On the Bardeen-Petterson Effect in black hole accretion discs

We investigate the effect of black hole spin on warped or misaligned accretion discs - in particular i) whether or not the inner disc edge aligns with the black hole spin and ii) whether the disc can maintain a smooth transition between an aligned inner disc and a misaligned outer disc, known as the Bardeen-Petterson effect. We employ high resolution 3D smoothed particle hydrodynamics simulations of $\alpha$-discs subject to Lense-Thirring precession, focussing on the bending wave regime where the disc viscosity is smaller than the aspect ratio $\alpha \lesssim H/R$. We first address the controversy in the literature regarding possible steady-state oscillations of the tilt close to the black hole. We successfully recover such oscillations in 3D at both small and moderate inclinations ($\lesssim 15^{\circ}$), provided both Lense-Thirring and Einstein precession are present, sufficient resolution is employed, and provided the disc is not so thick so as to simply accrete misaligned. Second, we find that discs inclined by more than a few degrees in general steepen and break rather than maintain a smooth transition, again in contrast to previous findings, but only once the disc scale height is adequately resolved. Finally, we find that when the disc plane is misaligned to the black hole spin by a large angle, the disc 'tears' into discrete rings which precess effectively independently and cause rapid accretion, consistent with previous findings in the diffusive regime ($\alpha \gtrsim H/R$). Thus misalignment between the disc and the spin axis of the black hole provides a robust mechanism for growing black holes quickly, regardless of whether the disc is thick or thin.Comment: 15 pages, 18 figures, movies available at http://users.monash.edu.au/~rnealon/ or YouTub

Tearing up the disc: misaligned accretion on to a binary

In a recent paper we have shown that the evolution of a misaligned disc around a spinning black hole can result in tearing the disc into many distinct planes. Tearing discs with random orientations produce direct dynamical accretion on to the hole in approximately 70% of all cases. Here we examine the evolution of a misaligned disc around a binary system. We show that these discs are susceptible to tearing for almost all inclinations. We also show that tearing of the disc can result in a significant acceleration of the disc evolution and subsequent accretion on to the binary - by factors up to 10,000 times that of a coplanar prograde disc with otherwise identical parameters. This provides a promising mechanism for driving mergers of supermassive black hole (SMBH) binaries on timescales much shorter than a Hubble time. Disc tearing also suggests new observational signatures of accreting SMBH binaries, and other systems such as protostellar binaries.Comment: 9 pages, 7 figures. Accepted to MNRAS. Simulation movies are available here: http://jila.colorado.edu/chrisnixon/tearing_binary.htm

Irrationality in Consumers’ Switching Decisions: When More Firms May Mean Less Benefit

We report evidence of three types of consumer switching decision errors within the UK electricity market. We identify consumers who do not switch despite substantial available savings, consumers who switch from a cheaper to a more expensive supplier and consumers who switch to a cheaper, but not the cheapest available supplier. Moreover, we find that consumers make more efficient decisions in markets with fewer competitors. This finding is consistent with theories of consumer confusion and “information-overload” rather than other “rational” explanations of consumer mistakes such as perceived differences in firm quality or uncertainty over consumers’ own demand.Consumer choice, Switching costs, Behavioural IO

A quantum key distribution protocol for rapid denial of service detection

We introduce a quantum key distribution protocol designed to expose fake users that connect to Alice or Bob for the purpose of monopolising the link and denying service. It inherently resists attempts to exhaust Alice and Bob's initial shared secret, and is 100% efficient, regardless of the number of qubits exchanged above the finite key limit. Additionally, secure key can be generated from two-photon pulses, without having to make any extra modifications. This is made possible by relaxing the security of BB84 to that of the quantum-safe block cipher used for day-to-day encryption, meaning the overall security remains unaffected for useful real-world cryptosystems such as AES-GCM being keyed with quantum devices.Comment: 13 pages, 3 figures. v2: Shifted focus of paper towards DoS and added protocol 4. v1: Accepted to QCrypt 201

Unexplored Opportunities: Multi-Sector Strategies for Collaboration in Underwater Unexploded Ordnance Remediation

Few global challenges are as ripe for multi-sector collaboration as underwater (UW) unexploded ordnance (UXO) remediation. Millions of metric tons of UXO are lying on and under the seabed corroding, decaying, and seeping toxic chemicals into the ecosystem—ultimately ending up in our food. Because most underwater UXO are from WWI and WWII, and given the corrosion rates of most metals from which ordnance is made, the inevitable problems with ordnance dumped, sunk, and fired into bodies of water (mostly in coastal regions) are catching up with us. The urgency is exacer-bated by biochemical changes in oceans due to climate change that increase rates of corrosion and other processes. Meanwhile, maritime technologies in several sectors have reduced the barrier of entry even for small companies and nongovernmental organizations (NGOs) to partner with militaries, corporations, and nations, large and small. With combined resources, expertise, and knowledge, these partnerships can remediate the ecological, economic, explosive, and human health hazards caused by underwater UXO. While some technological gaps exist in detection, disposal, and removal operations, the most significant barriers are political and economic. Nevertheless, multi-sector collaboration combined with innovation from maritime and explosives experts offers some unexplored opportunities to increase health, wealth, and safety for all

