynamics and gravity, with perhaps the most dramatic implication that the Einstein equations can be viewed as a thermodynamic equation of state. Recently Verlinde has proposed a model for gravity with a simple statistical mechanical interpretation that is applicable in the non-relatvistic regime. After critically analyzing the construction, we present a strong consistency check of the model. Specifically, we consider two well-motivated corrections to the area-entropy relation, the log correction and the volume correction, and follow Verlinde's construction to derive corrections to Newton's law of gravitation. We show that the deviations from Newton's law stemming from the log correction have the same form as the lowest order quantum effects of perturbative quantum gravity, and the deviations stemming from the volume correction have the same form as some modified Newtonian gravity models designed to explain the anomalous galactic rotation curves. 

 

Concluding Remarks:

Thus, the LCDM or standard/concordance-cosmological model (i.e. the CCM) relies to more than 95 per cent on unknown physics. The theoretical basis for the unknown physics is shaky at best. This is perhaps something one could live with, even though the whole construct is highly unsatisfying, if the actual predictions were consistent with reality.

But they are not (see Question C.I).

That the CMB and Big Bang nucleosynthesis as well as the motions of matter on galactic scales can be explained by a different cosmological model, one not based on the existence of cold or warm dark matter and thus probably not on Einstein's GR theory, is already well documented in the refereed scientific literature through the work of a young generation of very talented and imaginative physicists (for an account see Kroupa 2012: "The dark matter crisis: falsification of the current standard model of cosmology"). 

So, a part of the community is developing a better model. Such work is underway but is hindered not only by the complexity of its very nature, but alas also by disturbing human interventions (a book compiling personal experiences made by the mostly early-career researchers would be a valuable documentation of sociological issues at play even in our modern enlighted times).

 

Postscript: 

As a final word, it is useful to recall some hostorical records. The reader may also want to consult the very readable and detailed exposition on SciLogs on The Postulation of New Particles and the Pessimistic Meta-Induction by Josef Honerkamp

There were, in the past, a number of instances when unknown matter was postulated to exist, on the basis of existing knowledge, and then was indeed discovered:

  1. Atoms
  2. Electrons
  3. Anti-matter
  4. Neptune
  5. the neutrino.

But there were also at least 3 cases, when unknown matter was postulated to exist on the basis of existing theory, which were however later falsified:

  1. Phlogiston (solved by thermodynamics and atomic physics = new physical laws)
  2. aether (solved by special relativity = new physical laws)
  3. a planet within Mercury’s orbit (solved by general relativity = new laws of physics)

The case with phlogiston is an interesting parallel, because well before modern concepts were in place discrepancies had arisen within the phlogiston framework such that it became untenable centuries before quantum physics allowed oxidization for example to be understood at a fundamental level.

Concerning cold dark matter, we already have exactly this same situation at hand: within the LCDM framework insurmountable discrepancies have been arising despite a practically fantastic effort to solve these, only one of them being the Fritz Zwicky Paradox. We think this is the clear signal that the CCM is not viable, and we need to move on, whereby the success of MOND is giving essential clues. As with phlogiston, while the CCM may be ruled out already and while we do have Milgromian dynamics, we do not yet have a fundamental theory of space, time and matter. 


By Pavel Kroupa and Marcel Pawlowski  (31.03.2012): "Question C.III: Fundamental theoretical problems" on SciLogs. See the overview of topics in  The Dark Matter Crisis.


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Question C.II: MOND works far too well !

Pavel Kroupa | 21. March 2011, 12:00

Summary:

First try: Using only Solar System constraints, Newton and then Einstein developed the universal theory of gravitation. This Theory of General Relativity (GR) is then applied to model the universe. In order for it to fit the observational cosmological constraints, inflation, dark matter and dark energy need to be postulated to exist. Tests on scales of 10Mpc and less show this top-down modelling to fail despite major fine-tuning attempts. 

Second try: Using Solar System and galactic constraints Milgrom and then Bekenstein developed a new theory of gravitation. This MOND and TeVeS  approach is now being applied to model the universe. Cold dark matter is not needed, but applications to large-scale structure need to be developed. Tests on scales of 10Mpc and less show this bottom-up modelling to be successful without fine-tuning.

