205 research outputs found
Many worlds in one
A generic prediction of inflation is that the thermalized region we inhabit
is spatially infinite. Thus, it contains an infinite number of regions of the
same size as our observable universe, which we shall denote as \O-regions. We
argue that the number of possible histories which may take place inside of an
\O-region, from the time of recombination up to the present time, is finite.
Hence, there are an infinite number of \O-regions with identical histories up
to the present, but which need not be identical in the future. Moreover, all
histories which are not forbidden by conservation laws will occur in a finite
fraction of all \O-regions. The ensemble of \O-regions is reminiscent of
the ensemble of universes in the many-world picture of quantum mechanics. An
important difference, however, is that other \O-regions are unquestionably
real.Comment: 9 pages, 2 figures, comments and references adde
Effects of friction on cosmic strings
We study the evolution of cosmic strings taking into account the frictional
force due to the surrounding radiation. We consider small perturbations on
straight strings, oscillation of circular loops and small perturbations on
circular loops. For straight strings, friction exponentially suppresses
perturbations whose co-moving scale crosses the horizon before cosmological
time (in Planck units), where is the string tension.
Loops with size much smaller than will be approximately circular at the
time when they start the relativistic collapse. We investigate the possibility
that such loops will form black holes. We find that the number of black holes
which are formed through this process is well bellow present observational
limits, so this does not give any lower or upper bounds on . We also
consider the case of straight strings attached to walls and circular holes that
can spontaneously nucleate on metastable domain walls.Comment: 32 pages, TUTP-93-
Glimmers of a pre-geometric perspective
Space-time measurements and gravitational experiments are made by using
objects, matter fields or particles and their mutual relationships. As a
consequence, any operationally meaningful assertion about space-time is in fact
an assertion about the degrees of freedom of the matter (\emph{i.e} non
gravitational) fields; those, say for definiteness, of the Standard Model of
particle physics. As for any quantum theory, the dynamics of the matter fields
can be described in terms of a unitary evolution of a state vector in a Hilbert
space. By writing the Hilbert space as a generic tensor product of "subsystems"
we analyse the evolution of a state vector on an information theoretical basis
and attempt to recover the usual space-time relations from the information
exchanges between these subsystems. We consider generic interacting second
quantized models with a finite number of fermionic degrees of freedom and
characterize on physical grounds the tensor product structure associated with
the class of "localized systems" and therefore with "position". We find that in
the case of free theories no space-time relation is operationally definable. On
the contrary, by applying the same procedure to the simple interacting model of
a one-dimensional Heisenberg spin chain we recover the tensor product structure
usually associated with "position". Finally, we discuss the possible role of
gravity in this framework.Comment: 30 page
Reduced contextually induced muscle thermogenesis in rats with calorie restriction and lower aerobic fitness but not monogenic obesity
We have previously identified predator odor as a potent stimulus activating thermogenesis in skeletal muscle in rats. As this may prove relevant for energy balance and weight loss, the current study investigated whether skeletal muscle thermogenesis was altered with negative energy balance, obesity propensity seen in association with low intrinsic aerobic fitness, and monogenic obesity. First, weight loss subsequent to three weeks of 50% calorie restriction suppressed the muscle thermogenic response to predator odor. Next, we compared rats bred based on artificial selection for intrinsic aerobic fitness—high- and low-capacity runners (HCR, LCR)—that display robust leanness and obesity propensity, respectively. Aerobically fit HCR showed enhanced predator odor-induced muscle thermogenesis relative to the less-fit LCR. This contrasted with the profound monogenic obesity displayed by rats homozygous for a loss of function mutation in Melanocortin 4 receptor (Mc4rK314X/K314X rats), which showed no discernable deficit in thermogenesis. Taken together, these data imply that body size or obesity per se are not associated with deficient muscle thermogenesis. Rather, the physiological phenotype associated with polygenic obesity propensity may encompass pleiotropic mechanisms in the thermogenic pathway. Adaptive thermogenesis associated with weight loss also likely alters muscle thermogenic mechanisms.</p
A New Class of Inhomogeneous String Cosmological Models in General Relativity
A new class of solutions of Einstein field equations has been investigated
for inhomogeneous cylindrically symmetric space-time with string source. To get
the deterministic solution, it has been assumed that the expansion ()
in the model is proportional to the eigen value of the shear
tensor . Certain physical and geometric properties of the
models are also discussed.Comment: 12 pages, no figure. Submitted to Astrophys. Space Sci. arXiv admin
note: substantial text overlap with arXiv:0705.090
Cylindrically Symmetric Inhomogeneous Universes with a Cloud of Strings
Cylindrically symmetric inhomogeneous string cosmological models are
investigated in presence of string fluid as a source of matter. To get the
three types of exact solutions of Einstein's field equations we assume , and . Some physical and geometric
aspects of the models are discussed.Comment: 9 page
The Mathematical Universe
I explore physics implications of the External Reality Hypothesis (ERH) that
there exists an external physical reality completely independent of us humans.
