711 research outputs found
A simplified structure for the second order cosmological perturbation equations
Increasingly accurate observations of the cosmic microwave background and the
large scale distribution of galaxies necessitate the study of nonlinear
perturbations of Friedmann-Lemaitre cosmologies, whose equations are
notoriously complicated. In this paper we present a new derivation of the
governing equations for second order perturbations within the framework of the
metric-based approach that is minimal, as regards amount of calculation and
length of expressions, and flexible, as regards choice of gauge and
stress-energy tensor. Because of their generality and the simplicity of their
structure our equations provide a convenient starting point for determining the
behaviour of nonlinear perturbations of FL cosmologies with any given
stress-energy content, using either the Poisson gauge or the uniform curvature
gauge.Comment: 30 pages, no figures. Changed title to the one in published version
and some minor changes and addition
Modelling non-dust fluids in cosmology
Currently, most of the numerical simulations of structure formation use
Newtonian gravity. When modelling pressureless dark matter, or `dust', this
approach gives the correct results for scales much smaller than the
cosmological horizon, but for scenarios in which the fluid has pressure this is
no longer the case. In this article, we present the correspondence of
perturbations in Newtonian and cosmological perturbation theory, showing exact
mathematical equivalence for pressureless matter, and giving the relativistic
corrections for matter with pressure. As an example, we study the case of
scalar field dark matter which features non-zero pressure perturbations. We
discuss some problems which may arise when evolving the perturbations in this
model with Newtonian numerical simulations and with CMB Boltzmann codes.Comment: 5 pages; v2: typos corrected and refs added, submitted version; v3:
version to appear in JCA
Consistent perturbations in an imperfect fluid
We present a new prescription for analysing cosmological perturbations in a
more-general class of scalar-field dark-energy models where the energy-momentum
tensor has an imperfect-fluid form. This class includes Brans-Dicke models,
f(R) gravity, theories with kinetic gravity braiding and generalised galileons.
We employ the intuitive language of fluids, allowing us to explicitly maintain
a dependence on physical and potentially measurable properties. We demonstrate
that hydrodynamics is not always a valid description for describing
cosmological perturbations in general scalar-field theories and present a
consistent alternative that nonetheless utilises the fluid language. We apply
this approach explicitly to a worked example: k-essence non-minimally coupled
to gravity. This is the simplest case which captures the essential new features
of these imperfect-fluid models. We demonstrate the generic existence of a new
scale separating regimes where the fluid is perfect and imperfect. We obtain
the equations for the evolution of dark-energy density perturbations in both
these regimes. The model also features two other known scales: the Compton
scale related to the breaking of shift symmetry and the Jeans scale which we
show is determined by the speed of propagation of small scalar-field
perturbations, i.e. causality, as opposed to the frequently used definition of
the ratio of the pressure and energy-density perturbations.Comment: 40 pages plus appendices. v2 reflects version accepted for
publication in JCAP (new summary of notation, extra commentary on choice of
gauge and frame, extra references to literature
Infrared effects in inflationary correlation functions
In this article, I briefly review the status of infrared effects which occur
when using inflationary models to calculate initial conditions for a subsequent
hot, dense plasma phase. Three types of divergence have been identified in the
literature: secular, "time-dependent" logarithms, which grow with time spent
outside the horizon; "box-cutoff" logarithms, which encode a dependence on the
infrared cutoff when calculating in a finite-sized box; and "quantum"
logarithms, which depend on the ratio of a scale characterizing new physics to
the scale of whatever process is under consideration, and whose interpretation
is the same as conventional field theory. I review the calculations in which
these divergences appear, and discuss the methods which have been developed to
deal with them.Comment: Invited review for focus section of Classical & Quantum Gravity on
nonlinear and nongaussian perturbation theory. Some improvements compared to
version which will appear in CQG, especially in Sec. 2.3. 30 pages +
references
Vector and tensor contributions to the curvature perturbation at second order
We derive the evolution equation for the second order curvature perturbation
using standard techniques of cosmological perturbation theory. We do this for
