Monday, July 2, 2018

Dark Matter Has A Cluster Problem

One of the main flaws of the toy model version of MOND is that is underestimates dark matter effects in clusters. But, dark matter models also have trouble producing the right halos for clusters.
We explore a scenario where metal poor globular clusters (GCs) form at the centres of their own dark matter halos in the early universe before reionization. This hypothesis leads to predictions about the abundance, distribution and kinematics of GCs today that we explore using cosmological N-body simulations and analytical modelling. We find that selecting the massive tail of collapsed objects at z≳9 as GC formation sites leads to four main predictions: i) a highly clustered population of GCs around galaxies today, ii) a natural scaling between number of GCs and halo virial mass that follows roughly the observed trend, iii) a very low number of free floating GCs outside massive halos and iv) GCs should be embedded within massive and extended dark matter (sub)halos. We find that the strongest constraint to the model is given by the combination of (i) and (ii): a mass cut to tagged GCs halos which accounts for the number density of metal poor GCs today predicts a radial distribution that is too extended compared to recent observations. On the other hand, a mass cut sufficient to match the observed half number radius could only explain 60% of the metal poor population. In all cases, observations favour early redshifts for GC formation (z≥15) placing them as contributors to the early stages of reionization.
Peter Creasey, et al., "Globular Clusters Formed within Dark Halos I: present-day abundance, distribution and kinematics" (June 28, 2018).


neo said...

whats your theory on clusters, since regular MOND requires dark matter to work.

dark matter despite its issues explains a wider range of observations

andrew said...

Multiple modified gravity approaches including MOG and Deur's work correctly predict dark matter phenomena in clusters.

Dark matter does not explain a wider range of observations, so long as you recognize that you need not just a dark matter particle but a theory to explain how the dark matter is distributed.

neo said...

where does Deur explain how galaxy clusters are explained?

i've found extended mond which was extended to explain galaxy clusters, but authors admit it is something of a kludge and does not predict small scale structures

andrew said...

One of the earliest published papers where he does so is here:

Basically, the big difference between Deur's approach that gives it broader applicability than MOND is that the geometric shape of the matter distributions matters in his approach but not in MOND (apart from the external field effect). This assumption is supported by the differences in apparent dark matter phenomena in elliptical galaxies. Basically, the less spherical a system is, the greater the dark matter-like effects.

andrew said...

From the pertinent part of the paper: "Dark matter was first hypothesized to reconcile the motions of galaxies inside clusters with the observed luminous masses of those clusters. Estimating the non-Abelian effects in galaxy clusters with our technique is difficult: 1) the force outside the galaxy is suppressed since the binding of the galaxy components increases (this will be discuss further at the end of the Letter), but 2) the non-Abelian effects on the remaining outside field could balance this if the remaining outside field is strong enough. Since clusters are made mostly of elliptical galaxies for which the approximate sphericity suppresses the non-Abelian effects inside them, we ignore the first effect. We assume furthermore that the intergalactic gas is distributed homogeneously enough so that non-Abelian effects cancel (i.e. the gas does not influence our computation). Finally, we restrict the calculation to the interaction of two galaxies, assuming that others do not affect them. With these three assumptions, we can apply our calculations. Taking 1 Mpc as the distance between the two galaxies and M=40×109 M⊙ as the luminous mass of the two galaxies, we obtain b = −0.012 in lattice units. We express this from the dark matter standpoint by forcing gravity to obey a Newtonian form: V (r) = −GM2(1r-bar) ≡ −GM′21r(4) with M′/M = 1−r2b/a = 251. Gaseous mass in a cluster is typically 7 times larger than the total galaxy mass. Assuming that half of the cluster galaxies are spirals or flat ellipticals for which the non-Abelian effects on the remaining field are neglected, we obtain for the clustera ratio (M′/M)cluster = 18.0, that is our model of cluster is composed of 94% dark mass, to be compared with the observed 80-95%. Non-Abelian effects emerge in asymmetric mass distributions. This makes our mechanism naturally compatible with the Bullet cluster observation [15] (presented as a direct proof of dark matter existence since it is difficult to interpret in terms of modified gravity): Large non-Abelian effects should not be present in the center of the cluster collision where the intergalactic gas of the two clusters resides if the gas is homogeneous and does not show large asymmetric distributions. However, the large non-Abelian effects discussed in the preceding paragraph still accompany the galaxy systems.

