296 research outputs found
Discovering and quantifying nontrivial fixed points in multi-field models
We use the functional renormalization group and the -expansion
concertedly to explore multicritical universality classes for coupled
vector-field models in three Euclidean dimensions.
Exploiting the complementary strengths of these two methods we show how to make
progress in theories with large numbers of interactions, and a large number of
possible symmetry-breaking patterns. For the three- and four-field models we
find a new fixed point that arises from the mutual interaction between
different field sectors, and we establish the absence of infrared-stable fixed
point solutions for the regime of small . Moreover, we explore these
systems as toy models for theories that are both asymptotically safe and
infrared complete. In particular, we show that these models exhibit complete
renormalization group trajectories that begin and end at nontrivial fixed
points.Comment: 10 pages, 6 figures; minor changes, as published in EPJ
Coupled feedback loops maintain synaptic long-term potentiation: A computational model of PKMzeta synthesis and AMPA receptor trafficking
In long-term potentiation (LTP), one of the most studied types of neural
plasticity, synaptic strength is persistently increased in response to
stimulation. Although a number of different proteins have been implicated in
the sub-cellular molecular processes underlying induction and maintenance of
LTP, the precise mechanisms remain unknown. A particular challenge is to
demonstrate that a proposed molecular mechanism can provide the level of
stability needed to maintain memories for months or longer, in spite of the
fact that many of the participating molecules have much shorter life spans.
Here we present a computational model that combines simulations of several
biochemical reactions that have been suggested in the LTP literature and show
that the resulting system does exhibit the required stability. At the core of
the model are two interlinked feedback loops of molecular reactions, one
involving the atypical protein kinase PKM{\zeta} and its messenger RNA, the
other involving PKM{\zeta} and GluA2-containing AMPA receptors. We demonstrate
that robust bistability - stable equilibria both in the synapse's potentiated
and unpotentiated states - can arise from a set of simple molecular reactions.
The model is able to account for a wide range of empirical results, including
induction and maintenance of late-phase LTP, cellular memory reconsolidation
and the effects of different pharmaceutical interventions
Gravitational Wave Emission from Collisions of Compact Scalar Solitons
We numerically investigate the gravitational waves generated by the head-on
collision of equal-mass, self-gravitating, real scalar field solitons
(oscillatons) as a function of their compactness . We show that
there exist three different possible outcomes for such collisions: (1) an
excited stable oscillaton for low , (2) a merger and formation of
a black-hole for intermediate , and (3) a pre-merger collapse of
both oscillatons into individual black-holes for large . For (1),
the excited, aspherical oscillaton continues to emit gravitational waves. For
(2), the total energy in gravitational waves emitted increases with
compactness, and possesses a maximum which is greater than that from the merger
of a pair of equivalent mass black-holes. The initial amplitudes of the
quasi-normal modes in the post-merger ring-down in this case are larger than
that of corresponding mass black-holes -- potentially a key observable to
distinguish black-hole mergers with their scalar mimics. For (3), the
gravitational wave output is indistinguishable from a similar mass,
black-hole--black-hole merger.Comment: 8 Pages, 8 figures, movies :
https://www.youtube.com/playlist?list=PLSkfizpQDrcahgvc5TvBk5qtXAzkSyHP
Formation of Relativistic Axion Stars
Axions and axion-like particles are compelling candidates for the missing
dark matter of the universe. As they undergo gravitational collapse, they can
form compact objects such as axion stars or even black holes. In this paper, we
study the formation and distribution of such objects. First, we simulate the
formation of compact axion stars using numerical relativity with aspherical
initial conditions that could represent the final stages of axion dark matter
structure formation. We show that the final states of such collapse closely
follow the known relationship of initial mass and axion decay constant .
