Tuesday, September 20, 2011

Dark Energy, Black Holes, Dark Matter, Neutrinos and More

* Dark energy data are still consistent with a cosmological constant and a topologically flat universe.

* In galaxies, "black hole mass is not directly related to the mass of the dark matter halo but rather seems to be determined by the formation of the galaxy bulge."

* Black holes don't get bigger than about 10 billion times the mass of the sun. "One possible explanation . . . is that the black holes eventually reach the point when they radiate so much energy as they consume their surroundings that they end up interfering with the very gas supply that feeds them, which may interrupt nearby star formation. The new findings have implications for the future study of galaxy formation, since many of the largest galaxies in the Universe appear to co-evolve along with the black holes at their centres. . . . 'Evidence has been mounting for the key role that black holes play in the process of galaxy formation . . . But it now appears that they are likely the prima donnas of this space opera.'"

* Excessively high dark matter halo densities near black holes lead to results inconsistent with experiment, constraining the range of possible dark matter distributions and properties.

* "[G]alaxies are more clustered into groups than previously believed. The amount of galaxy clustering depends on the amount of dark matter." It takes about 300 billion suns of dark matter to form a single star-forming galaxy according to a survey of galaxies and cosmic background radiation patterns published in February of 2011.

* Newtonian approximations of astronomy scale many body problems aren't as horrible as one might suspect compared to a pure general relativity approach according to this analysis. It isn't entirely clear to me that its assumption that almost everything is moving at speeds much lower than the speed of light is accurate, or that this analysis properly accounts for angular momentum effects related to general relativity in galaxy scale systems.

* Experimental data rule out much of the dark matter parameter space.

* Warm dark matter models fit reality better than cold dark matter models:

Warm Dark Matter (WDM) research is progressing fast, the subject is new and WDM essentially works, naturally reproducing the astronomical observations over all scales: small (galactic) and large (cosmological) scales (LambdaWDM). Evidence that Cold Dark Matter (LambdaCDM) and its proposed tailored cures do not work at small scales is staggering. . . LambdaWDM simulations with keV particles remarkably reproduce the observations, small and large structures and velocity functions. Cored DM halos and WDM are clearly determined from theory and astronomical observations, they naturally produce the observed structures at all scales. keV sterile neutrinos are the leading candidates, they naturally appear extensions of the standard model of particle physics. Astrophysical constraints including Lyman alpha bounds put its mass in the range 1 < m < 13 keV.

For comparison sake, an electron has a mass of about 511 keV.  So, a keV scale sterile neutrino would be not more than about 3% of the mass of an electron, and possibly close to 0.2% of the mass of an electron, but would have a mass on the order of 1,000 to 50,000 times that of an ordinary electron neutrino, or more.  Warm dark matter candidates have much greater velocities than cold dark matter candidates.

[T]here are many signs that the simple picture of CDM in galaxies is not working. The most troubling signs of the failure of the CDM paradigm have to do with the tight coupling between baryonic matter and the dynamical signatures of DM in galaxies, e.g. the Tully-Fisher relation, the stellar disc-halo conspiracy, the maximaum disc phenomenon, the MOdified Newtonian Dynamics (MOND) phenomenon, the baryonic Tully-Fisher relation, the baryonic mass discrepancy-acceleration relation, the 1-parameter dimensionality of galaxies, and the presence of both a DM and a baryonic mean surface density.

The strangest of these relations is the “Bosma effect”: the centripetal contribution of the dynamically insigificant interstellar medium (ISM) in spiral galaxies is directly proportional to that of the dominant DM. The constant of proportionality has been determined for about 100 galaxies, with dwarf galaxies showing a smaller and late-type spirals showing a larger factor.

Hoekstra, van Albada & Sancisi set out to test the Bosma effect, showed that it indeed allowed a very detailed fit to the rotation curves of many well-studied galaxies, but concluded that it was not real. Reviewing their arguments, it is clear that their negative judgement was very conservative - by the normal standards of rotation curves, the results were, in fact, very convincing. Since their fits were performed by hand and not compared with the corresponding CDM model fits, it was not possible to make any formal conclusions and certainly not possible to reject the effect as non-physical.

Using the much better data made available by the Spitzer Infrared Nearby Galaxy Survey, The HI Nearby Galaxy Survey, and the analyses by de Block et al., we have tested the Bosma effect and compared the results against standard CDM models. The use of infrared photometry and colours permits formal fits to the stellar components nearly independent of extinction corrections and with reasonably reliable mass-to-light estimates. In addition to the standard bulge, stellar disk, and visible HI components, we fitted the rotation curves with the addition of either one or two Bosma components, using either the HI disc (so-called “simple Bosma” models) or both the stellar and the HI discs (“classic Bosma” models) as proxies, where the stellar disc is used as a proxy for the molecular gas obviously present in regions of previous and current star formation. For comparison, we also fit the data with self-consitent NFW models, where the compactness of the halo is a function of the halo mass or with the Burkert halo mass profiles used in the “Universal Rotation Curve” model. The “simple” Bosma models using only the HI as a proxy are remarkably good in the outer discs, as shown by Bosma, independent of the shape of the rotation curve. However, the inner disks are not well-fit by pure “HI-scaling”: the saturation of the HI surface densities above levels around 10M⊙/pc2 results in centripetal contributions which are clearly too small. This problem is not present in “classic” Bosma models, since the saturation of the HI profiles occurs exactly where the stellar component starts to dominate, permitting a perfect compensation.