In addition to reproducing the rotation curves and cluster dynamics and to explain the Tully-Fisher relation, our approach implies several consequences that can be tested: 1) Since the Non-Abelian distortions of the field are suppressed for spherically homogeneous distributions, rotation curves closer to Newtonian curves should be measured for spherical galaxies; 2) Two spiral galaxies should interact less than a similar system formed by two spherical galaxies. 3) In a two-body system, we expect a roughly linear potential for large enough effective coupling (& 10−3). This may be testable in a sparse galaxy cluster; 4) The past universe being more homogeneous, and density fluctuations being less massive, the non-Abelian effects should disappear at a time when the universe was homogeneous enough; 5) Structure formations would proceed differently than presently thought since dark matter is an ingredient of the current models, and since those assume an Abelian potential. Particularly, models of mergers of galaxies using a linear potential rather than dark matter constitute another test. Although the consequences of non-Abelian effects in gravity for galaxies are not familiar, similar observations (increases of a force’s strength at large distance) are well known in sub-nuclear physics. Those, closely related to the confinement of non-relativistic quarks inside hadrons, are fully explained by the theory of the strong nuclear force (Quantum Chromodynamics, QCD)."

neo said...

the only resevation i have as i've mentioned before is when i click on that link ttps://, the only citations to his paper are self-citations. there are plenty of papers like papers on preons to "mimetic gravity" to entropic mond, that haven't gotten much citations either, even if they sound convincing on their own terms. there are thousands of papers out there with all kinds of proposals.

if Milgrom or Stacy McGaugh or Verlinde or Smolin or dark matter theorists cited that paper, i'd feel a bit more comfortable.

this paper

Generalizing MOND to explain the missing mass in galaxy clusters
Alistair Hodson, Hongsheng Zhao

received 3 citations which are not self-citations

extending mond once again to explain galaxy clusters, i asked stacy mcgaugh about this but he didn't reply.

andrew said...

I don't disagree that Deur's work is not well known and doesn't really have anyone else who has joined his research program, although there are research programs that use similar methods.

This said Deur is a legitimate professional PhD physicist, albeit with his main work in QCD, the arguments appear solid, and there is a fairly substantial body of work on his trick of using a scalar graviton approximation to explore QC concepts that are intractable mathematically with a tensor graviton, which is equivalent to looking a static solutions of the tensor graviton case.

What is really remarkable is that he manages to find solutions to the phenomenological problems, and identify and solve more, with a single gravitational particle/field using the well established technique of analogy from QCD, in a way that is theoretically well grounded and in principle introduces no new physical constants not present in the SM and GR even though in practice it is useful to use a couple of additional constants without deriving them from first principles.

Solving all of the problems of DM, the impossible early galaxy problem, and all or most of dark energy with no new particles except the graviton and a sound well motivated theoretical basis is potentially an Einstein class breakthrough, even if it takes time to be accepted. Milgrom's theory is 30 years old, got no respect for a long time and still doesn't get much in most circles.

The fact that Deur's approach assumes a massless graviton as a mechanism also "automatically" solves a lot of problems not found in non-relativistic MOND, so it isn't exposed to the kind of theoretical landmines that a lot of other theories are.

neo said...

i kinda feel like this is sort of like preons or technicolor, sure there are papers and there are even searches for them in LHC, but not mainstream. string theorists arent' fans of LQG or even LQC.

as an aisde, mitchell porter and urs scheiber trash LQG for using non-standard quanitization. i've asked them what kind of quantization would they prefer on ashketar variables, if LQG is doing quantization of ashketar variables wrong, what's the right way to do it, and they gave some answer that quantization is a very hard thing to do.

what would be the "correct" way to quantize ashketar variables, or some version of gravity that reproduces GR.

i suspect one way to move forward is to create a theory of gravity that uses a dimensionless constant.