publications of chris clarkson
On scaling solutions with a dissipative fluid
J Ibanez C A Clarkson and A A Coley
We study the asymptotic behaviour of scaling solutions with a dissipative fluid and we show that, contrary to recent claims, the existence of stable accelerating attractor solution which solves the `energy' coincidence problem depends crucially on the chosen equations of state for the thermodynamical variables. We discuss two types of equations of state, one which contradicts this claim, and one which supports it.
Qualitative properties of magnetic fields in scalar field cosmology
C A Clarkson A A Coley and S D Quinlan
We study the qualitative properties of the class of spatially homogeneous Bianchi type-VIo cosmological models containing a perfect fluid with a linear equation of state, a scalar field with an exponential potential and a uniform cosmic magnetic field, using dynamical systems techniques. We find that all models evolve away from an expanding massless scalar field model in which the matter and the magnetic field are negligible dynamically. We also find that for a particular range of parameter values the models evolve towards the usual power-law inflationary model (with no magnetic field) and, furthermore, we conclude that inflation is not fundamentally affected by the presence of a uniform primordial magnetic field. We investigate the physical properties of the Bianchi type-I magnetic field models in some detail.
Cosmic microwave background and scalar-tensor theories of gravity
C A Clarkson A A Coley and E S D O'Neill
We show that if all observers see an isotropic cosmic microwave background in an expanding geodesic perfect fluid spacetime within a scalar-tensor theory of gravity, then that spacetime must be isotropic and spatially homogeneous. This result generalizes the Ehlers-Geren-Sachs theorem of general relativity, and serves to underpin the important result that any evolving cosmological model in a scalar-tensor theory that is compatible with observations must be almost Friedmann-Lemaître-Robertson-Walker.
Magnetic fields and the cosmic microwave background
C A Clarkson and A A Coley
Observations of the high degree of isotropy of the cosmic microwave background are commonly believed to indicate that the Universe is `almost' Friedmann-Lemaître-Robertson-Walker (at least since the time of last scattering). Theoretical support for this belief comes from the so-called Ehlers-Geren-Sachs theorem. We show that a generalization of this theorem rules out any strong magnetic fields in the Universe. Our theoretical result is model-independent and includes the case of inhomogeneous magnetic fields, complementing previous results. We thus prove that cosmic microwave background observations severely constrain all types of primordial and protogalactic magnetic fields in the universe.
Undermining the cosmological principle: almost isotropic observations in inhomogeneous cosmologies
Richard K Barrett and Chris A Clarkson
We challenge the widely held belief that the cosmological principle is an obvious consequence of the observed isotropy of the cosmic microwave background radiation (CMB), combined with the Copernican principle. We perform a detailed analysis of a class of inhomogeneous perfect fluid cosmologies admitting an isotropic radiation field, with a view to assessing their viability as models of the real universe. These spacetimes are distinguished from FLRW universes by the presence of inhomogeneous pressure, which results in an acceleration of the fluid (fundamental observers). We examine their physical, geometrical and observational characteristics for all observer positions in the spacetimes. To this end, we derive exact, analytic expressions for the distance-redshift relations and anisotropies for any observer, and compare their predictions with available observational constraints. As far as the authors are aware, this work represents the first exact analysis of the observational properties of an inhomogeneous cosmological model for all observer positions. Considerable attention is devoted to the anisotropy in the CMB. The difficulty of defining the surface of last scattering in exact, inhomogenous cosmological models is discussed; several alternative practical definitions are presented, and one of these is used to estimate the CMB anisotropy for any model. The isotropy constraints derived from `local' observations (redshift <1) are also considered, qualitatively. A crucial aspect of this work is the application of the Copernican principle: for a specific model to be acceptable we demand that it must be consistent with current observational constraints (especially anisotropy constraints) for all observer locations. The most important results of the paper are presented as exclusion plots in the two-dimensional parameter space of the models. We show that there is a region of parameter space not ruled out by the constraints we consider and containing models that are significantly inhomogeneous. It follows immediately from this that the cosmological principle cannot be assumed to hold on the basis of present observational constraints.
Does the isotropy of the CMB imply a homogeneous universe? Some generalized EGS theorems
Chris A Clarkson and Richard K Barrett
We demonstrate that the high isotropy of the cosmic microwave background (CMB), combined with the Copernican principle, is not sufficient to prove homogeneity of the universe - in contrast to previous results on this subject. The crucial additional factor not included in earlier work is the acceleration of the fundamental observers. We find the complete class of irrotational perfect fluid spacetimes admitting an exactly isotropic radiation field for every fundamental observer and show that they are Friedmann-Lemaître-Robertson-Walker (FLRW) if and only if the acceleration is zero. While inhomogeneous in general, these spacetimes all possess three-dimensional symmetry groups, from which it follows that they also admit a thermodynamic interpretation. In addition to perfect fluids models we also consider multi-component fluids containing non-interacting radiation, dust and a quintessential scalar field or cosmological constant in which the radiation is isotropic for the geodesic (dust) observers. It is shown that the non-acceleration of the fundamental observers forces these spacetimes to be FLRW. While it is plausible that fundamental observers (galaxies) in the real universe follow geodesics, it is strictly necessary to determine this from local observations for the cosmological principle to be more than an assumption. We discuss how observations may be used to test this.
On the observational characteristics of inhomogeneous cosmologies: undermining the cosmological principle; or, have cosmologists put all their EGS in one basket?
Chris A Clarkson
This thesis concerns the compatibility of inhomogeneous cosmologies with our present understanding of the universe. It is a problem of some interest to find the class of all relativistic cosmological models which are capable of providing a reasonable `fit' to the universe. This thesis, in some respects, is part of this process. We consider Stephani models, which are a generalisation of the standard Friedmann-Lemaitre-Robertson-Walker (FLRW) models, which can be thought of as FLRW models with acceleration and pressure gradients. Thus these models generalise the `dust' assumption of standard cosmology. The crucial aspect of this work is the retention of the Copernican principle -- an assumption regarded by many as crucial to cosmology. It states that we are not at a special location in the universe. This is a vital aspect of the original work in this thesis: consideration of an inhomogeneous model, while retaining the Copernican principle has, as far as the author is aware, not been considered in detail before. We start by generalising the Ehlers-Geren-Sachs Theorem to identify the class of inhomogeneous spacetimes which allow an isotropic radiation field for all observers in the spacetime. We then investigate observational and physical aspects of these models from all observer locations. We conclude that there exist spacetimes which conform to present observational constraints (especially anisotropy constraints) for any location in the spacetime, while at the same time being significantly inhomogeneous; ie, not `almost-FLRW'.
PhD thesis awarded by the university of glasgow 1999