Porous dust grains in debris disks

When modeling the density and grain size distribution in debris disks, the minimum particle size is often significantly larger than the corresponding blowout size. While the dust particles are usually modeled as compact, homogenous spheres, we instead investigate the impact of porosity. The optical properties of porous particles are determined, and the influences of porosity on the blowout size and dust temperatures investigated. Using the method of discrete dipole approximation, we calculate the scattering and absorption cross sections of porous particles and derive the blowout size and the behavior of the dust temperature. We investigate the influence on the beta-ratio. Blowout sizes are calculated for various stellar luminosities and porosities, and an approximation equation is derived to estimate the blowout size as a function of these parameters. Furthermore, we investigate the influence of the porosity on the dust equilibrium temperature. The blowout size increases with the particle porosity and stellar luminosity. The dust temperature of porous particles is lower than the one of the compact spheres, in particular the temperature of blowout grains decreases for porous particles.Comment: 10 pages, 18 figure

Challenges for Moduli Stabilization and String Cosmology near the Conifold

This brief article reviews a recently proposed scenario of moduli stabilization constructed in the vicinity of a conifold locus in the complex structure moduli space. We discuss typical features of moduli stabilization due to the logarithmic structure of the periods over the Calabi-Yau threefold. Integrating out heavy moduli implies exponential terms in the superpotential very reminiscent of non-perturbative contributions. Ultimately, these terms can lead to an alignment of axions potentially realizing large-field inflation. Our goal is to investigate common issues appearing for moduli stabilization near the conifold and subsequent applications to string cosmology. Even though this setup needs to be understood as a toy model, a closer look at the validity of the effective theories reveals characteristic obstacles, which are likely to occur in more serious scenarios as well.Comment: Proceedings prepared for the "Workshop on Geometry and Physics", November 2016, Ringberg Castle, German

The circumstellar disk of FS Tau B - A self-consistent model based on observations in the mid-infrared with NACO -

Protoplanetary disks are a byproduct of the star formation process. In the dense mid-plane of these disks, planetesimals and planets are expected to form. The first step in planet formation is the growth of dust particles from submicrometer-sized grains to macroscopic mm-sized aggregates. The grain growth is accompanied by radial drift and vertical segregation of the particles within the disk. To understand this essential evolutionary step, spatially resolved multi-wavelength observations as well as photometric data are necessary which reflect the properties of both disk and dust. We present the first spatially resolved image obtained with NACO at the VLT in the L$_\text{p}$ band of the near edge-on protoplanetary disk FS Tau B. Based on this new image, a previously published Hubble image in H band and the spectral energy distribution from optical to millimeter wavelengths, we derive constraints on the spatial dust distribution and the progress of grain growth. For this purpose we perform a disk modeling using the radiative transfer code MC3D. Radial drift and vertical sedimentation of the dust are not considered. We find a best-fit model which features a disk extending from $2\,\text{AU}$ to several hundreds AU with a moderately decreasing surface density and $M_\text{disk}=2.8\,\times\,10^{-2}\,\text{M}_\odot$. The inclination amounts to $i=80^\circ$. Our findings indicate that substantial dust grain growth has taken place and that grains of a size equal to or larger than $1\,\text{mm}$ are present in the disk. In conclusion, the parameters describing the vertical density distribution are better constrained than those describing the radial disk structure.Comment: 10 pages, 9 figures, 2 table

The Refined Swampland Distance Conjecture in Calabi-Yau Moduli Spaces

The Swampland Distance Conjecture claims that effective theories derived from a consistent theory of quantum gravity only have a finite range of validity. This will imply drastic consequences for string theory model building. The refined version of this conjecture says that this range is of the order of the naturally built in scale, namely the Planck scale. It is investigated whether the Refined Swampland Distance Conjecture is consistent with proper field distances arising in the well understood moduli spaces of Calabi-Yau compactification. Investigating in particular the non-geometric phases of Kahler moduli spaces of dimension $h^{11}\in\{1,2,101\}$, we always found proper field distances that are smaller than the Planck-length.Comment: 71 pages, 11 figures, v2: refs added, typos correcte

