1,181 research outputs found
Action of Singular Instantons of Hawking-Turok Type
Using Kaluza-Klein technique we show that the singularity of Hawking-Turok
type has a fixed point (bolt) contribution to the action in addition to the
usual boundary contribution. Interestingly by adding this contribution we can
obtain a simple expression for the total action which is feasible for both
regular and singular instantons. Our result casts doubt on the constraint
proposed by Turok in the recent calculation in which Vilenkin's instantons are
regarded as a limit of certain constrained instantons.Comment: 14 pages, LaTe
Coincident brane nucleation and the neutralization of \Lambda
Nucleation of branes by a four-form field has recently been considered in
string motivated scenarios for the neutralization of the cosmological constant.
An interesting question in this context is whether the nucleation of stacks of
coincident branes is possible, and if so, at what rate does it proceed. Feng et
al. have suggested that, at high ambient de Sitter temperature, the rate may be
strongly enhanced, due to large degeneracy factors associated with the number
of light species living on the worldsheet. This might facilitate the quick
relaxation from a large effective cosmological constant down to the observed
value. Here, we analyse this possibility in some detail. In four dimensions,
and after the moduli are stabilized, branes interact via repulsive long range
forces. Because of that, the Coleman-de Luccia (CdL) instanton for coincident
brane nucleation may not exist, unless there is some short range interaction
which keeps the branes together. If the CdL instanton exists, we find that the
degeneracy factor depends only mildly on the ambient de Sitter temperature, and
does not switch off even in the case of tunneling from flat space. This would
result in catastrophic decay of the present vacuum. If, on the contrary, the
CdL instanton does not exist, coindident brane nucleation may still proceed
through a "static" instanton, representing pair creation of critical bubbles --
a process somewhat analogous to thermal activation in flat space. In that case,
the branes may stick together due to thermal symmetry restoration, and the pair
creation rate depends exponentially on the ambient de Sitter temperature,
switching off sharply as the temperature approaches zero. Such static instanton
may be well suited for the "saltatory" relaxation scenario proposed by Feng et
al.Comment: 38 pages, 6 figures. Replaced with typos correcte
Second Order Perturbations of a Macroscopic String; Covariant Approach
Using a world-sheet covariant formalism, we derive the equations of motion
for second order perturbations of a generic macroscopic string, thus
generalizing previous results for first order perturbations. We give the
explicit results for the first and second order perturbations of a contracting
near-circular string; these results are relevant for the understanding of the
possible outcome when a cosmic string contracts under its own tension, as
discussed in a series of papers by Vilenkin and Garriga. In particular, second
order perturbations are necessaary for a consistent computation of the energy.
We also quantize the perturbations and derive the mass-formula up to second
order in perturbations for an observer using world-sheet time . The high
frequency modes give the standard Minkowski result while, interestingly enough,
the Hamiltonian turns out to be non-diagonal in oscillators for low-frequency
modes. Using an alternative definition of the vacuum, it is possible to
diagonalize the Hamiltonian, and the standard string mass-spectrum appears for
all frequencies. We finally discuss how our results are also relevant for the
problems concerning string-spreading near a black hole horizon, as originally
discussed by Susskind.Comment: New discussion about the quantum mass-spectrum in chapter
Solutions to the cosmological constant problems
We critically review several recent approaches to solving the two
cosmological constant problems. The "old" problem is the discrepancy between
the observed value of and the large values suggested by particle
physics models. The second problem is the "time coincidence" between the epoch
of galaxy formation and the epoch of -domination t_\L. It is
conceivable that the "old" problem can be resolved by fundamental physics
alone, but we argue that in order to explain the "time coincidence" we must
account for anthropic selection effects. Our main focus here is on the
discrete- models in which can change through nucleation of
branes. We consider the cosmology of this type of models in the context of
inflation and discuss the observational constraints on the model parameters.
