Asher Space Research Institute

Survey boundaries and nonlinear dynamics complicate reconstruction of large-scale matter-density and peculiar-velocity fields from galaxy redshift surveys. We develop and compare two reconstruction methods within a common Bayesian framework, based on linear perturbation theory and the Zel'dovich approximation, and apply them to the DESI Data Release 1 Bright Galaxy Survey. We infer the initial density field using a Gaussian prior and likelihood, with the Zel'dovich model displacing particles from a uniform Lagrangian grid. Both yield maximum a posteriori (MAP) fields; only the linear posterior is sampled. Our main findings are: (i) Both reconstructions yield similar large-scale density structures and coherent velocity patterns in the survey interior, beyond roughly 50 h^-1 Mpc from the footprint boundaries. (ii) Comparison with Cosmicflows-4 groups, predominantly from the SDSS Fundamental Plane sample in the DESI overlap, shows close agreement between mean observed radial velocities and MAP predictions when binned by the latter, despite substantial individual scatter. (iii) Both yield similar reflex-dipole profiles, defined as mean shell velocities relative to those at randomly sampled centres, out to ∼225 h^-1 Mpc. The profiles depend weakly on the prior normalization σ_8, but lie below the fiducial linear ΛCDM reference at r<50 h^-1 Mpc. Sparse sampling limits the largest radii. (iv) In an exploratory BAO test, shifting galaxies and randoms to a target redshift z=8 using reconstructed displacements sharpens the acoustic feature in both cases, with a slightly more pronounced peak for Zel'dovich. Survey geometry and galaxy bias remain the principal limitations.

We present a second-order calculation of relativistic large-scale-structure observables in cosmological perturbation theory, specifically the "cosmic rulers and clock", which are the building-blocks of any other large-scale-structure observable, including galaxy number counts, on large scales. We calculate the scalar rulers (longitudinal perturbation and magnification) and the cosmic clock to second order, using a fully non-linear covariant definition of the observables. We validate our formul ae on three non-trivial space-time metrics: two of them are null tests on metrics which are obtained by applying a gauge transformation to the background space-time, while the third is the "separate universe" curved background, for which we can also compute the observables exactly. We then illustrate the results by evaluating the second-order observables in a simplified symmetric setup. On large scales, they are suppressed over the linear contributions by similar to 10-4, while they become comparable to the linear contributions on mildly non-linear scales. The results of this paper form a significant (and the most complicated) part of the relativistic galaxy number density at second order.

We compare an autoencoder convolutional neural network (AE-CNN) with conventional maximum-likelihood estimation (MLE) for inferring cluster virial masses, Mv, directly from the galaxy distribution around clusters, without identifying members or interlopers. The AE-CNN is trained on mock galaxy catalogues, whereas the MLE assumes that clusters of similar mass share the same phase-space galaxy profile. Conceptually, the MLE returns an unbiased estimate of logMv at fixed true mass, whereas the AE-CNN approximates the posterior mean, so the true logMv is unbiased at fixed estimate. Using MultiDark Planck 2 mock clusters with redshift space number density as input, the AE-CNN attains an rms scatter of 0.10 dex between predicted and true logMv , compared with 0.16 dex for the MLE. With inputs based on mean peculiar velocities, binned in redshift space or observed distance, the AE-CNN achieves scatters of 0.12 and 0.16 dex, respectively, despite strong inhomogeneous Malmquist bias.

На основе результатов нескольких работ, посвященных исследованию средних спектров мощности (PDS) гамма-всплесков (GRB), составлен перечень особенностей PDS, которые необходимо объяснить. Используя доказанную в ряде публикаций возможность разложения временного профиля каждого гамма-всплеска в сумму нескольких двухсторонних импульсов, производится моделирование GRB во временной области и исследование средних спектров мощности моделей в частотной области. Сделаны краткие обзоры результатов ряда теоретических работ. Проведена Монте-Карло симуляция временных рядов, состоящих из совокупности двухсторонних импульсов с пуассоновским распределением положения импульсов на временной шкале, при различных формах импульсов и распределениях амплитуды и длительности импульсов. Все основные свойства среднего спектра мощности, имеющего форму квази-Лоренциана, теоретически выведенные в различных опубликованных исследованиях, подтверждены. PDS-суперпозиции двухсторонних импульсов, случайно распределенных во времени, не описываются единым степенным законом. В общем случае, форма PDS состоит из трех квазистепенных участков, разделенных двумя изломами. Положение двух изломов в PDS определяется параметрами асимметрии и эффективной длительности импульсов. Распределение длительности импульсов и их форма влияют на форму PDS, а распределение амплитуды импульсов практически не оказывает такого влияния. Величина перемежаемости (при больших значениях γ ≥ 1) влияет на форму PDS. Основываясь на предшествующих теоретических работах и проведенных в настоящем исследовании Монте-Карло симуляциях, можно утверждать, что все особенности средних спектров мощности PDS GRB объясняются с помощью простой модели кривой блеска в виде суперпозиции некоррелированных случайных двухсторонних импульсов. Основные особенности PDS GRB определяются только такими характеристиками импульсов, как параметр асимметрии, распределение длительности и форма импульсов.

We apply kinetic theory (KT) to solve for the vertical structure of an axisymmetric, self-gravitating, particulate ring around a spheroidal central body which may be either spherical or oblate. Our work improves upon the present KT models of rings by introducing a more general central body potential. We consider both dilute and dense rings. As an example for a dilute ring, we solve for the vertical structure of Saturn’s A ring including Saturn’s oblateness in our calculations. We compare our results with earlier works (Simon and Jenkins 1994) which assumed a spherical Saturn. Then, we calculate the corrections due to the oblateness on the ring stability condition obtained by them. We find that the planetary oblateness adversely affects ring stability due to the enhanced shearing. Next, we add self-gravity as an additional vertical force in dilute rings, where particle clumping is negligible, and find the combined stability boundary. Self-gravity is seen to have a stabilising effect on the ring as reported in earlier studies (Salo 1995), though, its stabilising effect is observed to be less than the destabilising effect of the oblateness of the central body, at least, for dilute rings. Then, we move to moderately dense rings. Oblateness of the central body is seen to perturb the ring properties, and, the results indicate a good qualitative comparison with DE simulations of Gupta et al., (2018). However, we note the emergence of strong self-gravity wakes in dense rings, an aspect that is usually ignored in kinetic theory for rings. Hence, we show the need for improved constitutive modelling of rings including the mesoscopic inhomogeneities in the form self-gravity wakes. Finally, we apply KT to solve for the vertical structure of Chariklo’s inner ring. Through this we estimate the 1/3 resonance torque of Chariklo that acts on the inner boundary of Chariklo’s inner ring by calculating the shear stress in the ring. Overall, our work demonstrates the structural differences of rings around minor planets like Chariklo that have highly oblate shapes.