17 research outputs found
Black Hole Superradiance in Dynamical Spacetime
We study the superradiant scattering of gravitational waves by a nearly
extremal black hole (dimensionless spin ) by numerically solving the
full Einstein field equations, thus including backreaction effects. This allows
us to study the dynamics of the black hole as it loses energy and angular
momentum during the scattering process. To explore the nonlinear phase of the
interaction, we consider gravitational wave packets with initial energies up to
of the mass of the black hole. We find that as the incident wave energy
increases, the amplification of the scattered waves, as well as the energy
extraction efficiency from the black hole, is reduced. During the interaction
the apparent horizon geometry undergoes sizable nonaxisymmetric oscillations.
The largest amplitude excitations occur when the peak frequency of the incident
wave packet is above where superradiance occurs, but close to the dominant
quasinormal mode frequency of the black hole.Comment: 5 pages, 4 figures; revised to match PRD versio
Surprises in the Evaporation of 2-Dimensional Black Holes
Quantum evaporation of Callan-Giddings-Harvey-Strominger (CGHS) black holes
is analyzed in the mean field approximation. This semi-classical theory
incorporates back reaction. Detailed analytical and numerical calculations show
that, while some of the assumptions underlying the standard evaporation
paradigm are borne out, several are not. Furthermore, if the black hole is
initially macroscopic, the evaporation process exhibits remarkable universal
properties (which are distinct from the features observed in the simplified,
exactly soluble models). Although the literature on CGHS black holes is quite
rich, these features had escaped previous analyses, in part because of lack of
required numerical precision, and in part because certain properties and
symmetries of the model were not fully recognized. Finally, our results provide
support for the full quantum gravity scenario recently developed by Ashtekar,
Taveras and Varadarajan.Comment: 4 pages, 3 figure
Treatments and placebos for the pathologies of effective field theories
We demonstrate some shortcomings of "fixing the equations," an increasingly
popular remedy for time evolution problems of effective field theories (EFTs).
We compare the EFTs and their "fixed" versions to the UV theories from which
they can be derived in two cases: K-essence and nonlinear Proca theory. We find
that when an EFT breaks down due to loss of hyperbolicity, fixing does not
approximate the UV theory well if the UV solution does not quickly settle down
to vacuum. We argue that this can be related to the EFT approximation itself
becoming invalid, which cannot be rectified by fixing.Comment: 5+4 pages. Version published as a PRD lette
Subtleties in constraining gravity theories with mass-radius data
Simultaneous measurements of neutron star masses and radii can be used to
constrain deviations from general relativity (GR) as was recently demonstrated
for the spontaneous scalarization model of Damour and Esposito-Far\`{e}se
(DEF). Here, we investigate the general applicability of the same procedure
beyond this single example. We first show that a simple variation of the DEF
model renders the same mass-radius measurements ineffective for obtaining
constraints. On the other hand, a recently popular and distinct model of
scalarization that arises in scalar-Gauss-Bonnet theory can be constrained
similarly to the original DEF model, albeit due to a slightly different
underlying mechanism. These establish that using the mass-radius data can
potentially constrain various theories of gravity, but the method also has
limitations.Comment: 18 pages, 11 figures. Published version. A larger dataset was used in
the analysis and a chapter explaining the effect of the dataset size is added
in this versio