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1. Relativistic Effects on Circumbinary Disk Evolution: Breaking the Polar Alignment around Eccentric Black Hole Binary Systems

   https://doi.org/10.3847/1538-4357/ad1a11  

   Childs, Anna C.; Martin, Rebecca G.; Nixon, C. J.; Geller, Aaron M.; Lubow, Stephen H.; Zhu, Zhaohuan; Lepp, Stephen (February 2024, The Astrophysical Journal)

   Abstract We study the effects of general relativity (GR) on the evolution and alignment of circumbinary disks around binaries on all scales. We implement relativistic apsidal precession of the binary into the hydrodynamics codephantom. We find that the effects of GR can suppress the stable polar alignment of a circumbinary disk, depending on how the relativistic binary apsidal precession timescale compares to the disk nodal precession timescale. Studies of circumbinary disk evolution typically ignore the effects of GR, which is an appropriate simplification for low-mass or widely separated binary systems. In this case, polar alignment occurs, provided that the disks initial misalignment is sufficiently large. However, systems with a very short relativistic precession timescale cannot polar align and instead move toward coplanar alignment. In the intermediate regime where the timescales are similar, the outcome depends upon the properties of the disk. Polar alignment is more likely in the wavelike disk regime (where the disk viscosity parameter is less than the aspect ratio,α<H/r), since the disk is in good radial communication. In the viscous disk regime, disk breaking is more likely. Multiple rings can destructively interact with one another, resulting in short disk lifetimes and the disk moving toward coplanar alignment. Around main-sequence star or stellar mass black hole binaries, polar alignment may be suppressed far from the binary, but in general, the inner parts of the disk can align to polar. Polar alignment may be completely suppressed for disks around supermassive black holes for close binary separations. 

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2. Calibration methods for a dual-wavelength interferometer system

   https://doi.org/10.1117/12.2323127  

   Zhang, Yanqi; Tangari Larrategui, Martin; Brown, Thomas G.; Ellis, Jonathan D. (September 2018, Proc. SPIE 10747, Optical System Alignment, Tolerancing, and Verification XII)

   Multiple wavelength interferometry has long been considered an option for the measurement of large aspheric slope departures. In particular, a synthetic wavelength offers a variety of approaches by which large phase excursions can be unwrapped. Using multiple wavelengths can create collimation and magnification mismatch errors between the individual wavelengths that arise during beam expansion and propagation. Here, we present and analyze alignment and calibration methods for a dual-wavelength interferometer that can significantly reduce both misalignment errors and chromatic aberrations in the system. To correct for misalignment, a general method is described for the alignment of a dual-wavelength interferometer, including the alignment of lasers, beam expanders, beam splitters for combining beams and for compensating errors in the reference surface, and the fringe imaging system. A Fourier transform test at the detector surface was conducted to validate that there is essentially no magnification difference between two wavelengths resulting from misalignment of optical system. For the chromatic aberration introduced by the optical elements in the system, a ray-trace model of the interferometer has been established, to simulate the chromatic effect that optical elements will have on the measurement results. As an experimental test, we examine an off-axis spherical mirror in a non-null condition using a highly aliased interferogram. The above alignment methods and the results are analyzed based on the simulated system errors. Using this method, we demonstrate a measured surface profile of deviation of λ/25 which is comparable to a direct measurement profile of the surface on axis using a Fizeau interferometer. 

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3. Quantifying Misalignment Between Agents

   Kierans, A.; Hazan, H.; Dori-Hacohen, S. (January 2022, ML Safety @ NeurIPS 2022)

   Growing concerns about the AI alignment problem have emerged in recent years, with previous work focusing mostly on (1) qualitative descriptions of the alignment problem; (2) attempting to align AI actions with human interests by focusing on value specification and learning; and/or (3) focusing on either a single agent or on humanity as a singular unit. However, the field as a whole lacks a systematic understanding of how to specify, describe and analyze misalignment among entities, which may include individual humans, AI agents, and complex compositional entities such as corporations, nation-states, and so forth. Prior work on controversy in computational social science offers a mathematical model of contention among populations (of humans). In this paper, we adapt this contention model to the alignment problem, and show how viewing misalignment can vary depending on the population of agents (human or otherwise) being observed as well as the domain or "problem area" in question. Our model departs from value specification approaches and focuses instead on the morass of complex, interlocking, sometimes contradictory goals that agents may have in practice. We discuss the implications of our model and leave more thorough verification for future work. 

