Quantum gravity and dark matter

We propose a connection between global physics and local galactic dynamics via quantum gravity. The salient features of cold dark matter (CDM) and modified Newtonian dynamics (MOND) are combined into a unified scheme by introducing the concept of MONDian dark matter which behaves like CDM at cluster and cosmological scales but emulates MOND at the galactic scale.     

Necessity of dark matter in modified Newtonian dynamics within galactic scales

To test modified Newtonian dynamics (MOND) on galactic scales, we study six strong gravitational lensing early-type galaxies from the CASTLES sample. Comparing the total mass (from lensing) with the stellar mass content (from a comparison of photometry and stellar population synthesis), we conclude that strong gravitational lensing on galactic scales requires a significant amount of dark matter, even within MOND. On such scales a 2 eV neutrino cannot explain the excess of matter in contrast with recent claims to explain the lensing data of the bullet cluster. The presence of dark matter is detected in regions with a higher acceleration than the characteristic MOND scale of approximately 10(-10) m/s(2). This is a serious challenge to MOND unless lensing is qualitatively different [possibly to be developed within a covariant, such as Tensor-Vector-Scalar (TeVeS), theory].     

The dark-baryonic matter mass relation for observational verification in Verlinde’s emergent gravity

Recently, a new interesting idea of origin of gravity has been developed by Verlinde. In this scheme of emergent gravity, where horizon entropy, microscopic de Sitter states and relevant contribution to gravity are involved, an entropy displacement resulting from matter behaves as a memory effect and can be exhibited at sub-Hubble scales, namely, the entropy displacement and its “elastic” response would lead to emergent gravity, which gives rise to an extra gravitational force. Then galactic dark matter effects may origin from such extra emergent gravity. We discuss some concepts in Verlinde’s theory of emergent gravity and point out some possible problems or issues, e.g., the gravitational potential caused by Verlinde’s emergent apparent dark matter may no longer be continuous in spatial distribution at ordinary matter boundary (such as a massive sphere surface). In order to avoid the unnatural discontinuity of the extra emergent gravity of Verlinde’s apparent dark matter, we suggest a modified dark-baryonic mass relation (a formula relating Verlinde’s apparent dark matter mass to ordinary baryonic matter mass) within this framework of emergent gravity. The modified mass relation is consistent with Verlinde’s result at relatively small scales (e.g., \(R<3h_{70}^{-1}\) Mpc). However, it seems that, compared with Verlinde’s relation, at large scales (e.g., gravitating systems with \(R>3h_{70}^{-1}\) Mpc), the modified dark-baryonic mass relation presented here might be in better agreement with the experimental curves of weak lensing analysis in the recent work of Brouwer et al. Galactic rotation curves are compared between Verlinde’s emergent gravity and McGaugh’s recent model of MOND (Modified Newtonian Dynamics established based on recent galaxy observations). It can be found that Verlinde rotational curves deviate far from those of McGaugh MOND model when the MOND effect (or emergent dark matter) dominates. Some applications of the modified dark-baryonic mass relation inspired by Verlinde’s emergent gravity will be addressed for galactic and solar scales. Potential possibilities to test this dark-baryonic mass relation as well as apparent dark matter effects, e.g., planetary perihelion precession at Solar System scale, will be considered. This may enable to place some constraints on the magnitudes of the MOND characteristic acceleration at the small solar scale.     

Recovering MOND from extended metric theories of gravity

We show that the Modified Newtonian Dynamics (MOND) regime can be fully recovered as the weak-field limit of a particular theory of gravity formulated in the metric approach. This is possible when Milgrom’s acceleration constant is taken as a fundamental quantity which couples to the theory in a very consistent manner. As a consequence, the scale invariance of the gravitational interaction is naturally broken. In this sense, Newtonian gravity is the weak-field limit of general relativity and MOND is the weak-field limit of that particular extended theory of gravity. We also prove that a Noether’s symmetry approach to the problem yields a conserved quantity coherent with this relativistic MONDian extension.     

Modified Newton's gravity in Finsler space as a possible alternative to dark matter hypothesis

A modified Newton's gravity is obtained as the weak field approximation of the Einstein's equation in Finsler space. It is found that a specified Finsler structure makes the modified Newton's gravity equivalent to the modified Newtonian dynamics (MOND). In the framework of Finsler geometry, the flat rotation curves of spiral galaxies can be deduced naturally without invoking dark matter.     

Perfect fluid dark matter

Taking the flat rotation curve as input and treating the matter content in the galactic halo region as perfect fluid we obtain a space–time metric at the galactic halo region in the framework of general relativity. We find that the resultant space–time metric is a non-relativistic dark matter induced space–time embedded in a static Friedmann–Lemaître–Robertson–Walker universe i.e. the flat rotation curve not only leads to the existence of dark matter but also suggests about the background geometry of the universe. Within its range of validity the flat rotation curve and the demand that the dark matter to be non-exotic together indicate for a (nearly) flat universe as favored by the modern cosmological observations. We obtain the expressions for energy density and pressure of dark matter there and consequently the equation of state of dark matter. Various other aspects of the solutions are also analyzed.     