different definitions of the gauge invariant curvature perturbation, arising
from different splits of the spatial metric, and compare the expressions. The
results are valid at all scales and include all contributions from scalar,
vector and tensor perturbations, as well as anisotropic stress, with all our
results written purely in terms of gauge invariant quantities. Taking the
large-scale approximation, we find that a conserved quantity exists only if, in
addition to the non-adiabatic pressure, the transverse traceless part of the
anisotropic stress tensor is also negligible. We also find that the version of
the gauge invariant curvature perturbation which is exactly conserved is the
one defined with the determinant of the spatial part of the inverse metric.Comment: 21 pages. Appendix added and conclusions extended. Updated to match
version published in JCA
New York: the animated city
The urban landscape of New York City is one that is familiar to many, but, through the medium of animation, this familiarity has been consistently challenged. Often metamorphic, and always meticulously constructed, animated imagery encourages reflective thinking. Focusing on the themes of construction, destruction, and interactivity, this article seeks to cast critical light upon the animated double life that New York City has lived through the following moving image texts: Disney’s Fantasia 2000 (1999), Patrick Jean’s computer-generated short Pixels (2009), and Rockstar Games’ open-world blockbuster Grand Theft Auto IV (2008)
Quantifying the behaviour of curvature perturbations during inflation
How much does the curvature perturbation change after it leaves the horizon,
and when should one evaluate the power spectrum? To answer these questions we
study single field inflation models numerically, and compare the evolution of
different curvature perturbations from horizon crossing to the end of
inflation. In particular we calculate the number of efolds it takes for the
curvature perturbation at a given wavenumber to settle down to within a given
fraction of their value at the end of inflation. We find that e.g. in chaotic
inflation, the amplitude of the comoving and the curvature perturbation on
uniform density hypersurfaces differ by up to 180 % at horizon crossing
assuming the same amplitude at the end of inflation, and that it takes
approximately 3 efolds for the curvature perturbation to be within 1 % of its
value at the end of inflation.Comment: Revtex4, 11 pages, 10 figures; v2: added results section E, added
references and acknowledgements; v3: clarification added to conclusions,
version to appear in CQ
Pure kinetic k-essence as the cosmic speed-up
In this paper, we consider three types of k-essence. These k-essence models
were presented in the parametric forms. The exact analytical solutions of the
corresponding equations of motion are found. It is shown that these k-essence
models for the presented solutions can give rise to cosmic acceleration.Comment: 10 pages, typos corrected, main results remain the same, minor
changes to match IJTP accepted versio
SMEs' Confidentiality Concerns for Security Information Sharing
Small and medium-sized enterprises are considered an essential part of the EU
economy, however, highly vulnerable to cyberattacks. SMEs have specific
characteristics which separate them from large companies and influence their
adoption of good cybersecurity practices. To mitigate the SMEs' cybersecurity
adoption issues and raise their awareness of cyber threats, we have designed a
self-paced security assessment and capability improvement method, CYSEC. CYSEC
is a security awareness and training method that utilises self-reporting
questionnaires to collect companies' information about cybersecurity awareness,
practices, and vulnerabilities to generate automated recommendations for
counselling. However, confidentiality concerns about cybersecurity information
have an impact on companies' willingness to share their information. Security
information sharing decreases the risk of incidents and increases users'
self-efficacy in security awareness programs. This paper presents the results
of semi-structured interviews with seven chief information security officers of
SMEs to evaluate the impact of online consent communication on motivation for
information sharing. The results were analysed in respect of the Self
Determination Theory. The findings demonstrate that online consent with
multiple options for indicating a suitable level of agreement improved
motivation for information sharing. This allows many SMEs to participate in
security information sharing activities and supports security experts to have a
better overview of common vulnerabilities. The final publication is available
at Springer via https://doi.org/10.1007/978-3-030-57404-8_22Comment: 10 pages, 2 figures, 14th International Symposium on Human Aspects of
Information Security & Assurance (HAISA 2020
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