419 research outputs found
Accelerating Universe from an Evolving Lambda in Higher Dimension
We find exact solutions in five dimensional inhomogeneous matter dominated
model with a varying cosmological constant. Adjusting arbitrary constants of
integration one can also achieve acceleration in our model. Aside from an
initial singularity our spacetime is regular everywhere including the centre of
the inhomogeneous distribution. We also study the analogous homogeneous
universe in (4+d) dimensions. Here an initially decelerating model is found to
give late acceleration in conformity with the current observational demands. We
also find that both anisotropy and number of dimensions have a role to play in
determining the time of flip, in fact the flip is delayed in multidimensional
models. Some astrophysical parameters like the age, luminosity distance etc are
also calculated and the influence of extra dimensions is briefly discussed.
Interestingly our model yields a larger age of the universe compared to many
other quintessential models.Comment: 18 pages, 9 figure
Lagrangian description of the fluid flow with vorticity in the relativistic cosmology
We develop the Lagrangian perturbation theory in the general relativistic
cosmology, which enables us to take into account the vortical effect of the
dust matter. Under the Lagrangian representation of the fluid flow, the
propagation equation for the vorticity as well as the density is exactly
solved. Based on this, the coupling between the density and vorticity is
clarified in a non-perturbative way. The relativistic correspondence to the
Lagrangian perturbation theory in the Newtonian cosmology is also emphasized.Comment: 14 pages (RevTeX); accepted for publication in Phys. Rev.
Three-body interactions in colloidal systems
We present the first direct measurement of three-body interactions in a
colloidal system comprised of three charged colloidal particles. Two of the
particles have been confined by means of a scanned laser tweezers to a
line-shaped optical trap where they diffused due to thermal fluctuations. Upon
the approach of a third particle, attractive three-body interactions have been
observed. The results are in qualitative agreement with additionally performed
nonlinear Poissson-Boltzmann calculations, which also allow us to investigate
the microionic density distributions in the neighborhood of the interacting
colloidal particles
The Plant Microbiome: From Ecology to Reductionism and beyond
Methodological advances over the past two decades have propelled plant microbiome research, allowing the field to comprehensively test ideas proposed over a century ago and generate many new hypotheses. Studying the distribution of microbial taxa and genes across plant habitats has revealed the importance of various ecological and evolutionary forces shaping plant microbiota. In particular, selection imposed by plant habitats strongly shapes the diversity and composition of microbiota and leads to microbial adaptation associated with navigating the plant immune system and utilizing plant-derived resources. Reductionist approaches have demonstrated that the interaction between plant immunity and the plant microbiome is, in fact, bidirectional and that plants, microbiota, and the environment shape a complex chemical dialogue that collectively orchestrates the plant microbiome. The next stage in plant microbiome research will require the integration of ecological and reductionist approaches to establish a general understanding of the assembly and function in both natural and managed environments
The Sachs-Wolfe Effect: Gauge Independence and a General Expression
In this paper we address two points concerning the Sachs-Wolfe effect: (i)
the gauge independence of the observable temperature anisotropy, and (ii) a
gauge-invariant expression of the effect considering the most general situation
of hydrodynamic perturbations. The first result follows because the gauge
transformation of the temperature fluctuation at the observation event only
contributes to the isotropic temperature change which, in practice, is absorbed
into the definition of the background temperature. Thus, we proceed without
fixing the gauge condition, and express the Sachs-Wolfe effect using the
gauge-invariant variables.Comment: 5 pages, closer to published versio
Dark Energy from structure: a status report
The effective evolution of an inhomogeneous universe model in any theory of
gravitation may be described in terms of spatially averaged variables. In
Einstein's theory, restricting attention to scalar variables, this evolution
can be modeled by solutions of a set of Friedmann equations for an effective
volume scale factor, with matter and backreaction source terms. The latter can
be represented by an effective scalar field (`morphon field') modeling Dark
Energy.
