19,254 research outputs found
Understanding Our Universe: Current Status and Open Issues
Last couple of decades have been the golden age for cosmology. High quality
data confirmed the broad paradigm of standard cosmology but have thrusted upon
us a preposterous composition for the universe which defies any simple
explanation, thereby posing probably the greatest challenge theoretical physics
has ever faced. Several aspects of these developments are critically reviewed,
concentrating on conceptual issues and open questions. [Topics discussed
include: Cosmological Paradigm, Growth of structures in the universe, Inflation
and generation of initial perturbations, Temperature anisotropies of the CMBR,
Dark energy, Cosmological Constant, Deeper issues in cosmology.]Comment: To appear in "100 Years of Relativity - Space-time Structure:
Einstein and Beyond", A.Ashtekar (Editor), World Scientific (Singapore,
2005); 30 pages; 4 figure
Divergence and convergence of inertial particles in high Reynolds number turbulence
Inertial particle data from three-dimensional direct numerical simulations of
particle-laden homogeneous isotropic turbulence at high Reynolds number are
analyzed using Voronoi tessellation of the particle positions, considering
different Stokes numbers. A finite-time measure to quantify the divergence of
the particle velocity by determining the volume change rate of the Voronoi
cells is proposed. For inertial particles the probability distribution function
(PDF) of the divergence deviates from that for fluid particles. Joint PDFs of
the divergence and the Voronoi volume illustrate that the divergence is most
prominent in cluster regions and less pronounced in void regions. For larger
volumes the results show negative divergence values which represent cluster
formation (i.e. particle convergence) and for small volumes the results show
positive divergence values which represents cluster destruction/void formation
(i.e. particle divergence). Moreover, when the Stokes number increases the
divergence takes larger values, which gives some evidence why fine clusters are
less observed for large Stokes numbers. Theoretical analyses further show that
the divergence for random particles in random flow satisfies a PDF
corresponding to the ratio of two independent variables following normal and
gamma distributions in one dimension. Extending this model to three dimensions,
the predicted PDF agrees reasonably well with Monte-Carlo simulations and DNS
data of fluid particles.Comment: 23 pages, 9 figure
Cosmological Tests of Gravity
Modifications of general relativity provide an alternative explanation to
dark energy for the observed acceleration of the universe. We review recent
developments in modified gravity theories, focusing on higher dimensional
approaches and chameleon/f(R) theories. We classify these models in terms of
the screening mechanisms that enable such theories to approach general
relativity on small scales (and thus satisfy solar system constraints). We
describe general features of the modified Friedman equation in such theories.
The second half of this review describes experimental tests of gravity in
light of the new theoretical approaches. We summarize the high precision tests
of gravity on laboratory and solar system scales. We describe in some detail
tests on astrophysical scales ranging from ~kpc (galaxy scales) to ~Gpc
(large-scale structure). These tests rely on the growth and inter-relationship
of perturbations in the metric potentials, density and velocity fields which
can be measured using gravitational lensing, galaxy cluster abundances, galaxy
clustering and the Integrated Sachs-Wolfe effect. A robust way to interpret
observations is by constraining effective parameters, such as the ratio of the
two metric potentials. Currently tests of gravity on astrophysical scales are
in the early stages --- we summarize these tests and discuss the interesting
prospects for new tests in the coming decade.Comment: Invited review for Annals of Physics; 58 pages, 8 figures
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