arXiv subjects

Ben David Normann

Publications and source records attributed to Ben David Normann.

12 recordsLinked to original sources

arXiv2025

A short review on joint weak and strong cluster lens-mass reconstruction

The distinction between weak and strong lensing is somewhat arbitrary, and both regimes are manifestations of the same physical phenomenon: gravity bending the path of light. Nevertheless, these two regimes have to a large extent been treated separately, since they require different approaches. This review traces the development of methods combining weak-lensing and strong-lensing data for joint lens-mass reconstruction, with a particular emphasis on cluster lenses, where both effects occur. We conclude that so-called inverse methods have been successful in merging the two regimes insofar data analysis is concerned. However, a number of improvements seem to be in place. First, not many studies include weak lensing data beyond shear. In light of the unprecedented quality of the data of JWST and future surveys, this is a clear point of improvement. Especially so since flexion terms have proven useful in determining sub-structures. Second, considering the amount of data available, and the complexity of non-parametric lenses, automating the processes of lens-mass reconstruction would be beneficial. Towards this end, invoking machine learning seems like a promising way forward.

astro-ph.CO↗

arXiv2024

Om å kartleggja mørk materie med maskinlæring

Gravitational lensing occurs as the path of light from distant celestial bodies is distorted due to gravitational attraction by other celestial bodies, whose mass is partly invisible, being so-called dark matter. When observed through a gravitational lens, distant galaxies appear distorted. A lot of research activity goes into mapping the dark matter in the universe through graviational lensing. However, the mathematical models are complicated, and calculations both time consuming and tideous, if manual. In this paper we discuss how we may combine the Roulette formalism due to Clarkson with machine learning for automatic, local estimation of the lense potential in strong lenses. We also present a framework of open source software for generating datasets and validating results. -- Gravitasjonslinsing er fenomenet der ljos frå fjerne himmellegeme vert avbøygd av tyngdekraften frå andre himmellegeme, som ofte ikkje er fullt synlege fordi mykje av massen er mørk materie. Observert gjennom ei gravitasjonslinse, framstår fjerne galaksar som forvrengde. Der er mykje forskingsaktivitet som freistar å karleggja mørk materie ved å studera linseeffektar, men dei matematiske modellane er kompliserte og utrekningane krev i dag mykje manuelt arbeide som er svært tidkrevjande. I denne artikkelen drøftar me korleis me kan kombinera rouletteformalismen åt Chris Clarkson med maskinlæring for automatisk, lokal estimering av linsepotentialet i sterke linser, og me presenterer eit rammeverk med programvare i open kjeldekode for å generera datasett og validera resultat.

astro-ph.IM↗

arXiv2023

On the simulation of gravitational lensing

Gravitational lensing refers to the deflection of light by the gravity of celestial bodies, often predominantly composed of dark matter. Seen through a gravitational lens, the images of distant galaxies appear distorted. In this paper we discuss simulation of the image distortion by gravitational lensing. The objective is to enhance our understanding of how gravitational lensing works through a simple tool to visualise hypotheses. The simulator can also generate synthetic data for the purpose of machine learning, which will hopefully allow us to invert the distortion function, something which is not analytically possible at present.

astro-ph.CO↗

arXiv2021

Can the Hubble tension be resolved by bulk viscosity?

We show that the cosmic bulk viscosity estimated in our previous works is sufficient to bridge the $H_0$ value inferred from observations of the early universe with the value inferred from the local (late) universe.

