5,128 research outputs found

### Large-scale filaments--Newtonian vs. modified dynamics

Eisenstein Loeb and Turner (ELT) have recently proposed a method for
estimating the dynamical masses of large-scale filaments, whereby the filament
is modeled by an axisymmetric, isothermal cylinder, for which ELT derive a
global relation between the (constant) velocity dispersion and the total line
density. We first show that the model assumptions of ELT can be relaxed
materially: an exact relation between the velocity and line density is derived
for any cylinder (not necessarily axisymmetric), with an arbitrary constituent
distribution function (so isothermality need not be assumed). We then consider
the same problem in the context of the modified dynamics (MOND). After a brief
comparison between scaling properties in the two theories, we study idealized
MOND model filaments. A preliminary application to the segment of the
Perseus-Pisces filament treated by ELT, gives MOND M/L estimates of order 10
s.u., compared with the Newtonian value of about 450, which ELT find. In spite
of the large uncertainties still besetting the analysis, this instance of MOND
application is of particular interest because: 1. Objects of this geometry have
not been dealt with before. 2. It pertains to large-scale structure. 3. The
typical accelerations involved are the lowest so far encountered in a
semi-virialized system.Comment: 12 page

### Galaxy groups and the modified dynamics

I estimate Modified-Dynamics (MOND), median M/L values for recently published
catalogues of galaxy groups. While the median, Newtonian M/L values quoted for
these catalogues are 110-200 solar units, the corresponding values for MOND are
less than 10 solar units.Comment: 5 pages, Latex, to appear in Astrophys. J. Let

### Cosmological fluctuation growth in bimetric MOND

I look at the growth of weak density inhomogeneities of nonrelativistic
matter, in bimetric-MOND (BIMOND) cosmology. I concentrate on
matter-twin-matter-symmetric versions of BIMOND, and assume that, on average,
the universe is symmetrically populated in the two sectors. MOND effects are
absent in an exactly symmetric universe, apart from the appearance of a
cosmological constant, Lambda~(a0/c)^2. MOND effects-local and cosmological-do
enter when density inhomogeneities that differ in the two sectors appear and
develop. MOND later takes its standard form in systems that are islands
dominated by pure matter. I derive the nonrelativistic equations governing
small-scale fluctuation growth. The equations split into two uncoupled systems,
one for the sum, the other for the difference, of the fluctuations in the two
sectors. The former is governed strictly by Newtonian dynamics. The latter is
governed by MOND dynamics, which entails stronger gravity, and nonlinearity
even for the smallest of perturbations. These cause the difference to grow
faster than the sum, conducing to matter-twin-matter segregation. The
nonlinearity also causes interaction between nested perturbations on different
scales. Because matter and twin matter (TM) repel each other in the MOND
regime, matter inhomogeneities grow not only by their own self gravity, but
also through shepherding by flanking TM overdensitie. The relative importance
of gravity and pressure in the MOND system depends also on the strength of the
perturbation. The development of structure in the universe, in either sector,
thus depends crucially on two initial fluctuation spectra: that of matter alone
and that of the matter-TM difference. I also discuss the back reaction on
cosmology of BIMOND effects that appear as "phantom matter" resulting from
inhomogeneity differences between the two sectors.Comment: 14 pages. Some clarifications added. Version published in Phys. Rev.

### Non-linear conformally invariant generalization of the Poisson equation to D>2 dimensions

I propound a non-linear generalization of the Poisson equation describing a
"medium" in D dimensions with a "dielectric constant" proportional to the field
strength to the power D-2. It is the only conformally invariant scalar theory
that is second order, and in which the scalar $phi$ couples to the sources
$\rho$ via a $\phi\rho$ contact term. The symmetry is used to generate
solutions for the field for some non-trivial configurations (e.g. for two
oppositely charged points). Systems comprising N point charges afford further
application of the symmetry. For these I derive e.g. exact expressions for the
following quantities: the general two-point-charge force; the energy function
and the forces in any three-body configuration with zero total charge; the
few-body force for some special configurations; the virial theorem for an
arbitrary, bound, many-particle system relating the time-average kinetic energy
to the particle charges. Possible connections with an underlying conformal
quantum field theory are mentioned.Comment: Revtex, 16 pages. To be published in Phys. Rev.

