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D. A. Steer

Publications and source records attributed to D. A. Steer.

51 records · Page 3Linked to original sources
arXiv2001

Brane Gas Inflation

We consider the brane gas picture of the early universe. At later stages, when there are no winding modes and the background is free to expand, we show that a moving 3-brane, which we identify with our universe, can inflate even though it is radiation-dominated. The crucial ingredients for successful inflation are the coupling to the dilaton and the equation of state of the bulk. If we suppose the brane initially forms in a collision of higher-dimensional branes, then the spectrum of primordial density fluctuations naturally has a thermal origin.

hep-ph↗
arXiv2001

Self-intersections and gravitational properties of chiral cosmic strings in Minkowski space

Chiral cosmic strings are naturally produced at the end of D-term inflation and they may have interesting cosmological consequences. As was first proved by Carter and Peter, the equations of motion for chiral cosmic strings in Minkowski space are integrable (just as for Nambu-Goto strings). Their solutions are labeled by a function $k(σ- t)$ where $t$ is time and $σ$ is the invariant length along the string, and the constraints on $k$, which determines the charge on the string, are that $0 \leq k^2 \leq 1$. We review the origin of this parameter and also discuss some general properties of such strings which can be deduced from the equations of motion. The metric around infinite chiral strings is then constructed in the weak field limit, and studied as a function of $k$. We also consider the angular momentum of circular chiral loops, and extend previous work to consider the evolution and self-intersection properties of a more general family of chiral cosmic string loops for which $k^2(σ-t)$ is not constant.

astro-ph↗
arXiv2000

Exploiting duality in a toy model of QCD at non-zero temperature and chemical potential: the massive Thirring model, sine-Gordon model and Coulomb gases

We focus on the massive Thirring model in 1+1 dimensions at finite temperature and non-zero chemical potential, and comment on some parallels between this model and QCD. In QCD, calculations of physical quantities such as transport coefficients are extremely difficult. In the massive Thirring model, similar calculations are greatly simplified by exploiting the duality which exists with the sine-Gordon model and its relation, at high temperature, to the exactly solvable classical Coulomb gas on the line.

hep-th↗
arXiv2000

Dynamics and properties of chiral cosmic strings in Minkowski space

Chiral cosmic strings are produced naturally at the end of inflation in supersymmetric models where the symmetry is broken via a D-term. Consequently in such theories, where both inflation and cosmic strings contribute to the density and CMBR (microwave background) perturbations, it is necessary to understand the evolution of chiral cosmic string networks. We study the dynamics of chiral cosmic strings in Minkowski space and comment on a number of differences with those of Nambu-Goto strings. To do this we follow the work of Carter and Peter who showed that the equations of motion for chiral cosmic strings reduce to a wave equation and two constraints, only one of which is different from the familiar Nambu-Goto constraints. We study chiral string loop solutions consisting of many harmonics and determine their self-intersection probabilities, and comment on the possible cosmological significance of these results.

astro-ph↗
arXiv2000

Cosmological parameter dependence in local string theories of structure formation

We perform a new and accurate study of the dependence on cosmological parameters of structure formation with local cosmic strings. The crucial new ingredients are the inclusion of the effects of gravitational backreaction on the evolution of the network, and the accurate evolution of the network through the radiation to matter transition. Our work re-iterates the fact that expanding Universe numerical simulations only probe a transient regime, and we incorporate our results into the unequal time correlators recently measured. We then compute the CMB and CDM fluctuations' power spectra for various values of the Hubble constant $H_0$ and baryon fraction $Ω_b$. We find that, whereas the dependence on $Ω_b$ is negligible, there is still a strong dependence on $H_0$.

astro-ph↗
arXiv1999

Non-equilibrium, time-dependent effective theory of a weakly coupled superconductor at finite temperature

We perform a well defined derivative expansion to obtain the time dependent effective theory for a BCS superconductor at finite temperature, using an arbitrary curve in the complex time plane. Our expansion is unique, being free of any problems related to the order in which analytical continuation and the derivative expansion are made, or equivalently to the order in which the zero momentum and zero energy limits are taken. In other words Landau damping terms are present but do not lead to the pathologies found in standard approaches. We discuss the reason for this, suggesting that these standard approaches may be internally inconsistent. The methods presented here are an attempt to overcome those inconsistencies, and finally, our results are compared to others in the literature.

cond-mat.supr-con↗
arXiv1999

Chiral Symmetry restoration in the massive Thirring model at finite T and $μ$: Dimensional reduction and the Coulomb gas

We show that in certain limits the (1+1)-dimensional massive Thirring model at finite temperature $T$ is equivalent to a one-dimensional Coulomb gas of charged particles at the same $T$. This equivalence is then used to explore the phase structure of the massive Thirring model. For strong coupling and $T>>m$ (the fermion mass) the system is shown to behave as a free gas of "molecules" (charge pairs in the Coulomb gas terminology) made of pairs of chiral condensates. This binding of chiral condensates is responsible for the restoration of chiral symmetry as $T\to\infty$. In addition, when a fermion chemical potential $μ\neq 0$ is included, the analogy with a Coulomb gas still holds with $μ$ playing the role of a purely imaginary external electric field. For small $T$ and $μ$ we find a typical massive Fermi gas behaviour for the fermion density, whereas for large $μ$ it shows chiral restoration by means of a vanishing effective fermion mass. Some similarities with the chiral properties of low-energy QCD at finite $T$ and baryon chemical potential are discussed.

