Can effects of quantum gravity be observed in the cosmic microwave background?

We investigate the question whether small quantum-gravitational effects can be observed in the anisotropy spectrum of the cosmic microwave background radiation. An observation of such an effect is needed in order to discriminate between different approaches to quantum gravity. Using canonical quantum gravity with the Wheeler-DeWitt equation, we find a suppression of power at large scales. Current observations only lead to an upper bound on the energy scale of inflation, but the framework is general enough to study other situations in which such effects might indeed be seen.Comment: 5 pages, 1 figure, essay awarded first prize in the Gravity Research Foundation essay competition 201

Quantum Gravitational Contributions to the CMB Anisotropy Spectrum

We derive the primordial power spectrum of density fluctuations in the framework of quantum cosmology. For this purpose we perform a Born-Oppenheimer approximation to the Wheeler-DeWitt equation for an inflationary universe with a scalar field. In this way we first recover the scale-invariant power spectrum that is found as an approximation in the simplest inflationary models. We then obtain quantum gravitational corrections to this spectrum and discuss whether they lead to measurable signatures in the CMB anisotropy spectrum. The non-observation so far of such corrections translates into an upper bound on the energy scale of inflation.Comment: 4 pages, v3: sign error in Eq. (5) and its consequences correcte

Quantum-gravitational effects for inflationary perturbations and the fate of mild singularities in quantum cosmology

In this dissertation, we investigate cosmological models within the framework of canonical quantum gravity based on the Wheeler–DeWitt equation with regard to whether it is possible to observe effects of quantum gravity in the Cosmic Microwave Background radiation and whether a specific class of mild singularities can be resolved by quantizing classical cosmological models in which they appear. The first part is motivated by the fact that there are several candidates for a theory of quantum gravity and it is therefore crucial to find tests in order to figure out which theory is closest to the truth. The main problem here is that quantum-gravitational effects are highly suppressed at the energy scales one can nowadays probe in experiments. However, the inflationary phase of the universe takes place at an energy scale where effects of quantum gravity could be sizeable. During inflation one can investigate primordial cosmological perturbations that are thought to be the seed for structure formation in the early universe as well as for primordial gravitational waves. Thus they have left their imprints in the anisotropies and the polarization of the Cosmic Microwave Background radiation, which have been measured by the space observatories COBE, WMAP and Planck. We investigate to which extent quantum-gravitational effects influence these perturbations by canonically quantizing inflationary models, in which a scalar inflaton field drives the exponential expansion of the universe. At first, we analyze a simplified model, where we only add perturbations to the scalar field. Secondly, we consider scalar and tensor perturbations in a gauge-invariant way for a de Sitter universe and a generic quasi-de Sitter slow-roll model. We perform a semiclassical Born–Oppenheimer type of approximation to the Wheeler–DeWitt equation of each model and recover a Schrödinger equation for the perturbation modes as well as a modified Schrödinger equation with a quantum-gravitational correction term. From the uncorrected Schrödinger equation, we derive the usual slow-roll power spectra. The quantum-gravitational correction term leads to a modification of the power spectra on the largest scales. This effect is, however, too small to be measurable, especially in light of the statistical uncertainty due to cosmic variance, which is most prominent on large scales. We also obtain a quantum-gravitational correction to the tensor-to-scalar ratio, which is, however, much more suppressed than the second-order slow-roll corrections. Finally, we compare our results to other methods in Wheeler–DeWitt quantum cosmology and to findings in other approaches to quantum gravity. The second part of this dissertation is based on the expectation that a quantum theory of gravity should resolve the singularities appearing in general relativity and in classical cosmology. We will focus on a specific set of cosmological singularities called type IV singularities that are of a mild nature in the sense that only higher derivatives of the Hubble parameter diverge. We model a universe with such a singularity by introducing a perfect fluid described by a generalized Chaplygin gas in the form of a scalar field, for which we consider both a standard as well as a phantom field with negative energy. After discussing the classical behavior, we can solve the Wheeler–DeWitt equation of this model analytically for a special case and can draw conclusions for the general case. We use the criterion that a singularity is avoided if the wave function vanishes in the region where the classical singularity is located. However, we obtain as a result that only particular solutions of the Wheeler–DeWitt equation of our model fulfill this criterion and therefore avoid the appearance of a type IV singularity. Lastly, we compare this result to earlier results finding an avoidance of other types of singularities and we discuss singularity resolution in other quantum gravity theories

Temperature steerable flows and Boltzmann generators

Boltzmann generators approach the sampling problem in many-body physics by combining a normalizing flow and a statistical reweighting method to generate samples in thermodynamic equilibrium. The equilibrium distribution is usually defined by an energy function and a thermodynamic state. Here, we propose temperature steerable flows (TSFs) which are able to generate a family of probability densities parametrized by a choosable temperature parameter. TSFs can be embedded in generalized ensemble sampling frameworks to sample a physical system across multiple thermodynamic states

Discriminating the trapped electron modes contribution in density fluctuation spectra

