I Is the Bullet Cluster Evidence for or Against Dark Matter?

[accdd]

Is the bullet cluster evidence for or against dark matter?
I understand the explanation that it is evidence in favor of the existence of dark matter, and it convinces me. However, some argue that it is evidence against its existence? Why?

 

[Orodruin]

However, some argue that it is evidence against its existence?

You have been here long enough to know that ”some peple say” is not a valid thread opening. Please provide references.

 

[accdd]

I found this:
https://arxiv.org/pdf/1003.0939.pdf

 

[George Jones]

I found this:
https://arxiv.org/pdf/1003.0939.pdf

Many folks here give arXiv pdf links, but I prefer, e.g.,
https://arxiv.org/abs/1003.0939

This give journal sbmission/publication information (which can used to see if the work has been cited), and the pdf is only one click away. In this case,
Astrophysical Journal 718 (2010) 60-65.

 

[Vanadium 50]

Is the bullet cluster evidence for or against dark matter?

Yes.
Both.

 

[alantheastronomer]

Yes.
Both.

Oh well THAT clears things up!

Is the bullet cluster evidence for or against dark matter?
I understand the explanation that it is evidence in favor of the existence of dark matter, and it convinces me. However, some argue that it is evidence against its existence? Why?

The paper you referenced is arguing that the dynamics needed to explain the observed properties of the collision, aren't statistically compatible with a universe with large scale structure determined by dark matter.

They're saying that the probability is too low for the clusters' relative velocity to be as high as 3000 km/s...

 

[ohwilleke]

There is not a consensus on the matter. Here are nine relevant papers. Additional papers are cited at the Wikipedia links for the Bullet Cluster and El Gordo.

Here are five papers discussing the issue in the context of the Bullet Cluster:

ONE

To quantify how rare the bullet-cluster-like high-velocity merging systems are in the standard LCDM cosmology, we use a large-volume 27 (Gpc/h)^3 MICE simulation to calculate the distribution of infall velocities of subclusters around massive main clusters. The infall-velocity distribution is given at (1-3)R_{200} of the main cluster (where R_{200} is similar to the virial radius), and thus it gives the distribution of realistic initial velocities of subclusters just before collision. These velocities can be compared with the initial velocities used by the non-cosmological hydrodynamical simulations of 1E0657-56 in the literature. The latest parameter search carried out recently by Mastropietro and Burkert showed that the initial velocity of 3000 km/s at about 2R_{200} is required to explain the observed shock velocity, X-ray brightness ratio of the main and subcluster, and displacement of the X-ray peaks from the mass peaks. We show that such a high infall velocity at 2R_{200} is incompatible with the prediction of a LCDM model: the probability of finding 3000 km/s in (2-3)R_{200} is between 3.3X10^{-11} and 3.6X10^{-9}. It is concluded that the existence of 1E0657-56 is incompatible with the prediction of a LCDM model, unless a lower infall velocity solution for 1E0657-56 with < 1800 km/s at 2R_{200} is found.

Jounghun Lee, Eiichiro Komatsu, "Bullet Cluster: A Challenge to LCDM Cosmology" (May 22, 2010). published in Astrophysical Journal 718 (2010) 60-65. https://arxiv.org/abs/1003.0939

TWO

The Bullet Cluster has provided some of the best evidence for the Λ cold dark matter (ΛCDM) model via direct empirical proof of the existence of collisionless dark matter, while posing a serious challenge owing to the unusually high inferred pairwise velocities of its progenitor clusters. Here, we investigate the probability of finding such a high-velocity pair in large-volume N-body simulations, particularly focusing on differences between halo-finding algorithms. We find that algorithms that do not account for the kinematics of infalling groups yield vastly different statistics and probabilities. When employing the ROCKSTAR halo finder that considers particle velocities, we find numerous Bullet-like pair candidates that closely match not only the high pairwise velocity, but also the mass, mass ratio, separation distance, and collision angle of the initial conditions that have been shown to produce the Bullet Cluster in non-cosmological hydrodynamic simulations. The probability of finding a high pairwise velocity pair among haloes with Mhalo ≥ 1014 M⊙ is 4.6 × 10−4 using ROCKSTAR, while it is ≈34 × lower using a friends-of-friends (FoF)-based approach as in previous studies. This is because the typical spatial extent of Bullet progenitors is such that FoF tends to group them into a single halo despite clearly distinct kinematics. Further requiring an appropriately high average mass among the two progenitors, we find the comoving number density of potential Bullet-like candidates to be of the order of ≈10−10 Mpc−3. Our findings suggest that ΛCDM straightforwardly produces massive, high relative velocity halo pairs analogous to Bullet Cluster progenitors, and hence the Bullet Cluster does not present a challenge to the ΛCDM model.