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4. Modeling of Human-Exoskeleton Alignment and Its Effect on the Elbow Flexor and Extensor Muscles during Rehabilitation

   https://doi.org/10.3390/modelling4030020  

   Rincon, Clarissa; Delgado, Pablo; Hakansson, Nils A.; Yihun, Yimesker (September 2023, Modelling)

   Human-exoskeleton misalignment could lead to permanent damages upon the targeted limb with long-term use in rehabilitation. Hence, achieving proper alignment is necessary to ensure patient safety and an effective rehabilitative journey. In this study, a joint-based and task-based exoskeleton for upper limb rehabilitation were modeled and assessed. The assessment examined and quantified the misalignment present at the elbow joint as well as its effects on the main flexor and extensor muscles’ tendon length during elbow flexion-extension. The effects of the misalignments found for both exoskeletons resulted to be minimal in most muscles observed, except the anconeus and brachialis. The anconeus muscle demonstrated a relatively higher variation in tendon length with the joint-based exoskeleton misalignment, indicating that the task-based exoskeleton is favored for tasks that involve this particular muscle. Moreover, the brachialis demonstrated a significantly higher variation with the task-based exoskeleton misalignment, indicating that the joint-based exoskeleton is favored for tasks that involve the muscle. 

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5. The evolution of galaxy intrinsic alignments in the MassiveBlackII universe

   https://doi.org/10.1093/mnras/stz3240  

   Bhowmick, Aklant K; Chen, Yingzhang; Tenneti, Ananth; Di Matteo, Tiziana; Mandelbaum, Rachel (January 2020, Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT We investigate the redshift evolution of the intrinsic alignments (IAs) of galaxies in the MassiveBlackII (MBII) simulation. We select galaxy samples above fixed subhalo mass cuts ($$M_h\gt 10^{11,12,13}\,\mathrm{M}_{\odot }\, h^{-1}$$) at z = 0.6 and trace their progenitors to z = 3 along their merger trees. Dark matter components of z = 0.6 galaxies are more spherical than their progenitors while stellar matter components tend to be less spherical than their progenitors. The distribution of the galaxy–subhalo misalignment angle peaks at ∼10 deg with a mild increase with time. The evolution of the ellipticity–direction (ED) correlation amplitude ω(r) of galaxies (which quantifies the tendency of galaxies to preferentially point towards surrounding matter overdensities) is governed by the evolution in the alignment of underlying dark matter (DM) subhaloes to the matter density of field, as well as the alignment between galaxies and their DM subhaloes. At scales $$\sim 1~\mathrm{Mpc}\, h^{-1}$$, the alignment between DM subhaloes and matter overdensity gets suppressed with time, whereas the alignment between galaxies and DM subhaloes is enhanced. These competing tendencies lead to a complex redshift evolution of ω(r) for galaxies at $$\sim 1~\mathrm{Mpc}\, h^{-1}$$. At scales $$\gt 1~\mathrm{Mpc}\, h^{-1}$$, alignment between DM subhaloes and matter overdensity does not evolve significantly; the evolution of the galaxy–subhalo misalignment therefore leads to an increase in ω(r) for galaxies by a factor of ∼4 from z = 3 to 0.6 at scales $$\gt 1~\mathrm{Mpc}\, h^{-1}$$. The balance between competing physical effects is scale dependent, leading to different conclusions at much smaller scales ($$\sim 0.1~\mathrm{Mpc}\, h^{-1}$$). 

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