88 research outputs found
Bouncing alternatives to inflation
Although the inflationary paradigm is the most widely accepted explanation
for the current cosmological observations, it does not necessarily correspond
to what actually happened in the early stages of our Universe. To decide on
this issue, two paths can be followed: first, all the possible predictions it
makes must be derived thoroughly and compared with available data, and second,
all imaginable alternatives must be ruled out. Leaving the first task to all
other contributors of this volume, we concentrate here on the second option,
focusing on the bouncing alternatives and their consequences.Comment: 11 pages, 3 figures, submitted as a contribution to the French
'Comptes Rendus de l'Academie des Sciences' on Inflatio
Production of non-gaussianities in a bouncing phase
We compute the level of non-gaussianities produced by a cosmological bouncing
phase in the minimal non-singular setup that lies within the context of General
Relativity when the matter content consists of a simple scalar field with a
standard kinetic term. Such a bouncing phase is obtained by requiring that the
spatial sections of the background spacetime be positively curved. We restrict
attention to the close vicinity of the bounce by Taylor expanding the scale
factor, the scalar field and its potential in powers of the conformal time
around the bounce. We find that possibly large non-gaussianities are
generically produced at the bounce itself and also discuss which shapes of
non-gaussianities are mostly likely to be produced.Comment: Matches published versio
Non-Gaussianity excess problem in classical bouncing cosmologies
The simplest possible classical model leading to a cosmological bounce is
examined in the light of the non-Gaussianities it can generate. Concentrating
solely on the transition between contraction and expansion, and assuming
initially purely Gaussian perturbations at the end of the contracting phase, we
find that the bounce acts as a source such that the resulting value for the
post-bounce may largely exceed all current limits, to the
point of potentially casting doubts on the validity of the perturbative
expansion. We conjecture that if one can assume that the non-Gaussianity
production depends only on the bouncing behavior of the scale factor and not on
the specifics of the model examined, then many realistic models in which a
nonsingular classical bounce takes place could exhibit a generic
non-Gaussianity excess problem that would need to be addressed for each case.Comment: 7 pages, 2 figures. Inclusion of additional results (equations and
plots) and more detailed discussion in the introduction and main body.
Accepted for publication in Physical Review
A classical bounce: constraints and consequences
We perform a detailed investigation of the simplest possible cosmological
model in which a bounce can occur, namely that where the dynamics is led by a
simple massive scalar field in a general self-interacting potential and a
background spacetime with positively curved spatial sections. By means of a
phase space analysis, we give the conditions under which an initially
contracting phase can be followed by a bounce and an inflationary phase lasting
long enough (i.e., at least 60-70 e-folds) to suppress spatial curvature in
today's observable universe. We find that, quite generically, this realization
requires some amount of fine-tuning of the initial conditions. We study the
effect of this background evolution on scalar perturbations by propagating an
initial power-law power spectrum through the contracting phase, the bounce and
the inflationary phase. We find that it is drastically modified, both
spectrally (k-mode mixing) and in amplitude. It also acquires, at leading
order, an oscillatory component, which, once evolved through the radiation and
matter dominated eras, happens to be compatible with the WMAP data.Comment: Updated references, improved figure resolutio
Stochastic gravitational wave background reconstruction for a non-equilateral and unequal-noise LISA constellation
We explore the impact of choosing different sets of Time-Delay Interferometry
(TDI) variables for detecting and reconstructing Stochastic Gravitational Wave
Background (SGWB) signals and estimating the instrumental noise in LISA. Most
works in the literature build their data analysis pipelines relying on a
particular set of TDI channels, the so-called AET variables, which are
orthogonal under idealized conditions. By relaxing the assumption of a
perfectly equilateral LISA configuration, we investigate to which degree these
channels remain orthogonal and compare them to other TDI channels. We show that
different sets of TDI variables are more robust under perturbations of the
perfect equilateral configuration, better preserving their orthogonality and,
thus, leading to a more accurate estimate of the instrumental noise. Moreover,
we investigate the impact of considering the noise levels associated with each
instrumental noise source to be independent of one another, generalizing the
analysis from two to twelve noise parameters. We find that, in this scenario,
the assumption of orthogonality is broken for all the TDI variables, leading to
a misestimation of measurement error for some of the noise parameters.
