352 research outputs found
Challenges in the hunt for dark energy dynamics
Includes bibliographical references. .One of the greatest challenges in modern cosmology is determining the origin of the observed acceleration of the Universe. The 'dark energy' believed to supply the negative pressure responsible for this cosmic acceleration remains elusive despite over a decade of investigation. Hunting for deviation from the 'vanilla' cosmological model, ACDM, and detecting dynamics with redshift in the equation of state remains a key research area, with many challenges. We introduce some of the challenges in the search for such dark energy dynamics. We illustrate that under the assumption of well-motivated scaling models for dark energy dynamics early universe constraints on the dark energy density imply that these models will be essentially indistinguishable from ACDM for the next decade. After introducing the Fisher Matrix formalism, we derive the Fisher Flex test as a measure of whether the assumption of Gaussianity in the likelihood is incorrect for parameter estimation. This formalism is general for any cosmological survey. Lastly, we study the degeneracies between dark energy and curvature and matter in a non-parametric approach, and show that incorrectly assuming values of cosmological components can exactly mimic dark energy dynamics. We connect to the parametric approach by showing how these uncertainties also degrade constraints on the dark energy parameters in an assumed functional form for w. Improving the accuracy of surveys and experiments to search for possible signatures of dark energy dynamics is the focus of much attention in contemporary cosmology; we highlight challenges in the hunt for dark energy dynamics
Molecular modeling of bacterial polysaccharide antigens to inform future vaccine development
Polysaccharide conjugate vaccines have been pivotal in reducing the prevalence and severity of bacterial infectious diseases worldwide, preventing countless deaths. The effectiveness of a vaccine can be extended if the selected vaccine strains in a multivalent vaccine cross-protect against non-vaccine strains. Detailed knowledge of antigen structure and conformation is required for vaccine components to be rationally selected. However, experimental methods may not be able to ascertain the conformations of polysaccharide chains. To address this, molecular dynamics simulations can provide key theoretical insights on molecular conformation to rationalize cross-protection data and inform vaccine development. In this work, we use molecular dynamics to investigate the conformations of glycan antigens of Neisseria meningitidis and Shigella flexneri bacteria - causative agents of meningitis and diarrheal disease. For N. meningitidis, our modeling indicates that serogroup A is unlikely to cross-protect against serogroup X infection, justifying the inclusion of serogroup X in future multivalent meningococcal vaccines. We also find that a chemically-stable carba-analogue of serogroup A has significant conformational differences to the native serogroup A chain, which does not support its use as a suitable serogroup A vaccine replacement. Our simulations of S. flexneri glycan antigens (serogroups Y, 2, 3, and 5) identify heuristics for the effects of substitution on backbone conformation and supports a proposed vaccine containing serotypes 2a (with O-acetylation) and 3a that will provide broad crossprotection. These findings can guide the rational selection of vaccine components to result in next-generation vaccines with greater cost-effectiveness and improved disease coverage
Tensor Detection Severely Constrains Axion Dark Matter
The recent detection of B-modes by BICEP2 has non-trivial implications for
axion dark matter implied by combining the tensor interpretation with
isocurvature constraints from Planck. In this paper the measurement is taken as
fact, and its implications considered, though further experimental verification
is required. In the simplest inflation models implies . If the axion decay constant constraints on
the dark matter (DM) abundance alone rule out the QCD axion as DM for (where accounts for theoretical
uncertainty). If then vacuum fluctuations of the axion field
place conflicting demands on axion DM: isocurvature constraints require a DM
abundance which is too small to be reached when the back reaction of
fluctuations is included. High QCD axions are thus ruled out. Constraints
on axion-like particles, as a function of their mass and DM fraction, are also
considered. For heavy axions with we find
, with stronger constraints on heavier
axions. Lighter axions, however, are allowed and (inflationary)
model-independent constraints from the CMB temperature power spectrum and large
scale structure are stronger than those implied by tensor modes.Comment: 6 pages, 1 figure. v2: Some discussion and references added. v3
Update on QCD discussion. Version accepted for publication in Physical Review
Letter
Extending BEAMS to incorporate correlated systematic uncertainties
New supernova surveys such as the Dark Energy Survey, Pan-STARRS and the LSST
will produce an unprecedented number of photometric supernova candidates, most
with no spectroscopic data. Avoiding biases in cosmological parameters due to
the resulting inevitable contamination from non-Ia supernovae can be achieved
with the BEAMS formalism, allowing for fully photometric supernova cosmology
studies. Here we extend BEAMS to deal with the case in which the supernovae are
correlated by systematic uncertainties. The analytical form of the full BEAMS
posterior requires evaluating 2^N terms, where N is the number of supernova
candidates. This `exponential catastrophe' is computationally unfeasible even
for N of order 100. We circumvent the exponential catastrophe by marginalising
numerically instead of analytically over the possible supernova types: we
augment the cosmological parameters with nuisance parameters describing the
covariance matrix and the types of all the supernovae, \tau_i, that we include
in our MCMC analysis. We show that this method deals well even with large,
unknown systematic uncertainties without a major increase in computational
time, whereas ignoring the correlations can lead to significant biases and
incorrect credible contours. We then compare the numerical marginalisation
technique with a perturbative expansion of the posterior based on the insight
that future surveys will have exquisite light curves and hence the probability
that a given candidate is a Type Ia will be close to unity or zero, for most
objects. Although this perturbative approach changes computation of the
posterior from a 2^N problem into an N^2 or N^3 one, we show that it leads to
biases in general through a small number of misclassifications, implying that
numerical marginalisation is superior.Comment: Resubmitted under married name Lochner (formally Knights). Version 3:
major changes, including a large scale analysis with thousands of MCMC
chains. Matches version published in JCAP. 23 pages, 8 figure
Towards the Future of Supernova Cosmology
For future surveys, spectroscopic follow-up for all supernovae will be
extremely difficult. However, one can use light curve fitters, to obtain the
probability that an object is a Type Ia. One may consider applying a
probability cut to the data, but we show that the resulting non-Ia
contamination can lead to biases in the estimation of cosmological parameters.
