3,158 research outputs found
Basin of attraction for turbulent thermalization and the range of validity of classical-statistical simulations
Different thermalization scenarios for systems with large fields have been
proposed in the literature based on classical-statistical lattice simulations
approximating the underlying quantum dynamics. We investigate the range of
validity of these simulations for condensate driven as well as fluctuation
dominated initial conditions for the example of a single component scalar field
theory. We show that they lead to the same phenomenon of turbulent
thermalization for the whole range of (weak) couplings where the
classical-statistical approach is valid. In the turbulent regime we establish
the existence of a dual cascade characterized by universal scaling exponents
and scaling functions. This complements previous investigations where only the
direct energy cascade has been studied for the single component theory. A
proposed alternative thermalization scenario for stronger couplings is shown to
be beyond the range of validity of classical-statistical simulations.Comment: 11 pages, 10 figures; version published in JHEP (minor revisions
Barriers of the McKeanâVlasov energy via a mountain pass theorem in the space of probability measures
We show that the empirical process associated with a system of weakly interacting diffusion processes exhibits a form of noise-induced metastability. The result is based on an analysis of the associated McKeanâVlasov free energy, which, for suitable attractive interaction potentials, has at least two distinct global minimisers at the critical parameter value . On the torus, one of these states is the spatially homogeneous constant state, and the other is a clustered state. We show that a third critical point exists at this value. As a result, we obtain that the probability of transition of the empirical process from the constant state scales like , with Î the energy gap at . The proof is based on a version of the mountain pass theorem for lower semicontinuous and λ-geodesically convex functionals on the space of probability measures equipped with the 2-Wasserstein metric, where M is a complete, connected, and smooth Riemannian manifold
Constraints on the Growth and Spin of the Supermassive Black Hole in M32 From High Cadence Visible Light Observations
We present 1-second cadence observations of M32 (NGC221) with the CHIMERA
instrument at the Hale 200-inch telescope of the Palomar Observatory. Using
field stars as a baseline for relative photometry, we are able to construct a
light curve of the nucleus in the g-prime and r-prime band with 1sigma=36
milli-mag photometric stability. We derive a temporal power spectrum for the
nucleus and find no evidence for a time-variable signal above the noise as
would be expected if the nuclear black hole were accreting gas. Thus, we are
unable to constrain the spin of the black hole although future work will use
this powerful instrument to target more actively accreting black holes. Given
the black hole mass of (2.5+/-0.5)*10^6 Msun inferred from stellar kinematics,
the absence of a contribution from a nuclear time-variable signal places an
upper limit on the accretion rate which is 4.6*10^{-8} of the Eddington rate, a
factor of two more stringent than past upper limits from HST. The low mass of
the black hole despite the high stellar density suggests that the gas liberated
by stellar interactions was primarily at early cosmic times when the low-mass
black hole had a small Eddington luminosity. This is at least partly driven by
a top-heavy stellar initial mass function at early cosmic times which is an
efficient producer of stellar mass black holes. The implication is that
supermassive black holes likely arise from seeds formed through the coalescence
of 3-100 Msun mass black holes that then accrete gas produced through stellar
interaction processes.Comment: 8 pages, 3 figures, submitted to the Astrophysical Journal, comments
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Wind and boundary layers in Rayleigh-Benard convection. I: analysis and modeling
The aim of this paper is to contribute to the understanding and to model the
processes controlling the amplitude of the wind of Rayleigh-Benard convection.
We analyze results from direct simulation of an L/H = 4 aspect-ratio domain
with periodic sidewalls at Ra = 1e5; 1e6; 1e7; 1e8 and at Pr = 1 by decomposing
independent realizations into wind and fluctuations. It is shown that deep
inside the thermal boundary layer, horizontal heat-fuxes exceed the average
vertical heat-fux by a factor 3 due to the interaction between the wind and the
mean temperature field. These large horizontal heat-fluxes are responsible for
spatial temperature differences that drive the wind by creating pressure
gradients. The wall fluxes and turbulent mixing in the bulk provide damping.
Using the DNS results to parameterise the unclosed terms, a simple model
capturing the essential processes governing the wind structure is derived. The
model consists of two coupled differential equations for wind velocity and
temperature amplitude. The equations indicate that the formation of a wind
structure is inevitable due to the positive feedback resulting from the
interaction between the wind and temperature field. Furthermore, the wind
velocity is largely determined by the turbulence in the bulk rather than by the
wall-shear stress. The model reproduces the Ra dependence of wind Reynolds
number and temperature amplitude
Kinetic and Structural Analysis of the Mg2+ -binding Site of the Guanine Nucleotide-binding Protein p21 H-ras.
The coordination and binding of the Mg2+ ion in the nucleotideâbinding site of p21 have been investigated using siteâdirected mutagenesis, kinetic methods, and phosphorous NMR. Mg2+ in the p21.nucleotide.Mg2+ complex appears to be in fast equilibrium with the solvent. The dissociation constant between Mg2+ and the p21.GDP complex was determined to be 2.8 microM. It decreases 30â or 16âfold on substituting Serâ17 or Aspâ57 with alanine, respectively, whereas the T35A mutation has no effect. All three mutations influence the dissociation constants and the association and dissociation rate constants of the interaction between guanine nucleotides and p21, but to a different degree. We conclude that Thrâ35 is only complexed to Mg2+ in the GTP conformation and both Aspâ57 and Serâ17 appear to be critical for both GDP and GTP binding. 31P NMR spectra of the GDP and Gpp(NH)p (guanosineâ5'â(beta,gammaâimido)triphosphate) complexes of mutated p21 show a remarkable perturbation of the guanine nucleotideâ binding site compared to wildâtype protein. The mutant proteins show reduced GTPase rates, which are not stimulated by the GTPaseâactivating protein GAP. p21(S17A) has been reported to function just as p21(S17N) as a dominant negative inhibitor of normal p21. We find that it inhibits oncogenic p21âinduced survival of primary neuron
Analytic study of the three-urn model for separation of sand
We present an analytic study of the three-urn model for separation of sand.
We solve analytically the master equation and the first-passage problem. We
find that the stationary probability distribution obeys the detailed balance
and is governed by the {\it free energy}. We find that the characteristic
lifetime of a cluster diverges algebraically with exponent 1/3 at the limit of
stability.Comment: 5pages, 4 figures include
The hippocampus and inferential reasoning: building memories to navigate future decisions
A critical aspect of inferential reasoning is the ability to form relationships between items or events that were not experienced together. This review considers different perspectives on the role of the hippocampus in successful inferential reasoning during both memory encoding and retrieval. Intuitively, inference can be thought of as a logical process by which elements of individual existing memories are retrieved and recombined to answer novel questions. Such flexible retrieval is sub-served by the hippocampus and is thought to require specialized hippocampal encoding mechanisms that discretely code events such that event elements are individually accessible from memory. In addition to retrieval-based inference, recent research has also focused on hippocampal processes that support the combination of information acquired across multiple experiences during encoding. This mechanism suggests that by recalling past events during new experiences, connections can be created between newly formed and existing memories. Such hippocampally mediated memory integration would thus underlie the formation of networks of related memories that extend beyond direct experience to anticipate future judgments about the relationships between items and events. We also discuss integrative encoding in the context of emerging evidence linking the hippocampus to the formation of schemas as well as prospective theories of hippocampal function that suggest memories are actively constructed to anticipate future decisions and actions
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