172 citations · 1.1k across the 34 of their papers we have counts for
30 papers · 1 filter
Mirror Symmetry and Double Signature Change
Tevian Dray, Charles Hellaby
The black mirror spacetime proposed by Tzanavaris, Boyle, and Turok [1] connects the two exterior regions of the extended Schwarzschild black hole directly to each other, with no i…
Rotation, Embedding and Topology for the Szekeres Geometry
Charles Hellaby, Robert G. Buckley
Recent work on the Szekeres inhomogeneous cosmological models uncovered a surprising rotation effect. Hellaby showed that the angular coordinates do not have a constant or…
Reversing the Null Limit of the Szekeres Metric
Charles Hellaby, Otakar Svítek
The null limits of the Lemaitre-Tolman and Szekeres spacetimes are known to be the Vaidya and news-free Robinson-Trautman metrics. We generalise this result to the case of non-zero…
Calculating Observables in Inhomogeneous Cosmologies I: General Framework
Charles Hellaby, Anthony Walters
We lay out a general framework for calculating the variation of a set of cosmological observables, down the past null cone of an arbitrarily placed observer, in a given arbitrary i…
Frame Rotation in the Szekeres Spacetimes
Charles Hellaby
The Szekeres metric is an inhomogeneous cosmological model without any symmetries. The standard Riemann-type coordinates can be transformed into spherical-type coordinates, but the…
Symmetry and Equivalence in Szekeres Models
Ira Georg, Charles Hellaby
We solve for all Szekeres metrics that have a single Killing vector. For quasi hyperboloidal () metrics, we find that translational symmetries are possible, but only in metr…
Ricci Time in the Lemaitre-Tolman Model and the Block Universe
Yasser Elmahalawy, Charles Hellaby, George F. R. Ellis
It is common to think of our universe according to the "block universe" concept, which says that spacetime consists of many "stacked" 3-surfaces, labelled by some kind of proper ti…
A New Type of Exact Arbitrarily Inhomogeneous Cosmology: Evolution of Deceleration in the Flat Homogeneous-On-Average Case
Charles Hellaby
A new method for constructing exact inhomogeneous universes is presented, that allows variation in 3 dimensions. The resulting spacetime may be statistically uniform on average, or…
Modelling Inhomogeneity in the Universe
Charles Hellaby
An overview of some recent developments in inhomogeneous models is presented. As the volume and precision of cosmological data improves, it will become more and more essential to u…
The Lemaitre Model and the Generalisation of the Cosmic Mass
Alnadhief A. H. Alfedeel, Charles Hellaby
We consider the spherically symmetric metric with a comoving perfect fluid and non-zero pressure -- the Lemaitre metric -- and present it in the form of a calculational algorithm.…
Imitating accelerated expansion of the Universe by matter inhomogeneities - corrections of some misunderstandings
Andrzej Krasinski, Charles Hellaby, Krzysztof Bolejko +1
A number of misunderstandings about modeling the apparent accelerated expansion of the Universe, and about the `weak singularity' are clarified: 1. Of the five definitions of the d…
Solving the Observer Metric
Charles Hellaby, Alnadhief H. A. Alfedeel
The analysis of modern cosmological data is becoming an increasingly important task as the amount of data multiplies. An important goal is to extract geometric information, i.e. th…
Physical and Geometrical Interpretation of the epsilon <= 0 Szekeres Models
Charles Hellaby, Andrzej Krasinski
We study the properties and behaviour of the quasi-pseudospherical and quasi-planar Szekeres models, obtain the regularity conditions, and analyse their consequences. The quantitie…
Determining the metric of the Cosmos: stability, accuracy, and consistency
M. L. McClure, Charles Hellaby
The ultimate application of Einstein's field equations is to empirically determine the geometry of the Universe from its matter content, rather than simply assuming the Universe ca…
Obtaining the spacetime metric from cosmological observations
Teresa Hui-Ching Lu, Charles Hellaby
Recent galaxy redshift surveys have brought in a large amount of accurate cosmological data out to redshift 0.3, and future surveys are expected to achieve a high degree of complet…
Alternative Methods of Describing Structure Formation in the Lemaitre-Tolman Model
Charles Hellaby, Andrzej Krasinski
We describe several new ways of specifying the behaviour of Lemaitre-Tolman (LT) models, in each case presenting the method for obtaining the LT arbitrary functions from the given…
Formation of voids in the Universe within the Lemaitre-Tolman model
Krzysztof Bolejko, Andrzej Krasinski, Charles Hellaby
We develop models of void formation starting from a small initial fluctuation at recombination and growing to a realistic present day density profile in agreement with observations…
Formation of a galaxy with a central black hole in the Lemaitre-Tolman model
Andrzej Krasinski, Charles Hellaby
We construct two models of the formation a galaxy with a central black hole, starting from a small initial fluctuation at recombination. This is an application of previously develo…
More examples of structure formation in the Lemaitre-Tolman model
Andrzej Krasinski, Charles Hellaby
In continuing our earlier research, we find the formulae needed to determine the arbitrary functions in the Lemaitre-Tolman model when the evolution proceeds from a given initial v…
You Can't Get Through Szekeres Wormholes - or - Regularity, Topology and Causality in Quasi-Spherical Szekeres Models
Charles Hellaby, Andrzej Krasinski
The spherically symmetric dust model of Lemaitre-Tolman can describe wormholes, but the causal communication between the two asymptotic regions through the neck is even less than i…
Structure formation in the Lemaitre-Tolman model
Andrzej Krasinski, Charles Hellaby
Structure formation within the Lemaitre-Tolman model is investigated in a general manner. We seek models such that the initial density perturbation within a homogeneous background…
Note on Signature Change and Colombeau Theory
Tevian Dray, George Ellis, Charles Hellaby
Recent work alludes to various `controversies' associated with signature change in general relativity. As we have argued previously, these are in fact disagreements about the (ofte…
On The Vaidya Limit of the Tolman Model
Charles Hellaby
We show that the only Tolman models which permit a Vaidya limit are those having a dust distribution that is hollow - such as the self-similar case. Thus the naked shell-focussing…
Large Scale Inhomogeneity Versus Source Evolution -- Can We Distinguish Them Observationally?
Nazeem Mustapha, Charles Hellaby, G. F. R. Ellis
We reconsider the issue of proving large scale spatial homogeneity of the universe, given isotropic observations about us and the possibility of source evolution both in numbers an…