26,288 research outputs found
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in the presence of a strongly entrenched standard paradigm.Comment: 25 pages, 9 figures, previous uploaded file was out of date, Canadian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
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 ; 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
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 or the negative pressure of cosmological dark energy (DE)
coincides with the positive pressure 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 . We propose to check whether a smooth
extension of GR with a simple kinetic Lagrangian of can be constructed,
and whether the pressure can bend space-time sufficiently to replace the roles
of a DE, Cold DM and heavy neutrinos in explaining anomalous
accelerations at all scales. As a specific proof of concept we find a
Vector-for- model (-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
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