Robert Thompson, et al., "The rise and fall of a challenger: the Bullet Cluster in Λ Cold Dark Matter simulations" (June 29, 2015) published at MNRAS.

THREE

We consider the orbit of the bullet cluster 1E 0657-56 in both CDM and 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 (~ 4700km/s) occur naturally in MOND. CDM can generate collision velocities of at most ~ 3800km/s, and is only consistent with the data provided that the shock velocity has been substantially enhanced by hydrodynamical effects.

Garry W. Angus and Stacy S. McGaugh, "The collision velocity of the bullet cluster in conventional and modified dynamics" (September 2, 2007) published at MNRAS.

FOUR

The Bullet Cluster (1E0657-56) is well-known as providing visual evidence of dark matter but it is potentially incompatible with the standard ΛCDM cosmology due to the high relative velocity of the two colliding clusters. Previous studies have focussed on the probability of such a high relative velocity amongst selected candidate systems. This notion of `probability' is however difficult to interpret and can lead to paradoxical results. Instead, we consider the expected number of Bullet-like systems on the sky up to a specified redshift, which allows for direct comparison with observations. Using a Hubble volume N-body simulation with high resolution we investigate how the number of such systems depends on the masses of the halo pairs, their separation, and collisional angle. This enables us to extract an approximate formula for the expected number of halo-halo collisions given specific collisional parameters. We use extreme value statistics to analyse the tail of the pairwise velocity distribution and demonstrate that it is fatter than the previously assumed Gaussian form. We estimate that the number of dark matter halo pairs as or more extreme than 1E0657-56 in mass, separation and relative velocity is 1.3+2.0−0.6 up to redshift z=0.3. However requiring the halos to have collided and passed through each other as is observed decreases this number to only 0.1. The discovery of more such systems would thus indeed present a challenge to the standard cosmology.

David Kraljic, Subir Sarkar, "How rare is the Bullet Cluster (in a ΛCDM universe)?" arXiv:1412.7719 (April 6, 2015) accepted for publication in 4 JCAP 50 (2015).

FIVE

We perform numerical simulations of the merging galaxy cluster 1E 0657-56 (the Bullet Cluster), including the effects of elastic dark matter scattering. In a similar manner to the stripping of gas by ram pressure, dark matter self-interactions would transfer momentum between the two galaxy cluster dark matter haloes, causing them to lag behind the collisionless galaxies. The absence of an observed separation between the dark matter and stellar components in the Bullet Cluster has been used to place upper limits on the cross-section for dark matter scattering. We emphasise the importance of analysing simulations in an observationally-motivated manner, finding that the way in which the positions of the various components are measured can have a larger impact on derived constraints on dark matter's self-interaction cross-section than reasonable changes to the initial conditions for the merger. In particular, we find that the methods used in previous studies to place some of the tightest constraints on this cross-section do not reflect what is done observationally, and overstate the Bullet Cluster's ability to constrain the particle properties of dark matter. We introduce the first simulations of the Bullet Cluster including both self-interacting dark matter and gas. We find that as the gas is stripped it introduces radially-dependent asymmetries into the stellar and dark matter distributions. As the techniques used to determine the positions of the dark matter and galaxies are sensitive to different radial scales, these asymmetries can lead to erroneously measured offsets between dark matter and galaxies even when they are spatially coincident.

Andrew Robertson, Richard Massey, Vincent Eke. "What does the Bullet Cluster tell us about self-interacting dark matter?" arXiv:1605.04307 (May 13, 2016) accepted for publication in MNRAS.