Quasi-coherent (QC) modes have been reported for more than 10 years in reflectometry fluctuations spectra in the core region of fusion plasmas. They have characteristics in-between coherent and broadband fluctuations as they oscillate at a marked frequency but have a wide spectrum. This work presents further evidences of the link recently established between QC modes and the trapped electron modes (TEM) instabilities (Arnichand et al 2014 Nucl. Fusion 54 123017). In electron cyclotron resonance heated discharges of Tore Supra, an enhancement of QC modes amplitude is observed in a region where TEM cause impurity transport and turbulence. In JET Ohmic plasmas, QC modes disappear during density ramp-up and current ramp-down. This is reminiscent of Tore Supra and TEXTOR observations during transitions from the linear Ohmic confinement (LOC) to the saturated Ohmic confinement (SOC) regimes. Evidencing TEM activity then becomes experimentally possible via analysis of fluctuation spectra.EURATOM 63305

Chest Compression-Related Flail Chest Is Associated with Prolonged Ventilator Weaning in Cardiac Arrest Survivors

Chest compressions during cardiopulmonary resuscitation (CPR) may be associated with iatrogenic chest wall injuries. The extent to which these CPR-associated chest wall injuries contribute to a delay in the respiratory recovery of cardiac arrest survivors has not been sufficiently explored. In a single-center retrospective cohort study, surviving intensive care unit (ICU) patients, who had undergone CPR due to medical reasons between 1 January 2018 and 30 June 2019, were analyzed regarding CPR-associated chest wall injuries, detected by chest radiography and computed tomography. Among 109 included patients, 38 (34.8%) presented with chest wall injuries, including 10 (9.2%) with flail chest. The multivariable logistic regression analysis identified flail chest to be independently associated with the need for tracheostomy (OR 15.5; 95% CI 2.77–86.27; p = 0.002). The linear regression analysis identified pneumonia (β 11.34; 95% CI 6.70–15.99; p < 0.001) and the presence of rib fractures (β 5.97; 95% CI 1.01–10.93; p = 0.019) to be associated with an increase in the length of ICU stay, whereas flail chest (β 10.45; 95% CI 3.57–17.33; p = 0.003) and pneumonia (β 6.12; 95% CI 0.94–11.31; p = 0.021) were associated with a prolonged duration of mechanical ventilation. Four patients with flail chest underwent surgical rib stabilization and were successfully weaned from the ventilator. The results of this study suggest that CPR-associated chest wall injuries, flail chest in particular, may impair the respiratory recovery of cardiac arrest survivors in the ICU. A multidisciplinary assessment may help to identify patients who could benefit from a surgical treatment approach

Back To The Roots: Tree-Based Algorithms for Weakly Supervised Anomaly Detection

Weakly supervised methods have emerged as a powerful tool for model-agnostic anomaly detection at the Large Hadron Collider (LHC). While these methods have shown remarkable performance on specific signatures such as di-jet resonances, their application in a more model-agnostic manner requires dealing with a larger number of potentially noisy input features. In this paper, we show that using boosted decision trees as classifiers in weakly supervised anomaly detection gives superior performance compared to deep neural networks. Boosted decision trees are well known for their effectiveness in tabular data analysis. Our results show that they not only offer significantly faster training and evaluation times, but they are also robust to a large number of noisy input features. By using advanced gradient boosted decision trees in combination with ensembling techniques and an extended set of features, we significantly improve the performance of weakly supervised methods for anomaly detection at the LHC. This advance is a crucial step towards a more model-agnostic search for new physics.Comment: 11 pages, 9 figure

Risk factors and outcomes of unrecognised endobronchial intubation in major trauma patients

Background Emergency tracheal intubation during major trauma resuscitation may be associated with unrecognised endobronchial intubation. The risk factors and outcomes associated with this issue have not previously been fully defined. Methods We retrospectively analysed adult patients admitted directly from the scene to the ED of a single level 1 trauma centre, who received either prehospital or ED tracheal intubation prior to initial whole-body CT from January 2008 to December 2019. Our objectives were to describe tube-to- carina distances (TCDs) via CT and to assess the risk factors and outcomes (mortality, length of intensive care unit stay and mechanical ventilation) of patients with endobronchial intubation (TCD <0 cm) using a multivariable model. Results We included 616 patients and discovered 26 (4.2%) cases of endobronchial intubation identified on CT. Factors associated with an increased risk of endobronchial intubations were short body height (OR per 1 cm increase 0.89; 95% CI 0.84 to 0.94; p≤0.001), a high body mass index (OR 1.14; 95% CI 1.04 to 1.25; p=0.005) and ED intubation (OR 3.62; 95% CI 1.39 to 8.90; p=0.006). Eight of 26 cases underwent tube thoracostomy, four of whom had no evidence of underlying chest injury on CT. There was no statistically significant difference in mortality or length of stay although the absolute number of endobronchial intubations was small. Conclusions Short body height and high body mass index were associated with endobronchial intubation. Before considering tube thoracostomy in intubated major trauma patients suspected of pneumothorax, the possibility of unrecognised endobronchial intubation should be considered

Low cost plastic optical fiber sensor based on surface plasmon resonance

Surface plasmon excitation using a variation of Kretschmann method based on light guiding through an optical fiber has been extensively studied in the literature. But, due to its particularly bad propagation conditions, plastic optical fiber was not taken into account in documented experiments. We propose a low cost sensor using this type of fiber, in which we try to avoid the problems both through careful design and signal processing. First of all we discuss the sample fabrication and measurement in section 2; then the results obtained are discussed in section 3, including the problems faced because of the multimode character of the fiber, for which we propose alternative sample shapes as a mean of reducing them. As a conclusion we propose a roadmap to design a low cost sensor based in the structures studied in this paper