In general it follows that the need for dark matter and perhaps for the other postulates depends on the gravitational theory being used. Since we do not yet understand gravitation it furthermore follows that these postulates probably only express our lack of understanding of cosmological physics.  

Indeed, there is no reasonable astronomical evidence for the existance of cosmologically relevant cold dark matter particles, and so searching for these would be futile. 

Background:

As introduced in the previous contribution to The Dark Matter Crisis, Question A: Galaxies do not work in LCDM, sociology and majority views, PK was recently contacted by a few people, and here are excerpts from some of the questions asked and the replies. These help to illustrate some of the issues at hand. The questions are

A) So the LCDM model fails on scales smaller than about 8 Mpc?

B1) What is a galaxy?

B2) What is a galaxy? (Addendum on the relaxation time)

C) What are the three best reasons for the failure of the LCDM model?

     I: Incompatibility with observations

    II: MOND works far too well ! (this contribution)

   III: Fundamental theoretical problems

D) What about the Bullet cluster?  And what about the Train-Wreck cluster Abell 520?

E) Why is the main stream community so reluctant to  go along with accepting the failure of LCDM?

This contribution deals with Question C, which may be taken to be central to The Dark Matter Crisis, while upcoming contributions will concentrate on the remaining questions.

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Question C.I: What are the three best reasons for the failure of the LCDM model? I: Incompatibility with observations

Pavel Kroupa | 08. March 2011, 20:00

Summary:

The development of the concordance cosmological model (CCM) over the past 40 years is based on the addition of at least three unknown ("dark") physical phenomena (inflation, cold dark matter, dark energy), in an attempt to make Einstein's field equation account for the distribution of matter on galactic and larger scales. None of these are understood nor experimentaly verified today. While these may constitute true discoveries of new physics, much as in the spirit of the past when for example Neptune and the neutrino were postulated to exist based on not understood observations, these dark additions also have a parallel in the Ptolomaic model which is based on a series of complex additions to circular motions in order to provide a calculation tool for the Solar System prior to the discovery of Kepler's and later Newton's laws. On close scrutiny the latter analogy appears to be the favourable one because the CCM is not able to account for the observed distribution of matter on scales of 10Mpc and less, where a massive computational effort by many groups has been able to quantify the theoretical distribution of matter. Meanwhile, new dynamical laws have been discovered which are extremely successful in accounting for the appearance and motion of matter on galactic scales and above. At the same time, it is emerging that the CCM is not unique in accounting for the large-scale matter distribution nor for Big Bang Nucleosynthesis nor for the cosmic microwave radiation. This suggests rather unambiguosly that our understanding of gravity is not complete. This conclusion, obtained purely from astronomical data, is nothing else but the statement that we do not have a good physical theory of matter, mass, space and time nor do we  know how and if they can be unified. 

 

Background:


As introduced in the previous contribution to The Dark Matter Crisis, Question A: Galaxies do not work in LCDM, sociology and majority views, PK was recently contacted by a few people, and here are excerpts from some of the questions asked and the replies. These help to illustrate some of the issues at hand. The questions are

A) So the LCDM model fails on scales smaller than about 8 Mpc?

B1) What is a galaxy?

B2) What is a galaxy? (Addendum on the relaxation time)

C) What are the three best reasons for the failure of the LCDM model?

     I: Incompatibility with observations (this contribution)

    II: MOND works far too well !

   III: Fundamental theoretical problems

D) What about the Bullet cluster?  And what about the Train-Wreck cluster Abell 520?

E) Why is the main stream community so reluctant to  go along with accepting the failure of LCDM?

This contribution deals with Question C, which may be taken to be central to The Dark Matter Crisis, while upcoming contributions will concentrate on the remaining questions.

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Question B2: What is a galaxy? (Addendum on the relaxation time)

Pavel Kroupa | 23. January 2011, 11:14

Background:

As introduced in the previous contribution to The Dark Matter Crisis, Question A: Galaxies do not work in LCDM, sociology and majority views, PK was recently contacted by a few people, and here are excerpts from some of the questions asked and the replies. These help to illustrate some of the issues at hand. The questions are

A) So the LCDM model fails on scales smaller than about 8 Mpc?

B1) What is a galaxy?

B2) What is a galaxy? (Addendum on the relaxation time) (this contribution)

C) What are the three best reasons for the failure of the LCDM model?

