Resolution of Curvature Singularities in Black Holes and the Early Universe

This thesis is concerned with two approaches on the singularity problem of the general theory of relativity. The first is of bottom-up nature. We start from Einstein's well established general relativity and make an educated guess for an asymptotically equivalent but non-singular theory. In the second approach we take the top-down perspective starting with the assumption that string theory gives the fundamental description of nature and analyse the resulting low energy effective theory. Our bottom-up approach is an application of the limiting curvature hypothesis to anisotropic cosmologies. This extends the success for isotropic cosmologies of Brandenberger et al. Applying the LCH, they constructed a theory in which all homogeneous and isotropic solutions are singularity free. Due to the non-analytic nature of the equations we were unable repeat the proof in the anisotropic case, but analytical and numerical analysis produce circumstantial evidence for a resolution of the singularity in this case as well. Generically this resolution seems not to involve a de Sitter phase as expected. Instead it would interpolate between a contracting anisotropic universe and a universe, that time-symmetrically expands anisotropically. During this transition spacetime evolves through a nearly flat, Minkowski phase. This solution could represent an alternative to the so-called bounce solutions as they appear in pre-big-bang scenarios. In our top-down approach we construct a simple model in type IIA super string theory. With a non-BPS D7 or D9 brane we introduce a tachyonic degree of freedom. Its potential is influenced by the compact background wrapped by the brane. In a way the mass can be tuned by the size of the compact dimension. We use a truncated action which was constructed in order to approximate the full string theory result for the dynamical creation and decay of non-BPS branes quite accurately. Taking the lowest order effective action for metric, dilaton and an effective action for the open tachyonic mode, we obtaine bounce solutions. The bounce results from the positivity of the pressure of the tachyon field in our Lagrangian. Both curvature and time derivative of the dilaton remain small during our bounce so that the gravitational sector behaves entirely classical. Asymptotically our bounce solutions are similar to pre-big bang and post-big bang solutions respectively. Thus there remain singularities in the curvature and the dilaton before or after the bounce. These asymptotic string frame curvature singularities can be resolved by the ad hoc addition of a potential term, that might result from alpha' corrections in the open string sector. Exact calculation of the corrections would be necessary in order to give a more precise picture.Diese Dissertation beschäftigt sich in zwei Ansätzen mit dem Problem von Singularitäten in der Allgemeinen Relativitätstheorie. Im ersten gehen wir von der einsteinschen Theorie aus und stellen eine Vermutung für eine asymptotisch äquivalente aber nichtsinguläre Theorie auf. Im zweiten Ansatz beginnen wir bei der Stringtheorie als fundamentaler Beschreibung der Welt und untersuchen die aus dieser Annahme resultierende effektive Theorie bei niedrigen Energien. Der erste Ansatz stellt eine Anwendung der Hypothese über die Krümmungsbegrenzung ("limiting curvature hypothesis") auf anisotrope Kosmologien dar. Dies erweitert die Betrachtung isotroper Kosmologien von Brandenberger et al. Diese konstruierten eine Theorie, in der alle homogenen und isotropen Lösungen frei von Singularitäten sind. Auf Grund der Nichtanalytizität der Gleichungen gelang es uns nicht, diesen Beweis im anisotropen Fall zu wiederholen. Dennoch deutet die analytische und numerische Untersuchung auf eine Auflösung der Singularitäten auch in diesem Fall hin. Generisch scheint die Auflösung nicht wie erwartet durch eine de-Sitter-Phase zu erfolgen. Stattdessen verbindet die Lösung ein kontrahierendes anisotropes Universum mit einem in zeitsymmetrischer Weise expandierenden. Der Übergang erfolgt in einer näherungsweise flachen Minkowski-Phase. Diese Lösung könnte eine Alternative zu den sogenannten Bounce-Lösungen darstellen, wie sie in Pre-Big-Bang-Modellen vorkommen. Im zweiten Ansatz konstruieren wir ein einfaches Modell in der Typ-IIA-Superstring-Theorie. Mit einer D7- oder D9-Bran, welche die BPS-Bedingung nicht erfüllt, führen wir einen tachyonischen Freiheitsgrad ein. Dessen Potenzial wird durch den kompakten Hintergund, auf den die Bran gewickelt ist, beeinflusst. In gewissem Sinn kann die Masse durch die Größe der kompakten Dimension eingestellt werden. Wir verwenden eine trunkierte Wirkung, welche so konstruiert ist, dass das Verhalten der vollen Stringtheorie bei dynamischer Erzeugnung und Zerfall von Nicht-BPS-Branen möglichst gut reproduziert wird. In niedrigster Ordnung von Metrik und Dilaton sowie der tachyonischen Anregung finden wir Bounce-Lösungen. Diese werden ermöglicht durch die Tatsache, dass das Tachyon in der verwendeten Wirkung stets mit positivem Druck auftritt. Sowohl Krümmung als auch die Zeitableitungen des Dilatons sind während des Bounces klein, so dass die Gravitation vollständig klassisch betrachtet werden kann. Die gefundenen Bounce-Lösungen nähern sich asymptotisch den Pre-Big-Bang- oder Post-Big-Bang-Lösungen an, so dass Singularitäten in Krümmung und Dilaton vor oder nach dem Bounce verbleiben. Diese Singularitäten im String-Bezugssystem können durch ein ad hoc eingeführtes, zusätzliches Potenzial aufgelöst werden. Ein solches könnte durch Alpha'-Korrekturen im Offenen-String-Sektor herrühren, deren exakte Berechnung für belastbare Aussagen erforderlich wäre