Remarkably, we find that for a flat power-law signal, the reconstruction of the
signal parameters is nearly unaffected in these various configurations.Comment: 31 pages, 13 figures, and supplementary materia
Comparison of dimethyl sulfoxide treated highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) electrodes for use in indium tin oxide-free organic electronic photovoltaic devices
Indium tin oxide (ITO)-free organic photovoltaic (OPV) devices were fabricated using highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as the transparent conductive electrode (TCE). The intrinsic conductivity of the PEDOT:PSS films was improved by two different dimethyl sulfoxide (DMSO) treatments â (i) DMSO was added directly to the PEDOT:PSS solution (PEDOT:PSSADD) and (ii) a pre-formed PEDOT:PSS film was immersed in DMSO (PEDOT:PSSIMM). X-ray photoelectron spectroscopy (XPS) and conductive atomic force microscopy (CAFM) studies showed a large amount of PSS was removed from the PEDOT:PSSIMM electrode surface. OPV devices based on a poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) bulk hetrojunction showed that the PEDOT:PSSIMM electrode out-performed the PEDOT:PSSADD electrode, primarily due to an increase in short circuit current density from 6.62 mA cmâ2 to 7.15 mA cmâ2. The results highlight the importance of optimising the treatment of PEDOT:PSS electrodes and demonstrate their potential as an alternative TCE for rapid processing and low-cost OPV and other organic electronic devices
Coupled currents in cosmic strings
We first examine the microstructure of a cosmic string endowed with two
simple Abelian currents. This microstructure depends on two state parameters.
We then provide the macroscopic description of such a string and show that it
depends on an additional Lorentz-invariant state parameter that relates the two
currents. We find that in most of the parameter space, the two-current string
is essentially equivalent to the single current-carrying string, i.e., only one
field condenses onto the defect. In the regions where two currents are present,
we find that as far as stability is concerned, one can approximate the dynamics
with good accuracy using an analytic model based on either a logarithmic (on
the electric side, i.e., for timelike currents) or a rational (on the magnetic
side, i.e., for spacelike currents) worldsheet Lagrangian.Comment: 25 pages, 9 figure
Observational signatures of a non-singular bouncing cosmology
We study a cosmological scenario in which inflation is preceded by a bounce.
In this scenario, the primordial singularity, one of the major shortcomings of
inflation, is replaced by a non-singular bounce, prior to which the universe
undergoes a phase of contraction. Our starting point is the bouncing cosmology
investigated in Falciano et al. (2008), which we complete by a detailed study
of the transfer of cosmological perturbations through the bounce and a
discussion of possible observational effects of bouncing cosmologies. We focus
on a symmetric bounce and compute the evolution of cosmological perturbations
during the contracting, bouncing and inflationary phases. We derive an
expression for the Mukhanov-Sasaki perturbation variable at the onset of the
inflationary phase that follows the bounce. Rather than being in the
Bunch-Davies vacuum, it is found to be in an excited state that depends on the
time scale of the bounce. We then show that this induces oscillations
superimposed on the nearly scale-invariant primordial spectra for scalar and
tensor perturbations. We discuss the effects of these oscillations in the
cosmic microwave background and in the matter power spectrum. We propose a new
way to indirectly measure the spatial curvature energy density parameter in the
context of this model.Comment: 40 pages, 5 figures, typos corrected and reference adde
The AIROPA software package - Milestones for testing general relativity in the strong gravity regime with AO
General relativity can be tested in the strong gravity regime by monitoring stars orbiting the supermassive black hole at the Galactic Center with adaptive optics. However, the limiting source of uncertainty is the spatial PSF variability due to atmospheric anisoplanatism and instrumental aberrations. The Galactic Center Group at UCLA has completed a project developing algorithms to predict PSF variability for Keck AO images. We have created a new software package (AIROPA), based on modified versions of StarFinder and Arroyo, that takes atmospheric turbulence profiles, instrumental aberration maps, and images as inputs and delivers improved photometry and astrometry on crowded fields. This software package will be made publicly available soon
CMB-S4 Science Book, First Edition
This book lays out the scientific goals to be addressed by the
next-generation ground-based cosmic microwave background experiment, CMB-S4,
envisioned to consist of dedicated telescopes at the South Pole, the high
Chilean Atacama plateau and possibly a northern hemisphere site, all equipped
with new superconducting cameras. CMB-S4 will dramatically advance cosmological
studies by crossing critical thresholds in the search for the B-mode
polarization signature of primordial gravitational waves, in the determination
of the number and masses of the neutrinos, in the search for evidence of new
light relics, in constraining the nature of dark energy, and in testing general
relativity on large scales
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