Hiding Lorentz Invariance Violation with MOND

Ho\v{r}ava gravity is a attempt to construct a renormalizable theory of gravity by breaking the Lorentz Invariance of the gravitational action at high energies. The underlying principle is that Lorentz Invariance is an approximate symmetry and its violation by gravitational phenomena is somehow hidden to present limits of observational precision. Here I point out that a simple modification of the low energy limit of Ho\v{r}ava gravity in its non-projectable form can effectively camouflage the presence of a preferred frame in regions where the Newtonian gravitational field gradient is higher than $cH_0$; this modification results in the phenomenology of MOND at lower accelerations.Comment: 5 pages, revised field equation, discussion added on potentials and dark energy, in press PR

A historical perspective on Modified Newtonian Dynamics

I review the history and development of Modified Newtonian Dynamics (MOND) beginning with the phenomenological basis as it existed in the early 1980s. I consider Milgrom's papers of 1983 introducing the idea and its consequences for galaxies and galaxy groups, as well as the initial reactions, both negative and positive. The early criticisms were primarily on matters of principle, such as the absence of conservation laws and perceived cosmological problems; an important step in addressing these issues was the development of the Lagrangian-based non-relativistic theory of Bekenstein and Milgrom. This theory led to the development of a tentative relativistic theory that formed the basis for later multi-field theories of gravity. On an empirical level the predictive success of the idea with respect to the phenomenology of galaxies presents considerable challenges for cold dark matter. For MOND the essential challenge remains the absence of a generally accepted theoretical underpinning of the idea and, thus, cosmological predictions. I briefly review recent progress in this direction. Finally I discuss the role and sociology of unconventional ideas in astronomy in the presence of a strongly entrenched standard paradigm.Comment: 25 pages, 9 figures, previous uploaded file was out of date, Canadian Journal of Physic

The Circumnuclear Material in the Galactic Centre: A Clue to the Accretion Process

On the basis of ``sticky particle'' calculations, it is argued that the gas features observed within 10 pc of the Galactic Centre-- the circumnuclear disk (CND) and the ionized gas filaments-- as well as the newly formed stars in the inner one parsec can be understood in terms of tidal capture and disruption of gas clouds on low angular momentum orbits in a potential containing a point mass. The calculations demonstrate that a dissipative component forms a ``dispersion ring'', an asymmetric elliptical torus precessing counter to the direction of rotation, and that this shape can be maintained for many orbital periods. For a range of plausible initial conditions, such a sturcture can explain the morphology and kinematics of the CND and of the most conspicuous ionized filament. While forming the dispersion ring, a small cloud with low specific angular momentum is drawn into a long filament which repeatedly collides with itself at high velocity. The compression in strong shocks is likely to lead to star formation even in the near tidal field of the point mass. This process may have general relevance to accretion onto massive black holes in normal and active galactic nuclei.Comment: 10 pg text, 14 figures, LaTex, mn.sty, accepted MNRA

The prediction of rotation curves in gas-dominated dwarf galaxies with modified dynamics

I consider the observed rotation curves of 12 gas-dominated low-surface-brightness galaxies -- objects in which the mass of gas ranges between 2.2 and 27 times the mass of the stellar disk (mean=9.4). This means that, in the usual decomposition of rotation curves into those resulting from various mass components, the mass-to-light ratio of the luminous stellar disk effectively vanishes as an additional adjustable parameter. It is seen that the observed rotation curves reflect the observed structure in gas surface density distribution often in detail. This fact is difficult to comprehend in the context of the dark matter paradigm where the dark halo completely dominates the gravitational potential in the low surface density systems; however it is expected result in the context of modified Newtonian dynamics (MOND) in which the baryonic matter is the only component. With MOND the calculated rotation curves are effectively parameter-free predictions.Comment: 9 pages, 4 figures, submitte

NGC 2419 does not challenge MOND, Part 2

I argue that, despite repeated claims of Ibata et al., the globular cluster NGC 2419 does not pose a problem for modified Newtonian dynamics (MOND). I present a new polytropic model with a running polytropic index. This model provides an improved representation of the radial distribution of surface brightness while maintaining a reasonable fit to the velocity dispersion profile. Although it may be argued that the differences with these observations remain large compared to the reported random errors, there are several undetectable systematic effects which render a formal likelihood analysis irrelevant. I comment generally upon these effects and upon the intrinsic limitations of pressure supported objects as tests of gravity.Comment: 3 page, 2 figure

