arXiv subjects

Michael Joyce

Publications and source records attributed to Michael Joyce.

At least 73 records · Page 4Linked to original sources

arXiv2002

Power-law correlation and discreteness in cosmological N-body simulations

We analyse with simple real-space statistics the Virgo consortium's cosmological N-body simulations. Significant clustering rapidly develops well below the initial mean interparticle separation Λ_i, where the gravitational force on a particle is dominated by that with its nearest neighbours. A power-law behaviour in the two point correlation function emerges, which in the subsequent evolution is continuously amplified and shifted to larger scales, in a roughly self-similar manner. We conclude that the density fluctuations at the smallest scales due to the particle-like nature of the distribution being evolved are thus essential in the development of these correlations, and not solely, as usually supposed, the very small continuous (fluid-like) fluctuations at scales larger than Λ_i >.

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arXiv2002

Initial conditions, Discreteness and non-linear structure formation in cosmology

In this lecture we address three different but related aspects of the initial continuous fluctuation field in standard cosmological models. Firstly we discuss the properties of the so-called Harrison-Zeldovich like spectra. This power spectrum is a fundamental feature of all current standard cosmological models. In a simple classification of all stationary stochastic processes into three categories, we highlight with the name ``super-homogeneous'' the properties of the class to which models like this, with $P(0)=0$, belong. In statistical physics language they are well described as glass-like. Secondly, the initial continuous density field with such small amplitude correlated Gaussian fluctuations must be discretised in order to set up the initial particle distribution used in gravitational N-body simulations. We discuss the main issues related to the effects of discretisation, particularly concerning the effect of particle induced fluctuations on the statistical properties of the initial conditions and on the dynamical evolution of gravitational clustering.

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arXiv2002

Real space statistical properties of standard cosmological models

After reviewing some basic relevant properties of stationary stochastic processes (SSP), we discuss the properties of the so-called Harrison-Zeldovich like spectra of mass density perturbations. These correlations are a fundamental feature of all current standard cosmological models. Examining them in real space we note they imply a "sub-poissonian" normalised variance in spheres $σ_M^2(R) \sim R^{-4} \ln R$. In particular this latter behaviour is at the limit of the most rapid decay ($\sim R^{-4}$) of this quantity possible for any stochastic distribution (continuous or discrete). In a simple classification of all SSP into three categories, we highlight with the name ``super-homogeneous'' the properties of the class to which models like this, with $P(0)=0$, belong. In statistical physics language they are well described as lattice or glass-like. We illustrate their properties through two simple examples: (i) the ``shuffled'' lattice and the One Component Plasma at thermal equilibrium.

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arXiv2002

The Glass-like Universe: Real-space correlation properties of standard cosmological models

After reviewing the basic relevant properties of stationary stochastic processes (SSP), defining basic terms and quantities, we discuss the properties of the so-called Harrison-Zeldovich like spectra. These correlations, usually characterized exclusively in k-space (i.e. in terms of power spectra P(k)), are a fundamental feature of all current standard cosmological models. Examining them in real space we note their characteristics to be a {\it negative} power law tail ξ(r) \sim - r^{-4} and a {\it sub-poissonian} normalised variance in spheres σ^2(R) \sim R^{-4} \ln R. We note in particular that this latter behaviour is at the limit of the most rapid decay (\sim R^{-4}) of this quantity possible for any stochastic distribution (continuous or discrete). This very particular characteristic is usually obscured in cosmology by the use of Gaussian spheres. In a simple classification of all SSP into three categories, we highlight with the name ``super-homogeneous'' the properties of the class to which models like this, with P(0)=0, belong. In statistical physics language they are well described as glass-like. They do not have either ``scale-invariant'' features, in the sense of critical phenomena, nor fractal properties. We illustrate their properties with some simple examples, in particular that of a ``shuffled'' lattice.

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arXiv2001

Erratum for "Supersymmetric Electroweak Baryogenesis"

We correct a numerical error which led to an overestimate of the baryon asymmetry from supersymmetric electroweak baryogenesis in our paper JHEP07(2000)018. Updated dependences of the baryon asymmetry on chargino mass parameters and the bubble wall velocity are shown. We also include LEP-II-constraints on the chargino mass parameters. Combined with our corrected results for the baryon asymmetry these constraints imply that the phase in the chargino mass matrix must violate CP nearly maximally in order to generate a large enough baryon asymmetry. A number of other typographical errors are also corrected.

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arXiv2001

Luminosity density estimation from redshift surveys and the mass density of the Universe

In most direct estimates of the mass density (visible or dark) of the Universe, a central input parameter is the luminosity density of the Universe. Here we consider the measurement of this luminosity density from red-shift surveys, as a function of the yet undetermined characteristic scale R_H at which the spatial distribution of visible matter tends to a well defined homogeneity. Making the canonical assumption that the cluster mass to luminosity ratio M/L is the universal one, we can estimate the total mass density as a function Ω_m(R_H,M/L). Taking the highest estimated cluster value M/L ~300h and a conservative lower limit R_H > 20 Mpc/h, we obtain the upper bound Ω_m < 0.1 . We note that for values of the homogeneity scale R_H in the range R_H ~ (90 +/- 45) hMpc, the value of Ω_m may be compatible with the nucleosynthesis inferred density in baryons.

