38,470 research outputs found
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 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in the presence of a strongly entrenched standard paradigm.Comment: 25 pages, 9 figures, previous uploaded file was out of date, Canadian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
'Is there no work in hand?' : the idle son theme at mid-century
This article presents work as the possible site of a generational conflict over the definition of masculinity between parents and children, focusing on the father and son dynamic. It tackles cases of idle sons, or educated young men who cannot establish themselves in any one career, in the middle classes around the middle of the 19th century. It discusses the changing attitudes to what forms masculine work in these classes. It gives emphasis to the case of widowed mother Margaret Oliphant who expressed mixed signals about the significance of work
Bounding quantum gate error rate based on reported average fidelity
Remarkable experimental advances in quantum computing are exemplified by
recent announcements of impressive average gate fidelities exceeding 99.9% for
single-qubit gates and 99% for two-qubit gates. Although these high numbers
engender optimism that fault-tolerant quantum computing is within reach, the
connection of average gate fidelity with fault-tolerance requirements is not
direct. Here we use reported average gate fidelity to determine an upper bound
on the quantum-gate error rate, which is the appropriate metric for assessing
progress towards fault-tolerant quantum computation, and we demonstrate that
this bound is asymptotically tight for general noise. Although this bound is
unlikely to be saturated by experimental noise, we demonstrate using explicit
examples that the bound indicates a realistic deviation between the true error
rate and the reported average fidelity. We introduce the Pauli distance as a
measure of this deviation, and we show that knowledge of the Pauli distance
enables tighter estimates of the error rate of quantum gates.Comment: New Journal of Physics Fast Track Communication. Gold open access
journa
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
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