71 research outputs found
The cosmological gravitational wave background from primordial density perturbations
We discuss the gravitational wave background generated by primordial density
perturbations evolving during the radiation era. At second-order in a
perturbative expansion, density fluctuations produce gravitational waves. We
calculate the power spectra of gravitational waves from this mechanism, and
show that, in principle, future gravitational wave detectors could be used to
constrain the primordial power spectrum on scales vastly different from those
currently being probed by large-scale structure. As examples we compute the
gravitational wave background generated by both a power-law spectrum on all
scales, and a delta-function power spectrum on a single scale.Comment: 8 Page
Instability of the massive Klein-Gordon field on the Kerr spacetime
We investigate the instability of the massive scalar field in the vicinity of
a rotating black hole. The instability arises from amplification caused by the
classical superradiance effect. The instability affects bound states: solutions
to the massive Klein-Gordon equation which tend to zero at infinity. We
calculate the spectrum of bound state frequencies on the Kerr background using
a continued fraction method, adapted from studies of quasinormal modes. We
demonstrate that the instability is most significant for the ,
state, for . For a fast rotating hole () we find
a maximum growth rate of ,
at . The physical implications are discussed.Comment: Added references. 27 pages, 7 figure
Black Hole Portal into Hidden Valleys
Superradiant instability turns rotating astrophysical black holes into unique
probes of light axions. We consider what happens when a light axion is coupled
to a strongly coupled hidden gauge sector. In this case superradiance results
in an adiabatic increase of a hidden sector CP-violating -parameter in
a near horizon region. This may trigger a first order phase transition in the
gauge sector. As a result a significant fraction of a black hole mass is
released as a cloud of hidden mesons and can be later converted into
electromagnetic radiation. This results in a violent electromagnetic burst. The
characteristic frequency of such bursts may range approximately from 100 eV to
100 MeV.Comment: 17 pages, 2 figure
Linearized f(R) Gravity: Gravitational Radiation & Solar System Tests
We investigate the linearized form of metric f(R)-gravity, assuming that f(R)
is analytic about R = 0 so it may be expanded as f(R) = R + a_2 R^2/2 + ... .
Gravitational radiation is modified, admitting an extra mode of oscillation,
that of the Ricci scalar. We derive an effective energy-momentum tensor for the
radiation. We also present weak-field metrics for simple sources. These are
distinct from the equivalent Kerr (or Schwarzschild) forms. We apply the
metrics to tests that could constrain f(R). We show that light deflection
experiments cannot distinguish f(R)-gravity from general relativity as both
have an effective post-Newtonian parameter \gamma = 1. We find that planetary
precession rates are enhanced relative to general relativity; from the orbit of
Mercury we derive the bound |a_2| < 1.2 \times 10^18 m^2. Gravitational wave
astronomy may be more useful: considering the phase of a gravitational waveform
we estimate deviations from general relativity could be measurable for an
extreme-mass-ratio inspiral about a 10^6 M_sol black hole if |a_2| > 10^17 m^2,
assuming that the weak-field metric of the black hole coincides with that of a
point mass. However Eot-Wash experiments provide the strictest bound |a_2| < 2
\times 10^-9 m^2. Although the astronomical bounds are weaker, they are still
of interest in the case that the effective form of f(R) is modified in
different regions, perhaps through the chameleon mechanism. Assuming the
laboratory bound is universal, we conclude that the propagating Ricci scalar
mode cannot be excited by astrophysical sources.Comment: 19 pages, 1 figure; typos in Sec. VIII. A. correcte
Exploring the String Axiverse with Precision Black Hole Physics
It has recently been suggested that the presence of a plenitude of light
axions, an Axiverse, is evidence for the extra dimensions of string theory. We
discuss the observational consequences of these axions on astrophysical black
holes through the Penrose superradiance process. When an axion Compton
wavelength is comparable to the size of a black hole, the axion binds to the
black hole "nucleus" forming a gravitational atom in the sky. The occupation
number of superradiant atomic levels, fed by the energy and angular momentum of
the black hole, grows exponentially. The black hole spins down and an axion
Bose-Einstein condensate cloud forms around it. When the attractive axion
self-interactions become stronger than the gravitational binding energy, the
axion cloud collapses, a phenomenon known in condensed matter physics as
"Bosenova". The existence of axions is first diagnosed by gaps in the mass vs
spin plot of astrophysical black holes. For young black holes the allowed
values of spin are quantized, giving rise to "Regge trajectories" inside the
