5,274 research outputs found
Observation of the phononic Lamb shift with a synthetic vacuum
The quantum vacuum fundamentally alters the properties of embedded particles.
In contrast to classical empty space, it allows for creation and annihilation
of excitations. For trapped particles this leads to a change in the energy
spectrum, known as Lamb shift. Here, we engineer a synthetic vacuum building on
the unique properties of ultracold atomic gas mixtures. This system makes it
possible to combine high-precision spectroscopy with the ability of switching
between empty space and quantum vacuum. We observe the phononic Lamb shift, an
intruiguing many-body effect orginally conjectured in the context of solid
state physics. Our study therefore opens up new avenues for high-precision
benchmarking of non-trivial theoretical predictions in the realm of the quantum
vacuum
Dynamics of Interacting Quintessence Models: Observational Constraints
Interacting quintessence models have been proposed to explain or, at least,
alleviate the coincidence problem of late cosmic acceleration. In this paper we
are concerned with two aspects of these kind of models: (i) the dynamical
evolution of the model of Chimento et al. [L.P. Chimento, A.S. Jakubi, D.
Pavon, and W. Zimdahl, Phys. Rev. D 67, 083513 (2003).], i.e., whether its
cosmological evolution gives rise to a right sequence of radiation, dark matter
and dark energy dominated eras, and (ii) whether the dark matter dark energy
ratio asymptotically evolves towards a non-zero constant. After showing that
the model correctly reproduces these eras, we correlate three data sets that
constrain the interaction at three redshift epochs: , ,
and . We discuss the model selection and argue that even if the model
under consideration fulfills both requirements, it is heavily constrained by
observation. The prospects that the coincidence problem can be explained by the
coupling of dark matter to dark energy are not clearly favored by the data.Comment: 16 pages, 7 figures and 3 tables. Modifications introduced to match
published versio
Substrate Stiffness Controls Osteoblastic and Chondrocytic Differentiation of Mesenchymal Stem Cells without Exogenous Stimuli
Stem cell fate has been linked to the mechanical properties of their underlying substrate, affecting mechanoreceptors and ultimately leading to downstream biological response. Studies have used polymers to mimic the stiffness of extracellular matrix as well as of individual tissues and shown mesenchymal stem cells (MSCs) could be directed along specific lineages. In this study, we examined the role of stiffness in MSC differentiation to two closely related cell phenotypes: osteoblast and chondrocyte. We prepared four methyl acrylate/methyl methacrylate (MA/MMA) polymer surfaces with elastic moduli ranging from 0.1 MPa to 310 MPa by altering monomer concentration. MSCs were cultured in media without exogenous growth factors and their biological responses were compared to committed chondrocytes and osteoblasts. Both chondrogenic and osteogenic markers were elevated when MSCs were grown on substrates with stiffnesschondrocytes, MSCs on lower stiffness substrates showed elevated expression of ACAN, SOX9, and COL2 and proteoglycan content; COMP was elevated in MSCs but reduced in chondrocytes. Substrate stiffness altered levels of RUNX2 mRNA, alkaline phosphatase specific activity, osteocalcin, and osteoprotegerin in osteoblasts, decreasing levels on the least stiff substrate. Expression of integrin subunits α1, α2, α5, αv, β1, and β3 changed in a stiffness- and cell type-dependent manner. Silencing of integrin subunit beta 1 (ITGB1) in MSCs abolished both osteoblastic and chondrogenic differentiation in response to substrate stiffness. Our results suggest that substrate stiffness is an important mediator of osteoblastic and chondrogenic differentiation, and integrin β1 plays a pivotal role in this process
On the order of BEC transition in weakly interacting gases predicted by mean-field theory
Predictions from Hartree-Fock (HF), Popov (P), Yukalov-Yukalova (YY) and
-matrix approximations regarding the thermodynamics from the normal to the
BEC phase in weakly interacting Bose gases are considered. By analyzing the
dependence of the chemical potential on temperature and particle
density we show that none of them predicts a second-order phase
transition as required by symmetry-breaking general considerations. In this
work we find that the isothermal compressibility predicted by
these theories does not diverge at criticality as expected in a true
second-order phase transition. Moreover the isotherms
typically exhibit a non-singled valued behavior in the vicinity of the BEC
transition, a feature forbidden by general thermodynamic principles. This
behavior can be avoided if a first order phase transition is appealed. The
facts described above show that although these mean field approximations give
correct results near zero temperature they are endowed with thermodynamic
anomalies in the vicinity of the BEC transition. We address the implications of
these results in the interpretation of current experiments with ultracold
trapped alkali gases.Comment: 16 pages, 5 figure
Factor copula models for item response data
Factor or conditional independence models based on copulas are proposed for multivariate discrete data such as item responses. The factor copula models have interpretations of latent maxima/minima (in comparison with latent means) and can lead to more probability in the joint upper or lower tail compared with factor models based on the discretized multivariate normal distribution (or multidimensional normal ogive model). Details on maximum likelihood estimation of parameters for the factor copula model are given, as well as analysis of the behavior of the log-likelihood. Our general methodology is illustrated with several item response data sets, and it is shown that there is a substantial improvement on existing models both conceptually and in fit to data
Static circularly symmetric perfect fluid solutions with an exterior BTZ metric
In this work we study static perfect fluid stars in 2+1 dimensions with an
exterior BTZ spacetime. We found the general expression for the metric
coefficients as a function of the density and pressure of the fluid. We found
the conditions to have regularity at the origin throughout the analysis of a
set of linearly independent invariants. We also obtain an exact solution of the
Einstein equations, with the corresponding equation of state , which
is regular at the origin.Comment: 10 pages, 1 figure, revtex 4. This paper is in honor of Alberto
Garcia's sixtieth birthday. Accepted by Gen. Rel. Gra
Optimal estimation of entanglement
Entanglement does not correspond to any observable and its evaluation always
corresponds to an estimation procedure where the amount of entanglement is
inferred from the measurements of one or more proper observables. Here we
address optimal estimation of entanglement in the framework of local quantum
estimation theory and derive the optimal observable in terms of the symmetric
logarithmic derivative. We evaluate the quantum Fisher information and, in
turn, the ultimate bound to precision for several families of bipartite states,
either for qubits or continuous variable systems, and for different measures of
entanglement. We found that for discrete variables, entanglement may be
efficiently estimated when it is large, whereas the estimation of weakly
entangled states is an inherently inefficient procedure. For continuous
variable Gaussian systems the effectiveness of entanglement estimation strongly
depends on the chosen entanglement measure. Our analysis makes an important
point of principle and may be relevant in the design of quantum information
protocols based on the entanglement content of quantum states.Comment: 9 pages, 2 figures, v2: minor correction
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