### A New interpretation of MOND based on Mach principle and an Unruh like effect

A new interpretation is introduced for MOND based on the Sciama's
interpretation of Mach principle and an Unruh like effect, in the context of a
generalized equivalence principle. It is argued that in a locally accelerated
frame with acceleration $a$ the appearance of a Rindler horizon may give rise
to a constant acceleration $a_0$ as the local properties of cosmological
horizon or Hubble length. The total gravitational acceleration inside this
frame becomes the combination of $a$ with $a_0$. For $a\gg a_0$, the
conventional gravitational mass $m_g$ interacts with the dominant acceleration
as $m_g a$ and application of Sciama's interpretation leads to the standard
Newtonian dynamics. For $a\ll a_0$, however, a reduced gravitational mass
$\bar{m}_g$ interacts with the dominant acceleration as $\bar{m}_g a_0$ and the
application of Sciama's interpretation on this reduced gravitational mass leads
to MOND. This introduces a third proposal for MOND: {\it The modification of
gravitational mass}.Comment: 11 pages, throughout revisio

### Gravitational Cherenkov losses in MOND theories

Survival of high-energy cosmic rays (HECRs) against gravitational Cherenkov
losses is shown not to cast strong constraints on MOND theories that are
compatible with general relativity (GR): theories that coincide with GR in the
high-acceleration limit. The energy-loss rate, L, is shown to be many orders
smaller than those derived in the literature for theories with no extra scale.
The gravitational acceleration produced by a HECR in its vicinity is much
higher than the MOND acceleration a0. So, modification to GR, which underlies
L, enters only beyond the MOND radius of the particle, within which GR holds
sway: r_M=sqrt(Gp/c a0). The spectral cutoff, which enters L quadratically, is
thus 1/r_M, not the particle's, much larger, de Broglie wavenumber: k_{dB}=
p/hbar. Thus, L is smaller than published rates, which use k_{dB}, by a factor
(r_M k_{dB})^2~10^{39}(cp/3.10^{11}Gev)^3. With 1/r_M as cutoff, the distance a
HECR can travel without major losses is q l_M, where l_M=c^2/a0 is the MOND
length, and q is a dimensionless function of parameters of the problem. Since
l_M is ~2 pi times the Hubble distance, survival of HECRs does not strongly
constrain GR-compatible, MOND theories. Such theories also easily satisfy
existing preferred-frame limits, inasmuch as these limits are gotten in
high-acceleration systems. I exemplify the results with MOND adaptations of
Einstein-Aether theories.Comment: Phys. Rev. Lett.; 4 pages; added some clarifications and reference

### Modelling the Pioneer anomaly as modified inertia

This paper proposes an explanation for the Pioneer anomaly: an unexplained
Sunward acceleration of 8.74 +/- 1.33 x 10^-10 m s^-2 seen in the behaviour of
the Pioneer probes. Two hypotheses are made: (1) Inertia is a reaction to Unruh
radiation and (2) this reaction is weaker for low accelerations because some
wavelengths in the Unruh spectrum do not fit within a limiting scale (twice the
Hubble distance) and are disallowed: a process similar to the Casimir effect.
When these ideas are used to model the Pioneer crafts' trajectories there is a
slight reduction in their inertial mass, causing an anomalous Sunward
acceleration of 6.9 +/- 3.5 x 10^-10 m s^-2 which agrees within error bars with
the observed Pioneer anomaly beyond 10 AU from the Sun. This new scheme is
appealingly simple and does not require adjustable parameters. However, it also
predicts an anomaly within 10 AU of the Sun, which has not been observed.
Various observational tests for the idea are proposed.Comment: 15 pages, 2 bw figures, accepted by MNRAS 19th December 200

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