gr-qc↗

arXiv2020

Remarks on cosmological bulk viscosity in different epochs

The intention of this paper is mainly two-fold. \textit{First}, we point out a striking numerical agreement between the bulk viscosity in the lepton era calculated by Husdal (2016) and our own calculations of the present-day bulk viscosity when the functional form is $ζ\,\sim\sqrtρ$. From a phenomenological point of view, we thus seem to have an ansatz for the viscosity which bridges the infancy of the Universe ($\sim 1$ s) with the present. This can also be looked upon as a kind of symmetry between the early-time cosmology and the present-day cosmology: it is quite remarkable that the kinetic theory-based bulk viscosity in the early universe and the experimentally-based bulk viscosity in the present universe can be covered by the same simple analytical formula. \textit{Second}, we consider the Kasner universe as a typical anisotropic model of Bianchi-type I, investigating whether this geometrical model is compatible with constant viscosity coefficients in the fluid. Perhaps surprisingly, the existence of a shear viscosity turns out to be incompatible with the Kasner model. By contrast, a bulk viscosity is non-problematic in the {\it isotropic} version of the model. In the special case of a Zel'dovich (stiff) fluid, the three equal exponents in the Kasner metric are even determined by the bulk viscosity alone, independent of the value of the fluid energy density. We also give a brief comparison with some other recent approaches to viscous cosmology.

gr-qc↗

arXiv2020

Recursion relations for gravitational lensing

The weak gravitational lensing formalism can be extended to the strong lensing regime by integrating a nonlinear version of the geodesic deviation equation. The resulting "roulette" expansion generalises the notion of convergence, shear and flexion to arbitrary order. The independent coefficients of this expansion are screen space gradients of the optical tidal tensor which approximates to the usual lensing potential in the weak field limit. From lensed images, knowledge of the roulette coefficients can in principle be inverted to reconstruct the mass distribution of a lens. In this paper, we simplify the roulette expansion and derive a family of recursion relations between the various coefficients, generalising the Kaiser-Squires relations beyond the weak-lensing regime.

gr-qc↗

arXiv2020

Approaching Wonderland

Continuing previous work, we show the existence of stable, anisotropic future attractors in Bianchi invariant sets with a $p$-form field ($p\,\in\,\{1,3\}$) and a perfect fluid. In particular, we consider the not previously investigated Bianchi invariant sets $\mathcal{B}$(II), $\mathcal{B}$(IV), $\mathcal{B}$(VII$_0$) and $\mathcal{B}$(VII$_{h})$ and examine their asymptotic behaviour. We find that the isolated equilibrium set Wonderland is a future attractor on all of its existence ($2/3<\,γ\,<2$) in all these sets except in $\mathcal{B}$(II), where the peculiar equilibrium sets Edge and Rope show up, taking over the stability for certain values of $γ$. In addition, in $\mathcal{B}$(IV) and $\mathcal{B}$(VII$_h$) plane gravitational wave solutions (with a non-zero $p$-form) serve as attractors whenever $2/3<\,γ\,<2$.

gr-qc↗

arXiv2020

Collins in Wonderland

What is the asymptotic future of a scalar-field model if the assumption of isotropy is relaxed in generic, homogeneous space-times with general relativity? This paper is a continuation of our previous work on Bianchi cosmologies with a $p$-form field (where $p\,\in\,\{1,3\}$)---or equivalently: an inhomogeneous, mass-less scalar gauge field with a homogeneous gradient. In this work we investigate such matter sector in General Relativity, and restrict to space-times of the particular Bianchi types VI$_0$ and VI$_{\tilde{h}}$, where $\tilde{h}=h<0\,\cap\,\neq\,-1/9\,\cup\,-1$. We show that the previously found fabric of exact solutions named Wonderland are future attractors in $\mathcal{B}$(VI$_0$) and $\mathcal{B}$(VI$_{\tilde{h}}$), extending the Collins perfect-fluid equilibrium set to include a $p$-form (with $p\,\in\,\{1,3\}$). We also write down the line-element corresponding to Wonderland in VI$_{\tilde{h}}$ and give explicit expressions for the underling gauge-potential $ϕ(t,\mathbf{x})$ corresponding to this solution. Simulation of a path approaching Wonderland in Bianchi type I is also given.

