11,248 research outputs found
Astrophysical signatures of boson stars: quasinormal modes and inspiral resonances
Compact bosonic field configurations, or boson stars, are promising dark
matter candidates which have been invoked as an alternative description for the
supermassive compact objects in active galactic nuclei. Boson stars can be
comparable in size and mass to supermassive black holes and they are hard to
distinguish by electromagnetic observations. However, boson stars do not
possess an event horizon and their global spacetime structure is different from
that of a black hole. This leaves a characteristic imprint in the
gravitational-wave emission, which can be used as a discriminant between black
holes and other horizonless compact objects. Here we perform a detailed study
of boson stars and their gravitational-wave signatures in a fully relativistic
setting, a study which was lacking in the existing literature in many respects.
We construct several fully relativistic boson star configurations, and we
analyze their geodesic structure and free oscillation spectra, or quasinormal
modes. We explore the gravitational and scalar response of boson star
spacetimes to an inspiralling stellar-mass object and compare it to its black
hole counterpart. We find that a generic signature of compact boson stars is
the resonant-mode excitation by a small compact object on stable quasi-circular
geodesic motion.Comment: 20 pages, 8 figures. v2: minor corrections, version to be published
in Phys. Rev. D. v3: final versio
Light rings as observational evidence for event horizons: long-lived modes, ergoregions and nonlinear instabilities of ultracompact objects
Ultracompact objects are self-gravitating systems with a light ring. It was
recently suggested that fluctuations in the background of these objects are
extremely long-lived and might turn unstable at the nonlinear level, if the
object is not endowed with a horizon. If correct, this result has important
consequences: objects with a light ring are black holes. In other words, the
nonlinear instability of ultracompact stars would provide a strong argument in
favor of the "black hole hypothesis," once electromagnetic or
gravitational-wave observations confirm the existence of light rings. Here we
explore in some depth the mode structure of ultracompact stars, in particular
constant-density stars and gravastars. We show that the existence of very
long-lived modes -- localized near a second, stable null geodesic -- is a
generic feature of gravitational perturbations of such configurations. Already
at the linear level, such modes become unstable if the object rotates
sufficiently fast to develop an ergoregion. Finally, we conjecture that the
long-lived modes become unstable under fragmentation via a
Dyson-Chandrasekhar-Fermi mechanism at the nonlinear level. Depending on the
structure of the star, it is also possible that nonlinearities lead to the
formation of small black holes close to the stable light ring. Our results
suggest that the mere observation of a light ring is a strong evidence for the
existence of black holes.Comment: 10 pages, RevTeX
Alternative scheme to generate a supersinglet state of three-level atoms
In this paper we propose an alternative scheme to generate a supersinglet
state of three three-level atoms via a single-mode of a cavity QED based on the
two-photon transitions described by the 'full microscopical Hamiltonian
approach'. In it, three three-level atoms prepared in suitable initial states
are sequentially sent through the cavity originally prepared in its vacuum
state. After an appropriate choice of the atom-cavity interaction times plus a
field detection the state that describes the whole atom-field system is
projected in the desired supersinglet state. The fidelity and success
probability of the state as well as the practical feasibility of the scheme are
discussed.Comment: 10 pages, 3 figures, 4 table
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