411 research outputs found

### Gravitational Microlensing by Globular Clusters

Stars in globular clusters can act either as sources for MACHOs (Massive
Astrophysical Compact Halo Objects) located along the line of sight or as
lenses for more distant background stars. Although the expected rate of
microlensing events is small, such observations can lead to very useful
results. In particular, one could get information on the shape of the galactic
halo along different lines of sight, allowing to better constrain its total
dark matter content. Moreover, on can also infer the total dark matter content
of globular clusters, which is presently not well known. To this latter
purpose, we analyse the microlensing events towards the galactic bulge, which
lie close to the three globular clusters NGC 6522, NGC 6528 and NGC 6540. We
find evidence that some microlensing events are indeed due to MACHOs located in
the globular clusters, suggesting, therefore, that these clusters contain a
significant amount of dark matter.Comment: 15 pages, 10 figures included. Accepted for publication in A &

### Dark matter vs. modifications of the gravitational inverse-square law. Results from planetary motion in the solar system

Dark matter or modifications of the Newtonian inverse-square law in the
solar-system are studied with accurate planetary astrometric data. From
extra-perihelion precession and possible changes in the third Kepler's law, we
get an upper limit on the local dark matter density, rho_{DM} < 3*10^{-16}
kg/m^3 at the 2-sigma confidence level. Variations in the 1/r^2 behavior are
considered in the form of either a possible Yukawa-like interaction or a
modification of gravity of MOND type. Up to scales of 10^{11} m,
scale-dependent deviations in the gravitational acceleration are really small.
We examined the MOND interpolating function mu in the regime of strong gravity.
Gradually varying mu suggested by fits of rotation curves are excluded, whereas
the standard form mu(x)= x/(1+x^2)^{1/2} is still compatible with data. In
combination with constraints from galactic rotation curves and theoretical
considerations on the external field effect, the absence of any significant
deviation from inverse square attraction in the solar system makes the range of
acceptable interpolating functions significantly narrow. Future radio ranging
observations of outer planets with an accuracy of few tenths of a meter could
either give positive evidence of dark matter or disprove modifications of
gravity.Comment: 7 pages, 4 figures, accepted for publication in MNRA

### Cosmological Parameter Determination in Free-Form Strong Gravitational Lens Modeling

We develop a novel statistical strong lensing approach to probe the
cosmological parameters by exploiting multiple redshift image systems behind
galaxies or galaxy clusters. The method relies on free-form mass inversion of
strong lenses and does not need any additional information other than
gravitational lensing. Since in free-form lensing the solution space is a
high-dimensional convex polytope, we consider Bayesian model comparison
analysis to infer the cosmological parameters. The volume of the solution space
is taken as a tracer of the probability of the underlying cosmological
assumption. In contrast to parametric mass inversions, our method accounts for
the mass-sheet degeneracy, which implies a degeneracy between the steepness of
the profile and the cosmological parameters. Parametric models typically break
this degeneracy, introducing hidden priors to the analysis that contaminate the
inference of the parameters. We test our method with synthetic lenses, showing
that it is able to infer the assumed cosmological parameters. Applied to the
CLASH clusters, the method might be competitive with other probes.Comment: 11 pages, 5 figures. Accepted for publication in MNRA

### The decay of highly excited open strings

The decay rates of leading edge Regge trajectory states are calculated for very high level number in open bosonic string theories, ignoring tachyon final states. The optical theorem simplifies the analysis while enabling identification of the different mass level decay channels. The main result is that (in four dimensions) the greatest single channel is the emission of a single photon and a state of the next mass level down. A simple asymptotic formula for arbitrarily high level number is given for this process. Also calculated is the total decay rate exactly up to N=100. It shows little variation over this range but appears to decrease for larger N. The formalism is checked in examples and the decay rate of the first excited level calculated for open superstring theories. The calculation may also have implications for high spin meson resonances

- â€¦