Here are two papers discussing El Gordo, which is a system similar in the important respects for astrophysics purposes to the Bullet Cluster:

SIX

El Gordo poses similar problems for dark matter models. See

The distinctive cometary X-ray morphology of the recently discovered massive galaxy cluster "El Gordo" (ACT-CT J0102−4915; z = 0.87) indicates that an unusually high-speed collision is ongoing between two massive galaxy clusters. A bright X-ray "bullet" leads a "twin-tailed" wake, with the Sunyaev–Zel'dovich (SZ) centroid at the end of the northern tail. We show how the physical properties of this system can be determined using our FLASH-based, N-body/hydrodynamic model, constrained by detailed X-ray, SZ, and Hubble lensing and dynamical data. The X-ray morphology and the location of the two dark matter components and the SZ peak are accurately described by a simple binary collision viewed about 480 million years after the first core passage. We derive an impact parameter of ≃300 kpc, and a relative initial infall velocity of ≃2250 km s−1 when separated by the sum of the two virial radii assuming an initial total mass of 2.15 × 1015 M☉ and a mass ratio of 1.9. Our model demonstrates that tidally stretched gas accounts for the northern X-ray tail along the collision axis between the mass peaks, and that the southern tail lies off axis, comprising compressed and shock heated gas generated as the less massive component plunges through the main cluster. The challenge for ΛCDM will be to find out if this physically extreme event can be plausibly accommodated when combined with the similarly massive, high-infall-velocity case of the Bullet cluster and other such cases being uncovered in new SZ based surveys.

Sandor M. Molnar, Tom Broadhurst. "A Hydrodynamical Solution For The “Twin-Tailed” Colliding Galaxy Cluster “El Gordo”. 800 (1) Astrophysical Journal 37 (2015). DOI: 10.1088/0004-637X/800/1/37

SEVEN

See also:

El Gordo (ACT-CL J0102-4915) is an extremely massive galaxy cluster (M200≈3×1015 M⊙) at redshift z=0.87 composed of two subclusters with mass ratio 3.6 merging at speed Vinfall≈2500 km/s. Such a fast collision between individually rare massive clusters is unexpected in Lambda cold dark matter (ΛCDM) cosmology at such high z. However, this is required for non-cosmological hydrodynamical simulations of the merger to match its observed properties (Zhang et al. 2015). Here, we determine the probability of finding a similar object in a ΛCDM context using the Jubilee simulation box with side length 6h−1 Gpc. We search for galaxy cluster pairs that have turned around from the cosmic expansion with properties similar to El Gordo in terms of total mass, mass ratio, redshift, and collision velocity relative to virial velocity. We fit the distribution of pair total mass quite accurately, with the fits used in two methods to infer the probability of observing El Gordo in the surveyed region. The more conservative (and detailed) method involves considering the expected distribution of pairwise mass and redshift for analogue pairs with similar dimensionless parameters to El Gordo in the past lightcone of a z=0 observer. Detecting one pair with its mass and redshift rules out ΛCDM cosmology at 6.16σ. We also use the results of Kraljic & Sarkar (2015) to show that the Bullet Cluster is in 2.78σ tension once the sky coverage of its discovery survey is accounted for. Using a χ2 approach, the combined tension can be estimated as 6.43σ. Both collisions arise naturally in a Milgromian dynamics (MOND) cosmology with light sterile neutrinos.

E. Asencio, I. Banik, P. Kroupa, "A massive blow for ΛCDM − the high redshift, mass, and collision velocity of the interacting galaxy cluster El Gordo contradicts concordance cosmology" arXiv:2012.03950 (December 7, 2020) published at 500 MNRAS 5249-5267 (2021).

Also, here are two papers illustrating proof of concept that gravitational theories can explain the Bullet Cluster:

EIGHT

A detailed analysis of the November 15, 2006 data release (Clowe et al., 2006) X-ray surface density Sigma-map and the strong and weak gravitational lensing convergence kappa-map for the Bullet Cluster 1E0657-558 is performed and the results are compared with the predictions of a modified gravity (MOG) and dark matter. Our surface density Sigma-model is computed using a King beta-model density, and a mass profile of the main cluster and an isothermal temperature profile are determined by the MOG. We find that the main cluster thermal profile is nearly isothermal. The MOG prediction of the isothermal temperature of the main cluster is T = 15.5 +- 3.9 keV, in good agreement with the experimental value T = 14.8{+2.0}{-1.7} keV. Excellent fits to the two-dimensional convergence kappa-map data are obtained without non-baryonic dark matter, accounting for the 8-sigma spatial offset between the Sigma-map and the kappa-map reported in Clowe et al. (2006). The MOG prediction for the kappa-map results in two baryonic components distributed across the Bullet Cluster 1E0657-558 with averaged mass-fraction of 83% intracluster medium (ICM) gas and 17% galaxies. Conversely, the Newtonian dark matter kappa-model has on average 76% dark matter (neglecting the indeterminant contribution due to the galaxies) and 24% ICM gas for a baryon to dark matter mass-fraction of 0.32, a statistically significant result when compared to the predicted Lambda-CDM cosmological baryon mass-fraction of 0.176{+0.019}{-0.012} (Spergel et al., 2006).

J. R. Brownstein, J. W. Moffat, "The Bullet Cluster 1E0657-558 evidence shows Modified Gravity in the absence of Dark Matter" arXiv:astro-ph/0702146 (September 13, 2007) published in MNRAS.

and

NINE

Our present understanding of the universe requires the existence of dark matter and dark energy. We describe here a natural mechanism that could make exotic dark matter and possibly dark energy unnecessary. Graviton-graviton interactions increase the gravitational binding of matter. This increase, for large massive systems such as galaxies, may be large enough to make exotic dark matter superfluous. Within a weak field approximation we compute the effect on the rotation curves of galaxies and find the correct magnitude and distribution without need for arbitrary parameters or additional exotic particles. The Tully-Fisher relation also emerges naturally from this framework. The computations are further applied to galaxy clusters.

A. Deur, “Implications of Graviton-Graviton Interaction to Dark Matter” (May 6, 2009) published at 676 Phys. Lett. B 21 (2009) (explaining the Bullet Cluster with the self-interaction of GR gravitational fields through an analysis of the geometry of the system).

 

[ohwilleke]

Some of the core issues underlying the disagreements are as follows:

1. Those who argue that the Bullet Cluster (and El Gordo) disfavor DM are arguing from a LambdaCDM type cosmology in which cold dark matter should make such high velocity collisions vanishingly rare, when they aren't vanishingly rare. DM proponents argue, however, that this just means that the galaxy assembly process that they use to model these quantities needs a little tweaking, rather than seeing it as a fundamental flaw in DM theories or even Cold Dark Matter theories. The gravity based MOND theory based cosmologies, however, naturally produce higher expected relative velocities of colliding galaxies. See PAPERS ONE, TWO, THREE, FOUR, SIX and SEVEN in Post #7 in this thread.

The Bullet Cluster (via the Wikipedia link in Post # 7).

El Gordo (via the Wikipedia link in Post # 7).

2. The perception that the Bullet Cluster (and El Gordo) help DM vis-a-vis gravity based explanations of dark matter phenomena is an intuitive reaction to the displacement of DM effects from some masses in theses systems. One study found using gravitational lensing evidence at a statistical significance of 8σ, that there was a spatial offset of the center of the total mass from the center of the baryonic mass peaks. See Clowe, Douglas; et al., "A Direct Empirical Proof of the Existence of Dark Matter". 648 (2) The Astrophysical Journal Letters L109–L113 (2006) arXiv:astro-ph/0608407.

This separation between the center of total mass and the center of baryonic mass is generically unsurprising in DM particle theories, but it is inconsistent with gravitational explanations that share certain properties with Newtonian gravity (basically, it is inconsistent with Abelian gauge theories - compare Sections 5 and 8 at this link, for an introduction to what that means).

But as the proof of concept papers illustrate, this perception is mostly due to a failure of researchers like Clowe, et al., to realize how non-linear behavior in non-Abelian gravitational based explanations can indeed successfully address these issues. See PAPER EIGHT and PAPER NINE in Post #7 in this thread.

3. Self-interacting Dark Matter (SIDM) theories that have a dark matter to dark matter fifth force mediated by a massive but much lighter than W or Z boson force carrying mediator particle has less well developed cosmology models to predict frequencies of relative velocities of galaxies relative than LambdaCDM. Proponents of these theories don't see these colliding clusters as a problem and instead think that they provide useful benchmarks with which to calibrate the parameter space of SIDM theories. See PAPER FIVE in Post #7 in this thread.