3,969 research outputs found
The Pioneer anomaly and the holographic scenario
Full text linkIn this paper we discuss the recently obtained relation between the
Verlinde's holographic model and the first phenomenological Modified Newtonian
dynamics. This gives also a promising possible explanation to the Pioneer
anomaly.Comment: 5 pages, Accepted for publication in Astrophysics & Space Scienc
N-body simulations in modified Newtonian dynamics
Full text linkWe describe some results obtained with N-MODY, a code for N-body simulations
of collisionless stellar systems in modified Newtonian dynamics (MOND). We
found that a few fundamental dynamical processes are profoundly different in
MOND and in Newtonian gravity with dark matter. In particular, violent
relaxation, phase mixing and galaxy merging take significantly longer in MOND
than in Newtonian gravity, while dynamical friction is more effective in a MOND
system than in an equivalent Newtonian system with dark matter.Comment: 4 pages, no figures. To appear in EAS Publication Series (Proceedings
of Symposium 7 of the JENAM 2008, Vienna
Modified gravity without dark matter
Full text linkOn an empirical level, the most successful alternative to dark matter in
bound gravitational systems is the modified Newtonian dynamics, or MOND,
proposed by Milgrom. Here I discuss the attempts to formulate MOND as a
modification of General Relativity. I begin with a summary of the
phenomenological successes of MOND and then discuss the various covariant
theories that have been proposed as a basis for the idea. I show why these
proposals have led inevitably to a multi-field theory. I describe in some
detail TeVeS, the tensor-vector-scalar theory proposed by Bekenstein, and
discuss its successes and shortcomings. This lecture is primarily pedagogical
and directed to those with some, but not a deep, background in General
RelativityComment: 28 pages, 10 figures, lecture given at Third Aegean Summer School,
The Invisible Universe: Dark Matter and Dark Energy, minor errors corrected,
references update
Probing Brownstein-Moffat Gravity via Numerical Simulations
Full text linkIn the standard scenario of the Newtonian gravity, a late-type galaxy (i.e.,
a spiral galaxy) is well described by a disk and a bulge embedded in a halo
mainly composed by dark matter. In Brownstein-Moffat gravity, there is a claim
that late-type galaxy systems would not need to have halos, avoiding as a
result the dark matter problem, i.e., a modified gravity (non-Newtonian) would
account for the galactic structure with no need of dark matter. In the present
paper, we probe this claim via numerical simulations. Instead of using a
"static galaxy," where the centrifugal equilibrium is usually adopted, we probe
the Brownstein-Moffat gravity dynamically via numerical -body simulations.Comment: 33 pages and 14 figures - To appear in The Astrophysical Journa
Gravitational anomalies signaling the breakdown of classical gravity
Full text linkRecent 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
Propagation of gravitational waves in multimetric gravity
Full text linkWe discuss the propagation of gravitational waves in a recently discussed
class of theories containing N >= 2 metric tensors and a corresponding number
of standard model copies. Using the formalism of gauge-invariant linear
perturbation theory we show that all gravitational waves propagate at the speed
of light. We then employ the Newman-Penrose formalism to show that two to six
polarizations of gravitational waves may exist, depending on the parameters
entering the equations of motion. This corresponds to E(2) representations N_2,
N_3, III_5 and II_6. We finally apply our general discussion to a recently
presented concrete multimetric gravity model and show that it is of class N_2,
i.e., it allows only two tensor polarizations, as it is the case for general
relativity. Our results provide the theoretical background for tests of
multimetric gravity theories using the upcoming gravitational wave experiments.Comment: 21 pages, no figures, journal versio
Massive particles in acoustic space-times emergent inertia and passive gravity
Full text linkI show that massive-particle dynamics can be simulated by a weak, spherical,
external perturbation on a potential flow in an ideal fluid. The effective
Lagrangian is of the form mc^2L(U^2/c^2), where U is the velocity of the
particle relative to the fluid and c the speed of sound. This can serve as a
model for emergent relativistic inertia a la Mach's principle with m playing
the role of inertial mass, and also of analog gravity where it is also the
