230 research outputs found
ASYMPTOTIC BEHAVIOR OF COMPLEX SCALAR FIELDS IN A FRIEDMAN-LEMAITRE UNIVERSE
We study the coupled Einstein-Klein-Gordon equations for a complex scalar
field with and without a quartic self-interaction in a curvatureless
Friedman-Lema\^{\i}\-tre Universe. The equations can be written as a set of
four coupled first order non-linear differential equations, for which we
establish the phase portrait for the time evolution of the scalar field. To
that purpose we find the singular points of the differential equations lying in
the finite region and at infinity of the phase space and study the
corresponding asymptotic behavior of the solutions. This knowledge is of
relevance, since it provides the initial conditions which are needed to solve
numerically the differential equations. For some singular points lying at
infinity we recover the expected emergence of an inflationary stage.Comment: uuencoded, compressed tarfile containing a 15 pages Latex file and 2
postscipt figures. Accepted for publication on Phys. Rev.
On Gravitational Waves in Spacetimes with a Nonvanishing Cosmological Constant
We study the effect of a cosmological constant on the propagation
and detection of gravitational waves. To this purpose we investigate the
linearised Einstein's equations with terms up to linear order in in a
de Sitter and an anti-de Sitter background spacetime. In this framework the
cosmological term does not induce changes in the polarization states of the
waves, whereas the amplitude gets modified with terms depending on .
Moreover, if a source emits a periodic waveform, its periodicity as measured by
a distant observer gets modified. These effects are, however, extremely tiny
and thus well below the detectability by some twenty orders of magnitude within
present gravitational wave detectors such as LIGO or future planned ones such
as LISA.Comment: 8 pages, 4 figures, accepted for publication in Physical Review
Numerical evidence for `multi-scalar stars'
We present a class of general relativistic soliton-like solutions composed of
multiple minimally coupled, massive, real scalar fields which interact only
through the gravitational field. We describe a two-parameter family of
solutions we call ``phase-shifted boson stars'' (parameterized by central
density rho_0 and phase delta), which are obtained by solving the ordinary
differential equations associated with boson stars and then altering the phase
between the real and imaginary parts of the field. These solutions are similar
to boson stars as well as the oscillating soliton stars found by Seidel and
Suen [E. Seidel and W.M. Suen, Phys. Rev. Lett. 66, 1659 (1991)]; in
particular, long-time numerical evolutions suggest that phase-shifted boson
stars are stable. Our results indicate that scalar soliton-like solutions are
perhaps more generic than has been previously thought.Comment: Revtex. 4 pages with 4 figures. Submitted to Phys. Rev.
What measurable zero point fluctuations can(not) tell us about dark energy
We show that laboratory experiments cannot measure the absolute value of dark
energy. All known experiments rely on electromagnetic interactions. They are
thus insensitive to particles and fields that interact only weakly with
ordinary matter. In addition, Josephson junction experiments only measure
differences in vacuum energy similar to Casimir force measurements. Gravity,
however, couples to the absolute value. Finally we note that Casimir force
measurements have tested zero point fluctuations up to energies of ~10 eV, well
above the dark energy scale of ~0.01 eV. Hence, the proposed cut-off in the
fluctuation spectrum is ruled out experimentally.Comment: 4 page
Are the singularities stable?
