294 research outputs found
Non-Gaussianities in Multifield Inflation: Superhorizon Evolution, Adiabaticity, and the Fate of fnl
We explore the superhorizon generation of large fnl of the local form in two
field inflation. We calculate the two- and three-point observables in a general
class of potentials which allow for an analytic treatment using the delta N
formalism. Motivated by the conservation of the curvature perturbation outside
the horizon in the adiabatic mode and also by the observed adiabaticity of the
power spectrum, we follow the evolution of fnl^{local} until it is driven into
the adibatic solution by passing through a phase of effectively single field
inflation. We find that although large fnl^{local} may be generated during
inflation, such non-gaussianities are transitory and will be exponentially
damped as the cosmological fluctuations approach adiabaticity.Comment: v3: Typos corrected, minor changes to match published version,
references added, 18 pages, 1 figure. v2: Changed sign of fnl to match WMAP
convention, minor changes throughout, references added, 18 pages, 1 figure.
v1: 17 pages, 1 figur
Machine Learning Classification of SDSS Transient Survey Images
We show that multiple machine learning algorithms can match human performance
in classifying transient imaging data from the Sloan Digital Sky Survey (SDSS)
supernova survey into real objects and artefacts. This is a first step in any
transient science pipeline and is currently still done by humans, but future
surveys such as the Large Synoptic Survey Telescope (LSST) will necessitate
fully machine-enabled solutions. Using features trained from eigenimage
analysis (principal component analysis, PCA) of single-epoch g, r and
i-difference images, we can reach a completeness (recall) of 96 per cent, while
only incorrectly classifying at most 18 per cent of artefacts as real objects,
corresponding to a precision (purity) of 84 per cent. In general, random
forests performed best, followed by the k-nearest neighbour and the SkyNet
artificial neural net algorithms, compared to other methods such as na\"ive
Bayes and kernel support vector machine. Our results show that PCA-based
machine learning can match human success levels and can naturally be extended
by including multiple epochs of data, transient colours and host galaxy
information which should allow for significant further improvements, especially
at low signal-to-noise.Comment: 14 pages, 8 figures. In this version extremely minor adjustments to
the paper were made - e.g. Figure 5 is now easier to view in greyscal
First On-Sky High Contrast Imaging with an Apodizing Phase Plate
We present the first astronomical observations obtained with an Apodizing
Phase Plate (APP). The plate is designed to suppress the stellar diffraction
pattern by 5 magnitudes from 2-9 lambda/D over a 180 degree region. Stellar
images were obtained in the M' band (4.85 microns) at the MMTO 6.5m telescope,
with adaptive wavefront correction made with a deformable secondary mirror
designed for low thermal background observations. The measured PSF shows a halo
intensity of 0.1% of the stellar peak at 2 lambda/D (0.36 arcsec), tapering off
as r^{-5/3} out to radius 9 lambda/D. Such a profile is consistent with
residual errors predicted for servo lag in the AO system.
We project a 5 sigma contrast limit, set by residual atmospheric
fluctuations, of 10.2 magnitudes at 0.36 arcsec separation for a one hour
exposure. This can be realised if static and quasi-static aberrations are
removed by differential imaging, and is close to the sensitivity level set by
thermal background photon noise for target stars with M'>3. The advantage of
using the phase plate is the removal of speckle noise caused by the residuals
in the diffraction pattern that remain after PSF subtraction. The APP gives
higher sensitivity over the range 2-5 lambda/D compared to direct imaging
techniques.Comment: 22 pages, 5 figures, 1 table, ApJ accepte
Axion Inflation and Gravity Waves in String Theory
The majority of models of inflation in string theory predict an absence of
measurable gravitational waves, r << 10^{-3}. The most promising proposals for
making string theoretic models that yield measurable tensor fluctuations
involve axion fields with slightly broken shift symmetry. We consider such
models in detail, with a particular focus on the N-flation scenario and on
axion valley/natural inflation models. We find that in Calabi-Yau threefold
compactifications with logarithmic Kahler potentials K it appears to be
difficult to meet the conditions required for axion inflation in the
supergravity regime. However, in supergravities with an (approximately)
quadratic shift-symmetric K, axion inflation may be viable. Such Kahler
potentials do arise in some string models, in specific limits of the moduli
space. We describe the most promising classes of models; more detailed study
will be required before one can conclude that working models exist.Comment: 30 + 16 pages, 5 figures. Added references, corrected typo
Features of deSitter Vacua in M-Theory
We compute the masses of all moduli in the unstable deSitter vacua arising in
the toy model of cosmological M-theory flux compactifications on the G2
holonomy manifolds of [1]. The slow-roll parameters in the tachyonic directions
are shown to be too large to be useful for conventional models of inflation.
However, it appears that we can find fast roll regimes which could, under
certain conditions, account for the current dark energy driven accelerated
expansion of the universe.Comment: 14 pages, 1 figur
Volume Weighted Measures of Eternal Inflation in the Bousso-Polchinski Landscape
We consider the cosmological dynamics associated with volume weighted
measures of eternal inflation, in the Bousso-Polchinski model of the string
theory landscape. We find that this measure predicts that observers are most
likely to find themselves in low energy vacua with one flux considerably larger
than the rest. Furthermore, it allows for a satisfactory anthropic explanation
of the cosmological constant problem by producing a smooth, and approximately
constant, distribution of potentially observable values of Lambda. The low
energy vacua selected by this measure are often short lived. If we require
anthropically acceptable vacua to have a minimum life-time of 10 billion years,
then for reasonable parameters a typical observer should expect their vacuum to
have a life-time of approximately 12 billion years. This prediction is model
dependent, but may point toward a solution to the coincidence problem of
cosmology.Comment: 35 pages, 8 figure
The Non-BPS Black Hole Attractor Equation
We study the attractor mechanism for extremal non-BPS black holes with an
infinite throat near horizon geometry, developing, as we do so, a physical
argument as to why such a mechanism does not exist in non-extremal cases. We
present a detailed derivation of the non-supersymmetric attractor equation.
This equation defines the stabilization of moduli near the black hole horizon:
the fixed moduli take values specified by electric and magnetic charges
corresponding to the fluxes in a Calabi Yau compactification of string theory.
They also define the so-called double-extremal solutions. In some examples,
studied previously by Tripathy and Trivedi, we solve the equation and show that
the moduli are fixed at values which may also be derived from the critical
points of the black hole potential.Comment: 32 Pages, 2 Figures, LaTeX; v2: typos corrected, references adde
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