A New Proposal for Galactic Dark Matter: Effect of f(T) Gravity

It is still a challenging problem to the theoretical physicists to know the exact nature of the galactic dark matter which causes the galactic rotational velocity to be more or less a constant. We have proposed that the dark matter as an effect of f(T) gravity. Assuming the flat rotation curves as input we have shown that f(T) gravity can explain galactic dynamics. Here, we don’t have to introduce dark matter. Spacetime metric inspired by f(T) gravity describes the region up to which the tangential velocity of the test particle is constant. This inherent property appears to be enough to produce stable circular orbits as well as attractive gravity.     

A Specific Case of Generalized Einstein-aether Theories

With the dark energy phenomena explored over a decade,in this present work we discuss a specific case of the generalized Einstein-aether theories,in which the modified Friedmann equation is similar to that in the Dvali-Gabadadze-Porrati(DGP) brane world model.We compute the joint statistic constraints on model parameters in this specific case by using the recent type Ia supernovae(SNe Ia) data,the cosmic microwave background(CMB) shift parameter data,and the baryonic acoustic oscillations(BAOs) data traced by the Sloan Digital Sky Survey(SDSS).Furthermore,we analyze other constrains from the observational Hubble parameter data(OHD).The comparison with the standard cosmological model(cosmological constant Λ cold dark matter(ΛCDM) model) is clearly shown;also we comment on the interesting relation between the coupling constant M in this model and the special accelerate scale in the modified Newtonian dynamics(MOND) model initially given by Milgrom with the hope for interpreting the galaxy rotation curves without introducing mysterious dark matter.     

Neutralino properties from a possible modulation signal in WIMP direct search

We report the main results of a paper by A. Bottino, F. Donato, N. Fornengo and S. Scopel, where the properties of neutralino are analysed under the hypothesis that some preliminary experimental results of the DAMA/NaI Collaboration may be indicative of a yearly modulation effect. The relevant supersymmetric configurations are analysed and the possibility of their investigation by indirect search for relic neutralinos and by accelerator measurements are discussed. It is shown that some of the configurations singled out by the DAMA/NaI results would have cosmological properties compatible with a neutralino as a dominant component of cold dark matter (on the average in the Universe and in our galactic halo).     

On the gravitodynamics of moving bodies

In the present work we propose a generalization of Newton’s gravitational theory from the original works of Heaviside and Sciama, that takes into account both approaches, and accomplishes the same result in a simpler way than the standard cosmological approach. The established formulation describes the local gravitational field related to the observables and effectively implements the Mach’s principle in a quantitative form that retakes Dirac’s large number hypothesis. As a consequence of the equivalence principle and the application of this formulation to the observable universe, we obtain, as an immediate result, a value of Ω = 2. We construct a dynamic model for a galaxy without dark matter, which fits well with recent observational data, in terms of a variable effective inertial mass that reflects the present dynamic state of the universe and that replicates from first principles, the phenomenology proposed in MOND. The remarkable aspect of these results is the connection of the effect dubbed dark matter with the dark energy field, which makes it possible for us to interpret it as longitudinal gravitational waves.     

Comparing dark matter models,modified Newtonian dynamics and modified gravity in accounting for galaxy rotation curves

We compare six models(including the baryonic model,two dark matter models,two modified Newtonian dynamics models and one modified gravity model) in accounting for galaxy rotation curves.For the dark matter models,we assume NFW profile and core-modified profile for the dark halo,respectively.For the modified Newtonian dynamics models,we discuss Milgrom's MOND theory with two different interpolation functions,the standard and the simple interpolation functions.For the modified gravity,we focus on Moffat's MSTG theory.We fit these models to the observed rotation curves of 9 high-surface brightness and 9 low-surface brightness galaxies.We apply the Bayesian Information Criterion and the Akaike Information Criterion to test the goodness-of-fit of each model.It is found that none of the six models can fit all the galaxy rotation curves well.Two galaxies can be best fitted by the baryonic model without involving nonluminous dark matter.MOND can fit the largest number of galaxies,and only one galaxy can be best fitted by the MSTG model.Core-modified model fits about half the LSB galaxies well,but no HSB galaxies,while the NFW model fits only a small fraction of HSB galaxies but no LSB galaxies.This may imply that the oversimplified NFW and core-modified profiles cannot model the postulated dark matter haloes well.     

Mirror matter, mirror gravity and galactic rotational curves

We discuss astrophysical implications of the modified gravity model in which the two matter components, ordinary and dark, couple to separate gravitational fields that mix to each other through small mass terms. There are two spin-2 eigenstates: the massless graviton, which induces universal Newtonian attraction, and the massive one, which gives rise to the Yukawa-like potential which is repulsive between the ordinary and dark bodies. As a result for distances much smaller than the Yukawa radius r _m the gravitation strength between the two types of matter becomes vanishing. If r _m ～10 kpc, the typical size of a galaxy, there are interesting implications for the nature of dark matter. In particular, one can avoid the problem of the cusp that is typical for the cold dark matter halos. Interestingly, the flat shape of the rotational curves can be explained even in the case of the collisional and dissipative dark matter (as e.g. mirror matter), which cannot give the extended halos but instead must form galactic discs similarly to the visible matter. The observed rotational curves for the large, medium-size and dwarf galaxies can be nicely reproduced. We also briefly discuss possible implications for the direct search of dark matter.     

