10,516 research outputs found
Precision of Hubble constant derived using black hole binary absolute distances and statistical redshift information
Measured gravitational waveforms from black hole binary inspiral events
directly determine absolute luminosity distances. To use these data for
cosmology, it is necessary to independently obtain redshifts for the events,
which may be difficult for those without electromagnetic counterparts. Here it
is demonstrated that certainly in principle, and possibly in practice,
clustering of galaxies allows extraction of the redshift information from a
sample statistically for the purpose of estimating mean cosmological
parameters, without identification of host galaxies for individual events. We
extract mock galaxy samples from the 6th Data Release of the Sloan Digital Sky
Survey resembling those that would be associated with inspiral events of
stellar mass black holes falling into massive black holes at redshift z ~ 0.1
to 0.5. A simple statistical procedure is described to estimate a likelihood
function for the Hubble constant H_0: each galaxy in a LISA error volume
contributes linearly to the log likelihood for the source redshift, and the log
likelihood for each source contributes linearly to that of H_0. This procedure
is shown to provide an accurate and unbiased estimator of H_0. It is estimated
that a precision better than one percent in H_0 may be possible if the rate of
such events is sufficiently high, on the order of 20 to z = 0.5.Comment: 9 pages, 4 figures, submitted to Phys. Rev. D; new references adde
The Full Re-Ionization of Helium
Observations of resolved HeII Lyman alpha absorption in spectra of two QSO's
suggest that the epoch of helium ionization occurred at z~3. Proximity zones in
the spectra of the quasars (z=3.18, 3.285) at 304 A resemble Stromgren spheres,
suggesting that the intergalactic medium is only singly ionized in helium. We
present models of the proximity effect which include the full physics of the
ionization, heating and cooling and an accurately simulated inhomogeneous gas
distribution. In these models the underdense intergalactic medium is heated to
at least 10,000-20,000 K after cooling to as low as a few 1000 K due to
cosmological expansion, with higher temperatures achieved farther away from the
quasar due to absorption-hardened ionizing spectra. The quasars turn on for a
few times 10^7 years with a fairly steady flux output at 228 A comparable to
the 304 A flux output directly observed with HST. The recoveries in the spectra
occur naturally due to voids in the IGM and may provide a fairly
model-independent probe of the baryon density.Comment: 5 pages, 3 figures, to appear in the proceedings of "After the Dark
Ages: When Galaxies were Young (the Universe at 2<z<5)", 9th Annual October
Astrophysics Conference in Marylan
Light-like Signals in General relativity and Cosmology
The modelling of light-like signals in General Relativity taking the form of
impulsive gravitational waves and light-like shells of matter is examined.
Systematic deductions from the Bianchi identities are made. These are based
upon Penrose's hierarchical classification of the geometry induced on the null
hypersurface history of the surface by its imbedding in the space-times to the
future and to the past of it. The signals are not confined to propagate in a
vacuum and thus their interaction with matter (a burst of radiation propagating
through a cosmic fluid, for example) is also studied. Results are accompanied
by illustrative examples using cosmological models, vacuum space-times, the de
sitter univers and Minkowskian space-time.Comment: 21 pages, latex, no figure
Gravitational Waves from Mesoscopic Dynamics of the Extra Dimensions
Recent models which describe our world as a brane embedded in a higher
dimensional space introduce new geometrical degrees of freedom: the shape
and/or size of the extra dimensions, and the position of the brane. These modes
can be coherently excited by symmetry breaking in the early universe even on
``mesoscopic'' scales as large as 1 mm, leading to detectable gravitational
radiation. Two sources are described: relativistic turbulence caused by a
first-order transition of a radion potential, and Kibble excitation of
Nambu-Goldstone modes of brane displacement. Characteristic scales and spectral
properties are estimated and the prospects for observation by LISA are
discussed. Extra dimensions with scale between 10 \AA and 1 mm, which enter the
3+1-D era at cosmic temperatures between 1 and 1000 TeV, produce backgrounds
with energy peaked at observed frequencies in the LISA band, between
and Hz. The background is detectable above instrument and
astrophysical foregrounds if initial metric perturbations are excited to a
fractional amplitude of or more, a likely outcome for the
Nambu-Goldstone excitations.Comment: Latex, 5 pages, plus one figure, final version to appear in Phys.
Rev. Let
Space missions to detect the cosmic gravitational-wave background
It is thought that a stochastic background of gravitational waves was
produced during the formation of the universe. A great deal could be learned by
measuring this Cosmic Gravitational-wave Background (CGB), but detecting the
CGB presents a significant technological challenge. The signal strength is
expected to be extremely weak, and there will be competition from unresolved
astrophysical foregrounds such as white dwarf binaries. Our goal is to identify
the most promising approach to detect the CGB. We study the sensitivities that
can be reached using both individual, and cross-correlated pairs of space based
interferometers. Our main result is a general, coordinate free formalism for
calculating the detector response that applies to arbitrary detector
configurations. We use this general formalism to identify some promising
designs for a GrAvitational Background Interferometer (GABI) mission. Our
conclusion is that detecting the CGB is not out of reach.Comment: 22 pages, 7 figures, IOP style, References Adde
Kangaroo Bond Issuance in Australia
The issue of kangaroo bonds (foreign A$-denominated bonds) has become a significant part of the Australian bond market. The Australian experience offers some lessons to other countries interested in developing their domestic bond markets
Results of the Mariner 6 and 7 Mars occultation experiments
Final profiles of temperature, pressure, and electron density on Mars were obtained for the Mariner 6 and 7 entry and exit cases, and results are presented for both the lower atmosphere and ionosphere. The results of an analysis of the systematic and formal errors introduced at each stage of the data-reduction process are also included. At all four occulation points, the lapse rate of temperature was subdadiabatic up to altitudes in excess of 20 km. A pronounced temperature inversion was present above the surface at the Mariner 6 exit point. All four profiles exhibit a sharp, superadiabatic drop in temperature at high altitudes, with temperatures falling below the frost point of CO2. These results give a strong indication of frozen CO2 in the middle atmosphere of Mars
High-power operation of a K-band second harmonic gyroklystron
Amplification studies of a two-cavity second-harmonic gyroklystron are reported. A magnetron injection gun produces a 440 kV, 200–245 A, 1 μs beam with an average perpendicular-to-parallel velocity ratio slightly less than 1. The TE011 input cavity is driven near 9.88 GHz and the TE021 output cavity resonates near 19.76 GHz. Peak powers exceeding 21 MW are achieved with an efficiency near 21% and a large signal gain above 25 dB. This performance represents the current state of the art for gyroklystrons in terms of the peak power normalized to the output wavelength squared
Interplanetary propulsion using inertial fusion
Inertial fusion can be used to power spacecraft within the solar system and beyond. Such spacecraft have the potential for short-duration manned-mission performance exceeding other technologies. We are conducting a study to assess the systems aspects of inertial fusion as applied to such missions, based on the conceptual engine design of Hyde (1983) we describe the required systems for an entirely new spacecraft design called VISTA that is based on the use of DT fuel. We give preliminary design details for the power conversion and power conditioning systems for manned missions to Mars of total duration of about 100 days. Specific mission performance results will be published elsewhere, after the study has been completed
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