Towards a future singularity?

We discuss whether the future extrapolation of the present cosmological state may lead to a singularity even in case of "conventional" (negative) pressure of the dark energy field, namely $w=p/\rho \geq -1$. The discussion is based on an often neglected aspect of scalar-tensor models of gravity: the fact that different test particles may follow the geodesics of different metric frames, and the need for a frame-independent regularization of curvature singularities.Comment: 8 pages. Essay written for the "2004 Awards for Essays on Gravitation" (Gravity Research Foundation, Wellesley Hills, MA, USA), and selected for "Honorable Mention

Plant canopy shape and the influences on UV exposures to the canopy

The solar spectra at selected sites over hemispherical, conical and pinnacle plant canopy models has been evaluated with a dosimetric technique. The irradiance at the sites varies by up to a factor of 0.31 compared to the irradiance on a horizontal plane. The biologically effective (UVBE) exposures evaluated with the dosimetric technique at sites over the plant canopy are up to 19% of that on a horizontal plane. Compared to a spectroradiometer, the technique provides a more practicable method of measuring the UVBE exposures at multiple sites over a plant canopy. Usage of a dosimeter at one site to provide the exposures at that site for different sun angles introduces an error of more than 50%. Knowledge of the spectra allowed the UV and UVBE exposures to be calculated at each site along with the exposures to the entire canopies. These were dependent on the sun angle and the canopy shape. For plant damage, the UVBE was a maximum of about 1.4 mJ cm-2/min. Compared to the hemispherical canopy, the UVBE exposure for generalised plant damage was 45% less for the pinnacle canopy and 23% less for the conical canopy. The canopy exposures could not be determined from measurements of the ambient exposure

A method for selecting an efficient diagnostic protocol for classification of perceptive and cognitive impairments in neurological patients

"Published in final edited form as: Conf Proc IEEE Eng Med Biol Soc. 2011 ; 2011: 1129–1132. doi:10.1109/IEMBS.2011.6090264."An important and unresolved problem in the assessment of perceptual and cognitive deficits in neurological patients is how to choose from the many existing behavioral tests, a subset that is sufficient for an appropriate diagnosis. This problem has to be dealt with in clinical trials, as well as in rehabilitation settings and often even at bedside in acute care hospitals. The need for efficient, cost effective and accurate diagnostic-evaluations, in the context of clinician time constraints and concerns for patients’ fatigue in long testing sessions, make it imperative to select a set of tests that will provide the best classification of the patient’s deficits. However, the small sample size of the patient population complicates the selection methodology and the potential accuracy of the classifier. We propose a method that allows for ordering tests based on having progressive increases in classification using cross-validation to assess the classification power of the chosen test set. This method applies forward linear regression to find an ordering of the tests with leave-one-out cross-validation to quantify, without biasing to the training set, the classification power of the chosen tests.R01 NS064100 - NINDS NIH HHS; R01NS064100 - NINDS NIH HHSAccepted manuscrip

Physics case of the very high energy electron--proton collider, VHEeP

The possibility of a very high energy electron-proton (VHEeP) collider with a centre-of-mass energy of 9 TeV has been presented at previous workshops. These proceedings briefly summarise the VHEeP concept, which was recently published, and developments since then, as well as future directions. At the VHEeP collider, with a centre-of-mass energy 30 times greater than HERA, parton momentum fractions, $x$, down to about $10^{-8}$ are accessible for photon virtualities, $Q^2$, of 1 GeV$^2$. This extension in the kinematic range to low $x$ complements proposals for other electron-proton or electron-ion colliders.Comment: 6 pages, 2 figures, for proceedings of DIS 2017 worksho

Nanoscale-targeted patch-clamp recordings of functional presynaptic ion channels

Important modulatory roles have been attributed to presynaptic NMDA receptors (NMDARs) located on cerebellar interneuron terminals. Evidence supporting a presynaptic location includes an increase in the frequency of mini events following the application of NMDA and gold particle-labelled NMDA receptor antibody localisation. However, more recent work, using calcium indicators, casts doubt on the idea of presynaptic NMDARs because basket cell varicosities did not show the expected calcium rise following either the local iontophoresis of L-aspartate or the two-photon uncaging of glutamate. (In theory such calcium imaging is sensitive enough to detect the calcium rise from even a single activated receptor.) It has therefore been suggested that the effects of NMDA are mediated via the activation of somatodendritic channels, which subsequently cause a subthreshold depolarization of the axon. Here we report results from a vibrodissociated preparation of cerebellar Purkinje cells, in which the interneuron cell bodies are no longer connected but many of their terminal varicosities remain attached and functional. This preparation can retain both inhibitory and excitatory inputs. We find that the application of NMDA increases the frequency of both types of synaptic event. The characteristics of these events suggest they can originate from interneuron, parallel fiber and even climbing fiber terminals. Interestingly, retrograde signalling seems to activate only the inhibitory terminals. Finally, antibody staining of these cells shows NMDAR-like immunoreactivity co-localised with synaptic markers. Since the Purkinje cells show no evidence of postsynaptic NMDAR-mediated currents, we conclude that functional NMDA receptors are located on presynaptic terminals