710 research outputs found
No-boundary measure and preference for large e-foldings in multi-field inflation
The no-boundary wave function of quantum gravity usually assigns only very
small probability to long periods of inflation. This was a reason to doubt
about the no-boundary wave function to explain the observational universe. We
study the no-boundary proposal in the context of multi-field inflation to see
whether the number of fields changes the situation. For a simple model, we find
that indeed the no-boundary wave function can give higher probability for
sufficient inflation, but the number of fields involved has to be very high.Comment: 16 pages, 2 figure
The causal structure of dynamical charged black holes
We study the causal structure of dynamical charged black holes, with a
sufficient number of massless fields, using numerical simulations. Neglecting
Hawking radiation, the inner horizon is a null Cauchy horizon and a curvature
singularity due to mass inflation. When we include Hawking radiation, the inner
horizon becomes space-like and is separated from the Cauchy horizon, which is
parallel to the out-going null direction. Since a charged black hole must
eventually transit to a neutral black hole, we studied the neutralization of
the black hole and observed that the inner horizon evolves into a space-like
singularity, generating a Cauchy horizon which is parallel to the in-going null
direction. Since the mass function is finite around the inner horizon, the
inner horizon is regular and penetrable in a general relativistic sense.
However, since the curvature functions become trans-Planckian, we cannot
saymore about the region beyond the inner horizon, and it is natural to say
that there is a 'physical' space-like singularity. However, if we assume an
exponentially large number of massless scalar fields, our results can be
extended beyond the inner horizon. In this case, strong cosmic censorship and
black hole complementarity can be violated.Comment: 23 pages, 23 figure
TCP throughput guarantee in the DiffServ Assured Forwarding service: what about the results?
Since the proposition of Quality of Service architectures by the IETF, the
interaction between TCP and the QoS services has been intensively studied. This
paper proposes to look forward to the results obtained in terms of TCP
throughput guarantee in the DiffServ Assured Forwarding (DiffServ/AF) service
and to present an overview of the different proposals to solve the problem. It
has been demonstrated that the standardized IETF DiffServ conditioners such as
the token bucket color marker and the time sliding window color maker were not
good TCP traffic descriptors. Starting with this point, several propositions
have been made and most of them presents new marking schemes in order to
replace or improve the traditional token bucket color marker. The main problem
is that TCP congestion control is not designed to work with the AF service.
Indeed, both mechanisms are antagonists. TCP has the property to share in a
fair manner the bottleneck bandwidth between flows while DiffServ network
provides a level of service controllable and predictable. In this paper, we
build a classification of all the propositions made during these last years and
compare them. As a result, we will see that these conditioning schemes can be
separated in three sets of action level and that the conditioning at the
network edge level is the most accepted one. We conclude that the problem is
still unsolved and that TCP, conditioned or not conditioned, remains
inappropriate to the DiffServ/AF service
Dynamics of false vacuum bubbles in Brans-Dicke theory
We study the dynamics of false vacuum bubbles in the Brans-Dicke theory of
gravity by using the thin shell or thin wall approximation. We consider a false
vacuum bubble that has a different value for the Brans-Dicke field between the
inside false vacuum region and the outside true vacuum region. Within a certain
limit of field values, the difference of field values makes the effective
tension of the shell negative. This allows new expanding false vacuum bubbles
to be seen by the outside observer, which are disallowed in Einstein gravity.Comment: 29 pages, 20 figure
Dynamical formation and evolution of (2+1)-dimensional charged black holes
In this paper, we investigate the dynamical formation and evolution of 2 +
1-dimensional charged black holes. We numerically study dynamical collapses of
charged matter fields in an anti de Sitter background and note the formation of
black holes using the double-null formalism. Moreover, we include re-normalized
energy-momentum tensors assuming the S-wave approximation to determine
thermodynamical back-reactions to the internal structures. If there is no
semi-classical effects, the amount of charge determines the causal structures.
If the charge is sufficiently small, the causal structure has a space-like
singularity. However, as the charge increases, an inner Cauchy horizon appears.
If we have sufficient charge, we see a space-like outer horizon and a time-like
inner horizon, and if we give excessive charge, black hole horizons disappear.