gr-qc↗

arXiv2019

A study of inhomogeneous massless scalar gauge fields in cosmology

Why is the Universe so homogeneous and isotropic? We summarize a general study of a $γ$-law perfect fluid alongside an inhomogeneous, massless scalar gauge field (with homogeneous gradient) in anisotropic spaces with General Relativity. The anisotropic matter sector is implemented as a $j$-form (field-strength level), where $j\,\in\,\{1,3\}$, and the spaces studied are Bianchi space-times of solvable type. Wald's no-hair theorem is extended to include the $j$-form case. We highlight three new self-similar space-times: the Edge, the Rope and Wonderland. The latter solution is so far found to exist in the physical state space of types I,II, IV, VI$_0$, VI$_h$, VII$_0$ and VII$_h$, and is a global attractor in I and V. The stability analysis of the other types has not yet been performed. This paper is a summary of ~[1], with some remarks towards new results which will be further laid out in upcoming work.

gr-qc↗

arXiv2018

Bianchi cosmologies with $p$-form gauge fields

In this paper the dynamics of free gauge fields in Bianchi type I-VII$_{h}$ space-times is investigated. The general equations for a matter sector consisting of a $p$-form field strength ($p\,\in\,\{1,3\}$), a cosmological constant ($4$-form) and perfect fluid in Bianchi type I-VII$_{h}$ space-times are computed using the orthonormal frame method. The number of independent components of a $p$-form in all Bianchi types I-IX are derived and, by means of the dynamical systems approach, the behaviour of such fields in Bianchi type I and V are studied. Both a local and a global analysis are performed and strong global results regarding the general behaviour are obtained. New self-similar cosmological solutions appear both in Bianchi type I and Bianchi type V, in particular, a one-parameter family of self-similar solutions,"Wonderland ($λ$)" appears generally in type V and in type I for $λ=0$. Depending on the value of the equation of state parameter other new stable solutions are also found ("The Rope" and "The Edge") containing a purely spatial field strength that rotates relative to the co-moving inertial tetrad. Using monotone functions, global results are given and the conditions under which exact solutions are (global) attractors are found.

gr-qc↗

arXiv2016

Characteristic Properties of Two Different Viscous Cosmology Models for the Future Universe

We analyze characteristic properties of two different cosmological models: (i) a one-component dark energy model where the bulk viscosity $ζ$ is associated with the fluid as a whole, and (ii) a two-component model where $ζ$ is associated with a dark matter component $ρ_{\rm m}$ only, the dark energy component considered inviscid. Shear viscosity is omitted. We assume throughout the simple equation of state $p=wρ$, with $w$ a constant. In the one-component model we consider two possibilities, either to take $ζ$ proportional to the scalar expansion (equivalent to the Hubble parameter), in which case the evolution becomes critically dependent on the value of the small constant $α=1+w$ and the magnitude of $ζ$. Second, we consider the case $ζ=~$const., where a de Sitter final stage is reached in the future. In the two-component model we consider only the case where the dark matter viscosity $ζ_{\rm m}$ is proportional to the square of $ρ_{\rm m}$, where again a de Sitter form is found in the future. In this latter case the formalism is supplemented by a phase space analysis. As a general result of our considerations we suggest that a value $ζ_0\sim 10^6~$Pa s for the present viscosity is reasonable, and that the two-component model seems to be favored.

gr-qc↗

arXiv2016

General Bulk-Viscous Solutions and Estimates of Bulk Viscosity in the Cosmic Fluid

We derive a general formalism for bulk viscous solutions of the energy-conservation-equation for $ρ(a,ζ)$, both for a single-component and a multicomponent fluid in the Friedmann universe. For our purposes these general solutions become valuable in estimating order of magnitude of the phenomenological viscosity in the cosmic fluid at present. $H(z)$ observations are found to put an upper limit on the magnitude of the modulus of the present day bulk viscosity. It is found to be $ζ_0\sim 10^6~$Pa s, in agreement with previous works. We point out that this magnitude is acceptable from a hydrodynamic point of view. Finally, we bring new insight by using our estimates of $ζ$ to analyse the fate of the future universe. Of special interest is the case $ζ\propto \sqrtρ$ for which the fluid, originally situated in the quintessence region, may slide through the phantom barrier and inevitably be driven into a big rip. Typical rip times are found to be a few hundred Gy.

gr-qc↗