The spacetime singularities play a useful role in gravitational theories by
distinguishing physical solutions from non-physical ones. The problem, we
studying in this paper is: are these singularities stable? To answer this
question, we have analyzed the general problem of stability of the family of
the static spherically symmetric solutions of the standard Einstein-Maxwell
model coupled to an extra free massless scalar field. We have obtained the
equations for the axial and polar perturbations. The stability against axial
perturbations has been proven.Comment: 13 pages, LaTeX, no figure
Spontaneous Scalarization and Boson Stars
We study spontaneous scalarization in Scalar-Tensor boson stars. We find that
scalarization does not occur in stars whose bosons have no self-interaction. We
introduce a quartic self-interaction term into the boson Lagrangian and show
that when this term is large, scalarization does occur. Strong self-interaction
leads to a large value of the compactness (or sensitivity) of the boson star, a
necessary condition for scalarization to occur, and we derive an analytical
expression for computing the sensitivity of a boson star in Brans-Dicke theory
from its mass and particle number. Next we comment on how one can use the
sensitivity of a star in any Scalar-Tensor theory to determine how its mass
changes when it undergoes gravitational evolution. Finally, in the Appendix, we
derive the most general form of the boson wavefunction that minimises the
energy of the star when the bosons carry a U(1) charge.Comment: 23 pages, 5 postscript figures. Typing errors corrected. Includes
some new text that relates the paper to several previous results. Accepted
for publication in PR
Galactic Halos As Boson Stars
We investigate the boson star with the self-interacting scalar field as a
model of galactic halos. The model has slightly increasing rotation curves and
allows wider ranges of the mass() and coupling() of the halo dark
matter particle than the non-interacting model previously
suggested(ref.\cite{sin1}). Two quantities are related by
\lambda^{\frac{1}{2}} (m_p/m)^2\st{>}{\sim} 10^{50}.Comment: 15 pages. Standard Latex file with 2 tex figures. Revised version to
be published in Phy. Rev. D. (Stability arguments are added.
Charged Scalar-Tensor Boson Stars: Equilibrium, Stability and Evolution
We study charged boson stars in scalar-tensor (ST) gravitational theories. We
analyse the weak field limit of the solutions and analytically show that there
is a maximum charge to mass ratio for the bosons above which the weak field
solutions are not stable. This charge limit can be greater than the GR limit
for a wide class of ST theories. We numerically investigate strong field
solutions in both the Brans Dicke and power law ST theories. We find that the
charge limit decreases with increasing central boson density. We discuss the
gravitational evolution of charged and uncharged boson stars in a cosmological
setting and show how, at any point in its evolution, the physical properties of
the star may be calculated by a rescaling of a solution whose asymptotic value
of the scalar field is equal to its initial asymptotic value. We focus on
evolution in which the particle number of the star is conserved and we find
that the energy and central density of the star decreases as the cosmological
time increases. We also analyse the appearance of the scalarization phenomenon
recently discovered for neutron stars configurations and, finally, we give a
short discussion on how making the correct choice of mass influences the
argument over which conformal frame, the Einstein frame or the Jordan frame, is
physical.Comment: RevTeX, 27 pages, 9 postscript figures. Minor revisions and updated
references. Accepted for publication in Phys. Rev.
Rotating Boson Star with Large Self-interaction in (2+1) dimensions
Solutions for rotating boson stars in (2+1) dimensional gravity with a
negative cosmological constant are obtained numerically. The mass, particle
number, and radius of the (2+1) dimensional rotating boson star are shown.
Consequently we find the region where the stable boson star can exist.Comment: 14 pages, 6 figures, RevTe
Dynamical Evolution of Boson Stars II: Excited States and Self-Interacting Fields
The dynamical evolution of self-gravitating scalar field configurations in
numerical relativity is studied. The previous analysis on ground state boson
stars of non-interacting fields is extended to excited states and to fields
with self couplings.
Self couplings can significantly change the physical dimensions of boson
stars, making them much more astrophysically interesting (e.g., having mass of
order 0.1 solar mass). The stable () and unstable () branches of
equilibrium configurations of boson stars of self-interacting fields are
studied; their behavior under perturbations and their quasi-normal oscillation
frequencies are determined and compared to the non-interacting case.
Excited states of boson stars with and without self-couplings are studied and
compared. Excited states also have equilibrium configurations with and
branch structures; both branches are intrinsically unstable under a generic
perturbation but have very different instability time scales. We carried out a
detailed study of the instability time scales of these configurations. It is
found that highly excited states spontaneously decay through a cascade of
intermediate states similar to atomic transitions.Comment: 16 pages+ 13 figures . All figures are available at
http://wugrav.wustl.edu/Paper
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