89 research outputs found
Cosmic string scaling in flat space
We investigate the evolution of infinite strings as a part of a complete
cosmic string network in flat space. We perform a simulation of the network
which uses functional forms for the string position and thus is exact to the
limits of computer arithmetic. Our results confirm that the wiggles on the
strings obey a scaling law described by universal power spectrum. The average
distance between long strings also scales accurately with the time. These
results suggest that small-scale structure will also scale in expanding
universe, even in the absence of gravitational damping.Comment: 13 pages,7 figure
Energy conditions outside a dielectric ball
We show analytically that the vacuum electromagnetic stress-energy tensor
outside a ball with constant dielectric constant and permeability always obeys
the weak, null, dominant, and strong energy conditions. There are still no
known examples in quantum field theory in which the averaged null energy
condition in flat spacetime is violated.Comment: 12 pages, RevTex
The quantum inequalities do not forbid spacetime shortcuts
A class of spacetimes (comprising the Alcubierre bubble, Krasnikov tube, and
a certain type of wormholes) is considered that admits `superluminal travel' in
a strictly defined sense. Such spacetimes (they are called `shortcuts' in this
paper) were suspected to be impossible because calculations based on `quantum
inequalities' suggest that their existence would involve Planck-scale energy
densities and hence unphysically large values of the `total amount of negative
energy' E_tot. I argue that the spacetimes of this type may not be unphysical
at all. By explicit examples I prove that: 1) the relevant quantum inequality
does not (always) imply large energy densities; 2) large densities may not lead
to large values of E_tot; 3) large E_tot, being physically meaningless in some
relevant situations, does not necessarily exclude shortcuts.Comment: Minor corrections and addition
Volume Weighted Measures of Eternal Inflation in the Bousso-Polchinski Landscape
We consider the cosmological dynamics associated with volume weighted
measures of eternal inflation, in the Bousso-Polchinski model of the string
theory landscape. We find that this measure predicts that observers are most
likely to find themselves in low energy vacua with one flux considerably larger
than the rest. Furthermore, it allows for a satisfactory anthropic explanation
of the cosmological constant problem by producing a smooth, and approximately
constant, distribution of potentially observable values of Lambda. The low
energy vacua selected by this measure are often short lived. If we require
anthropically acceptable vacua to have a minimum life-time of 10 billion years,
then for reasonable parameters a typical observer should expect their vacuum to
have a life-time of approximately 12 billion years. This prediction is model
dependent, but may point toward a solution to the coincidence problem of
cosmology.Comment: 35 pages, 8 figure
Prediction and explanation in the multiverse
Probabilities in the multiverse can be calculated by assuming that we are
typical representatives in a given reference class. But is this class well
defined? What should be included in the ensemble in which we are supposed to be
typical? There is a widespread belief that this question is inherently vague,
and that there are various possible choices for the types of reference objects
which should be counted in. Here we argue that the ``ideal'' reference class
(for the purpose of making predictions) can be defined unambiguously in a
rather precise way, as the set of all observers with identical information
content. When the observers in a given class perform an experiment, the class
branches into subclasses who learn different information from the outcome of
that experiment. The probabilities for the different outcomes are defined as
the relative numbers of observers in each subclass. For practical purposes,
wider reference classes can be used, where we trace over all information which
is uncorrelated to the outcome of the experiment, or whose correlation with it
is beyond our current understanding. We argue that, once we have gathered all
practically available evidence, the optimal strategy for making predictions is
to consider ourselves typical in any reference class we belong to, unless we
have evidence to the contrary. In the latter case, the class must be
correspondingly narrowed.Comment: Minor clarifications adde
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