Furthermore, hot dark matter and cold dark matter produce results contrary to the observed large scale structure of the universe (same source):

WDM refers to keV scale DM particles. This is not Hot DM (HDM). (HDM refers to eV scale DM particles, which are already ruled out). CDM refers to heavy DM particles (so called wimps of GeV scale or any scale larger than keV).

It should be recalled that the connection between small scale structure features and the mass of the DM particle follows mainly from the value of the free-streaming length lfs. Structures smaller than lfs are erased by free-streaming. WDM particles with mass in the keV scale produce lfs ∼ 100 kpc while 100 GeV CDM particles produce an extremely small lfs ∼ 0.1 pc. While the keV WDM lfs ∼ 100 kpc is in nice agreement with the astronomical observations, the GeV CDM lfs is a million times smaller and produces the existence of too many small scale structures till distances of the size of the Oort’s cloud in the solar system. No structures of such type have ever been observed. Also, the name CDM precisely refers to simulations with heavy DM particles in the GeV scale. Most of the literature on CDM simulations do not make explicit the relevant ingredient which is the mass of the DM particle (GeV scale wimps in the CDM case).

The mass of the DM particle with the free-streaming length naturally enters in the initial power spectrum used in the N-body simulations and in the initial velocity. The power spectrum for large scales beyond 100 kpc is identical for WDM and CDM particles, while the WDM spectrum is naturally cut off at scales below 100 kpc, corresponding to the keV particle mass free-streaming length. In contrast, the CDM spectrum smoothly continues for smaller and smaller scales till ∼ 0.1 pc, which gives rise to the overabundance of predicted CDM structures at such scales.

CDM particles are always non-relativistic, the initial velocities are taken zero in CDM simulations, (and phase space density is unrealistically infinity in CDM simulations), while all this is not so for WDM.

Since keV scale DM particles are non relativistic for z < 10^6 they could also deserve the name of cold dark matter, although for historical reasons the name WDM is used. Overall, seen in perspective today, the reasons why CDM does not work are simple: the heavy wimps are excessively non-relativistic (too heavy, too cold, too slow), and thus frozen, which preclude them to erase the structures below the kpc scale, while the eV particles (HDM) are excessively relativistic, too light and fast, (its free streaming length is too large), which erase all structures below the Mpc scale; in between, WDM keV particles produce the right answer.

The big picture evidence for warm dark matter as opposed to cold dark matter also suggests that the recent experimental signs of 10 GeV dark matter (contradicted by other experiments that should have validity in that mass range) are probably wrong. But, plain old electron neutrinos are too light (comparable to hot dark matter models), and the tau neutrinos that would seem to be closest to the right mass range aren't believed to be very stable.

* It may be possible to detect traces of a sterile neurtino warm dark matter candidate in beta decays of radioactive elements like Rhenium 187 and Tritium.

* The cosmic microwave background data appear to be inconsistent with a cold dark matter model.

* Some decent empirical fits to observed dark matter profiles are obtained with the Einasto halo model (which has two continous and one low whole number parameters): "This is not in agreement with the predictions from ΛCDM simulations."

* New data disfavor previous results that suggests that there were five generations of neutrinos but can fit a 3+1 generation neutrino model with a fourth generation sterile neutrino as heavy as 5.6 eV (which is still lighter than one might hope to fit warm dark matter models).

* An electron neutrino mass estimate of 0.25-1 eV is obtained based upon consideration of the observed diffraction behavior of neutrinos. This approach "resolves anomalies of LSND and two neutrino experiment."

* A four generation extension of the Standard Model (SM4) with two Higgs doublets can produce a sterile neutrino that could account for 1% of cold dark matter.

* A seesaw mass model for neutrinos is explored in a noncommunitive geometric context.

* Distinctions between what a PMNS matrix for neutrinos should look like in the cases of Dirac and Majorana masses are compared.

* Dynamical mass generation in gluons and its link to chiral symmetry breaking is explored.

* An effort to compute fundamental particle masses from a preon model is set forth.

* An ad hoc effort to look at gravitational effects on gauge couplings suggests that there is "a nontrivial gravitational contribution to the gauge coupling constant with an asymptotic free power-law running." Thus, one reason for the "running coupling constants" of electroweak theory and QCD may be quantum gravity effects. Coupling constants could also be caused by a fractal structure of spacetime.

* Quantum entanglement could be disfavors the existence of a "long range force" that operates on scales greater than the strong and weak forces but not greater than the micrometer scale.


Andrew Oh-Willeke said...

The latest finds are consistent with the 10 billion sun mass limit on black hole size.

Andrew Oh-Willeke said...

There are only two proposed examples of intermediate mass black holes with masses between 30 and 300,000 times the mass of the sun. The first, with a mass of about 1,300 times that of the sun was found in 2004. A second possibility, identified in 2006, is a disputed conclusion. All other black holes observed are either stellar black holes formed from the collapse of a single star, or supermassive black holes at the center of galaxies.