Towards underlying quantum gravity constraints on string inflation

Inflation is a widely accepted concept in cosmology proposing an accelerated expansion of the very early universe. For the class of large-field inflation models the energy driving the expansion arises from a scalar inflaton field that traverses trans-Planckian distances in a suitable potential. This thesis aims to discuss whether there exist underlying string theory or quantum gravity principles constraining/forbidding large-field inflation. Our framework is axion inflation and its interplay with moduli stabilization in string theory. Axionic inflaton fields appear naturally in string compactifications and are protected from UV corrections due to their shift symmetry. The thesis is basically organized as follows: first, attempting to engineer a fully-fledged model of large-field inflation within string theory and second, analyzing possible underlying quantum gravity reasons to explain the ubiquitous control issues. More precisely, we investigate aligned inflation in the vicinity of a conifold in the complex structure moduli space as well as axion monodromy inflation for a D7-brane position modulus. The ultimate failure of all scenarios boils down to the violation of a sophisticated mass hierarchy that is required to justify the employed effective field theories. These obstacles can be traced back to the swampland conjectures which had been claimed to hold generically for effective theories deduced from quantum gravity. In order to gather more evidence for these conjectures we investigate geodesic distances in moduli spaces of various Calabi-Yau manifolds. Our results strongly support one of the swampland conjectures that predicts a break down of the effective theory of inflation as soon as one moves trans-Planckian distances. If true, parametrically controllable models of large single field inflation seem to be impossible in string theory

The influence of dust grain porosity on the analysis of debris disc observations

Debris discs are often modelled assuming compact dust grains, but more and more evidence for the presence of porous grains is found. We aim at quantifying the systematic errors introduced when modelling debris discs composed of porous dust with a disc model assuming spherical, compact grains. We calculate the optical dust properties derived via the fast, but simple effective medium theory. The theoretical lower boundary of the size distribution -- the so-called 'blowout size' -- is compared in the cases of compact and porous grains. Finally, we simulate observations of hypothetical debris discs with different porosities and feed them into a fitting procedure using only compact grains. The deviations of the results for compact grains from the original model based on porous grains are analysed. We find that the blowout size increases with increasing grain porosity up to a factor of two. An analytical approximation function for the blowout size as a function of porosity and stellar luminosity is derived. The analysis of the geometrical disc set-up, when constrained by radial profiles, are barely affected by the porosity. However, the determined minimum grain size and the slope of the grain size distribution derived using compact grains are significantly overestimated. Thus, the unexpectedly high ratio of minimum grain size to blowout size found by previous studies using compact grains can be partially described by dust grain porosity, although the effect is not strong enough to completely explain the trend.Comment: accepted by MNRA

Towards underlying quantum gravity constraints on string inflation

Inflation is a widely accepted concept in cosmology proposing an accelerated expansion of the very early universe. For the class of large-field inflation models the energy driving the expansion arises from a scalar inflaton field that traverses trans-Planckian distances in a suitable potential. This thesis aims to discuss whether there exist underlying string theory or quantum gravity principles constraining/forbidding large-field inflation. Our framework is axion inflation and its interplay with moduli stabilization in string theory. Axionic inflaton fields appear naturally in string compactifications and are protected from UV corrections due to their shift symmetry. The thesis is basically organized as follows: first, attempting to engineer a fully-fledged model of large-field inflation within string theory and second, analyzing possible underlying quantum gravity reasons to explain the ubiquitous control issues. More precisely, we investigate aligned inflation in the vicinity of a conifold in the complex structure moduli space as well as axion monodromy inflation for a D7-brane position modulus. The ultimate failure of all scenarios boils down to the violation of a sophisticated mass hierarchy that is required to justify the employed effective field theories. These obstacles can be traced back to the swampland conjectures which had been claimed to hold generically for effective theories deduced from quantum gravity. In order to gather more evidence for these conjectures we investigate geodesic distances in moduli spaces of various Calabi-Yau manifolds. Our results strongly support one of the swampland conjectures that predicts a break down of the effective theory of inflation as soon as one moves trans-Planckian distances. If true, parametrically controllable models of large single field inflation seem to be impossible in string theory

ModDrop: adaptive multi-modal gesture recognition

We present a method for gesture detection and localisation based on multi-scale and multi-modal deep learning. Each visual modality captures spatial information at a particular spatial scale (such as motion of the upper body or a hand), and the whole system operates at three temporal scales. Key to our technique is a training strategy which exploits: i) careful initialization of individual modalities; and ii) gradual fusion involving random dropping of separate channels (dubbed ModDrop) for learning cross-modality correlations while preserving uniqueness of each modality-specific representation. We present experiments on the ChaLearn 2014 Looking at People Challenge gesture recognition track, in which we placed first out of 17 teams. Fusing multiple modalities at several spatial and temporal scales leads to a significant increase in recognition rates, allowing the model to compensate for errors of the individual classifiers as well as noise in the separate channels. Futhermore, the proposed ModDrop training technique ensures robustness of the classifier to missing signals in one or several channels to produce meaningful predictions from any number of available modalities. In addition, we demonstrate the applicability of the proposed fusion scheme to modalities of arbitrary nature by experiments on the same dataset augmented with audio.Comment: 14 pages, 7 figure