Recommendations for high intensity upper body exercise testing

Introduction: For given submaximal and maximal peak power outputs aerobic responses to upper body exercise are different to those for lower body exercise (Sawka, 1986: Exercise & Sport Sciences Reviews, 14, 175-211). However, much less is known regarding responses to exercise intensities at and around peak oxygen up take (VO2peak). Purpose: The purpose of this study was to determine the metabolic responses during arm crank ergometry (ACE) below, at and above peak oxygen uptake and to help establish exercise testing guidelines for high intensity upper body exercise. Methods: Following institutional ethical approval fourteen male students (Age 21.1, s = 6.1 years and 2.44 s=0.44 VO2peak) volunteered to take part in this study. Each participant exercised on a table mounted cycle ergometer (Monark 894E, Monark Exercise AB, Sweden). After habituation peak minute power (PMP) was calculated from an incremental test. Subsequently each participant completed four continuous work tests (CWT) to volitional exhaustion at 80%, 90%, 100% and 110% of PMP. All tests were completed at 70 rev∙min-1 with a minimum of 48-h between tests and the order was counterbalanced. Each CWT was preceded by a 5 min warm-up, loaded with a mass corresponding to the participants 80% PMP for 20 s at minutes 2, 3 and 4. Oxygen uptake (VO2), respiratory exchange ratio (RER), heart rate (HR) and ratings of perceived exertion for the arms (local (RPEL) and cardiorespiratory strain (RPECR) were recorded at 1 min, 2 min and at volitional exhaustion. The EMG responses at three sites (flexor carpi ulnaris, biceps brachii and triceps brachii lateral) were recorded using double-differential (16-3000 Hz bandwidth, x300 gain), bipolar, active electrodes (MP-2A, Linton, Norfolk, UK). Electromyographic data were sampled at 1000 Hz and filtered using a 20 to 500 Hz band-pass filter (MP150 Data Acquisition and AcqKnowledge 4.0, Biopac, Goleta, CA). The EMG signals for each muscle were root mean squared (RMS) with a 500-ms sample window. The signal was then normalised, prior to each CWT, as a percentage of the mean of 3 sets of 10 duty cycles completed during the warm-up (see above) when the participants 80% PMP for 20 s was applied. Time to exhaustion (Tlim) was recorded as the performance outcome measure. Data for Tlim were analysed using one-way analysis of variance. Differences in EMG, VO2, RER, HR, RPEL and RPECR were analysed using separate two-way analysis of variance with repeated measures (trial x time). All analyses were performed using the Statistical Package for Social Sciences ( 17.0; SPSS Inc., Chicago, IL). Individual differences in means were located using Bonferroni post-hoc correction. Significance was accepted at P < 0.05. Results: As resistive load increased Tlim decreased (611 s=194, 397 s=99, 268 s=90, 206 s=67s, respectively; P < 0.001, ES = 0.625). Post-hoc analysis revealed that Tlim using 80%PMP was longer than for 90%, 100% and 110% PMP trials (P < 0.001) and 90% was longer than both 100% and 110% PMP trials (P = 0.079, P = 0.001). At exhaustion VO2 was similar across trials (P = 0.413, ES = 0.053), although 80% PMP VO2 tended to be less (2.10 s=0.32 l·min-1) than for 90% (2.29 s=0.37), 100% (2.33 s=0.49) and 110% (2.26 s=0.34). Also, 80% PMP VO2 was less than VO2peak (P = 0.013). There were differences in RER at Tlim (P < 0.001, ES = 0.593) with values increasing with % PMP (1.15 s=0.07, 1.26 s=0.07, 1.36 s=0.10, 1.40 s=0.09, respectively). There were no differences across trials for HR at Tlim (~173 (12); P = 0.834, ES = 0.016) and HR was proportional to %PMP at 1 min, and 2 min. For flexor carpi ulnaris there was an increase in activation as exercise intensity increased (P < 0.001, ES = 0.245). There were a similar responses for biceps brachii and triceps brachii demonstrating an increase in activation with exercise intensity (P <0.001, ES = 0.137, P < 0.001, ES = 0.163, respectively). No differences for RPEL and RPECR were observed at Tlim. Discussion: There was a clear response of Tlim with intensity as expected for lower body exercise (Hill et al., 2002: Medicine and Science in Sports and Exercise, 34(4), 709-714). Despite differences in Tlim across exercise intensities VO2, HR and RPE were similar at exhaustion indicating a functional cardiorespiratory maximum had been reached. As indicated by the RER an increased activation of the anaerobic metabolism with greater exercise intensities (100% and 110%) is likely and therefore this may represent a greater anaerobic component at these two intensities. The increase in EMG activity with intensity could indicate an increase activity with an increase in exercise intensity. Conclusion: It is recommended that due to the combination of muscle activation, oxygen uptake and Tlim that an exercise intensity of 90% or 100% of PMP could be used for high intensity upper body exercise testing