It's nearly frictionless, so it has no mechanism to cluster significantly around a supermassive black hole.Saul said:1) How does one keep the dark matter from clustering around the super massive object? It appears dark matter works in one direction only. (i.e. It is invoked when required to explain anomalous motion and must cluster to explain the anomalous motion. It does not cluster around super massive black holes.)
It's not just about anomalous motion. Today, a wide body of mutually-corroborating evidence all points towards dark matter.Saul said:2) See thread in astrophysics section "Dark matter on the ropes?" I agree the explanation for the anomalous motion is not MOND.
Chalnoth said:It's nearly frictionless, so it has no mechanism to cluster significantly around a supermassive black hole.
It's not just about anomalous motion. Today, a wide body of mutually-corroborating evidence all points towards dark matter.
The cuspy halo problem arises from cosmological simulations that seem to indicate cold dark matter would form cuspy distributions — that is, increasing sharply to a high value at a central point — in the most dense areas of the universe. This would imply that the center of our galaxy, for example, should exhibit a higher dark-matter density than other areas. However, it seems rather that the centers of these galaxies likely have no cusp in the dark-matter distribution at all.
This remains an intractable problem. Speculation that the distribution of baryonic matter may somehow displace cold dark matter in the dense cores of spiral galaxies has not been substantiated by any plausible explanation or computer simulation.
There are two problems with your presumptions:Saul said:The cuspy halo problem is that dark matter density should increase around mass centers such as super massive black holes. If the super massive object is a classical BH it will continue to grow if dark matter in falls into it.
Chalnoth said:There are two problems with your presumptions:
1. Simulations aren't very good at modeling baryonic matter, and the "cuspy halo problem" is critically dependent upon the baryon matter model.
2. Because we currently don't know how dark matter interacts, simulations ignore any interactions that dark matter does have. For instance, a small annihilation cross section may easily solve the "cuspy halo problem" because dark matter annihilations could prevent high density regions from forming.
It is not reasonable to throw out dark matter when simply implementing expected non-idealities of dark matter may well solve the "cuspy halo problem".
Chronos said:Dark matter survives these issues when it is resistant to accretion by black holes [links already provided]. It strongly suggests dark matter either does not exist [unlikely], or has properties not yet understood. The fact that eddington [and mass] limits are obeyed in all suspected black holes observed to date is not insignificant.
BDM Dark Matter: CDM with a core profile and a free streaming scale
We present a new dark matter model BDM which is an hybrid between hot dark matter HDM and cold dark matter CDM, in which the BDM particles behave as HDM above the energy scale Ec and as CDM below this scale. Evolution of structure formation is similar to that of CDM model but BDM predicts a nonvanishing free streaming _fs scale and a inner galaxy core radius rcore, both quantities determined in terms of a single parameter Ec, which corresponds to the phase transition energy scale of the subjacent elementary particle model. For energies above Ec or for a scale factor a smaller then ac, with a < ac < aeq, the particles are massless and _ redshifts as radiation. However, once the energy becomes E ≤ Ec or a > ac then the BDM particles acquire a large mass through a non perturbative mechanism, as baryons do, and _ redshifts as matter with the particles having a vanishing velocity. Typical energies are Ec = O(10 − 100)eV giving a _fs ∝ E−4/3 c < _Mpc and Mfs ∝ E−4 c < _ 109M⊙. A _fs 6= 0, rcore 6= 0 help to resolve some of the shortcomings of CDM such as overabundance substructure in CDM halos and numerical fit to rotation curves in dwarf spheroidal and LSB galaxies. Finally, our BDM model and the phase transition scale Ec can be derived from particle physics.
The model simply consist of particles that at high energy densities are massless relativistic particles with a velocity of light, v = c, but at low densities they acquire a large mass, due to nonperturbative quantum field effects, and become non relativistic with a vanishing (small) dispersion velocity. We will name this type of dark matter BDM, from bound states dark matter. The name is motivated by the particle physics model, discussed in section III, but we would like to stress out that the cosmological properties of BDM do not depend on this particle model but on the different behavior of the BDM particles. The phase transition energy density is defined pc ≡ E4 c and its value can be determined theoretical by the particle physics model or phenomenological by consistency with the cosmological data.