     I: Incompatibility with observations

    II: MOND works far too well ! 

   III: Fundamental theoretical problems

D) What about the Bullet cluster?  And what about the Train-Wreck cluster Abell 520?

E) Why is the main stream community so reluctant to  go along with accepting the failure of LCDM?

This contribution deals with Question B2, while upcoming contributions will concentrate on the remaining questions.

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Question B1: What is a galaxy?

Pavel Kroupa | 19. January 2011, 17:00



Question B1: "What is a galaxy?  - vote here!"

 

Answer: The astronomical object we commonly call a "galaxy" has no formal definition yet. This issue is now raised to a more formal problem by Forbes & Kroupa (2011)(F&K).

Here is the associated press release.

Science and New Scientist also report on this question.

Your vote is of interest: Being motivated by the vote at the General Assembly of the International Astronomical Union in Prague in the year 2006 on Pluto's status in the Solar System and given the lack of a formal definition of what constitutes a "galaxy", Prof. Duncan Forbes from Swinburne University in Melbourne has organised a poll to seek if a consensus may emerge how a galaxy could perhaps be defined. To contribute to the poll, feel free to cast your vote here as to what you think a galaxy is. But please read the above F&K paper first.

And, feel free to post your own views on what a galaxy is in the comments section below.

The results of the poll and of the discussion will be reported at conferences. 

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Question A: Galaxies do not work in LCDM, sociology and majority views

Pavel Kroupa | 17. January 2011, 13:00

Independently of any dark-matter detections or the success or failure of dark-matter searches

(it is notable that the originally favoured dark matter particles have long ago been excluded through direct searches, as summarised by Prof. Stacy McGaugh),

it is a well known problem that galaxies cannot be reproduced in the standard cosmological (i.e. the LCDM) scenario. In the LCDM model, the mass of the universe consists to about 4 % out of normal (baryonic) matter which we observe, while 96% is in an unknown dark form (about 22 % being the exotic cold dark matter and about 74% being in dark energy).

Since the large fraction of astronomers (about 95 % as is often stated) and physicists are convinced that the LCDM model is the correct description of the cosmological universe, a vast industry has been established world-wide to try to solve the failure of LCDM on galaxy scales. Each year hundreds of research papers are spurned out by excellently funded reseach groups claiming to solve some aspect of the problem.

For example, many research papers deal with the question why there are only about two dozen satellite galaxies around the Milky Way, while there ought to be many thousands of satellite dark matter halos around the Milky Way (the Missing Satellite Problem). Why are the satellite galaxies of the Milky Way distributed so unevenly about the Milky Way, forming a giant disk-like distribution which is nearly perpendicularly oriented to the disk of the Milky Way (the Satellite Anisotropy Problem)? Another issue of focuss is why the dark matter halos inferred from observations have cores, while the LCDM model predicts them to have dense central regions, i.e. cusps (the Core-Cusp Problem). Another issue being worked on in much detail is why there are large thin disk galaxies with no bulges (the Angular-Momentum Problem). Why are disk galaxies, which by the way are the majority of all galaxies, all so simple (the Invariant Galaxy Problem)?  Why do large elliptical galaxies appear so rapidly after the Big Bang even though they must have been build-up from pre-existing galaxies in the LCDM model, according to which galaxies form through myriads of mergers of smaller dark matter halos that already have a normal-matter content in the form of stars and gas. Why are the predicted building blocks (these dwarf galaxies) observed to be younger than the large elliptical galaxies, although they should be older because the small dark matter halos form before the large ones (the Downsizing Probem)? Or, why is there an observed strong conspiracy between the distribution of normal (baryonic) matter and dark matter in all disk galaxies? This Conspiracy Problem has not been solved although it is known since decades. And, related to this, why is there no observational evidence for dark matter in dwarf-elliptical galaxies and large elliptical galaxies within their visible regions? Why does dark matter always only appear when the surface density of normal matter falls below a critical level? The surface density is but the gravitational acceleration - so, why should dark matter decide to appear when the acceleration falls below a critical threshold, which happens to be the same for all galaxies (the Dark Matter Emergence Probem)? Note that this is a purely observationally established fact, highlighted recently by Gentile et al.(2009, Nature) without any implication, as yet, of hidden deeper physics. 

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