225 research outputs found
A Quantum Window Onto Early Inflation
Inflation in the early Universe is one of the most promising probes of
gravity in the high-energy regime. However, observable scales give access to a
limited window in the inflationary dynamics. In this essay, we argue that
quantum corrections to the classical dynamics of cosmological fields allow us
to probe much earlier epochs of the inflationary phase and extend this window
by many orders of magnitude. We point out that both the statistics of
cosmological fluctuations at observable scales, and the field displacements
acquired by spectator fields that play an important role in many
post-inflationary processes, are sensitive to a much longer phase of the
inflationary epoch.Comment: 5 pages, 2 figures. Honourable Mention from the Gravity Research
Foundation 2017 Awards for Essays on Gravitation. Version to appear in
October 2017 Special Issue of the International Journal of Modern Physics D.
Updated to match the published versio
Renormalisation group improvement in the stochastic formalism
We investigate compatibility between the stochastic infrared (IR) resummation
of light test fields on inflationary spacetimes and renormalisation group
running of the ultra-violet (UV) physics. Using the Wilsonian approach, we
derive improved stochastic Langevin and Fokker-Planck equations which
consistently include the renormalisation group effects. With the exception of
stationary solutions, these differ from the naive approach of simply replacing
the classical potential in the standard stochastic equations with the
renormalisation group improved potential. Using this new formalism, we
exemplify the IR dynamics with the Yukawa theory during inflation, illustrating
the differences between the consistent implementation of the UV running and the
naive approach.Comment: 18 pages, 1 figure, 1 appendix, matches accepted version in JCA
Multiple spectator condensates from inflation
We investigate the development of spectator (light test) field condensates
due to their quantum fluctuations in a de Sitter inflationary background,
making use of the stochastic formalism to describe the system. In this context,
a condensate refers to the typical field value found after coarse-graining
using the Hubble scale , which can be essential to seed the initial
conditions required by various post-inflationary processes. We study models
with multiple coupled spectators and for the first time we demonstrate that new
forms of stationary solution exist (distinct from the standard exponential
form) when the potential is asymmetric. Furthermore, we find a critical value
for the inter-field coupling as a function of the number of fields above which
the formation of stationary condensates collapses to . Considering some
simple two-field example potentials, we are also able to derive a lower limit
on the coupling, below which the fluctuations are effectively decoupled, and
the standard stationary variance formulae for each field separately can be
trusted. These results are all numerically verified by a new publicly available
python class (https://github.com/umbralcalc/nfield) to solve the coupled
Langevin equations over a large number of fields, realisations and timescales.
Further applications of this new tool are also discussed.Comment: 18 pages, 10 figures, paragraph added to match published version in
JCA
Surface Structural Disordering in Graphite upon Lithium Intercalation/Deintercalation
We report on the origin of the surface structural disordering in graphite
anodes induced by lithium intercalation and deintercalation processes. Average
Raman spectra of graphitic anodes reveal that cycling at potentials that
correspond to low lithium concentrations in LixC (0 \leq x < 0.16) is
responsible for most of the structural damage observed at the graphite surface.
The extent of surface structural disorder in graphite is significantly reduced
for the anodes that were cycled at potentials where stage-1 and stage-2
compounds (x > 0.33) are present. Electrochemical impedance spectra show larger
interfacial impedance for the electrodes that were fully delithiated during
cycling as compared to electrodes that were cycled at lower potentials (U <
0.15 V vs. Li/Li+). Steep Li+ surface-bulk concentration gradients at the
surface of graphite during early stages of intercalation processes, and the
inherent increase of the LixC d-spacing tend to induce local stresses at the
edges of graphene layers, and lead to the breakage of C-C bonds. The exposed
graphite edge sites react with the electrolyte to (re)form the SEI layer, which
leads to gradual degradation of the graphite anode, and causes reversible
capacity loss in a lithium-ion battery.Comment: 12 pages, 5 figure
Non-Gaussianity from Axion-Gauge Fields Interactions during Inflation
We study the scalar-tensor-tensor non-Gaussian signal in an inflationary
model comprising also an axion coupled with SU(2) gauge fields. In this set-up,
metric fluctuations are sourced by the gauge fields already at the linear level
providing an enhanced chiral gravitational waves spectrum. The same mechanism
is at work in generating an amplitude for the three-point function that is
parametrically larger than in standard single-field inflation.Comment: References added. 19 pages, 5 figure
A novel way to determine the scale of inflation
We show that in the Feebly Interacting Massive Particle (FIMP) model of Dark Matter (DM), one may express the inflationary energy scale H-* as a function of three otherwise unrelated quantities, the DM isocurvature perturbation amplitude, its mass and its self-coupling constant, independently of the tensor-to-scalar ratio. The FIMP model assumes that there exists a real scalar particle that alone constitutes the DM content of the Universe and couples to the Standard Model via a Higgs portal. We consider carefully the various astrophysical, cosmological and model constraints, accounting also for variations in inflationary dynamics and the reheating history, to derive a robust estimate for H-* that is con fined to a relatively narrow range. We point out that, within the context of the FIMP DM model, one may thus determine H-* reliably even in the absence of observable tensor perturbations.Peer reviewe
Lithium Diffusion in Graphitic Carbon
Graphitic carbon is currently considered the state-of-the-art material for
the negative electrode in lithium-ion cells, mainly due to its high
reversibility and low operating potential. However, carbon anodes exhibit
mediocre charge/discharge rate performance, which contributes to severe
transport-induced surface-structural damage upon prolonged cycling, and limits
the lifetime of the cell. Lithium bulk diffusion in graphitic carbon is not yet
completely understood, partly due to the complexity of measuring bulk transport
properties in finite-sized, non-isotropic particles. To solve this problem for
graphite, we use the Devanathan-Stachurski electrochemical methodology combined
with ab-initio computations to deconvolute, and quantify the mechanism of
lithium-ion diffusion in highly oriented pyrolytic graphite (HOPG). The results
reveal inherent high lithium-ion diffusivity in the direction parallel to the
graphene plane (ca. 10^-7 - 10^-6 cm2 s-1), as compared to sluggish lithium-ion
transport along grain boundaries (ca. 10^-11 cm^2 s^-1), indicating the
possibility of rational design of carbonaceous materials and composite
electrodes with very high rate capability.Comment: 9 pages, 3 figure
Bayesian evidence of the post-Planck curvaton
We perform a Bayesian model comparison for scenarios within the quadratic curvaton model, determining the degree to which both are disfavoured with respect to the ΛCDM concordance model and single-field quadratic inflation, using the recent Planck data release. Despite having three additional model parameters, the simplest curvaton scenario is not disfavoured relative to single-field quadratic inflation, and it becomes favoured against this single-field model when we include the joint BICEP/Keck/Planck analysis. In all cases we assume an instantaneous inflaton decay and no surviving isocurvature perturbations. Despite the success of Planck reaching its forecast measurement accuracy, we show that the current constraints on local non-Gaussianity are insufficiently precise to have any significant impact on the evidence ratios so far. We also determine the precision σ(fNL) required by future measurements assuming a fiducial value of fNL = −5/4 or 10.8 to no longer disfavour the curvaton against the ΛCDM parametrisation, and we discuss the effect that the predicted increase in precision from future measurements on fNL may have. We show that our results are not very sensitive to our choice of priors
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