1,550 research outputs found
Cold Dark Matter with MOND Scaling
We provide a holographic dual description of Milgrom's scaling associated
with galactic rotation curves. Our argument is based on the recent entropic
reinterpretation of Newton's laws of motion. We propose a duality between cold
dark matter and modified Newtonian dynamics (MOND). We introduce the concept of
MONDian dark matter, and discuss some of its phenomenological implications. At
cluster as well as cosmological scales, the MONDian dark matter would behave as
cold dark matter, but at the galactic scale, the MONDian dark matter would act
as MOND.Comment: 5 pages, v3: version to appear in PLB with the title changed and more
discussion
Gravitational anomalies signaling the breakdown of classical gravity
Recent observations for three types of astrophysical systems severely
challenge the GR plus dark matter scenario, showing a phenomenology which is
what modified gravity theories predict. Stellar kinematics in the outskirts of
globular clusters show the appearance of MOND type dynamics on crossing the
threshold. Analysis shows a ``Tully-Fisher'' relation in these systems,
a scaling of dispersion velocities with the fourth root of their masses.
Secondly, an anomaly has been found at the unexpected scales of wide binaries
in the solar neighbourhood. Binary orbital velocities cease to fall along
Keplerian expectations, and settle at a constant value, exactly on crossing the
threshold. Finally, the inferred infall velocity of the bullet cluster
is inconsistent with the standard cosmological scenario, where much smaller
limit encounter velocities appear. This stems from the escape velocity limit
present in standard gravity; the ``bullet'' should not hit the ``target'' at
more than the escape velocity of the joint system, as it very clearly did.
These results are consistent with extended gravity, but would require rather
contrived explanations under GR, each. Thus, observations now put us in a
situation where modifications to gravity at low acceleration scales cease to be
a matter of choice, to now become inevitable.Comment: 10 pages, 5 figures, Astrophysics and Space Science Proceedings 38,
4
Remarks on the properties of elliptical galaxies in modified Newtonian dynamics
Two incorrect arguments against MOND in elliptical galaxies could be that the
equivalent circular velocity curves tend to become flat at much larger
accelerations than in spiral galaxies, and that the Newtonian dark matter halos
are more concentrated than in spirals. Here, we compare published scaling
relations for the dark halos of elliptical galaxies to the scaling relations
expected for MONDian phantom halos. We represent the baryonic content of
galaxies by spherical profiles, and their corresponding MONDian phantom halos
by logarithmic halos. We then derive the surface densities, central densities,
and phase space densities and compare them with published scaling relations. We
conclude that it is possible to get flat circular velocity curves at high
acceleration in MOND, and that this happens for baryonic distributions
described by Jaffe profiles in the region where the circular velocity curve is
flat. Moreover, the scaling relations of dark halos of ellipticals are
remarkably similar to the scaling relations of phantom halos of MOND.Comment: Accepted for publication in A and
Gravitational waves and lensing of the metric theory proposed by Sobouti
We investigate in detail two physical properties of the metric f(R) theory
developed by Sobouti (2007). We first look for the possibility of producing
gravitational waves that travel at the speed of light. We then check the
possibility of producing extra bending in the lenses produced by the theory. We
do this by using standard weak field approximations to the gravitational field
equations that appear in Sobouti's theory. We show in this article that the
metric theory of gravitation proposed by Sobouti (2007) predicts the existence
of gravitational waves travelling at the speed of light in vacuum. In fact,
this is proved in general terms for all metric theories of gravity which can be
expressed as powers of Ricci's scalar. We also show that an extra additional
lensing as compared to the one predicted by standard general relativity is
produced. These two points are generally considered to be of crucial importance
in the development of relativistic theories of gravity that could provide an
alternative description to the dark matter paradigm.Comment: 10 pages, 2 figures. Added a comment on the recent article by Saffari
(arXiv:0704.3345v1) and small typos as well as general comments in the
introuduction and conclusio
Spiral galaxy rotation curves determined from Carmelian general relativity
Equations of motion, in cylindrical co-ordinates, for the observed rotation
of gases within the gravitational potential of spiral galaxies have been
derived from Carmeli's Cosmological General Relativity theory. A Tully-Fisher
type relation results and rotation curves are reproduced without the need for
non-baryonic halo dark matter. Two acceleration regimes are discovered that are
separated by a critical accleration .
For accelerations larger than the critical value the Newtonian force law
applies, but for accelerations less than the critical value the Carmelian
regime applies. In the Newtonian regime the accelerations fall off as ,
but in the Carmelian regime the accelerations fall off as . This is new
physics but is exactly what is suggested by Migrom's phenomenological MOND
theory.Comment: 9 pages, 10 figures, Int. J. Theor. Phys, corrected typo
The dark matter halo of NGC 1399 - CDM or MOND?
