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R. A. Battye

Publications and source records attributed to R. A. Battye.

At least 55 records · Page 3Linked to original sources
arXiv2005

Rigidity and stability of cold dark solid universe model

Observational evidence suggests that the large scale dynamics of the universe is presently dominated by dark energy, meaning a non-luminous cosmological constituent with a negative value of the pressure to density ratio $w=P/ρ$, which would be unstable if purely fluid, but could be stable if effectively solid with sufficient rigidity. It was suggested by Bucher and Spergel that such a solid constituent might be constituted by an effectively cold (meaning approximately static) distribution of cosmic strings with $w=-1/3$, or membranes with the observationally more favoured value $w=-2/3$, but it was not established whether the rigidity in such models actually would be sufficient for stabilisation. The present article provides an explicit evaluation of the rigidity to density ratio, which is shown to be given in both string and membrane cases by $μ/ρ=4/15$, and it is confirmed that this is indeed sufficient for stabilisation.

hep-th↗
arXiv2003

Regularization of the linearized gravitational self-force for branes

We discuss the linearized, gravitational self-interaction of a brane of arbitrary codimension in a spacetime of arbitrary dimension. We find that in the codimension two case the gravitational self-force is exactly zero for a Nambu-Goto equation of state, generalizing a previous result for a string in four dimensions. For the case of a 3-brane, this picks out the case of a six-dimensional brane-world model as having special properties which we discuss. In particular, we see that bare tension on the brane has no effect locally, suppressing the cosmological constant problem.

hep-th↗
arXiv2001

Cosmological expansion on a dilatonic brane-world

In this paper we study brane-world scenarios with a bulk scalar field, using a covariant formalism to obtain a 4D Einstein equation via projection onto the brane. We discuss, in detail, the effects of the bulk on the brane and how the scalar field contribute to the gravitational effects. We also discuss choice of conformal frame and show that the frame selected by the induced metric provides a natural choice. We demonstrate our formalism by applying it to cosmological scenarios of Randall-Sundrum and Horava-Witten type models. Finally we consider the cosmology of models where the scalar field couples non-minimally to the matter on the brane. This gives rise to a novel scenario where the universe expands from a finite scale factor with an initial period of accelerated expansion, thus avoiding the singularity and flatness problem of the standard big bang model.

hep-th↗
arXiv2001

Solitonic fullerene structures in light atomic nuclei

The Skyrme model is a classical field theory which has topological soliton solutions. These solitons are candidates for describing nuclei, with an identification between the numbers of solitons and nucleons. We have computed numerically, using two different minimization algorithms, minimum energy configurations for up to 22 solitons. We find, remarkably, that the solutions for seven or more solitons have nucleon density isosurfaces in the form of polyhedra made of hexagons and pentagons. Precisely these structures arise, though at the much larger molecular scale, in the chemistry of carbon shells, where they are known as fullerenes.

hep-th↗
arXiv2001

Generic junction conditions in brane-world scenarios

We present the generic junction conditions obeyed by a co-dimension one brane in an arbitrary background spacetime. As well as the usual Darmois-Israel junction conditions which relate the discontinuity in the extrinsic curvature to the to the energy-momentum tensor of matter which is localized to the brane, we point out that another condition must also be obeyed. This condition, which is the analogous to Newton's second law for a point particle, is trivially satisfied when $Z_2$ symmetry is enforced by hand, but in more general circumstances governs the evolution of the brane world-volume. As an illustration of its effect we compute the force on the brane due to a form field.

hep-th↗
arXiv2000

Cosmic concordance and the fine structure constant

Recent measurements of a peak in the angular power spectrum of the cosmic microwave background appear to suggest that geometry of the universe is close to being flat. But if other accepted indicators of cosmological parameters are also correct then the best fit model is marginally closed, with the peak in the spectrum at larger scales than in a flat universe. Such observations can be reconciled with a flat universe if the fine structure constant had a lower value at earlier times, which would delay the recombination of electrons and protons and also act to suppress secondary oscillations as observed. We discuss evidence for a few percent increase in the fine structure constant between the time of recombination and the present.

astro-ph↗
arXiv1999

Axion string cosmology and its controversies

Understanding axion cosmology has important experimental consequences since it constrains the range of allowed values for the axion mass. In the standard thermal scenario, which assumes Peccei-Quinn symmetry restoration after inflation, an axion string network forms at the phase transition $T\sim\fa$ and then radiatively decays into a cosmological background of axions. Under standard assumptions for the evolution of this string network and the radiation from it, axions must have a mass of $\ma\sim 100μ{\rm eV}$ with large specified uncertainties. We discuss critically the various suggestions in the literature that the axion mass migh be lighter.

astro-ph↗
arXiv1998

Second order Lagrangian and symplectic current for gravitationally perturbed Dirac-Goto-Nambu strings and branes

We extend a recent analysis of gravitational perturbations on Dirac-Nambu-Goto strings, membranes and higher dimensional branes. In an arbitrary gauge, it is shown that the relevant first order equations governing the displacement vector of the worldsheet and metric perturbation are obtainable from a variational principle whose Lagrangian is constructed as a second order perturbation of the standard Dirac-Goto-Nambu action density. A symplectic current functional is obtained as a by-product that is potentially useful for the derivation of conservation laws in particular circumstances.

