843 research outputs found

### Accretion of a Symmetry Breaking Scalar Field by a Schwarzschild Black Hole

We simulate the behaviour of a Higgs-like field in the vicinity of a
Schwarzschild black hole using a highly accurate numerical framework. We
consider both the limit of the zero-temperature Higgs potential, and a toy
model for the time-dependent evolution of the potential when immersed in a
slowly cooling radiation bath. Through these numerical investigations, we aim
to improve our understanding of the non-equilibrium dynamics of a symmetry
breaking field (such as the Higgs) in the vicinity of a compact object such as
a black hole. Understanding this dynamics may suggest new approaches for
studying properties of scalar fields using black holes as a laboratory.Comment: 16 pages, 5 figure

### Cosmic bubble and domain wall instabilities III: The role of oscillons in three-dimensional bubble collisions

We study collisions between pairs of bubbles nucleated in an ambient false
vacuum. For the first time, we include the effects of small initial (quantum)
fluctuations around the instanton profiles describing the most likely initial
bubble profile. Past studies of this problem neglect these fluctuations and
work under the assumption that the collisions posess an exact SO(2,1) symmetry.
We use three-dimensional lattice simulations to demonstrate that for
double-well potentials, small initial perturbations to this symmetry can be
amplified as the system evolves. Initially the amplification is well-described
by linear perturbation theory around the SO(2,1) background, but the onset of
strong nonlinearities amongst the fluctuations quickly leads to a drastic
breaking of the original SO(2,1) symmetry and the production of oscillons in
the collision region. We explore several single-field models, and we find it is
hard to both realize inflation inside of a bubble and produce oscillons in a
collision. Finally, we extend our results to a simple two-field model. The
additional freedom allowed by the second field allows us to construct viable
inflationary models that allow oscillon production in collisions. The breaking
of the SO(2,1) symmetry allows for a new class of observational signatures from
bubble collisions that do not posess azimuthal symmetry, including the
production of gravitational waves which cannot be supported by an SO(2,1)
spacetime.Comment: 35 pages + references, 26 figures. Submitted to JCAP. v2:
Acknowledgments updates, no other change

### Cosmic bubble and domain wall instabilities I: parametric amplification of linear fluctuations

This is the first paper in a series where we study collisions of nucleated
bubbles taking into account the effects of small initial (quantum) fluctuations
in a fully 3+1-dimensional setting. In this paper, we consider the evolution of
linear fluctuations around highly symmetric though inhomogeneous backgrounds.
We demonstrate that a large degree of asymmetry develops over time from tiny
fluctuations superposed upon planar and SO(2,1) symmetric backgrounds. These
fluctuations arise from zero-point vacuum oscillations, so excluding them by
enforcing a spatial symmetry is inconsistent in a quantum treatment. We
consider the limit of two colliding planar walls, with fluctuation mode
functions characterized by the wavenumber transverse to the collision direction
and a longitudinal shape along the collision direction $x$, which we solve for.
Initially, the fluctuations obey a linear wave equation with a time- and
space-dependent mass $m_{eff}(x,t)$. When the walls collide multiple times,
$m_{eff}$ oscillates in time. We use Floquet theory to study the fluctuations
and generalize techniques familiar from preheating to the case with many
coupled degrees of freedom. This inhomogeneous case has bands of unstable
transverse wavenumbers $k_\perp$ with exponentially growing mode functions.
From the detailed spatial structure of the mode functions in $x$, we identify
both broad and narrow parametric resonance generalizations of the homogeneous
$m_{eff}(t)$ case of preheating. The unstable $k_\perp$ modes are
longitudinally localized, yet can be described as quasiparticles in the
Bogoliubov sense. We define an effective occupation number to show they are
created in bursts for the case of well-defined collisions in the background.
The transverse-longitudinal coupling accompanying nonlinearity radically breaks
this localized particle description, with nonseparable 3D modes arising.Comment: 37 pages + references, 20 figures, submitted to JCA

### Cosmic bubble and domain wall instabilities II: Fracturing of colliding walls

We study collisions between nearly planar domain walls including the effects
of small initial nonplanar fluctuations. These perturbations represent the
small fluctuations that must exist in a quantum treatment of the problem. In a
previous paper, we demonstrated that at the linear level a subset of these
fluctuations experience parametric amplification as a result of their coupling
to the planar symmetric background. Here we study the full three-dimensional
nonlinear dynamics using lattice simulations, including both the early time
regime when the fluctuations are well described by linear perturbation theory
as well as the subsequent stage of fully nonlinear evolution. We find that the
nonplanar fluctuations have a dramatic effect on the overall evolution of the
system. Specifically, once these fluctuations begin to interact nonlinearly the
split into a planar symmetric part of the field and the nonplanar fluctuations
loses its utility. At this point the colliding domain walls dissolve, with the
endpoint of this being the creation of a population of oscillons in the
collision region. The original (nearly) planar symmetry has been completely
destroyed at this point and an accurate study of the system requires the full
three-dimensional simulation.Comment: 23 pages + references, 13 figures. Submitted to JCAP. v2:
Acknowledgements updated, no other change