A large number of candidates have been proposed for DM of which cold dark matter (CDM) has been the most popular. CDM model has been successful on large scales in explaining structure formation in the early universe as well as abundances of galaxy clusters [1]. However, CDM predicts steeply cusped density profiles and causing a large fraction of haloes to survive as substructure inside larger haloes [4, 5]. These characteristics of CDM haloes, however, seem to disagree with a number of observations. The number of sub-haloes around a typical Milky Way galaxy, as identified by satellite galaxies, is an order of magnitude smaller than predicted by CDM [6] and the observed rotation curves for dwarf spheriodal dSph and low surface brightness (LSB) galaxies seem to indicate that their dark matter haloes have constant density cores [7, 8] instead of steep cusps as predicted by the NFW profile. Low surface brightness galaxies are diffuse, low luminosity systems, with a total mass believed to be dominated by their host dark matter halos [9]. Assuming that LSB galaxies are in dynamical equilibrium, the stars act as tracers of the gravitational potential, and can therefore be used as a probe of the dark matter density profile [10]. Much better fits to dSph and LSB observations are found when using a cored halo model [11]. Cored halos have a mass-density that remains at an approximately constant value towards the center.
Because dark matter is weakly-interacting, it is unlikely to have the same matter/anti-matter asymmetry that baryonic matter has (the primordial asymmetry in baryonic matter/anti-matter was very small, but nearly all of it annihilated due to the fast reaction time of baryonic matter).Saul said:The are two problems with what you are proposing. The first is why would dark matter annihilate itself?
The expansion rate dampens the annihilation. Given a particular model, it isn't that difficult to compute the expected abundance of dark matter. Yes, there is a parameter space where it all self-annihilates (a region of parameter space that is therefore excluded). But there is also substantial parameter space where there is self annihilation, but enough of it survives in the early universe for it to be as abundant as we observe today.Saul said:The second is when the universe was formed there would be high density and all the dark matter would annihilate itself.
Chalnoth said:Because dark matter is weakly-interacting, it is unlikely to have the same matter/anti-matter asymmetry that baryonic matter has (the primordial asymmetry in baryonic matter/anti-matter was very small, but nearly all of it annihilated due to the fast reaction time of baryonic matter).
A majority of dark matter models, as a result, predict annihilation to occur (albeit slowly).The expansion rate dampens the annihilation. Given a particular model, it isn't that difficult to compute the expected abundance of dark matter. Yes, there is a parameter space where it all self-annihilates (a region of parameter space that is therefore excluded). But there is also substantial parameter space where there is self annihilation, but enough of it survives in the early universe for it to be as abundant as we observe today.
An upper limit to the central density of dark matter haloes from consistency with the presence of massive central black holes
Since reaching runaway accretion would strongly distort the host dark matter halo, the inferences of QSO black holes in this mass range lead to an upper limit on the central dark matter densities of their host haloes of po (Dark matter density) < 250 solar masses/pc^3. This limit scales inversely with the assumed central black hole mass. However, thinking of dark matter profiles as universal across galactic populations, as cosmological studies imply, we obtain a firm upper limit for the central density of dark matter in such structures.
CMB data constraint on self-annihilation of dark matter particles
Recently, self-annihilation of dark matter particles is proposed to explain the “WMAP Haze” and excess of energetic positrons and electrons in ATIC and PAMELA results. If self-annihilation of dark matter occurs around the recombination of cosmic plasma, energy release from self-annihilation of dark matter delays the recombination, and hence affects CMB anisotropy. By using the recent CMB data, we have investigated the self-annihilation of dark matter particles. In this investigation, we do not found statistically significant evidence, and impose an upper bound on hvi/m. The upcoming data from Planck surveyor and the Fermi Gamma-ray telescope will allow us to break some of parameter degeneracy and improve constraints on self-annihilation of dark matter particles.
By analyzing the recent CMB data, we have constrained the self-annihilation of dark matter particles. We do not find statistically significant evidence on self-annihilation, and impose an upper bound on Fdm < 0.7314 at 95% confidence level. Due to the parameter degeneracy (i.e. Fdm hvi/m) in our analysis, significant self-annihilation is still possible, provided a dark matter particle is very massive (i.e. ≫1GeV). Therefore, a dark matter particle should be quite massive (i.e. m≫1GeV), if the excess of energetic positrons and electrons in PAMELA/ATIC data is attributed to self-annihilation of dark matter particles. Self-annihilation of dark matter particles leads to high level of gamma-ray emission from the region around the Galactic halo. Therefore, Fermi Gamma-ray telescope will allow us to break some of parameter degeneracy and impose independent constraints on self-annihilation of dark matter. Using the upcoming Planck data as well as Fermi Gammaray telescope data, we shall be able to impose important constraints on self-annihilation properties of dark matter particles.
