It is possible to modify or reinterpret the laws of gravity to reproduce essentially all galactic scale phenomena associated with dark matter, although no widely accepted total gravitational modification solution exists at this point.
Dark matter models, meanwhile, are plagued by the absence of any discovered particles that can serve as exotic dark matter, and as required in many models, by the absence of a mechanism for self-interactions within the dark sector or the creation of dark matter particles. Cold dark matters have even more problems.
The latest issue with dark matter models is their failure to produce two separate classes of dwarf galaxies - one with lots of dark matter and one with almost none, with the latter seeming to predominate, contrary to inferred dark matter halo observations.
The existence of exotic dark matter particles outside the standard model of particle physics constitutes a central hypothesis of the current standard model of cosmology (SMoC). Using a wide range of observational data I outline why this hypothesis cannot be correct for the real Universe. Assuming the SMoC to hold,
(i) the two types of dwarf galaxies, the primordial dwarfs with dark matter and the tidal dwarf galaxies without dark matter, ought to present clear observational differences. But there is no observational evidence for two separate families of dwarfs, neither in terms of their location relative to the baryonic Tully-Fisher relation nor in terms of their radius--mass relation. And, the arrangements in rotating disk-of-satellites, in particular around the Milky Way and Andromeda, has been found to be only consistent with most if not all dwarf satellite galaxies being tidal dwarf galaxies. The highly symmetric structure of the entire Local Group too is inconsistent with its galaxies stemming from a stochastic merger-driven hierarchical buildup over cosmic time.
(ii) Dynamical friction on the expansive and massive dark matter halos is not evident in the data.
Taking the various lines of evidence together, the hypothesis that dynamically relevant exotic dark matter exists needs to be firmly rejected.- Pavel Kroupa, "Lessons from the Local Group (and beyond) on dark matter" (September 22, 2014).
Kroupa argues that both Cold Dark Matter (CDM) and Warm Dark Matter (WDM) are ruled out by this analysis and by elimination argues that a flaw in the equations of general relativity that describe gravity must account for observed dark matter phenomena.
I don't find Kroupa's analysis to be such an obvious and unequivocal rejection of exotic dark matter. But, the analysis does reveal yet another issue with conventional dark matter theories that must be resolved in any correct theory explaining dark matter phenomena.
Dark Matter Behavior and Modeling
Another new paper suggests important flaws in how the dynamics of galaxies have been modeled in the past. Christiane Frigerio Martins, Jose' Ademir Sales Lima, and Pietro Chimenti argue in "Rotation Curves and Nonextensive Statistics" (September 22, 2014) that the prevailing analysis uses a methodology appropriate only for short-range forces that is beyond its domain of applicability in modeling the gravitational interactions of particles and needs to be revised accordingly.
Another study by Bernardo Cervantes Sodi, Cheng Li and Changbom Park entitled "Dark matter halos of barred disk galaxies" (September 23, 2014) refines existing empirical data regarding the inferred shape of dark matter halos in barred disk galaxies based upon new data. The core result is that "in agreement with previous studies, confirms that the bar fraction does not directly depend on the group/cluster environment, but the dependence exists through its dependence on internal morphology."
So does a study of 13 galaxies for which precision measurements are available by Marc S. Seigar, Benjamin L. Davis, Joel Berrier, Daniel Kennefick, in "Constraining dark matter halo profiles and galaxy formation models using spiral arm morphology. II. Dark and stellar mass concentrations for 13 nearby face-on galaxies" (September 23, 2014). Most notably, the abstract of the paper reports that: "we show that there appears to be a trend (albeit a weak correlation) between spiral arm pitch angle and halo concentration. We discuss implications for the suggested link between supermassive black hole (SMBH) mass and dark halo concentration, using pitch angle as a proxy for SMBH mass." These trends are a fit for those suggested by Deur in his graviton self-interaction model (i.e. a relationship between the magnitude of dark matter effects and overall mass and deviation from spherical symmetry).
Meanwhile, Gerasimos Rigopoulos, in "A Relativistic view on large scale N-body simulations" (September 23, 2014), notes that while Newtonian N-body simulations approximate the general relativistic behavior accurately at the first order, that there are material second order non-linear effects in general relativity that are not captured by such simulations making them inappropriate to use, particularly in very large simulations, without modification.
Dark Matter Creation
One of the strongest warm dark matter candidates is suggested by X-ray observations from galaxy clusters.
The recent observation of an X-ray line at an energy of 3.5 keV mainly from galaxy clusters has generated a buzz in the Dark Matter community. If confirmed, this signal could stem from a decaying sterile neutrino of a mass of 7.1 keV. Such a particle could make up all the Dark Matter, but it is not clear how it was produced in the early Universe.
In this paper we show that it is possible to discriminate between different production mechanisms with present-day astronomical data. The most stringent constraint comes from the Lyman-α forest and seems to disfavor all but one of the main production mechanisms proposed in the literature, which is the production via decay of heavy scalar singlets. Pinning down the production mechanism will help to decide whether the X-ray signal indeed comprises an indirect detection of Dark Matter.- Alexander Merle and Aurel Schneider entitled "Production of Sterile Neutrino Dark Matter and the 3.5 keV line" (September 22, 2014).
The surprise ending to this paper is that the only means of production consistent with the Lyman alpha forest produces unusually slow moving (i.e. "cold") dark matter particles that have the same "small scale" problems as cold dark matter. Despite its low mass, particles produced in this fashion would be cold dark matter, rather than the expected warm dark matter, which is disfavored by astronomy observations such as satellite galaxy abundance.
Thus, the bottom line of the paper is really to disfavor the 3.5 keV X-ray line as a true signal of dark matter decay, because that interpretation, taken to its logical conclusion, produces contradictory data.
BICEP-2 Questioned Again
Finally, a new analysis of the BICEP-2 data in light of Planck dust polarization data adds to the chorus of doubt about the validity of the signal that the experiment claims it observed of primordial gravitational tensor waves. The paper is Marina Cortês, Andrew R Liddle, and David Parkinson, "Tensors, BICEP2, prior dependence, and dust" (September 23, 2014). They state:
When foregrounds are accounted for, the BICEP2 results no longer require non-standard inflationary parameter regions. We present limits on primordial AT and nT, adopting foreground scenarios put forward by Mortonson & Seljak and motivated by Planck 353 GHz observations, and assess what dust contribution leaves a detectable cosmological signal. We find that if there is sufficient dust for the signal to be compatible with standard inflation, then the primordial signal is too weak to be robustly detected by BICEP2 if Planck+WMAP upper limits from temperature and E-mode polarization are correct.Planck dust and polarization data has been trickling out over the last few months, and this paper is probably still not the definitive one. But, the prospects for the BICEP2 result holding up don't look particularly good.