hep-th↗
arXiv1998

Cosmic structure formation in Hybrid Inflation models

A wide class of inflationary models, known as Hybrid Inflation models, may produce topological defects during a phase transition at the end of the inflationary epoch. We point out that, if the energy scale of these defects is close to that of Grand Unification, then their effect on cosmic structure formation and the generation of microwave background anisotropies cannot be ignored. Therefore, it is possible for structure to be seeded by a combination of the adiabatic perturbations produced during inflation and active isocurvature perturbations produced by defects. Since the two mechanisms are uncorrelated the power spectra can be computed by a weighted average of the individual contributions. We investigate the possible observational consequences of this with reference to general Hybrid Inflation models and also a specific model based on Supergravity. These mixed perturbation scenarios have some novel observational consequences and these are discussed qualitatively.

astro-ph↗
arXiv1998

On the origin of dark matter axions

We discuss the possible sources of dark matter axions in the early universe. In the standard thermal scenario, an axion string network forms at the Peccei-Quinn phase transition $T\sim \fa$ and then radiatively decays into a cosmological background of axions; to be the dark matter, these axions must have a mass $\ma \sim 100 μeV$ with specified large uncertainties. An inflationary phase with a reheat temperature below the PQ-scale $T_{reh} \lapp \fa$ can also produce axion strings through quantum fluctuations, provided that the Hubble parameter during inflation is large $H_1 \gapp \fa$; this case again implies a dark matter axion mass $\ma \sim 100 μeV$. For a smaller Hubble parameter during inflation $H_1 \lapp \fa$, `anthropic tuning' allows dark matter axions to have any mass in a huge range below $\ma\lapp 1 meV$.

astro-ph↗
arXiv1998

Spectrum of radiation from axion strings

In the wide variety of axion cosmologies in which axion strings form, their radiative decay is the dominant mechanism for the production of axions, imposing a tight constraint on the axion mass. Here, we focus on the mechanism by which axions are produced in this scenario and, in particular, the key issue of the axion spectrum emitted by an evolving network of strings.

astro-ph↗
arXiv1998

Cosmic strings in a universe with non-critical matter density

In the light of recent work which suggests that structure formation by cosmic strings in a critical matter density universe requires large biases, we discuss the effects of reducing the matter density. Using a full linear Einstein-Boltzmann solver, we com pute the cold dark matter density field and the microwave background anisotropies/polarization in a flat universe with a non-zero cosmological constant. We find that universes with $Ω_Λ=0.7-0.8$ give a good fit to the shape of the observed ga laxy distribution, albeit with a bias of around 2. This has interesting implications for the anisotropy and polarization of the microwave background. Finally, we discuss the effects of relaxing the assumption that the universe is flat.

astro-ph↗
arXiv1998

Non-divergence of gravitational self-interactions for Goto-Nambu strings

The classical linearised gravitational self interaction of a Goto-Nambu string is examined in four spacetime dimensions. Using a conveniently gauge independent tensorial treatment, the divergent part of the self-force is shown to be exactly zero. This is due to cancelation by a contribution that was neglected in the previous treatments. This result has implications for many applications.

hep-th↗
arXiv1998

Cosmic Axions

The current cosmological constraints on a dark matter axion are reviewed. We describe the basic mechanisms by which axions are created in the early universe, both in the standard thermal scenario in which axion strings form and in inflationary models. In the thermal scenario, the dominant process for axion production is through the radiative decay of an axion string network, which implies a dark matter axion of mass m_a ~ 100 microeV with specified large uncertainties. An inflationary phase does not affect this string bound if the reheat temperature is high T_reh > f_a or, conversely, for T_reh < f_a, if the Hubble parameter during inflation is large H_1 > f_a; in both cases, strings form and we return to the standard picture with a m_a ~ 100 microeV dark matter axion. Inflationary models with f_a > H_1 > T_reh face strong CMBR constraints and require `anthropic misalignment' fine-tuning in order to produce a dark matter axion; in this case, some inflation models are essentially incompatible with a detectable axion, while others can be engineered to allow a dark matter axion anywhere in a huge mass range below m_a < 1 meV. We endeavour to clarify the sometimes confusing and contradictory literature on axion cosmology.

astro-ph↗
arXiv1997

A detail study of defect models for cosmic structure formation

We calculate predictions from wide class of `active' models of cosmic structure formation which allows us to scan the space of possible defect models. We calculate the linear cold dark matter power spectrum and Cosmic Microwave Background (CMB) anisotropies over all observable scales using a full linear Einstein-Boltzmann code. Our main result, which has already been reported, points to a serious problem reconciling the observed amplitude of the large-scale galaxy distribution with the COBE normalization. Here, we describe our methods and results in detail. The problem is present for a wide range of defect parameters, which can be used to represent potential differences among defect models, as well as possible systematic numerical errors. We explicitly examine the impact of varying the defect model parameters and we show how the results substantiate these conclusions. The standard scaling defect models are in serious conflict with the current data, and we show how attempts to resolve the problem by considering non-scaling defects or modified stress-energy components would require radical departures from what has become the standard picture.

astro-ph↗
arXiv1997

Gravitational Waves From Cosmic Strings

Gravitational waves from cosmic strings are generated in the first fractions of a second after the Big Bang, potentially providing a unprecedented probe of the early universe. We discuss the key dynamical processes underlying calculations of the stochasti c background produced by a string network and we detail the parameter dependencies of the resulting spectral density $Ω_{gr}(f)$. The present constraints on the cosmic string mass scale $μ$ arising from the millisecond pulsar timings and primor dial nucleosynthesis are discussed, justifying our present conservative bound $Gμ/c^2 < 5.4(\pm 1.1)\times 10^{-6}$. We then discuss the strong prospect of detecting (or ruling out) cosmic string background with the next generation of gravitational wave experiments. Comparison is also made with alternative cosmological sources of gravitational waves such as inflation and hybrid topological defects.

astro-ph↗