198 research outputs found
Response of a spaceborne gravitational wave antenna to solar oscillations
We investigate the possibility of observing very small amplitude low
frequency solar oscillations with the proposed laser interferometer space
antenna (LISA). For frequencies below the
dominant contribution is from the near zone time dependent gravitational
quadrupole moments associated with the normal modes of oscillation. For
frequencies above the dominant contribution
is from gravitational radiation generated by the quadrupole oscillations which
is larger than the Newtonian signal by a factor of the order , where is the distance to the Sun, and is the velocity of light.
The low order solar quadrupole pressure and gravity oscillation modes have
not yet been detected above the solar background by helioseismic velocity and
intensity measurements. We show that for frequencies , the signal due to solar oscillations will have a higher
signal to noise ratio in a LISA type space interferometer than in
helioseismology measurements. Our estimates of the amplitudes needed to give a
detectable signal on a LISA type space laser interferometer imply surface
velocity amplitudes on the sun of the order of 1-10 mm/sec in the frequency
range . If such modes exist with
frequencies and amplitudes in this range they could be detected with a LISA
type laser interferometer.Comment: 16 pages, 6 figures, 1 table. A reworked and considerably improved
version of ArXiv:astro-ph/0103472, Published in PR
Hydrodynamics of primordial black hole formation
The hydrodynamic picture of the formation of primordial black holes (PBH) at the early stages of expansion of the Universe is considered. It is assumed that close to singularity, expansion occurs in a quasi-isotropic way. Using an EVM, a spherically symmetrical nonlinear problem of the evolution of primary strong deviation from the Fridman solution was solved. What these deviations must be, so that the formation of PBH occurred was clarified. Attention was devoted to the role of pressure gradients. It is pointed out that at the moment of formation of PBH, only a small part of matter enters into it, primarily the component of perturbation. It is also pointed out that at this moment, the mass of PBH essentially is smaller than the mass considered within the cosmic horizon. The possibility of changing the mass of the PBH as a result of accretion is analyzed
Comment on "Formation of primordial black holes by cosmic strings"
We show that in a pioneering paper by Polnarev and Zembowicz, some
conclusions concerning the characteristics of the Turok-strings are generally
not correct. In addition we show that the probability of string collapse given
there, is off by a large prefactor (~1000).Comment: 5 pages, LaTeX and 1 figure, postscript. To appear in PR
The evolution of a binary in a retrograde circular orbit embedded in an accretion disk
Supermassive black hole binaries may form as a consequence of galaxy mergers.
Both prograde and retrograde orbits have been proposed. We study a binary of a
small mass ratio, q, in a retrograde orbit immersed in and interacting with a
gaseous accretion disk in order to estimate time scales for inward migration
leading to coalescence and the accretion rate to the secondary component. We
employ both semi-analytic methods and two dimensional numerical simulations,
focusing on the case where the binary mass ratio is small but large enough to
significantly perturb the disk. We develop the theory of type I migration for
this case and determine conditions for gap formation finding that then inward
migration occurs on a time scale equal to the time required for one half of the
secondary mass to be accreted through the unperturbed disk, with accretion onto
the secondary playing only a minor role. The semi-analytic and fully numerical
approaches are in good agreement, the former being applicable over long time
scales. Inward migration induced by interaction with the disk alleviates the
final parsec problem. Accretion onto the secondary does not significantly
affect the orbital evolution, but may have observational consequences for high
accretion efficiency. The binary may then appear as two sources of radiation
rotating around each other. This study should be extended to consider orbits
with significant eccentricity and the effects of gravitational radiation at
small length scales. Note too that torques acting between a circumbinary disk
and a retrograde binary orbit may cause the mutual inclination to increase on a
timescale that can be similar to, or smaller than that for orbital evolution,
depending on detailed parameters. This is also an aspect for future study
(abridged).Comment: 24 pages, 18 figures, accepted for publication in A&A. For movies of
the simulations see
http://astro.qmul.ac.uk/people/sijme-jan-paardekooper/publication
Probability of primordial black hole formation and its dependence on the radial profile of initial configurations
In this paper we derive the probability of the radial profiles of spherically
symmetric inhomogeneities in order to provide an improved estimation of the
number density of primordial black holes (PBHs). We demonstrate that the
probability of PBH formation depends sensitively on the radial profile of the
initial configuration. We do this by characterising this profile with two
parameters chosen heuristically: the amplitude of the inhomogeneity and the
second radial derivative, both evaluated at the centre of the configuration. We
calculate the joint probability of initial cosmological inhomogeneities as a
function of these two parameters and then find a correspondence between these
parameters and those used in numerical computations of PBH formation. Finally,
we extend our heuristic study to evaluate the probability of PBH formation
taking into account for the first time the radial profile of curvature
inhomogeneities.Comment: Version 2 with corrections from referees included, changes mostly
improve the presentatio
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