Baryonic matter in a star is basically all normal matter. Obviously it can't annihilate. But due to the weak interactions of dark matter, those annihilations in the early universe would generally have been rather slow, leaving nearly equal parts matter and anti-matter around (if not exactly equal, as the asymmetry that produced the overabundance of normal matter for baryons wouldn't work for dark matter).Saul said:I do not see why dark matter would self annihilate when its density exceeds 250 Solar masses/pc^3. (The point is 250 solar masses/pc^3 is not a high density.) Think of baryonic matter in a star or a planet. Does it self annihilate?
This density is about 1.7*10^9 times the current critical density. Thus the dark matter would have been around this dense at around a redshift of z=2000, when the universe was a mere 133,000 years old. So the annihilation rate only needs to be slow enough that it would have taken longer than a few hundred thousand years or so to annihilate at those densities (possibly less, depending: the primordial abundance could have been very very high).Saul said:However, setting aside the physics question of why dark matter would self annihilate at low densities, the self annihilation hypothesis appears to fail as the density of dark matter in the early universe would be higher than 250 Solar masses/pc^3, therefore all of the dark matter would have self annihilated.
Constraints on Dark Matter Annihilation in Clusters of Galaxies with the Fermi Large Area Telescope
Nearby clusters and groups of galaxies are potentially bright sources of high-energy gamma-ray emission resulting from the pair-annihilation of dark matter particles. However, no significant gamma-ray emission has been detected so far from clusters in the first 11 months of observations with the Fermi Large Area Telescope. We interpret this non-detection in terms of constraints on dark matter particle properties. In particular for leptonic annihilation final states and particle masses greater than 200 GeV, gamma-ray emission from inverse Compton scattering of CMB photons is expected to dominate the dark matter annihilation signal from clusters, and our gamma-ray limits exclude large regions of the parameter space that would give a good fit to the recent anomalous Pamela and Fermi-LAT electron-positron measurements. We also present constraints on the annihilation of more standard dark matter candidates, such as the lightest neutralino of supersymmetric models. The constraints are particularly strong when including the fact that clusters are known to contain substructure at least on galaxy scales, increasing the expected gamma-ray flux by a factor of 5 over a smooth-halo assumption. We also explore the effect of uncertainties in cluster dark matter density profiles, finding a systematic uncertainty in the constraints of roughly a factor of two, but similar overall conclusions. In this work, we focus on deriving limits on dark matter models; a more general consideration of the Fermi-LAT data on clusters and clusters as gamma-ray sources is forthcoming.
Clusters of galaxies are the most massive collapsed objects in the Universe and are very dark matter dominated, making them potentially bright sources of gamma-ray emission from dark matter annihilation. No significant gamma-ray emission has been detected from clusters of galaxies in the first 11 months of Fermi-LAT survey mode observations [10]. In this paper, we explored the implications of the non-detection of clusters by Fermi-LAT in terms of constraints on models of dark matter annihilation. In particular, we focused on the six best candidate clusters and groups of galaxies in the context of searches for gamma-ray emission from dark matter pair annihilation [2] after excluding from the sample clusters which host bright central AGN or lie close to the Galactic plane. We analyzed the Fermi-LAT data to derive upper limits on the gamma-ray flux from dark matter annihilation in specific models, self-consistently incorporating the expected spectral shape for a given particle mass and annihilation final state. We conservatively assume only gamma-ray emission from dark matter annihilation when interpreting the ...
Earth-mass dark-matter haloes as the first structures in the early Universe
The Universe was nearly smooth and homogeneous before a redshift of z = 100, about 20 million years after the Big Bang1. After this epoch, the tiny fluctuations imprinted upon the matter distribution during the initial expansion began to collapse because of gravity. The properties of these fluctuations depend on the unknown nature of dark matter2, 3, 4, the determination of which is one of the biggest challenges in present-day science5, 6, 7. Here we report supercomputer simulations of the concordance cosmological model, which assumes neutralino dark matter (at present the preferred candidate), and find that the first objects to form are numerous Earth-mass dark-matter haloes about as large as the Solar System. They are stable against gravitational disruption, even within the central regions of the Milky Way. We expect over 10^15 to survive within the Galactic halo, with one passing through the Solar System every few thousand years. The nearest structures should be among the brightest sources of γ-rays (from particle–particle annihilation).
Ich said:Huh?
Nothing passes right through a black hole.