1,118 research outputs found
Considerations in the Interpretation of Cosmological Anomalies
Anomalies drive scientific discovery -- they are associated with the cutting
edge of the research frontier, and thus typically exploit data in the low
signal-to-noise regime. In astronomy, the prevalence of systematics --- both
"known unknowns" and "unknown unknowns" --- combined with increasingly large
datasets, the widespread use of ad hoc estimators for anomaly detection, and
the "look-elsewhere" effect, can lead to spurious false detections. In this
informal note, I argue that anomaly detection leading to discoveries of new
physics requires a combination of physical understanding, careful experimental
design to avoid confirmation bias, and self-consistent statistical methods.
These points are illustrated with several concrete examples from cosmology.Comment: An informal summary of an invited talk given at the IAU symposium
"Statistical Challenges in 21st Century Cosmology", accepted in the
Proceedings IAU Symposium No. 306, 2014 (A.F. Heavens, J.-L. Starck, A.
Krone-Martins eds.). 7 pages, 1 figure. Comments welcom
Exploiting the full potential of photometric quasar surveys: Optimal power spectra through blind mitigation of systematics
We present optimal measurements of the angular power spectrum of the XDQSOz
catalogue of photometric quasars from the Sloan Digital Sky Survey. These
measurements rely on a quadratic maximum likelihood estimator that
simultaneously measures the auto- and cross-power spectra of four redshift
samples, and provides minimum-variance, unbiased estimates even at the largest
angular scales. Since photometric quasars are known to be strongly affected by
systematics such as spatially-varying depth and stellar contamination, we
introduce a new framework of extended mode projection to robustly mitigate the
impact of systematics on the power spectrum measurements. This technique
involves constructing template maps of potential systematics, decorrelating
them on the sky, and projecting out modes which are significantly correlated
with the data. Our method is able to simultaneously process several thousands
of nonlinearly-correlated systematics, and mode projection is performed in a
blind fashion. Using our final power spectrum measurements, we find a good
agreement with theoretical predictions, and no evidence for further
contamination by systematics. Extended mode projection not only obviates the
need for aggressive sky and quality cuts, but also provides control over the
level of systematics in the measurements, enabling the search for small signals
of new physics while avoiding confirmation bias.Comment: 13 pages, 8 figures. v2: version accepted by MNRAS. v3: systematics
templates publicly available on www.earlyuniverse.org/code, no change to
pape
The cut-sky cosmic microwave background is not anomalous
The observed angular correlation function of the cosmic microwave background
has previously been reported to be anomalous, particularly when measured in
regions of the sky uncontaminated by Galactic emission. Recent work by
Efstathiou et al. presents a Bayesian comparison of isotropic theories, casting
doubt on the significance of the purported anomaly. We extend this analysis to
all anisotropic Gaussian theories with vanishing mean ( = 0), using
the much wider class of models to confirm that the anomaly is not likely to
point to new physics. On the other hand if there is any new physics to be
gleaned, it results from low-l alignments which will be better quantified by a
full-sky statistic.
We also consider quadratic maximum likelihood power spectrum estimators that
are constructed assuming isotropy. The underlying assumptions are therefore
false if the ensemble is anisotropic. Nonetheless we demonstrate that, for
theories compatible with the observed sky, these estimators (while no longer
optimal) remain statistically superior to pseudo-C_l power spectrum estimators.Comment: PRD in press. Extremely minor updates, mirroring typographical
changes made in proo
No new cosmological concordance with massive sterile neutrinos
It has been claimed recently that massive sterile neutrinos could bring about
a new concordance between observations of the cosmic microwave background
(CMB), the large-scale structure (LSS) of the Universe, and local measurements
of the Hubble constant, . We demonstrate that this apparent concordance
results from combining datasets which are in significant tension, even within
this extended model, possibly indicating remaining systematic biases in the
measurements. We further show that this tension remains when the cosmological
model is further extended to include significant tensor modes, as suggested by
the recent BICEP2 results. Using the Bayesian evidence, we show that the
minimal CDM model is strongly favoured over its neutrino extensions by
various combinations of datasets. Robust data combinations yield stringent
limits of eV and eV at CL for the sum of active and sterile neutrinos,
respectively.Comment: 6 pages, version accepted by PR
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
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