The present work provides an overview over the Dark Energy debate in
connection with the impact of inhomogeneities, and formulates strategies for a
comprehensive quantitative evaluation of backreaction effects both in
theoretical and observational cosmology. We recall the basic steps of a
description of backreaction effects in relativistic cosmology that lead to
refurnishing the standard cosmological equations, but also lay down a number of
challenges and unresolved issues in connection with their observational
interpretation.
The present status of this subject is intermediate: we have a good
qualitative understanding of backreaction effects pointing to a global
instability of the standard model of cosmology; exact solutions and
perturbative results modeling this instability lie in the right sector to
explain Dark Energy from inhomogeneities. It is fair to say that, even if
backreaction effects turn out to be less important than anticipated by some
researchers, the concordance high-precision cosmology, the architecture of
current N-body simulations, as well as standard perturbative approaches may all
fall short in correctly describing the Late Universe.Comment: Invited Review for a special Gen. Rel. Grav. issue on Dark Energy, 59
pages, 2 figures; matches published versio
A Two-Dimensional Electron Gas as a Sensitive Detector for Time-Resolved Tunneling Measurements on Self-Assembled Quantum Dots
A two-dimensional electron gas (2DEG) situated nearby a single layer of self-assembled quantum dots (QDs) in an inverted high electron mobility transistor (HEMT) structure is used as a detector for time-resolved tunneling measurements. We demonstrate a strong influence of charged QDs on the conductance of the 2DEG which allows us to probe the tunneling dynamics between the 2DEG and the QDs time resolved. Measurements of hysteresis curves with different sweep times and real-time conductance measurements in combination with an boxcar-like evaluation method enables us to unambiguously identify the transients as tunneling events between the s- and p-electron QD states and the 2DEG and rule out defect-related transients
Knowledge-based energy functions for computational studies of proteins
This chapter discusses theoretical framework and methods for developing
knowledge-based potential functions essential for protein structure prediction,
protein-protein interaction, and protein sequence design. We discuss in some
details about the Miyazawa-Jernigan contact statistical potential,
distance-dependent statistical potentials, as well as geometric statistical
potentials. We also describe a geometric model for developing both linear and
non-linear potential functions by optimization. Applications of knowledge-based
potential functions in protein-decoy discrimination, in protein-protein
interactions, and in protein design are then described. Several issues of
knowledge-based potential functions are finally discussed.Comment: 57 pages, 6 figures. To be published in a book by Springe
Are biological systems poised at criticality?
Many of life's most fascinating phenomena emerge from interactions among many
elements--many amino acids determine the structure of a single protein, many
genes determine the fate of a cell, many neurons are involved in shaping our
thoughts and memories. Physicists have long hoped that these collective
behaviors could be described using the ideas and methods of statistical
mechanics. In the past few years, new, larger scale experiments have made it
possible to construct statistical mechanics models of biological systems
directly from real data. We review the surprising successes of this "inverse"
approach, using examples form families of proteins, networks of neurons, and
flocks of birds. Remarkably, in all these cases the models that emerge from the
data are poised at a very special point in their parameter space--a critical
point. This suggests there may be some deeper theoretical principle behind the
behavior of these diverse systems.Comment: 21 page
Search for the standard model Higgs boson decaying into two photons in pp collisions at sqrt(s)=7 TeV
A search for a Higgs boson decaying into two photons is described. The
analysis is performed using a dataset recorded by the CMS experiment at the LHC
from pp collisions at a centre-of-mass energy of 7 TeV, which corresponds to an
integrated luminosity of 4.8 inverse femtobarns. Limits are set on the cross
section of the standard model Higgs boson decaying to two photons. The expected
exclusion limit at 95% confidence level is between 1.4 and 2.4 times the
standard model cross section in the mass range between 110 and 150 GeV. The
analysis of the data excludes, at 95% confidence level, the standard model
Higgs boson decaying into two photons in the mass range 128 to 132 GeV. The
largest excess of events above the expected standard model background is
observed for a Higgs boson mass hypothesis of 124 GeV with a local significance
of 3.1 sigma. The global significance of observing an excess with a local
significance greater than 3.1 sigma anywhere in the search range 110-150 GeV is
estimated to be 1.8 sigma. More data are required to ascertain the origin of
this excess.Comment: Submitted to Physics Letters
- …