The issue of multiple brane nucleation raised by Feng {\it et. al.} is
discussed in some detail. We also review continuous-\L models in which the
role of the cosmological constant is played by a slowly varying potential of a
scalar field. We find that both continuous and discrete models can in principle
solve both cosmological constant problems, although the required values of the
parameters do not appear very natural. M-theory-motivated brane models, in
which the brane tension is determined by the brane coupling to the four-form
field, do not seem to be viable, except perhaps in a very tight corner of the
parameter space. Finally, we point out that the time coincidence can also be
explained in models where is fixed, but the primordial density
contrast is treated as a random variable.Comment: 30 pages, 3 figures, two notes adde
Spectrum from the warped compactifications with the de Sitter universe
We discuss the spectrum of the tensor metric perturbations and the stability
of warped compactifications with the de Sitter spacetime in the
higher-dimensional gravity. The spacetime structure is given in terms of the
warped product of the non-compact direction, the spherical internal dimensions
and the four-dimensional de Sitter spacetime. To realize a finite bulk volume,
we construct the brane world model, using the cut-copy-paste method. Then, we
compactify the spherical directions on the brane. In any case, we show the
existence of the massless zero mode and the mass gap of it with massive
Kaluza-Klein modes. Although the brane involves the spherical dimensions, no
light massive mode is excited. We also investigate the scalar perturbations,
and show that the model is unstable due to the existence of a tachyonic bound
state, which seems to have the universal negative mass square, irrespective of
the number of spacetime dimensions.Comment: Journal version (JHEP
Singular Instantons Made Regular
The singularity present in cosmological instantons of the Hawking-Turok type
is resolved by a conformal transformation, where the conformal factor has a
linear zero of codimension one. We show that if the underlying regular manifold
is taken to have the topology of , and the conformal factor is taken to
be a twisted field so that the zero is enforced, then one obtains a
one-parameter family of solutions of the classical field equations, where the
minimal action solution has the conformal zero located on a minimal volume
noncontractible submanifold. For instantons with two singularities, the
corresponding topology is that of a cylinder with D=4
analogues of `cross-caps' at each of the endpoints.Comment: 23 pages, compressed and RevTex file, including nine postscript
figure files. Submitted versio
Bigravity and Lorentz-violating Massive Gravity
Bigravity is a natural arena where a non-linear theory of massive gravity can
be formulated. If the interaction between the metrics and is
non-derivative, spherically symmetric exact solutions can be found. At large
distances from the origin, these are generically Lorentz-breaking bi-flat
solutions (provided that the corresponding vacuum energies are adjusted
appropriately). The spectrum of linearized perturbations around such
backgrounds contains a massless as well as a massive graviton, with {\em two}
physical polarizations each. There are no propagating vectors or scalars, and
the theory is ghost free (as happens with certain massive gravities with
explicit breaking of Lorentz invariance). At the linearized level, corrections
to GR are proportional to the square of the graviton mass, and so there is no
vDVZ discontinuity. Surprisingly, the solution of linear theory for a static
spherically symmetric source does {\em not} agree with the linearization of any
of the known exact solutions. The latter coincide with the standard
Schwarzschild-(A)dS solutions of General Relativity, with no corrections at
all. Another interesting class of solutions is obtained where and are
proportional to each other. The case of bi-de Sitter solutions is analyzed in
some detail.Comment: 25 pages. v3 Typos corrected, references added. v4 Introduction
extende
Cosmological Perturbations from the No Boundary Euclidean Path Integral
We compute, from first principles, the quantum fluctuations about instanton
saddle points of the Euclidean path integral for Einstein gravity coupled to a
scalar field. The Euclidean two-point correlator is analytically continued into
the Lorentzian region where it describes the quantum mechanical vacuum
fluctuations in the state described by no boundary proposal initial conditions.
We concentrate on the density perturbations in open inflationary universes
produced from cosmological instantons, describing the differences between
non-singular Coleman-De Luccia and singular Hawking-Turok instantons. We show
how the Euclidean path integral uniquely specifies the fluctuations in both
cases.Comment: 21 pages, RevTex file, including five postscript figure file
Black Holes from Nucleating Strings
We evaluate the probability that a loop of string that has spontaneously
nucleated during inflation will form a black hole upon collapse, after the end
of inflation. We then use the observational bounds on the density of primordial
black holes to put constraints on the parameters of the model. Other
constraints from the distortions of the microwave background and emission of
gravitational radiation by the loops are considered. Also, observational
constraints on domain wall nucleation and monopole pair production during
inflation are briefly discussed.Comment: 27 pages, tutp-92-
Defect CFTs and holographic multiverse
We investigate some aspects of a recent proposal for a holographic
description of the multiverse. Specifically, we focus on the implications on
the suggested duality of the fluctuations of a bubble separating two universes
with different cosmological constants. We do so by considering a similar
problem in a 2+1 CFT with a codimension one defect, obtained by an M5-brane
probe embedding in AdS_4x S^7, and studying its spectrum of fluctuations. Our
results suggest that the kind of behavior required by the spectrum of bubble
fluctuations is not likely to take place in defect CFTs with an AdS dual,
although it might be possible if the defect supports a non-unitary theory.Comment: 19 pages; v2: typos fixed, minor changes
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