passive gravitational mass. m depends on the particle type and intrinsic
structure, while L is universal: For D dimensional particles L is proportional
to the hypergeometric function F(1,1/2;D/2;U^2/c^2). Particles fall in the same
way in the analog gravitational field independent of their internal structure,
thus satisfying the weak equivalence principle. For D less or equal 5 they all
have a relativistic limit with the acquired energy and momentum diverging as U
approaches c. For D less or equal 7 the null geodesics of the standard acoustic
metric solve our equation of motion. Interestingly, for D=4 the dynamics is
very nearly Lorentzian. The particles can be said to follow the geodesics of a
generalized acoustic metric of a Finslerian type that shares the null geodesics
with the standard acoustic metric. In vortex geometries, the ergosphere is
automatically the static limit. As in the real world, in ``black hole''
geometries circular orbits do not exist below a certain radius that occurs
outside the horizon. There is a natural definition of antiparticles; and I
describe a mock particle vacuum in whose context one can discuss, e.g.,
particle Hawking radiation near event horizons.Comment: 15 page: version published in Physical Review
Mapping the galactic gravitational potential with peculiar acceleration
Get PDFIt has been suggested recently that the change in cosmological redshift (the
Sandage test of expansion) could be observed in the next generation of large
telescopes and ultra-stable spectrographs. In a recent paper we estimated the
change of peculiar velocity, i.e. the peculiar acceleration, in nearby galaxies
and clusters and shown it to be of the same order of magnitude as the typical
cosmological signal. Mapping the acceleration field allows for a reconstruction
of the galactic gravitational potential without assuming virialization. In this
paper we focus on the peculiar acceleration in our own Galaxy, modeled as a
Kuzmin disc and a dark matter spherical halo. We estimate the peculiar
acceleration for all known Galactic globular clusters and find some cases with
an expected velocity shift in excess of 20 cm/sec for observations fifteen
years apart, well above the typical cosmological acceleration. We then compare
the predicted signal for a MOND (modified Newtonian dynamics) model in which
the spherical dark matter halo is absent. We find that the signal pattern is
qualitatively different, showing that the peculiar acceleration field could be
employed to test competing theories of gravity. However the difference seems
too small to be detectable in the near future.Comment: 11 pages, 10 figures, 3 tables, minor changes, accepted for
publication by MNRA
Virial mass in DGP brane cosmology
Full text linkWe study the virial mass discrepancy in the context of a DPG brane-world
scenario and show that such a framework can offer viable explanations to
account for the mass discrepancy problem. This is done by defining a
geometrical mass that we prove to be proportional to the virial
mass. Estimating using observational data, we show that it
behaves linearly with and has a value of the order of , pointing
to a possible resolution of the virial mass discrepancy. We also obtain the
radial velocity dispersion of galaxy clusters and show that it is compatible
with the radial velocity dispersion profile of such clusters. This velocity
dispersion profile can be used to differentiate various models predicting the
virial mass.Comment: 12 pages, 1 figure, to appear in CQ
A Nonlocal Metric Formulation of MOND
Get PDFWe study a class of nonlocal, but causal, covariant and conserved field
equations for the metric. Although nonlocal, these equations do not seem to
possess extra graviton solutions in weak field perturbation theory. Indeed, the
equations reduce to those of general relativity when the Ricci scalar vanishes
throughout spacetime. When a static matter source is present we show how these
equations can be adjusted to reproduce Milgrom's Modified Newtonian Dynamics in
the weak field regime, while reducing to general relativity for strong fields.
We compute the angular deflection of light in the weak field regime and
demonstrate that it is the same as for general relativity, resulting in far too
little lensing if no dark matter is present. We also study the field equations
for a general Robertson-Walker geometry. An interesting feature of our
equations is that they become conformally invariant in the MOND limit.Comment: 22 pages, LaTeX 2 epsilon, no figure
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