A tensor-vector-scalar framework for modified dynamics and cosmic dark matter

I describe a tensor-vector-scalar theory that reconciles the galaxy scale success of modified Newtonian dynamics (MOND) with the cosmological scale evidence for CDM. The theory provides a cosmological basis for MOND in the sense that the predicted phenomenology only arises in a cosmological background. The theory contains an evolving effective potential, and scalar field oscillations in this potential comprise the cold dark matter; the de Broglie wavelength of these soft bosons, however, is sufficiently large that they cannot accumulate in galaxies. The theory predicts, inevitably, a constant anomalous acceleration in the outer solar system which, depending upon the choice of parameters, can be consistent with that detected by the Pioneer spacecrafts.Comment: minor corrections, numerical error corrected in eq. 37 and subsequent equations, accepted MNRA

Coincidences of Dark Energy with Dark Matter -- Clues for a Simple Alternative?

A rare coincidence of scales in standard particle physics is needed to explain why $\Lambda$ or the negative pressure of cosmological dark energy (DE) coincides with the positive pressure $P_0$ of random motion of dark matter (DM) in bright galaxies. Recently Zlosnik et al. (2007) propose to modify the Einsteinian curvature by adding a non-linear pressure from a medium flowing with a four-velocity vector field $U^\mu$. We propose to check whether a smooth extension of GR with a simple kinetic Lagrangian of $U^\mu$ can be constructed, and whether the pressure can bend space-time sufficiently to replace the roles of a $w=-1$ DE, $w=0$ Cold DM and heavy neutrinos in explaining anomalous accelerations at all scales. As a specific proof of concept we find a Vector-for-$\Lambda$ model (${\mathbf V\Lambda}$-model) and its variants. With essentially {\it no free parameters}, these appear broadly consistent with the solar system, gravitational potentials in dwarf spiral galaxies and the bullet cluster of galaxies, early universe with inflation, structure formation and BBN, and late acceleration with a 1:3 ratio of DM:DE.Comment: to appear in ApJ Letters, 4 page

Confrontation of MOND with the rotation curves of early-type disc galaxies

We extend the MOND analysis to a sample of 17 high surface brightness, early-type disc galaxies with rotation curves derived from a combination of 21cm HI line observations and optical spectroscopic data. A number of these galaxies have asymptotic rotation velocities between 250 and 350 km/s making them among the most massive systems (in terms of baryonic mass) considered in the context of MOND. We find that the general MOND prediction for such galaxies -- a rotation curve which gradually declines to the asymptotic value -- is confirmed, and in most cases the MOND rotation curve, determined from the mean radial light and gas distribution, agrees in detail with the observed rotation curve. In the few cases where MOND appears not to work well, the discrepancies can generally be understood in terms of various observational errors -- such as incorrect orientation angles and/or distances -- or of unmodelled physical effects -- such as non-circular motions. The implied mass-to-light ratios for the stellar disc and bulge constrain the MOND interpolating function; the form recently suggested by Zhao & Famaey (2005) yields more sensible values than the one traditionally used in MOND determinations of galaxy rotation curves.Comment: 9 pages, 2 figures, submitted MNRA

Galactic metric, dark radiation, dark pressure and gravitational lensing in brane world models

In the braneworld scenario, the four dimensional effective Einstein equation has extra terms which arise from the embedding of the 3-brane in the bulk. These non-local effects, generated by the free gravitational field of the bulk, may provide an explanation for the dynamics of the neutral hydrogen clouds at large distances from the galactic center, which is usually explained by postulating the existence of the dark matter. We obtain the exact galactic metric, the dark radiation and the dark pressure in the flat rotation curves region in the brane world scenario. Due to the presence of the bulk effects, the flat rotation curves could extend several hundred kpc. The limiting radius for which bulk effects are important is estimated and compared with the numerical values of the truncation parameter of the dark matter halos, obtained from weak lensing observations. There is a relatively good agreement between the predictions of the model and observations. The deflection of photons is also considered and the bending angle of light is computed. The bending angle predicted by the brane world models is much larger than that predicted by standard general relativistic and dark matter models. The angular radii of the Einstein rings are obtained in the small angles approximation. The predictions of the brane world model for the tangential shear are compared with the observational data obtained in the weak lensing of galaxies in the Red-Sequence Cluster Survey. Therefore the study of the light deflection by galaxies and the gravitational lensing could discriminate between the different dynamical laws proposed to model the motion of particles at the galactic level and the standard dark matter models.Comment: 33 pages, 3 figures, accepted for publication in Ap