1,046 research outputs found
Towards the solution of the relativistic gravitational radiation reaction problem for binary black holes
Here we present the results of applying the generalized Riemann zeta-function
regularization method to the gravitational radiation reaction problem. We
analyze in detail the headon collision of two nonspinning black holes with
extreme mass ratio. The resulting reaction force on the smaller hole is
repulsive. We discuss the possible extensions of these method to generic orbits
and spinning black holes. The determination of corrected trajectories allows to
add second perturbative corrections with the consequent increase in the
accuracy of computed waveforms.Comment: Contribution to the Proceedings of the 3rd LISA Symposiu
Matter density perturbations in modified gravity models with arbitrary coupling between matter and geometry
We consider theories with an arbitrary coupling between matter and gravity
and obtain the perturbation equation of matter on subhorizon scales. Also, we
derive the effective gravitational constant and two parameters
and , which along with the perturbation equation of the matter
density are useful to constrain the theory from growth factor and weak lensing
observations. Finally, we use a completely solvable toy model which exhibits
nontrivial phenomenology to investigate specific features of the theory. We
obtain the analytic solution of the modified Friedmann equation for the scale
factor in terms of time and use the age of the oldest star clusters and
the primordial nucleosynthesis bounds in order to constrain the parameters of
our toy model.Comment: 9 pages, 3 figures, uses revtex4, added Appendix and references,
minor changes, accepted in Phys. Rev. D (to appear
Open timelike curves violate Heisenberg's uncertainty principle
Toy models for quantum evolution in the presence of closed timelike curves
(CTCs) have gained attention in the recent literature due to the strange
effects they predict. The circuits that give rise to these effects appear quite
abstract and contrived, as they require non-trivial interactions between the
future and past which lead to infinitely recursive equations. We consider the
special case in which there is no interaction inside the CTC, referred to as an
open timelike curve (OTC), for which the only local effect is to increase the
time elapsed by a clock carried by the system. Remarkably, circuits with access
to OTCs are shown to violate Heisenberg's uncertainty principle, allowing
perfect state discrimination and perfect cloning of coherent states. The model
is extended to wave-packets and smoothly recovers standard quantum mechanics in
an appropriate physical limit. The analogy with general relativistic
time-dilation suggests that OTCs provide a novel alternative to existing
proposals for the behaviour of quantum systems under gravity
Improved limits on short-wavelength gravitational waves from the cosmic microwave background
The cosmic microwave background (CMB) is affected by the total radiation
density around the time of decoupling. At that epoch, neutrinos comprised a
significant fraction of the radiative energy, but there could also be a
contribution from primordial gravitational waves with frequencies greater than
~ 10^-15 Hz. If this cosmological gravitational wave background (CGWB) were
produced under adiabatic initial conditions, its effects on the CMB and matter
power spectrum would mimic massless non-interacting neutrinos. However, with
homogenous initial conditions, as one might expect from certain models of
inflation, pre big-bang models, phase transitions and other scenarios, the
effect on the CMB would be distinct. We present updated observational bounds
for both initial conditions using the latest CMB data at small scales from the
South Pole Telescope (SPT) in combination with Wilkinson Microwave Anisotropy
Probe (WMAP), current measurements of the baryon acoustic oscillations, and the
Hubble parameter. With the inclusion of the data from SPT the adiabatic bound
on the CGWB density is improved by a factor of 1.7 to 10^6 Omega_gw < 8.7 at
the 95% confidence level (C.L.), with weak evidence in favor of an additional
radiation component consistent with previous analyses. The constraint can be
converted into an upper limit on the tension of horizon-sized cosmic strings
that could generate this gravitational wave component, with Gmu < 2 10^-7 at
95% C.L., for string tension Gmu. The homogeneous bound improves by a factor of
3.5 to 10^6 Omega_gw < 1.0 at 95% C.L., with no evidence for such a component
from current data.Comment: 5 pages, 3 figure
Gravitational Waves in Bianchi Type-I Universes I: The Classical Theory
The propagation of classical gravitational waves in Bianchi Type-I universes
is studied. We find that gravitational waves in Bianchi Type-I universes are
not equivalent to two minimally coupled massless scalar fields as it is for the
Robertson-Walker universe. Due to its tensorial nature, the gravitational wave
is much more sensitive to the anisotropy of the spacetime than the scalar field
is and it gains an effective mass term. Moreover, we find a coupling between
the two polarization states of the gravitational wave which is also not present
in the Robertson-Walker universe.Comment: 34 papers, written in ReVTeX, submitted to Physical Review
Hadamard States and Adiabatic Vacua
Reversing a slight detrimental effect of the mailer related to TeXabilityComment: 10pages, LaTeX (RevTeX-preprint style
Analogue Cosmological Particle Creation: Quantum Correlations in Expanding Bose Einstein Condensates
We investigate the structure of quantum correlations in an expanding Bose
Einstein Condensate (BEC) through the analogue gravity framework. We consider
both a 3+1 isotropically expanding BEC as well as the experimentally relevant
case of an elongated, effectively 1+1 dimensional, expanding condensate. In
this case we include the effects of inhomogeneities in the condensate, a
feature rarely included in the analogue gravity literature. In both cases we
link the BEC expansion to a simple model for an expanding spacetime and then
study the correlation structure numerically and analytically (in suitable
approximations). We also discuss the expected strength of such correlation
patterns and experimentally feasible BEC systems in which these effects might
be detected in the near future.Comment: Reference adde
Not all adiabatic vacua are physical states
Adiabatic vacua are known to be Hadamard states. We show, however that the
energy-momentum tensor of a linear Klein-Gordon field on Robertson-Walker
spaces developes a generic singularity on the initial hypersurface if the
adiabatic vacuum is of order less than four. Therefore, adiabatic vacua are
physically reasonable only if their order is at least four.
A certain non-local large momentum expansion of the mode functions has
recently been suggested to yield the subtraction terms needed to remove the
ultraviolet divergences in the energy-momentum tensor. We find that this scheme
fails to reproduce the trace anomaly and therefore is not equivalent to
adiabatic regularisation.Comment: 13 pages, LaTex2
Dispersive fields in de Sitter space and event horizon thermodynamics
When Lorentz invariance is violated at high energy, the laws of black hole
thermodynamics are apparently no longer satisfied. To shed light on this
observation, we study dispersive fields in de Sitter space. We show that the
Bunch-Davies vacuum state restricted to the static patch is no longer thermal,
and that the Tolman law is violated. However we also show that, for free fields
at least, this vacuum is the only stationary stable state, as if it were in
equilibrium. We then present a precise correspondence between dispersive
effects found in de Sitter and in black hole metrics. This indicates that the
consequences of dispersion on thermodynamical laws could also be similar.Comment: 19 pages. Black and White version on Phys.Rev.D serve
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