A different method, which allows the use of the full dataset and results in
unbiased cosmological parameter estimation, is Bayesian Estimation Applied to
Multiple Species (BEAMS). BEAMS is a Bayesian approach to the problem which
includes the uncertainty in the types in the evaluation of the posterior. Here
we outline the theory of BEAMS and demonstrate its effectiveness using both
simulated datasets and SDSS-II data. We also show that it is possible to use
BEAMS if the data are correlated, by introducing a numerical marginalisation
over the types of the objects. This is largely a pedagogical introduction to
BEAMS with references to the main BEAMS papers.Comment: Replaced under married name Lochner (formally Knights). 3 pages, 2
figures. To appear in the Proceedings of 13th Marcel Grossmann Meeting
(MG13), Stockholm, Sweden, 1-7 July 201
Bayesian estimation applied to multiple species
Observed data are often contaminated by undiscovered interlopers, leading to biased parameter estimation. Here we present BEAMS (Bayesian estimation applied to multiple species) which significantly improves on the standard maximum likelihood approach in the case where the probability for each data point being “pure” is known. We discuss the application of BEAMS to future type-Ia supernovae (SNIa) surveys, such as LSST, which are projected to deliver over a million supernovae light curves without spectra. The multiband light curves for each candidate will provide a probability of being Ia (pure) but the full sample will be significantly contaminated with other types of supernovae and transients. Given a sample of N supernovae with mean probability, ⟨P⟩, of being Ia, BEAMS delivers parameter constraints equal to N⟨P⟩ spectroscopically confirmed SNIa. In addition BEAMS can be simultaneously used to tease apart different families of data and to recover properties of the underlying distributions of those families (e.g. the type-Ibc and II distributions). Hence BEAMS provides a unified classification and parameter estimation methodology which may be useful in a diverse range of problems such as photometric redshift estimation or, indeed, any parameter estimation problem where contamination is an issue
Using the full power of the cosmic microwave background to probe axion dark matter
The cosmic microwave background (CMB) places strong constraints on models of
dark matter (DM) that deviate from standard cold DM (CDM), and on initial
conditions beyond the scalar adiabatic mode. Here, the full \textit{Planck}
data set (including temperature, -mode polarisation, and lensing deflection)
is used to test the possibility that some fraction of the DM is composed of
ultralight axions (ULAs). This represents the first use of CMB lensing to test
the ULA model. We find no evidence for a ULA component in the mass range
. We put percent-level constraints on
the ULA contribution to the DM, improving by up to a factor of two compared to
the case with temperature anisotropies alone. Axion DM also provides a
low-energy window onto the high-energy physics of inflation through the
interplay between the vacuum misalignment production of axions and isocurvature
perturbations. We perform the first systematic investigation into the parameter
space of ULA isocurvature, using an accurate isocurvature transfer function at
all values. We precisely identify a "window of co-existence" for
where the data allow,
simultaneously, a contribution of ULAs to the DM, and
contributions of isocurvature and tensors to the CMB power. ULAs in this window
(and \textit{all} lighter ULAs) are shown to be consistent with a large
inflationary Hubble parameter, . The window of
co-existence will be fully probed by proposed CMB-S4 observations with
increased accuracy in the high- lensing power and low- and
-mode polarisation. If ULAs in the window exist, this could allow for two
independent measurements of in the CMB using the axion DM content and
isocurvature, and the tensor contribution to -modes.Comment: 15+8 pages, 12+4 figures, chains available online at
http://www.dunlap.utoronto.ca/~hlozek/AxiChains, code at
https://github.com/dgrin1/axionCAM
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