I argue that with a sufficiently broad definition of mathematics, it implies
the Mathematical Universe Hypothesis (MUH) that our physical world is an
abstract mathematical structure. I discuss various implications of the ERH and
MUH, ranging from standard physics topics like symmetries, irreducible
representations, units, free parameters, randomness and initial conditions to
broader issues like consciousness, parallel universes and Godel incompleteness.
I hypothesize that only computable and decidable (in Godel's sense) structures
exist, which alleviates the cosmological measure problem and help explain why
our physical laws appear so simple. I also comment on the intimate relation
between mathematical structures, computations, simulations and physical
systems.Comment: Replaced to match accepted Found. Phys. version, 31 pages, 5 figs;
more details at http://space.mit.edu/home/tegmark/toe.htm
Modeling what we sample and sampling what we model: challenges for zooplankton model assessment
Zooplankton are the intermediate trophic level between phytoplankton and fish, and are an important component of carbon and nutrient cycles, accounting for a large proportion of the energy transfer to pelagic fishes and the deep ocean. Given zooplankton's importance, models need to adequately represent zooplankton dynamics. A major obstacle, though, is the lack of model assessment. Here we try and stimulate the assessment of zooplankton in models by filling three gaps. The first is that many zooplankton observationalists are unfamiliar with the biogeochemical, ecosystem, size-based and individual-based models that have zooplankton functional groups, so we describe their primary uses and how each typically represents zooplankton. The second gap is that many modelers are unaware of the zooplankton data that are available, and are unaccustomed to the different zooplankton sampling systems, so we describe the main sampling platforms and discuss their strengths and weaknesses for model assessment. Filling these gaps in our understanding of models and observations provides the necessary context to address the last gap—a blueprint for model assessment of zooplankton. We detail two ways that zooplankton biomass/abundance observations can be used to assess models: data wrangling that transforms observations to be more similar to model output; and observation models that transform model outputs to be more like observations. We hope that this review will encourage greater assessment of zooplankton in models and ultimately improve the representation of their dynamics
Hamiltonian dynamics and Noether symmetries in Extended Gravity Cosmology
We discuss the Hamiltonian dynamics for cosmologies coming from Extended
Theories of Gravity. In particular, minisuperspace models are taken into
account searching for Noether symmetries. The existence of conserved quantities
gives selection rule to recover classical behaviors in cosmic evolution
according to the so called Hartle criterion, that allows to select correlated
regions in the configuration space of dynamical variables. We show that such a
statement works for general classes of Extended Theories of Gravity and is
conformally preserved. Furthermore, the presence of Noether symmetries allows a
straightforward classification of singularities that represent the points where
the symmetry is broken. Examples of nonminimally coupled and higher-order
models are discussed.Comment: 20 pages, Review paper to appear in EPJ
Supersymmetric Dark Matter and Yukawa Unification
An analysis of supersymmetric dark matter under the Yukawa unification
constraint is given. The analysis utilizes the recently discovered region of
the parameter space of models with gaugino mass nonuniversalities where large
negative supersymmetric corrections to the b quark mass appear to allow
unification for a positive sign consistent with the and constraints. In the present analysis we use the
revised theoretical determination of ()
in computing the difference which takes account of
a reevaluation of the light by light contribution which has a positive sign.
The analysis shows that the region of the parameter space with
nonuniversalities of the gaugino masses which allows for unification of Yukawa
couplings also contains regions which allow satisfaction of the relic density
constraint. Specifically we find that the lightest neutralino mass consistent
with the relic density constraint, unification for SU(5) and
unification for SO(10) in addition to other constraints lies in the region
below 80 GeV. An analysis of the maximum and the minimum neutralino-proton
scalar cross section for the allowed parameter space including the effect of a
new determination of the pion-nucleon sigma term is also given. It is found
that the full parameter space for this class of models can be explored in the
next generation of proposed dark matter detectors.Comment: 28 pages,nLatex including 5 fig
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