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arXiv2000

Supersymmetric Electroweak Baryogenesis

We re-examine the generation of the baryon asymmetry in the minimal supersymmetric standard model (MSSM) during the electroweak phase transition. We find that the dominant source for baryogenesis arises from the chargino sector. The CP-violation comes from the complex phase in the mu parameter, which provides CP-odd contributions to the particle dispersion relations. This leads to different accelerations for particles and antiparticles in the wall region which, combined with diffusion, leads to the separation of Higgsinos and their antiparticles in the front of the wall. These asymmetries get transported to produce perturbations in the left-handed chiral quarks, which then drive sphaleron interactions to create the baryon asymmetry. We present a complete derivation of the semiclassical WKB formalism, including the chargino dispersion relations and a self-consistent derivation of the diffusion equations starting from semiclassical Boltzmann equations for WKB-excitations. We stress the advantages of treating the transport equations in terms of the manifestly gauge invariant physical energy and kinetic momentum, rather than in the gauge variant canonical variables used in previous treatments. We show that a large enough baryon asymmetry can be created for the phase of the complex mu parameter as small as ~ 0.001, which is consistent with bounds from the neutron electric dipole moment.

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arXiv2000

Fermion propagator in a nontrivial background field

We study the fermion propagator in a spatially varying classical background field, and show that, contrary to common wisdom, it may get nontrivial gradient corrections already at the first order in derivative expansion. This occurs whenever the fermion self-energy acquires a spatially (or temporally) varying pseudoscalar term, a simple example of which is given by a complex mass term m(x)= m_R + i gamma_5 m_I. Such effective mass terms arise for example in extensions of the Standard Model during the electroweak transition, and they are crucial in providing the CP-violation necessary for electroweak baryogenesis.

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arXiv2000

The Quantum Boltzmann Equation in a Nontrivial Background

This talk is a status report on our study of quantum transport equations relevant for baryogenesis computations. Our main finding is that, as a consequence of localization in space, the quasiparticle picture of the plasma dynamics breaks down at first non-trivial order in gradient expansion. While in this talk we focus on bosons, we expect that a similar picture holds for fermions. We then argue that the quasiparticle picture is recovered in the adiabatic limit of frequent scattering.

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arXiv1999

The semiclassical propagator in field theory

We consider scalar field theory in a changing background field. As an example we study the simple case of a spatially varying mass for which we construct the semiclassical approximation to the propagator. The semiclassical dispersion relation is obtained by consideration of spectral integrals and agrees with the WKB result. Further we find that, as a consequence of localization, the semiclassical approximation necessarily contains quantum correlations in momentum space.

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arXiv1999

Quantum Transport Equations for a Scalar Field

We derive quantum Boltzmann equations from Schwinger-Dyson equations in gradient expansion for a weakly coupled scalar field theory with a spatially varying mass. We find that at higher order in gradients a full description of the system requires specifying not only an on shell distribution function but also a finite number of its derivatives, or equivalently its higher moments. These derivatives describe quantum coherence arising as a consequence of localization in position space. We then show that in the limit of frequent scatterings coherent quantum effects are suppressed, and the transport equations reduce to the single Boltzmann equation for particle density, in which particles flow along modified semiclassical trajectories in phase space.

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arXiv1999

Quantum Boltzmann equations for mixing scalar fields

We report on a work in progress, whose goal is a systematic field theoretical derivation of the quantum transport equations for baryon production in the electroweak plasma at a first order phase transition in the limit of slowly varying background fields (thick wall limit). We start with the Schwinger-Dyson equations for the two point Green function written in the closed time contour (CTC) formalism. The quantum Boltzmann equations for the density matrix arise when the SD-equations are expanded to the first order in the gradients in the on-shell limit. In this paper we consider only scalar fields, but the formalism can easily be extended to fermions.

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arXiv1998

Cosmology with a Primordial Scaling Field

A weakly coupled scalar field $Φ$ with a simple exponential potential $V=M_P^4\exp(-λΦ/M_P)$ where $M_P$ is the reduced Planck mass, and $λ> 2$, has an attractor solution in a radiation or matter dominated universe in which it mimics the scaling of the dominant component, contributing a fixed fraction $Ω_ϕ$ (determined by $λ$) to the energy density. Such fields arise generically in particle physics theories involving compactified dimensions, with values of $λ$ which give a cosmologically relevant $Ω_ϕ$. For natural initial conditions on the scalar field in the early universe the attractor solution is established long before the epoch of structure formation, and in contrast to the solutions used in other scalar field cosmologies, it is one which does not involve an energy scale for the scalar field characteristic of late times . We study in some detail the evolution of matter and radiation perturbations in a standard inflation-motivated $Ω=1$ dark-matter dominated cosmology with this extra field. Using a full Einstein-Boltzmann calculation we compare observable quantities with current data. We find that, for $Ω_ϕ\simeq 0.08-0.12$, these models are consistent with large angle cosmic microwave background anisotropies as detected by COBE, the linear mass variance as compiled from galaxy surveys, big bang nucleosynthesis, the abundance of rich clusters and constraints from the Lyman-$α$ systems at high redshift. Given the simplicity of the model, its theoretical motivation and its success in matching observations, we argue that it should be taken on a par with other currently viable models of structure formation.