hep-th↗
arXiv1998

Kinks versus fermions or the 2D sine-Gordon versus massive Thirring models, at nonzero temperature and chemical potential

We study bosonisation in the massive Thirring and sine-Gordon models at finite temperature and nonzero fermion chemical potential. Both canonical operator and path integral approaches are used to prove the equality of the partition functions of the two models at finite $T$ and zero chemical potential, as it has been recently shown. This enables the relationship between thermal normal ordering and path-integral renormalisation to be specified. Furthermore, we prove that thermal averages of zero-charge operators can also be identified. At nonzero chemical potential and temperature we show, in perturbation theory around the massless case, that the bosonised theory is the sine-Gordon model plus an additional topological term, accounting for the existence of zero charge excitations (the fermions or the kinks) in the thermal bath. This result is the 2D version of the low-energy lagrangian at finite baryon density.

hep-th↗
arXiv1998

Thermal bosonisation in the sine-Gordon and massive Thirring models

We study bosonisation in the massive Thirring and sine-Gordon models at finite temperature T and nonzero fermion chemical potential $μ$. For that purpose we use both canonical operator and path integral approaches, paying particular attention to the issues of thermal normal ordering and renormalisation. At T>0 and $μ=0$, the massive Thirring model bosonises to the sine-Gordon model with the same T=0 identification between coupling constants. We prove that not only the partition functions of the two models coincide, as was recently shown, but also that thermal averages of zero-charge operators can be identified. In particular, analysis of the point-split regularised fermion current then leads to the thermal equivalence between sine-Gordon kinks and Thirring fermions. At $μ\neq 0$, T>0 and working in perturbation theory about the massless Thirring model, we show that the bosonised theory is the sine-Gordon model plus an additional topological term which accounts for the existence of net fermion charge excitations (the fermions or the kinks) in the thermal bath. This result generalises one recently obtained for the massless case, and it is the two-dimensional version of the low-energy QCD chiral Lagrangian at finite baryon density.

hep-ph↗
arXiv1998

The evolution of a network of cosmic string loops

We set up and analyse a model for the non-equilibrium evolution of a network of cosmic strings initially containing only loops and no infinite strings. Due to this particular initial condition, our analytical approach differs significantly from existing ones. We describe the average properties of the network in terms of the distribution function n(l,t) dl, the average number of loops per unit volume with physical length between l and l + dl at time t. The dynamical processes which change the length of loops are then estimated and an equation, which we call the `rate equation', is derived for (dn/dt). In a non-expanding universe, the loops should reach the equilibrium distribution predicted by string statistical mechanics. Analysis of the rate equation gives results consistent with this. We then study the rate equation in an expanding universe and suggest that three different final states are possible for the evolving loop network, each of which may well be realised for some initial conditions. If the initial energy density in loops in the radiation era is low, then the loops rapidly disappear. For large initial energy densities, we expect that either infinite strings are formed or that the loops tend towards a scaling solution in the radiation era and then rapidly disappear in the matter era. Such a scenario may be relevant given recent work highlighting the problems with structure formation from the standard cosmic string scenario.

hep-ph↗
arXiv1998

Evolution of a Network of Cosmic String Loops

We discuss and summarise the predictions of a model for the non-equilibrium evolution of a network of cosmic strings initially containing {\em only loops} and {\em no infinite strings}. The results are of interest given recent work highlighting the problems with structure formation from the standard cosmic string scenario.

hep-ph↗
arXiv1998

On Normal ordering and Canonical transformations in Thermal Field Theory

We look at a real scalar field in thermal equilibrium in the context of the new normal ordering and field split defined by Evans and Steer. We show that the field split defines a natural canonical transformation, but that this transformation differs from others known in thermal field theory.

hep-ph↗
arXiv1998

Wick's Theorem for non-symmetric normal ordered products and contractions

We consider arbitrary splits of field operators into two parts, and use the corresponding definition of normal ordering introduced by Evans and Steer. In this case the normal ordered products and contractions have none of the special symmetry properties assumed in existing proofs of Wick's theorem. Despite this, we prove that Wick's theorem still holds in its usual form as long as the contraction is a c-number. Wick's theorem is thus shown to be much more general than existing derivations suggest, and we discuss possible simplifying applications of this result.

hep-ph↗
arXiv1996

Wick's Theorem at Finite Temperature

We consider Wick's Theorem for finite temperature and finite volume systems. Working at an operator level with a path ordered approach, we show that contrary to claims in the literature, expectation values of normal ordered products can be chosen to be zero and that results obtained are independent of volume. Thus the path integral and operator approaches to finite temperature and finite volume quantum field theories are indeed seen to be identical. The conditions under which normal ordered products have simple symmetry properties are also considered.

hep-ph↗