133,027 research outputs found
Computational Cosmology: from the Early Universe to the Large Scale Structure
In order to account for the observable Universe, any comprehensive theory or
model of cosmology must draw from many disciplines of physics, including gauge
theories of strong and weak interactions, the hydrodynamics and microphysics of
baryonic matter, electromagnetic fields, and spacetime curvature, for example.
Although it is difficult to incorporate all these physical elements into a
single complete model of our Universe, advances in computing methods and
technologies have contributed significantly towards our understanding of
cosmological models, the Universe, and astrophysical processes within them. A
sample of numerical calculations (and numerical methods) applied to specific
issues in cosmology are reviewed in this article: from the Big Bang singularity
dynamics to the fundamental interactions of gravitational waves; from the
quark-hadron phase transition to the large scale structure of the Universe. The
emphasis, although not exclusively, is on those calculations designed to test
different models of cosmology against the observed Universe.Comment: appearing, Living Reviews in Relativit
Boundary Inflation and the WMAP Data
Inflation in a five-dimensional brane world model with two boundary branes is
studied. We make use of the moduli space approximation whereby the low energy
theory reduces to a four-dimensional biscalar-tensor gravity plus a minimally
coupled scalar field. After a detailed analysis of the inflationary solutions,
we derive the evolution equations of the linear perturbations separating the
adiabatic mode from two entropy modes. We then examine the primordial scalar
and tensor power spectra and show that their tilt depends on the scalar-tensor
coupling constant. Finally, the induced CMB anisotropies are computed and we
present a Monte Carlo Markov Chains exploration of the parameter space using
the first year WMAP data. We find a marginalized probability bound for the
associated Eddington parameter at the end of inflation 1 - gamma < 0.002, at
95% confidence level. This suggests that future CMB data could provide crucial
information helping to distinguish scalar-tensor and standard inflationary
scenarios.Comment: 24 pages, 19 figures, uses RevTex. Qualitative discussions added,
matches published versio
No entailing laws, but enablement in the evolution of the biosphere
Biological evolution is a complex blend of ever changing structural
stability, variability and emergence of new phenotypes, niches, ecosystems. We
wish to argue that the evolution of life marks the end of a physics world view
of law entailed dynamics. Our considerations depend upon discussing the
variability of the very "contexts of life": the interactions between organisms,
biological niches and ecosystems. These are ever changing, intrinsically
indeterminate and even unprestatable: we do not know ahead of time the "niches"
which constitute the boundary conditions on selection. More generally, by the
mathematical unprestatability of the "phase space" (space of possibilities), no
laws of motion can be formulated for evolution. We call this radical emergence,
from life to life. The purpose of this paper is the integration of variation
and diversity in a sound conceptual frame and situate unpredictability at a
novel theoretical level, that of the very phase space. Our argument will be
carried on in close comparisons with physics and the mathematical constructions
of phase spaces in that discipline. The role of (theoretical) symmetries as
invariant preserving transformations will allow us to understand the nature of
physical phase spaces and to stress the differences required for a sound
biological theoretizing. In this frame, we discuss the novel notion of
"enablement". This will restrict causal analyses to differential cases (a
difference that causes a difference). Mutations or other causal differences
will allow us to stress that "non conservation principles" are at the core of
evolution, in contrast to physical dynamics, largely based on conservation
principles as symmetries. Critical transitions, the main locus of symmetry
changes in physics, will be discussed, and lead to "extended criticality" as a
conceptual frame for a better understanding of the living state of matter
Active Galactic Nuclei and Transformation of Dark Matter into Visible Matter
The hypothesis that dark matter is converted into visible particles in active
galactic nuclei is investigated. If dark matter consists of stable superheavy
neutral particles and active galactic nuclei are rotating black holes, then,
due to the Penrose process, superheavy particles can decay into unstable
particles with larger mass, whose decay into quarks and leptons leads to events
in cosmic rays observed by the Auger group. Similar processes of decay of
superheavy particles of dark matter into visible matter occurred in the early
Universe. Numerical estimates of the processes in active galactic nuclei and in
the early Universe are given.Comment: 5 pages, LaTeX; v2: one reference added, published versio
Strangeness in Astrophysics and Cosmology
Some recent developments concerning the role of strange quark matter for
astrophysical systems and the QCD phase transition in the early universe are
addressed. Causality constraints of the soft nuclear equation of state as
extracted from subthreshold kaon production in heavy-ion collisions are used to
derive an upper mass limit for compact stars. The interplay between the
viscosity of strange quark matter and the gravitational wave emission from
rotation-powered pulsars are outlined. The flux of strange quark matter nuggets
in cosmic rays is put in perspective with a detailed numerical investigation of
the merger of two strange stars. Finally, we discuss a novel scenario for the
QCD phase transition in the early universe, which allows for a small
inflationary period due to a pronounced first order phase transition at large
baryochemical potential.Comment: 8 pages, invited talk given at the International Conference on
Strangeness in Quark Matter (SQM2009), Buzios, Brasil, September 28 - October
2, 200
How dissipation constrains fluctuations in nonequilibrium liquids: Diffusion, structure and biased interactions
The dynamics and structure of nonequilibrium liquids, driven by
non-conservative forces which can be either external or internal, generically
hold the signature of the net dissipation of energy in the thermostat. Yet,
disentangling precisely how dissipation changes collective effects remains
challenging in many-body systems due to the complex interplay between driving
and particle interactions. First, we combine explicit coarse-graining and
stochastic calculus to obtain simple relations between diffusion, density
correlations and dissipation in nonequilibrium liquids. Based on these results,
we consider large-deviation biased ensembles where trajectories mimic the
effect of an external drive. The choice of the biasing function is informed by
the connection between dissipation and structure derived in the first part.
Using analytical and computational techniques, we show that biasing
trajectories effectively renormalizes interactions in a controlled manner, thus
providing intuition on how driving forces can lead to spatial organization and
collective dynamics. Altogether, our results show how tuning dissipation
provides a route to alter the structure and dynamics of liquids and soft
materials.Comment: 21 pages, 7 figure
On the Detectability of CMB Anisotropies Induced by de Sitter-G\"odel-de Sitter Phase Transition
A geometrical phase transition in the very early Universe, from de Sitter to
G\"odel and back to de Sitter (dGd) spacetimes, can explain the universal
phenomenon of rotation of many large scale structures. This phase transition is
shown to induce fluctuations on the matter and radiation fields with possibly
observable traces. In this work we simulate the dGd-induced inhomogeneities and
use their power spectrum, parametrized by the parameter pair (), as
possible seeds of CMB anisotropies along with the standard inflationary
perturbations. With the Planck 2018 observations, we find
and consistent
with pure inflationary power spectrum and no hint for the dGd transition.
Future large scale surveys can further tighten the constraints and probe the
physics of the early Universe
Phenomemology of a Realistic Accelerating Universe Using Tracker Fields
We present a realistic scenario of tracking of scalar fields with varying
equation of state. The astrophysical constraints on the evolution of scalar
fields in the physical universe are discussed. The nucleosynthesis and the
galaxy formation constraints have been used to put limits on and
estimate during cosmic evolution. Interpolation techniques have been
applied to estimate at the present epoch. The epoch of
transition from matter to quintessence dominated era and consequent onset of
acceleration in cosmic expansion is calculated and taking the lower limit
as estimated from data, it is shown that the
supernova observations beyond redshift would reveal deceleration in
cosmic expansion.Comment: 10 pages, 4 figures, late
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