Non-thermal dark matter,high energy cosmic rays and late-decaying particles from inflationary quantum fluctuations

It has been suggested that the origin of cosmic rays above the GZK limit might be explained by the decay of particles, X, with mass of the order of 10¹² GeV. Generation of heavy particles from inflationary quantum fluctuations is a prime candidate for the origin of the decaying X particles. It has also been suggested that the problem of non-singular galactic halos might be explained if dark matter originates non-thermally from the decay of particles, Y, such that there is a free-streaming length of the order of 0.1 Mpc. Here we explore the possibility that quantum fluctuations might account for the Y particles as well as the X particles. For the case of non-thermal WIMP dark matter with unsuppressed weak interactions we find that there is a general problem with deuterium photo-dissociation, disfavouring WIMP dark matter candidates. For the case of more general dark matter particles, which may have little or no interaction with conventional matter, we discuss the conditions under which X and Y scalars or fermions can account for non-thermal dark matter and cosmic rays. For the case where X and Y scalars are simultaneously produced, we show that galactic halos are likely to have a dynamically significant component of X scalar cold dark matter in addition to the dominant non-thermal dark matter component.     

Dark matter spikes and annihilation radiation from the galactic center

The annihilation rate of weakly interacting cold dark matter particles at the galactic center could be greatly enhanced by the growth of a density spike around the central supermassive black hole (SBH). Here we discuss the effects of hierarchical mergers on the central spike. Mergers between halos containing SBHs lead to the formation of SBH binaries which transfer energy to the dark matter particles, lowering their density. The predicted flux of annihilation photons from the galactic center is several orders of magnitude smaller than in models that ignore the effects of SBHs and mergers. Measurement of the annihilation radiation could in principle be used to constrain the merger history of the galaxy.     

Recent developments in dark matter searches

A brief review is first given of the forms of dark matter that are hypothesized, and a summary of the basic observational evidence for dark matter is provided. Then a summary of recent results from indirect and direct detection dark matter search experiments is given. Some discussion is also done of MOND theories along with recent analysis of galaxy surface density data that provides some support for such theories.     

Squeezing MOND into a Cosmological Scenario

Explaining the effects of dark matter using modified gravitational dynamics (MOND) has for decades been both an intriguing and controversial possibility. By insisting that the gravitational interaction that accounts for the Newtonian force also drives cosmic expansion, one may kinematically identify which cosmologies are compatible with MOND, without explicit reference to the underlying theory so long as the theory obeys Birkhoff's law. We find that the critical acceleration a(0) must have a slight source-mass dependence (a(0) approximately M(1/3)) and that MOND cosmologies are naturally compatible with observed late-time expansion history. However, cosmologies that can produce enough density perturbations to account for structure formation are contrived and fine tuned. Even then, they may be marginally ruled out by evidence of early (z approximately 20) reionization.     

Gamma ray line constraints on effective theories of dark matter

A monochromatic gamma ray line results when dark matter particles in the galactic halo annihilate to produce a two body final state which includes a photon. Such a signal is very distinctive from astrophysical backgrounds, and thus represents an incisive probe of theories of dark matter. We compare the recent null results of searches for gamma ray lines in the galactic center and other regions of the sky with the predictions of effective theories describing the interactions of dark matter particles with the Standard Model. We find that the null results of these searches provide constraints on the nature of dark matter interactions with ordinary matter which are complementary to constraints from other observables, and stronger than collider constraints in some cases.     

A useful guide for gravitational wave observers to test modified gravity models

We present an extension of a previously suggested test of all modified theories of gravity that would reproduce MOND at low accelerations. In a class of models, called “dark matter emulators”, gravitational waves and other particles couple to different metrics. This leads to a detectable time lag between their detection at Earth from the same source. We calculate this time lag numerically for any event that occurs in our galaxy up to 400 kpc, and present a graph of this possible time lag. This suggests that, gravitational wave observers might have to consider the possibility of extending their analysis to non-coincident gravitational and electromagnetic signals, and the graph that we present might be a useful guideline for this effort.     

Do magnetic fields prevent mirror particles from entering the galactic disk?

Recently it has been suggested that magnetic fields prevent mirror particles from entering the galactic disk, thereby disfavouring the mirror dark matter explanation of the dark matter direct detection experiments. We show that mirror particle self interactions will typically randomize the directions of heavy mirror particles on length scales much shorter than their gyroradius. This means that heavy mirror particles are free to enter the galactic disk and consequently mirror dark matter remains consistent with all experiments and observations.