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arXiv1997

Turning Around the Sphaleron Bound: Electroweak Baryogenesis in an Alternative Post-inflationary Cosmology

The usual sphaleron bound and the statement of the impossibility of baryon production at a second order phase transition or analytic cross-over are reformulated in the first part of the paper as requirements of the expansion rate of the Universe at the electroweak scale. With an (exact or effective) additional contribution to the energy density scaling as 1/a^6, which dominates until just before nucleosynthesis, the observed baryon asymmetry may be produced at the electroweak scale in simple extensions of the Minimal Standard Model, even in the case that the phase transition is not first order. We focus our attention on one such cosmology, in which the Universe goes through a period termed `kination' in which its energy is dominated by the kinetic energy of a scalar field. The required kinetic energy dominated modes can occur either as a field rolls down an exponential (or steeper) potential, or in the oscillation of a field about the minimum of a steep power-law potential. We implement in detail the former case with a single exponential field first driving inflation, and then rolling into a kinetic energy dominated mode. Reheating is achieved using an alternative to the usual mechanism due to Spokoiny, in which the Universe is `reheated' by particle creation in the expanding background. Density perturbations of the magnitude required for structure formation may also be generated. We show that the analogous model for the power-law potential cannot be consistently implemented. In models with inflation driven by a second field and the usual mechanism of reheating (by decay of the inflaton) the required kinetic energy dominated cosmology is viable in both types of potential.

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arXiv1997

Electroweak Baryogenesis without the Phase Transition

Radiation domination at the electroweak epoch is a simplifying assumption, but one for which there is no observational basis. Treating the expansion rate as a variable, I re-examine electroweak baryogenesis in various scenarios. At a first order phase transition the main effect is on the sphaleron bound, which becomes a lower bound on the expansion rate in any given theory. At a second-order or cross-over phase transition, the created baryon asymmetry is directly proportional to the expansion rate. I sketch an alternative post-inflationary cosmology, in which the kinetic energy of a scalar field dominates the Universe until shortly before nucleosynthesis, and argue that the observed baryon asymmetry could be produced in this case even at an analytic cross-over.

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arXiv1997

Structure formation with a self-tuning scalar field

A scalar field with an exponential potential has the particular property that it is attracted into a solution in which its energy scales as the dominant component (radiation or matter) of the Universe, contributing a fixed fraction of the total energy density. We study the growth of perturbations in a CDM dominated $Ω=1$ universe with this extra field, with an initial flat spectrum of adiabatic fluctuations. The observational constraints from structure formation are satisfied as well, or better, than in other models, with a contribution to the energy density from the scalar field $Ω_ϕ\sim 0.1$ which is small enough to be consistent with entry into the attractor prior to nucleosynthesis.

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arXiv1996

Electroweak Baryogenesis and the Expansion Rate of the Universe

The standard requirement for the production of baryons at the electroweak phase transition, that the phase transition be first order and the sphaleron bound be satisfied, is predicated on the assumption of a radiation dominated universe at that epoch. One simple alternative - domination by the energy in a kinetic mode of a scalar field which scales as $1/a^6$ - gives a significantly weakened sphaleron bound for the preservation of a baryon asymmetry produced at a first-order phase transition, and allows the possibility that the observed baryon asymmetry be produced when the phase transition is second-order or cross-over. Such a phase of `kination' at the electroweak scale can occur in various ways as a scalar field evolves in an exponential potential after inflation.

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arXiv1995

Non-local Electroweak Baryogenesis Part I : Thin Wall Regime

We investigate `non-local' schemes for baryogenesis at a first order electroweak phase transition, in which the effects of a $CP$ violating condensate on the bubble wall propagate into the unbroken phase where the sphaleron rate is unsupressed. Such a condensate exists in multi-Higgs extensions of the standard model, and may exist due to an instability in the minimal standard model. In this paper we first discuss the general problem of determining the $CP$ violating perturbations, distinguishing two regimes (quantum and classical). We then give an analytic treatment of quantum mechanical reflection in the thin wall regime, in which interactions with the plasma can be neglected as a particle propagates across the wall. We focus on leptons because of their much weaker coupling to the plasma. We argue that they are likely to be accurately described by this calculation, but quarks are not. The relative magnitude of the baryon asymmetry produced for different fermions depends on their relative Yukawa couplings ({\it not} their zero temperature masses), their transport properties and their interactions. We calculate the baryon asymmetry for various parameter ranges and conclude that asymmetries comparable with observations can be generated.

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