Eyeing up the Future of the Pupillary Light Reflex in Neurodiagnostics

This is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).The pupillary light reflex (PLR) describes the constriction and subsequent dilation of the pupil in response to light as a result of the antagonistic actions of the iris sphincter and dilator muscles. Since these muscles are innervated by the parasympathetic and sympathetic nervous systems, respectively, different parameters of the PLR can be used as indicators for either sympathetic or parasympathetic modulation. Thus, the PLR provides an important metric of autonomic nervous system function that has been exploited for a wide range of clinical applications. Measurement of the PLR using dynamic pupillometry is now an established quantitative, non-invasive tool in assessment of traumatic head injuries. This review examines the more recent application of dynamic pupillometry as a diagnostic tool for a wide range of clinical conditions, varying from neurodegenerative disease to exposure to toxic chemicals, as well as its potential in the non-invasive diagnosis of infectious disease.Peer reviewedFinal Published versio

Being proven wrong elicits learning in children - but only in those with higher executive function skills

This study investigated whether prompting children to generate predictions about an outcome facilitates activation of prior knowledge and improves belief revision. 51 children aged 9-12 were tested on two experimental tasks in which generating a prediction was compared to closely matched control conditions, as well as on a test of executive functions (EF). In Experiment 1, we showed that children exhibited a pupillary surprise response to events that they had predicted incorrectly, hypothesized to reflect the transient release of noradrenaline in response to cognitive conflict. However, children\u27s surprise response was not associated with better belief revision, in contrast to a previous study involving adults. Experiment 2 revealed that, while generating predictions helped children activate their prior knowledge, only those with better inhibitory control skills learned from incorrectly predicted outcomes. Together, these results suggest that good inhibitory control skills are needed for learning through cognitive conflict. Thus, generating predictions benefits learning - but only among children with sufficient EF capacities to harness surprise for revising their beliefs. (DIPF/Orig.

Pupil Size as a Gateway Into Conscious Interpretation of Brightness

Although retinal illumination is the main determinant of pupil size, evidence indicates that extra-retinal factors, including attention and contextual information, also modulate the pupillary response. For example, stimuli that evoke the idea of brightness (e.g., pictures of the sun) induce pupillary constriction compared to control stimuli of matched luminance. Is conscious appraisal of these stimuli necessary for the pupillary constriction to occur? Participants' pupil diameter was recorded while sun pictures and their phase-scrambled versions were shown to the left eye. A stream of Mondrian patterns was displayed to the right eye to produce continuous flash suppression, which rendered the left-eye stimuli invisible on some trials. Results revealed that when participants were aware of the sun pictures their pupils constricted relative to the control stimuli. This was not the case when the pictures were successfully suppressed from awareness, demonstrating that pupil size is highly sensitive to the contents of consciousness

Portable infrared pupillometry in critical care

Infrared pupillometry was introduced in 1962 but portable instruments that use this technology have only recently become available in the hospital setting. Questions surrounding the accuracy of these instruments have been addressed by documenting the inter-observer agreement on pupillary measurements and also by comparisons with standard pen-light examinations. The following commentary summarizes the development of these devices and provides a wider perspective on how the pupil and its reflexes might be used in providing care for patients with critical illness

The Circadian Response of Intrinsically Photosensitive Retinal Ganglion Cells

Intrinsically photosensitive retinal ganglion cells (ipRGC) signal environmental light level to the central circadian clock and contribute to the pupil light reflex. It is unknown if ipRGC activity is subject to extrinsic (central) or intrinsic (retinal) network-mediated circadian modulation during light entrainment and phase shifting. Eleven younger persons (18–30 years) with no ophthalmological, medical or sleep disorders participated. The activity of the inner (ipRGC) and outer retina (cone photoreceptors) was assessed hourly using the pupil light reflex during a 24 h period of constant environmental illumination (10 lux). Exogenous circadian cues of activity, sleep, posture, caffeine, ambient temperature, caloric intake and ambient illumination were controlled. Dim-light melatonin onset (DLMO) was determined from salivary melatonin assay at hourly intervals, and participant melatonin onset values were set to 14 h to adjust clock time to circadian time. Here we demonstrate in humans that the ipRGC controlled post-illumination pupil response has a circadian rhythm independent of external light cues. This circadian variation precedes melatonin onset and the minimum ipRGC driven pupil response occurs post melatonin onset. Outer retinal photoreceptor contributions to the inner retinal ipRGC driven post-illumination pupil response also show circadian variation whereas direct outer retinal cone inputs to the pupil light reflex do not, indicating that intrinsically photosensitive (melanopsin) retinal ganglion cells mediate this circadian variation

Pupillary Stroop effects

We recorded the pupil diameters of participants performing the words’ color-naming Stroop task (i.e., naming the color of a word that names a color). Non-color words were used as baseline to firmly establish the effects of semantic relatedness induced by color word distractors. We replicated the classic Stroop effects of color congruency and color incongruency with pupillary diameter recordings: relative to non-color words, pupil diameters increased for color distractors that differed from color responses, while they reduced for color distractors that were identical to color responses. Analyses of the time courses of pupil responses revealed further differences between color-congruent and color-incongruent distractors, with the latter inducing a steep increase of pupil size and the former a relatively lower increase. Consistent with previous findings that have demonstrated that pupil size increases as task demands rise, the present results indicate that pupillometry is a robust measure of Stroop interference, and it represents a valuable addition to the cognitive scientist’s toolbox