Central galaxies in galaxy clusters may be key discriminants in the
competition between the cold dark matter (CDM) paradigm and modified Newtonian
dynamics (MOND). We investigate the dark halo of NGC 1399, the central galaxy
of the Fornax cluster, out to a galactocentric distance of 80 kpc. The data
base consists of 656 radial velocities of globular clusters obtained with
MXU/VLT and GMOS/Gemini, which is the largest sample so far for any galaxy. We
performed a Jeans analysis for a non-rotating isotropic model. An NFW halo with
the parameters r_s = 50 kpc and rho_s = 0.0065 M_sun/pc^3 provides a good
description of our data, fitting well to the X-ray mass. More massive halos are
also permitted that agree with the mass of the Fornax cluster as derived from
galaxy velocities. We compare this halo with the expected MOND models under
isotropy and find that additional dark matter on the order of the stellar mass
is needed to get agreement. A fully radial infinite globular cluster system
would be needed to change this conclusion. Regarding CDM, we cannot draw firm
conclusions. To really constrain a cluster wide halo, more data covering a
larger radius are necessary. The MOND result appears as a small-scale variant
of the finding that MOND in galaxy clusters still needs dark matter.Comment: 4 pages, 2 figures, accepted for publication as a Letter in A&
The Evolutionary Status of SS433
We consider possible evolutionary models for SS 433. We assume that
common-envelope evolution is avoided if radiation pressure is able to expel
most of a super-Eddington accretion flow from a region smaller than the
accretor's Roche lobe. This condition is satisfied, at least initially, for
largely radiative donors with masses in the range 4-12 solar masses. For donors
more massive than about 5 solar masses, moderate mass ratios q = M_2/M_1 > 1
are indicated, thus tending to favor black-hole accretors. For lower mass
donors, evolutionary considerations do not distinguish between a neutron star
or black hole accretor. In all cases the mass transfer (and mass loss) rates
are much larger than the likely mass-loss rate in the precessing jets. Almost
all of the transferred mass is expelled at radii considerably larger than the
jet acceleration region, producing the "stationary" H-alpha line, the infrared
luminosity, and accounting for the low X-ray luminosity.Comment: 13 pages, Astrophysical Journal Letters, accepte
Wide binaries as a critical test of Classical Gravity
Modified gravity scenarios where a change of regime appears at acceleration
scales have been proposed. Since for systems the
acceleration drops below at scales of around 7000 AU, a statistical
survey of wide binaries with relative velocities and separations reaching
AU and beyond should prove useful to the above debate. We apply the
proposed test to the best currently available data. Results show a constant
upper limit to the relative velocities in wide binaries which is independent of
separation for over three orders of magnitude, in analogy with galactic flat
rotation curves in the same acceleration regime. Our results are
suggestive of a breakdown of Kepler's third law beyond
scales, in accordance with generic predictions of modified gravity theories
designed not to require any dark matter at galactic scales and beyond.Comment: accepted for publication in EPJ
Nonlinear electrodynamics and the Pioneer 10/11 spacecraft anomaly
The occurrence of the phenomenon known as photon acceleration is a natural
prediction of nonlinear electrodynamics (NLED). This would appear as an
anomalous frequency shift in any modeling of the electromagnetic field that
only takes into account the classical Maxwell theory. Thus, it is tempting to
address the unresolved anomalous, steady; but time-dependent, blueshift of the
Pioneer 10/11 spacecrafts within the framework of NLED. Here we show that
astrophysical data on the strength of the magnetic field in both the Galaxy and
the local (super)cluster of galaxies support the view on the major Pioneer
anomaly as a consequence of the phenomenon of photon acceleration. If
confirmed, through further observations or lab experiments, the reality of this
phenomenon should prompt to take it into account in any forthcoming research on
both cosmological evolution and origin and dynamical effects of primordial
magnetic fields, whose seeds are estimated to be very weak.Comment: Final version accepted for publication in Europhysics Letters, uses
EPL style, 7 page
Gravitational redshift of galaxies in clusters as predicted by general relativity
The theoretical framework of cosmology is mainly defined by gravity, of which
general relativity is the current model. Recent tests of general relativity
within the \Lambda Cold Dark Matter (CDM) model have found a concordance
between predictions and the observations of the growth rate and clustering of
the cosmic web. General relativity has not hitherto been tested on cosmological
scales independent of the assumptions of the \Lambda CDM model. Here we report
observation of the gravitational redshift of light coming from galaxies in
clusters at the 99 per cent confidence level, based upon archival data. The
measurement agrees with the predictions of general relativity and its
modification created to explain cosmic acceleration without the need for dark
energy (f(R) theory), but is inconsistent with alternative models designed to
avoid the presence of dark matter.Comment: Published in Nature issued on 29 September 2011. This version
includes the Letter published there as well as the Supplementary Information.
23 pages, 7 figure
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