295 research outputs found
Supermassive black holes or boson stars? Hair counting with gravitational wave detectors
The evidence for supermassive Kerr black holes in galactic centers is strong
and growing, but only the detection of gravitational waves will convincingly
rule out other possibilities to explain the observations. The Kerr spacetime is
completely specified by the first two multipole moments: mass and angular
momentum. This is usually referred to as the ``no-hair theorem'', but it is
really a ``two-hair'' theorem. If general relativity is the correct theory of
gravity, the most plausible alternative to a supermassive Kerr black hole is a
rotating boson star. Numerical calculations indicate that the spacetime of
rotating boson stars is determined by the first three multipole moments
(``three-hair theorem''). LISA could accurately measure the oscillation
frequencies of these supermassive objects. We propose to use these measurements
to ``count their hair'', unambiguously determining their nature and properties.Comment: 8 pages. This essay received an honorable mention in the Gravity
Research Foundation Essay Competition, 200
Oscillatons formed by non linear gravity
Oscillatons are solutions of the coupled Einstein-Klein-Gordon (EKG)
equations that are globally regular and asymptotically flat. By means of a
Legendre transformation we are able to visualize the behaviour of the
corresponding objects in non-linear gravity where the scalar field has been
absorbed by means of the conformal mapping.Comment: Revtex file, 6 pages, 3 eps figure; matches version published in PR
Rotating Boson Stars in 5 Dimensions
We study rotating boson stars in five spacetime dimensions. The boson fields
consist of a complex doublet scalar field. Considering boson stars rotating in
two orthogonal planes with both angular momenta of equal magnitude, a special
ansatz for the boson field and the metric allows for solutions with nontrivial
dependence on the radial coordinate only. The charge of the scalar field equals
the sum of the angular momenta. The rotating boson stars are globally regular
and asymptotically flat. For our choice of a sixtic potential the rotating
boson star solutions possess a flat spacetime limit. We study the solutions in
flat and curved spacetime.Comment: 17 pages, 6 figure
Boson Stars as Gravitational Lenses
We discuss boson stars as possible gravitational lenses and study the lensing
effect by these objects made of scalar particles. The mass and the size of a
boson star may vary from an individual Newtonian object similar to the Sun to
the general relativistic size and mass of a galaxy close to its Schwarzschild
radius. We assume boson stars to be transparent which allows the light to pass
through them though the light is gravitationally deflected. We assume boson
stars of the mass to be on non-cosmological distance from
the observer. We discuss the lens equation for these stars as well as the
details of magnification. We find that there are typically three images of a
star but the deflection angles may vary from arcseconds to even degrees. There
is one tangential critical curve (Einstein ring) and one radial critical curve
for tangential and radial magnification, respectively. Moreover, the deflection
angles for the light passing in the gravitational field of boson stars can be
very large (even of the order of degrees) which reflects the fact they are very
strong relativistic objects. We also propose a suitable formula for the lens
equation for such large deflection angles, and with the reservation that large
deflection angle images are highly demagnified but in the area of the
tangential critical curve, their existence may help in observational detection
of suitable lenses possessing characteristic features of boson stars which
could also serve as a direct evidence for scalar fields in the universe.Comment: accepted by Astrophys. J., 31 pages, AASTeX, 6 figure
Exact inflationary solutions
We present a new class of exact inflationary solutions for the evolution of a
universe with spatial curvature, filled with a perfect fluid, a scalar field
with potential and a cosmological
constant . With the potential and a negative cosmological
constant, the scale factor experiments a graceful exit.
We give a brief discussion about the physical meaning of the solutions.Comment: 10 pages, revtex file, 6 figures included with epsf. To be published
in IJMP-
Rotating Boson Stars and Q-Balls
We consider axially symmetric, rotating boson stars. Their flat space limits
represent spinning Q-balls. We discuss their properties and determine their
domain of existence. Q-balls and boson stars are stationary solutions and exist
only in a limited frequency range. The coupling to gravity gives rise to a
spiral-like frequency dependence of the boson stars. We address the flat space
limit and the limit of strong gravitational coupling. For comparison we also
determine the properties of spherically symmetric Q-balls and boson stars.Comment: 22 pages, 18 figure
Cerenkov radiation and scalar stars
We explore the possibility that a charged particle moving in the
gravitational field generated by a scalar star could radiate energy via a
recently proposed gravitational \v{C}erenkov mechanism. We numerically prove
that this is not possible for stable boson stars. We also show that soliton
stars could have \v{C}erenkov radiation for particular values of the boson
mass, although diluteness of the star grows and actual observational
possibility decreases for the more usually discussed boson masses. These
conclusions diminish, although do not completely rule out, the observational
possibility of actually detecting scalar stars using this mechanism, and lead
us to consider other forms, like gravitational lensing.Comment: Accepted for publication in Class. Quantum Gra
Boson Stars with Generic Self-Interactions
We study boson star configurations with generic, but not nontopological, self-interaction terms. When compared with the usual λ|ψ|4 potential, similar results for masses and number of particles appear. However, changes are noticed in the order of a few percent of the star mass. We explore, using catastrophe theory, the stability properties of the configurations and look for possible observational outputs that may differentiate one model from the other: gravitational redshifts, rotation curves of accreted particles, and lensing phenomena are computed for our new cases.Facultad de Ciencias Exacta
Evolution of 3D Boson Stars with Waveform Extraction
Numerical results from a study of boson stars under nonspherical
perturbations using a fully general relativistic 3D code are presented together
with the analysis of emitted gravitational radiation. We have constructed a
simulation code suitable for the study of scalar fields in space-times of
general symmetry by bringing together components for addressing the initial
value problem, the full evolution system and the detection and analysis of
gravitational waves. Within a series of numerical simulations, we explicitly
extract the Zerilli and Newman-Penrose scalar gravitational waveforms
when the stars are subjected to different types of perturbations. Boson star
systems have rapidly decaying nonradial quasinormal modes and thus the complete
gravitational waveform could be extracted for all configurations studied. The
gravitational waves emitted from stable, critical, and unstable boson star
configurations are analyzed and the numerically observed quasinormal mode
frequencies are compared with known linear perturbation results. The
superposition of the high frequency nonspherical modes on the lower frequency
spherical modes was observed in the metric oscillations when perturbations with
radial and nonradial components were applied. The collapse of unstable boson
stars to black holes was simulated. The apparent horizons were observed to be
slightly nonspherical when initially detected and became spherical as the
system evolved. The application of nonradial perturbations proportional to
spherical harmonics is observed not to affect the collapse time. An unstable
star subjected to a large perturbation was observed to migrate to a stable
configuration.Comment: 26 pages, 12 figure
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