We have some circumstantial evidences that weak cosmic censorship is still
satisfied, even for such excessive charge cases. Also, we confirm that there is
mass inflation along the inner horizon, although the properties are quite
different from those of four-dimensional cases. Semi-classical back-reactions
will not affect the outer horizon, but they will affect the inner horizon. Near
the center, there is a place where negative energy is concentrated. Thus,
charged black holes in three dimensions have two types of curvature
singularities in general: via mass inflation and via a concentration of
negative energy. Finally, we classify possible causal structures.Comment: 40 pages, 15 figure
Responses of the Brans-Dicke field due to gravitational collapses
We study responses of the Brans-Dicke field due to gravitational collapses of
scalar field pulses using numerical simulations. Double-null formalism is
employed to implement the numerical simulations. If we supply a scalar field
pulse, it will asymptotically form a black hole via dynamical interactions of
the Brans-Dicke field. Hence, we can observe the responses of the Brans-Dicke
field by two different regions. First, we observe the late time behaviors after
the gravitational collapse, which include formations of a singularity and an
apparent horizon. Second, we observe the fully dynamical behaviors during the
gravitational collapse and view the energy-momentum tensor components. For the
late time behaviors, if the Brans-Dicke coupling is greater (or smaller) than
-1.5, the Brans-Dicke field decreases (or increases) during the gravitational
collapse. Since the Brans-Dicke field should be relaxed to the asymptotic value
with the elapse of time, the final apparent horizon becomes time-like (or
space-like). For the dynamical behaviors, we observed the energy-momentum
tensors around ~ -1.5. If the Brans-Dicke coupling is greater than
-1.5, the component can be negative at the outside of the black hole.
This can allow an instantaneous inflating region during the gravitational
collapse. If the Brans-Dicke coupling is less than -1.5, the oscillation of the
component allows the apparent horizon to shrink. This allows a
combination that violates weak cosmic censorship. Finally, we discuss the
implications of the violation of the null energy condition and weak cosmic
censorship.Comment: 28 pages, 14 figure
The no-boundary measure in string theory: Applications to moduli stabilization, flux compactification, and cosmic landscape
We investigate the no-boundary measure in the context of moduli
stabilization. To this end, we first show that for exponential potentials,
there are no classical histories once the slope exceeds a critical value. We
also investigate the probability distributions given by the no-boundary wave
function near maxima of the potential. These results are then applied to a
simple model that compactifies 6D to 4D (HBSV model) with fluxes. We find that
the no-boundary wave function effectively stabilizes the moduli of the model.
Moreover, we find the a priori probability for the cosmological constant in
this model. We find that a negative value is preferred, and a vanishing
cosmological constant is not distinguished by the probability measure. We also
discuss the application to the cosmic landscape. Our preliminary arguments
indicate that the probability of obtaining anti de Sitter space is vastly
greater than for de Sitter.Comment: 27 pages, 8 figure
Bubble collision with gravitation
In this paper, we study vacuum bubble collisions with various potentials
including gravitation, assuming spherical, planar, and hyperbolic symmetry. We
use numerical calculations from double-null formalism. Spherical symmetry can
mimic the formation of a black hole via multiple bubble collisions. Planar and
especially hyperbolic symmetry describes two bubble collisions. We study both
cases, when two true vacuum regions have the same field value or different
field values, by varying tensions. For the latter case, we also test symmetric
and asymmetric bubble collisions, and see details of causal structures. If the
colliding energy is sufficient, then the vacuum can be destabilized, and it is
also demonstrated. This double-null formalism can be a complementary approach
in the context of bubble collisions.Comment: 31 pages, 19 figure
The horizon-entropy increase law for causal and quasi-local horizons and conformal field redefinitions
We explicitly prove the horizon-entropy increase law for both causal and
quasi-locally defined horizons in scalar-tensor and gravity theories.
Contrary to causal event horizons, future outer trapping horizons are not
conformally invariant and we provide a modification of trapping horizons to
complete the proof, using the idea of generalised entropy. This modification
means they are no longer foliated by marginally outer trapped surfaces but
fixes the location of the horizon under a conformal transformation. We also
discuss the behaviour of horizons in "veiled" general relativity and show,
using this new definition, how to locate cosmological horizons in flat
Minkowski space with varying units, which is physically identified with a
spatially flat FLRW spacetime.Comment: 23 page
- âŠ