### Constraining cosmological ultra-large scale structure using numerical relativity

Cosmic inflation, a period of accelerated expansion in the early universe,
can give rise to large amplitude ultra-large scale inhomogeneities on distance
scales comparable to or larger than the observable universe. The cosmic
microwave background (CMB) anisotropy on the largest angular scales is
sensitive to such inhomogeneities and can be used to constrain the presence of
ultra-large scale structure (ULSS). We numerically evolve nonlinear
inhomogeneities present at the beginning of inflation in full General
Relativity to assess the CMB quadrupole constraint on the amplitude of the
initial fluctuations and the size of the observable universe relative to a
length scale characterizing the ULSS. To obtain a statistically significant
number of simulations, we adopt a toy model in which inhomogeneities are
injected along a preferred direction. We compute the likelihood function for
the CMB quadrupole including both ULSS and the standard quantum fluctuations
produced during inflation. We compute the posterior given the observed CMB
quadrupole, finding that when including gravitational nonlinearities, ULSS
curvature perturbations of order unity are allowed by the data, even on length
scales not too much larger than the size of the observable universe. Our
results illustrate the utility and importance of numerical relativity for
constraining early universe cosmology.Comment: 14 pages, 6 figures v3: Clarifications added regarding the generality
of results - conclusions unchanged, version accepted for publication in PRD,
v2: updated with minor clarifications, submitte

### The Impact of Peculiar Velocities on the Estimation of the Hubble Constant from Gravitational Wave Standard Sirens

In this work we investigate the systematic uncertainties that arise from the
calculation of the peculiar velocity when estimating the Hubble constant
($H_0$) from gravitational wave standard sirens. We study the GW170817 event
and the estimation of the peculiar velocity of its host galaxy, NGC 4993, when
using Gaussian smoothing over nearby galaxies. NGC 4993 being a relatively
nearby galaxy, at $\sim 40 \ {\rm Mpc}$ away, is subject to a significant
effect of peculiar velocities. We demonstrate a direct dependence of the
estimated peculiar velocity value on the choice of smoothing scale. We show
that when not accounting for this systematic, a bias of $\sim 200 \ {\rm km \ s
^{-1}}$ in the peculiar velocity incurs a bias of $\sim 4 \ {\rm km \ s ^{-1} \
Mpc^{-1}}$on the Hubble constant. We formulate a Bayesian model that accounts
for the dependence of the peculiar velocity on the smoothing scale and by
marginalising over this parameter we remove the need for a choice of smoothing
scale. The proposed model yields$H_0 = 68.6 ^{+14.0}_{-8.5}~{\rm km\ s^{-1}\
Mpc^{-1}}$. We demonstrate that under this model a more robust unbiased
estimate of the Hubble constant from nearby GW sources is obtained.Comment: 9 pages, 5 figure

### Restricted Quantum Theory of Affine Toda Solitons

We quantise the reduced theory obtained by substituting the soliton solutions
of affine Toda theory into its symplectic form. The semi-classical S-matrix is
found to involve the classical Euler dilogarithm.Comment: 10pp, LaTe

### Dimensional deformation of sine-Gordon breathers into oscillons

Oscillons are localized field configurations oscillating in time with
lifetimes orders of magnitude longer than their oscillation period. In this
paper, we simulate non-travelling oscillons produced by deforming the breather
solutions of the sine-Gordon model. Such a deformation treats the
dimensionality of the model as a real parameter to produce spherically
symmetric oscillons. After considering the post-transient oscillation frequency
as a control parameter, we probe the initial parameter space to show how the
availability of oscillons depends on the number of spatial dimensions. For
small dimensional deformations, our findings are consistent with the lack of a
minimal amplitude bound to form oscillons. In $D\gtrsim 2$ spatial dimensions,
we observe solutions undergoing intermittent phases of contraction and
expansion in their cores. Knowing that stable and unstable configurations can
be mapped to disjoint regions of the breather parameter space, we find that
amplitude modulated solutions are located in the middle of both stability
regimes. This displays the dynamics of critical behavior for solutions around
the stability limit.Comment: 18+7 pages, 20 figures. Minor typos fixed. Comments are welcom

### Affine Toda Solitons and Vertex Operators

Affine Toda theories with imaginary couplings associate with any simple Lie
algebra ${\bf g}$ generalisations of Sine Gordon theory which are likewise
integrable and possess soliton solutions. The solitons are \lq\lq created" by
exponentials of quantities $\hat F^i(z)$ which lie in the untwisted affine
Kac-Moody algebra ${\bf\hat g}$ and ad-diagonalise the principal Heisenberg
subalgebra. When ${\bf g}$ is simply-laced and highest weight irreducible
representations at level one are considered, $\hat F^i(z)$ can be expressed as
a vertex operator whose square vanishes. This nilpotency property is extended
to all highest weight representations of all affine untwisted Kac-Moody
algebras in the sense that the highest non vanishing power becomes proportional
to the level. As a consequence, the exponential series mentioned terminates and
the soliton solutions have a relatively simple algebraic expression whose
properties can be studied in a general way. This means that various physical
properties of the soliton solutions can be directly related to the algebraic
structure. For example, a classical version of Dorey's fusing rule follows from
the operator product expansion of two $\hat F$'s, at least when ${\bf g}$ is
simply laced. This adds to the list of resemblances of the solitons with
respect to the particles which are the quantum excitations of the fields.Comment: Imperial/TP/92-93/29 SWAT/92-93/5 PU-PH-93/1392, requires newma

### A Social Network Study To Improve Collaborative Partnerships Among the Southeastern Health Equity Council (SHEC)

This report presents research conducted on the relationships among and attributes of members of the Southeastern Health Equity Council (SHEC, herein Council) to provide recommendations for partnerships, collaboration, and the recruitment of new members. The background, methods, results, and recommendations are outlined in detail throughout this report. Social networks are measured and defined as connections among people, organization, and/or other units. SNA is a valuable and innovative tool for recognizing strengths and weaknesses in collaborative partnerships. The evaluative study presented herein can be replicated in other councils within the Regional Health Equity Councils to improve collaborations not only among SHEC partnerships, but also the nine remaining regions as well. Among the SHEC, social networking models will be designed in an efforts to better understand partnerships, reach the desire goal to analyze partnerships among SHEC, and develop a better understanding of the broad-based constituency served by the Council for the purposes of improving collaborative partnerships

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