gap region. The axion cloud can also be observed directly either through
precision mapping of the near horizon geometry or through gravitational waves
coming from the Bosenova explosion, as well as axion transitions and
annihilations in the gravitational atom. Our estimates suggest that these
signals are detectable in upcoming experiments, such as Advanced LIGO, AGIS,
and LISA. Current black hole spin measurements imply an upper bound on the QCD
axion decay constant of 2 x 10^17 GeV, while Advanced LIGO can detect signals
from a QCD axion cloud with a decay constant as low as the GUT scale. We
finally discuss the possibility of observing the gamma-rays associated with the
Bosenova explosion and, perhaps, the radio waves from axion-to-photon
conversion for the QCD axion.Comment: 54 pages, 15 figures; v2: PRD version, small correction in eq. 48 and
53 as well as fig. 10, conclusions unchange
On the consistency of a repulsive gravity phase in the early Universe
We exploit the possibility of existence of a repulsive gravity phase in the
evolution of the Universe. A toy model with a free scalar field minimally
coupled to gravity, but with the "wrong sign" for the energy and negative
curvature for the spatial section, is studied in detail. The background
solutions display a bouncing, non-singular Universe. The model is well-behaved
with respect to tensor perturbations. But, it exhibits growing models with
respect to scalar perturbations whose maximum occurs in the bouncing. Hence,
large inhomogeneties are produced. At least for this case, a repulsive phase
may destroy homogeneity, and in this sense it may be unstable. A newtonian
analogous model is worked out; it displays qualitatively the same behaviour.
The generality of this result is discussed. In particular, it is shown that the
addition of an attractive radiative fluid does not change essentially the
results. We discuss also a quantum version of the classical repulsive phase,
through the Wheeler-de Witt equation in mini-superspace, and we show that it
displays essentially the same scenario as the corresponding attractive phase.Comment: Latex file, 15 pages, 7 figures. There is a new figure, a new section
and some other minor correction
Nonthermal radiation of rotating black holes
Nonthermal radiation of a Kerr black hole is considered as tunneling of
created particles through an effective Dirac gap. In the leading semiclassical
approximation this approach is applicable to bosons as well. Our semiclassical
results for photons and gravitons do not contradict those obtained previously.
For neutrinos the result of our accurate quantum mechanical calculation is
about two times larger than the previous one.Comment: 10 pages, 2 figures; 2 references added, few typos correcte
Late-time oscillatory behaviour for self-gravitating scalar fields
This paper investigates the late-time behaviour of certain cosmological
models where oscillations play an essential role. Rigorous results are proved
on the asymptotics of homogeneous and isotropic spacetimes with a linear
massive scalar field as source. Various generalizations are obtained for
nonlinear massive scalar fields, -essence models and gravity. The
effect of adding ordinary matter is discussed as is the case of nonlinear
scalar fields whose potential has a degenerate zero.Comment: 17 pages, additional reference
Chaos in Anisotropic Pre-Inflationary Universes
We study the dynamics of anisotropic Bianchi type-IX models with matter and
cosmological constant. The models can be thought as describing the role of
anisotropy in the early stages of inflation. The concurrence of the
cosmological constant and anisotropy are sufficient to produce a chaotic
dynamics in the gravitational degrees of freedom, connected to the presence of
a critical point of saddle-center type in the phase space of the system. The
invariant character of chaos is guaranteed by the topology of the cylinders
emanating from unstable periodic orbits in the neighborhood of the
saddle-center. We discuss a possible mechanism for amplification of specific
wavelengths of inhomogeneous fluctuations in the models. A geometrical
interpretation is given for Wald's inequality in terms of invariant tori and
their destruction by increasing values of the cosmological constant.Comment: 14 pages, figures available under request. submitted to Physical
Review
The Imprint of Gravitational Waves in Models Dominated by a Dynamical Cosmic Scalar Field
An alternative to the standard cold dark matter model has been recently
proposed in which a significant fraction of the energy density of the universe
is due to a dynamical scalar field () whose effective equation-of-state
differs from that of matter, radiation or cosmological constant (). In
this paper, we determine how the Q-component modifies the primordial inflation
gravitational wave (tensor metric) contribution to the cosmic microwave
background anisotropy and, thereby, one of the key tests of inflation.Comment: 15 pages, 14 figures, revtex, submitted to Phys. Rev.
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