65 research outputs found
Age-dependent decay in the landscape
The picture of the "multiverse" arising in diverse cosmological scenarios
involves transitions between metastable vacuum states. It was pointed out by
Krauss and Dent that the transition rates decrease at very late times, leading
to a dependence of the transition probability between vacua on the age of each
vacuum region. I investigate the implications of this non-Markovian,
age-dependent decay on the global structure of the spacetime in landscape
scenarios. I show that the fractal dimension of the eternally inflating domain
is precisely equal to 3, instead of being slightly below 3 in scenarios with
purely Markovian, age-independent decay. I develop a complete description of a
non-Markovian landscape in terms of a nonlocal master equation. Using this
description I demonstrate by an explicit calculation that, under some technical
assumptions about the landscape, the probabilistic predictions of our position
in the landscape are essentially unchanged, regardless of the measure used to
extract these predictions. I briefly discuss the physical plausibility of
realizing non-Markovian vacuum decay in cosmology in view of the possible
decoherence of the metastable quantum state.Comment: 10 pages, RevTeX4, 1 figure included. Clarification of approximation
used, conclusions weakene
Non-Gaussianity in Island Cosmology
In this paper we fully calculate the non-Gaussianity of primordial curvature
perturbation of island universe by using the second order perturbation
equation. We find that for the spectral index , which is
favored by current observations, the non-Gaussianity level seen in
island will generally lie between 30 60, which may be tested by the
coming observations. In the landscape, the island universe is one of
anthropically acceptable cosmological histories. Thus the results obtained in
some sense means the coming observations, especially the measurement of
non-Gaussianity, will be significant to make clear how our position in the
landscape is populated.Comment: 5 pages, 1 eps figure, some discussions added, published versio
CMB Bispectrum from Active Models of Structure Formation
We propose a new method for the numerical computation of the angular bispectrum of the CMB anisotropies arising from active models such as cosmic topological defects, using a modified Boltzmann code. The method, similarly to CMBFAST, does not use CMB sky maps and requires moderate computational power. As a first implementation, we apply our method to a recently proposed model of simulated cosmic strings and find that the observability of the non-Gaussian signal is negligible
Fisher's arrow of `time' in cosmological coherent phase space
Fisher's arrow of `time' in a cosmological phase space defined as in quantum
optics (i.e., whose points are coherent states) is introduced as follows.
Assuming that the phase space evolution of the universe starts from an initial
squeezed cosmological state towards a final thermal one, a Fokker-Planck
equation for the time-dependent, cosmological Q phase space probability
distribution can be written down. Next, using some recent results in the
literature, we derive an information arrow of time for the Fisher phase space
cosmological entropy based on the Q function. We also mention the application
of Fisher's arrow of time to stochastic inflation modelsComment: 10 pages, LaTex, Honorable Mention at GRF-199
Counting Pockets with World Lines in Eternal Inflation
We consider the long standing puzzle of how to obtain meaningful
probabilities in eternal inflation. We demonstrate a new algorithm to compute
the probability distribution of pocket universe types, given a multivacua
inflationary potential. The computed probability distribution is finite and
manifestly gauge-independent. We argue that in some scenarios this technique
can be applied to disfavor some eternally inflating potentials.Comment: 16 pages, 9 figures, added references, minor elaboration on a few
points to match published versio
Morphological characterization of shocked porous material
Morphological measures are introduced to probe the complex procedure of shock
wave reaction on porous material. They characterize the geometry and topology
of the pixelized map of a state variable like the temperature. Relevance of
them to thermodynamical properties of material is revealed and various
experimental conditions are simulated. Numerical results indicate that, the
shock wave reaction results in a complicated sequence of compressions and
rarefactions in porous material. The increasing rate of the total fractional
white area roughly gives the velocity of a compressive-wave-series.
When a velocity is mentioned, the corresponding threshold contour-level of
the state variable, like the temperature, should also be stated. When the
threshold contour-level increases, becomes smaller. The area increases
parabolically with time during the initial period. The curve goes
back to be linear in the following three cases: (i) when the porosity
approaches 1, (ii) when the initial shock becomes stronger, (iii) when the
contour-level approaches the minimum value of the state variable. The area with
high-temperature may continue to increase even after the early
compressive-waves have arrived at the downstream free surface and some
rarefactive-waves have come back into the target body. In the case of energetic
material ... (see the full text)Comment: 3 figures in JPG forma
Predictability crisis in inflationary cosmology and its resolution
Models of inflationary cosmology can lead to variation of observable
parameters ("constants of Nature") on extremely large scales. The question of
making probabilistic predictions for today's observables in such models has
been investigated in the literature. Because of the infinite thermalized volume
resulting from eternal inflation, it has proven difficult to obtain a
meaningful and unambiguous probability distribution for observables, in
particular due to the gauge dependence. In the present paper, we further
develop the gauge-invariant procedure proposed in a previous work for models
with a continuous variation of "constants". The recipe uses an unbiased
selection of a connected piece of the thermalized volume as sample for the
probability distribution. To implement the procedure numerically, we develop
two methods applicable to a reasonably wide class of models: one based on the
Fokker-Planck equation of stochastic inflation, and the other based on direct
simulation of inflationary spacetime. We present and compare results obtained
using these methods.Comment: 23 pages, 13 figure
Cosmological particle production and the precision of the WKB approximation
Particle production by slow-changing gravitational fields is usually
described using quantum field theory in curved spacetime. Calculations require
a definition of the vacuum state, which can be given using the adiabatic (WKB)
approximation. I investigate the best attainable precision of the resulting
approximate definition of the particle number. The standard WKB ansatz yields a
divergent asymptotic series in the adiabatic parameter. I derive a novel
formula for the optimal number of terms in that series and demonstrate that the
error of the optimally truncated WKB series is exponentially small. This
precision is still insufficient to describe particle production from vacuum,
which is typically also exponentially small. An adequately precise
approximation can be found by improving the WKB ansatz through perturbation
theory. I show quantitatively that the fundamentally unavoidable imprecision in
the definition of particle number in a time-dependent background is equal to
the particle production expected to occur during that epoch. The results are
illustrated by analytic and numerical examples.Comment: 14 pages, RevTeX, 5 figures; minor changes, a clarification in Sec.
II
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