302 research outputs found
Massive graviton as a testable cold dark matter candidate
We construct a consistent model of gravity where the tensor graviton mode is
massive, while linearized equations for scalar and vector metric perturbations
are not modified. The Friedmann equation acquires an extra dark-energy
component leading to accelerated expansion. The mass of the graviton can be as
large as , being constrained by the pulsar timing
measurements. We argue that non-relativistic gravitational waves can comprise
the cold dark matter and may be detected by the future gravitational wave
searches.Comment: 4 pages, final version to appear in PR
Peaks in the Cosmic Microwave Background: flat versus open models
We present properties of the peaks (maxima) of the CMB anisotropies expected
in flat and open CDM models. We obtain analytical expressions of several
topological descriptors: mean number of maxima and the probability distribution
of the gaussian curvature and the eccentricity of the peaks. These quantities
are calculated as functions of the radiation power spectrum, assuming a
gaussian distribution of temperature anisotropies. We present results for
angular resolutions ranging from 5' to 20' (antenna FWHM), scales that are
relevant for the MAP and COBRAS/SAMBA space missions and the ground-based
interferometer experiments. Our analysis also includes the effects of noise. We
find that the number of peaks can discriminate between standard CDM models, and
that the gaussian curvature distribution provides a useful test for these
various models, whereas the eccentricity distribution can not distinguish
between them.Comment: 13 pages latex file using aasms4.sty + 3 tables + 2 postscript
figures, to appear in ApJ (March 1997
Gauging the dark matter fraction in a S0 galaxy at z=0.47 through gravitational lensing from deep HST/ACS imaging
We analyze a new gravitational lens, OAC-GL J1223-1239, serendipitously found
in a deep I-band image of the Hubble Space Telescope (HST) Advanced Camera for
Surveys (ACS). The lens is a L_*, edge-on S0 galaxy at z=0.4656. The
gravitational arc has a radius of 0.42 arcsec. We have determined the total
mass and the dark matter (DM) fraction within the Einstein radius as a function
of the lensed source redshift, which is presently unknown. For z ~ 1.3, which
is in the middle of the redshift range plausible for the source according to
some external constraints, we find the central velocity dispersion to be ~180
km/s. With this value, close to that obtained by means of the Faber-Jackson
relation at the lens redshift, we compute a 30% DM fraction within the Einstein
radius (given the uncertainty in the source redshift, the allowed range for the
DM fraction is 25-35 % in our lensing model). When compared with the galaxies
in the local Universe, the lensing galaxy, OAC-GL J1223-1239 seems to fall in
the transition regime between massive DM dominated galaxies and lower-mass, DM
deficient systems.Comment: 18 pages, 5 figures; accepted for publication in Ap
Status Update of the Parkes Pulsar Timing Array
The Parkes Pulsar Timing Array project aims to make a direct detection of a
gravitational-wave background through timing of millisecond pulsars. In this
article, the main requirements for that endeavour are described and recent and
ongoing progress is outlined. We demonstrate that the timing properties of
millisecond pulsars are adequate and that technological progress is timely to
expect a successful detection of gravitational waves within a decade, or
alternatively to rule out all current predictions for gravitational wave
backgrounds formed by supermassive black-hole mergers.Comment: 10 pages, 3 figures, Amaldi 8 conference proceedings, accepted by
Classical & Quantum Gravit
Gravitational wave detection using pulsars: status of the Parkes Pulsar Timing Array project
The first direct detection of gravitational waves may be made through
observations of pulsars. The principal aim of pulsar timing array projects
being carried out worldwide is to detect ultra-low frequency gravitational
waves (f ~ 10^-9 to 10^-8 Hz). Such waves are expected to be caused by
coalescing supermassive binary black holes in the cores of merged galaxies. It
is also possible that a detectable signal could have been produced in the
inflationary era or by cosmic strings. In this paper we review the current
status of the Parkes Pulsar Timing Array project (the only such project in the
Southern hemisphere) and compare the pulsar timing technique with other forms
of gravitational-wave detection such as ground- and space-based interferometer
systems.Comment: Accepted for publication in PAS
Faraday rotation, stochastic magnetic fields and CMB maps
The high- and low-frequency descriptions of the pre-decoupling plasma are
deduced from the Vlasov-Landau treatment generalized to curved space-times and
in the presence of the relativistic fluctuations of the geometry. It is
demonstrated that the interplay between one-fluid and two-fluid treatments is
mandatory for a complete and reliable calculation of the polarization
observables. The Einstein-Boltzmann hierarchy is generalized to handle the
dispersive propagation of the electromagnetic disturbances in the
pre-decoupling plasma. Given the improved physical and numerical framework, the
polarization observables are computed within the magnetized CDM
paradigm (mCDM). In particular, the Faraday-induced B-mode is
consistently estimated by taking into account the effects of the magnetic
fields on the initial conditions of the Boltzmann hierarchy, on the dynamical
equations and on the dispersion relations. The complete calculations of the
angular power spectra constitutes the first step for the derivation of
magnetized maps of the CMB temperature and polarization which are here obtained
for the first time and within the minimal mCDM model. The obtained
results set the ground for direct experimental scrutiny of large-scale
magnetism via the low and high frequency instruments of the Planck explorer
satellite.Comment: 53 pages, 15 included figure
Reconnection of Non-Abelian Cosmic Strings
Cosmic strings in non-abelian gauge theories naturally gain a spectrum of
massless, or light, excitations arising from their embedding in color and
flavor space. This opens up the possibility that colliding strings miss each
other in the internal space, reducing the probability of reconnection. We study
the topology of the non-abelian vortex moduli space to determine the outcome of
string collision. Surprisingly we find that the probability of classical
reconnection in this system remains unity, with strings passing through each
other only for finely tuned initial conditions. We proceed to show how this
conclusion can be changed by symmetry breaking effects, or by quantum effects
associated to fermionic zero modes, and present examples where the probability
of reconnection in a U(N) gauge theory ranges from 1/N for low-energy
collisions to one at higher energies.Comment: 25 Pages, 3 Figures. v2: comment added, reference adde
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