Second, we demonstrate with a toy model how this information can be used to
scan a model density field to predict the number densities and masses of such
compact objects. In addition to being detectable by the LIGO/VIRGO
gravitational wave interferometer network for axion mass of eV, we show using peak statistics that for , there
exists a "mass gap" between the masses of axion stars and black holes formed
from collapse
General Relativistic Polarized Proca Stars
Massive vector fields can form spatially localized, non-relativistic,
stationary field configurations supported by gravitational interactions. The
ground state configurations (p-solitons/vector solitons/dark photon
stars/polarized Proca stars) have a time-dependent vector field pointing in the
same spatial direction throughout the configuration at any instant of time, can
carry macroscopic amounts of spin angular momentum, and are spherically
symmetric and monotonic in the energy density. In this paper, we include
general relativistic effects, and numerically investigate the stability of
compact polarized Proca stars (linear and circularly polarized) and compare
them to hedgehog-like field configurations (with radially pointing field
directions). Starting with approximate field profiles of such stars, we evolve
the system numerically using 3+1 dimensional numerical simulations in general
relativity. We find that these initial conditions lead to stable
configurations. However, at sufficiently large initial compactness, they can
collapse to black holes. We find that the initial compactness that leads to
black hole formation is higher for circularly polarized stars (which carry
macroscopic spin angular momentum), compared to linearly polarized ones, which
in turn is higher than that for hedgehog configurations.Comment: 10 pages, 6 figure
Cosmic String Loop Collapse in Full General Relativity
We present the first fully general relativistic dynamical simulations of
Abelian Higgs cosmic strings using 3+1D numerical relativity. Focusing on
cosmic string loops, we show that they collapse due to their tension and can
either (i) unwind and disperse or (ii) form a black hole, depending on their
tension and initial radius. We show that these results can be predicted
using an approximate formula derived using the hoop conjecture, and argue that
it is independent of field interactions. We extract the gravitational waveform
produced in the black hole formation case and show that it is dominated by the
and mode. We also compute the total gravitational wave energy
emitted during such a collapse, being of the initial total
cosmic string loop mass, for a string tension of and
radius . We use our results to put a bound on the production
rate of planar cosmic strings loops as .Comment: Movies:
https://www.youtube.com/playlist?list=PLSkfizpQDrcaAxkuQ3BtjILn_tJu-jXx
Coherent Gravitational Waveforms and Memory from Cosmic String Loops
We construct, for the first time, the time-domain gravitational wave strain
waveform from the collapse of a strongly gravitating Abelian Higgs cosmic
string loop in full general relativity. We show that the strain exhibits a
large memory effect during merger, ending with a burst and the characteristic
ringdown as a black hole is formed. Furthermore, we investigate the waveform
and energy emitted as a function of string width, loop radius and string
tension . We find that the mass normalized gravitational wave energy
displays a strong dependence on the inverse of the string tension
, with at the percent level, for the regime where .
Conversely, we show that the efficiency is only weakly dependent on the initial
string width and initial loop radii. Using these results, we argue that
gravitational wave production is dominated by kinematical instead of
geometrical considerations.Comment: 15 pages, 16 figures, 2 YouTube movies: https://youtu.be/-dhYA2788LA
https://youtu.be/0sSH54gXu4
Spatially Averaged Quantum Inequalities Do Not Exist in Four-Dimensional Spacetime
We construct a particular class of quantum states for a massless, minimally
coupled free scalar field which are of the form of a superposition of the
vacuum and multi-mode two-particle states. These states can exhibit local
negative energy densities. Furthermore, they can produce an arbitrarily large
amount of negative energy in a given region of space at a fixed time. This
class of states thus provides an explicit counterexample to the existence of a
spatially averaged quantum inequality in four-dimensional spacetime.Comment: 13 pages, 1 figure, minor corrections and added comment
CLARIAH-DE cross-service search - prospects and benefits of merging subject-specific services
CLARIAH-DE combines services and offerings of CLARIN-D and DARIAH-DE. This includes various search applications which are made directly available to researchers. These search applications are presented in this working paper based on their main characteristics and compared with a focus on possible harmonizations. Opportunities and risks of different forms of technical integration are highlighted. Identified challenges can be explained in particular considering the background of different organizational and technical frameworks as well as highly specific and discipline-dependent requirements. The integration work that has already been carried out and the experiences gained with regard to future work and possible integration of further applications are also discussed. The experiences made in CLARIAH-DE can especially be of interest for other projects in the field of digital research infrastructures
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