Anne-Christine Davis

Detecting dark energy in orbit: The cosmological chameleon

We show that the chameleon scalar field can drive the current phase of cosmic acceleration for a large class of scalar potentials that are also consistent with local tests of gravity. These provide explicit realizations of a quintessence model where the quintessence scalar field couples directly to baryons and dark matter with gravitational strength. We analyze the cosmological evolution of the chameleon field and show the existence of an attractor solution with the chameleon following the minimum of its effective potential. For a wide range of initial conditions, spanning many orders of magnitude in initial chameleon energy density, the attractor is reached before nucleosynthesis. Surprisingly, the range of allowed initial conditions leading to a successful cosmology is wider than in normal quintessence. We discuss applications to the cyclic model of the universe and show how the chameleon mechanism weakens some of the constraints on cyclic potentials.

Brane world cosmology

Recent developments in the physics of extra dimensions have opened up new avenues to test such theories. We review cosmological aspects of brane world scenarios such as the Randall–Sundrum brane model and two-brane systems with a bulk scalar field. We start with the simplest brane world scenario leading to a consistent cosmology: a brane embedded in an anti-de Sitter space–time. We generalize this setting to the case with a bulk scalar field and then to two-brane systems.We discuss different ways of obtaining a low-energy effective theory for two-brane systems, such as the moduli space approximation and the low-energy expansion. A comparison between the different methods is given. Cosmological perturbations are briefly discussed as well as early universe scenarios such as the cyclic model and the born-again brane world model. Finally we also present some physical consequences of brane world scenarios on the cosmic microwave background and the variation of constants.

Chiral Symmetry Breaking in Coulomb Gauge QCD

Abstract We analyze chiral symmetry breaking in QCD in Coulomb gauge. Using the Ward identities, we derive the renormalized gap equation from the renormalized Dyson equation for the vector and axial-vector vertices. We work within the ladder approximation, in which the Bethe-Salpeter kernel is a sum of longitudinal and transverse terms, depending only on momentum transfer. This relates the chiral symmetry breaking parameters to the static quark potential. When transverse gluon exchange is neglected, our gap equation agrees in the infrared with that obtained by Amer et al. from a non-normal-ordered Coulomb gauge hamiltonian, while disagreeing with the gap equation obtained by Finger and Mandula using a normal-ordering prescription. The corrected gap equation leads to infrared-finite formulas for the effective quark and pion parameters, in which integrals for physical quantities converge for an infrared-singular confining potential V c ∞ q −4 ; we present the results of a numerical solution in this case.

Cosmological creation of D-branes and anti-D-branes

We argue that the early universe may be described by an initial state of space-filling branes and anti-branes. At high temperature this system is stable. At low temperature tachyons appear and lead to a phase transition, dynamics, and the creation of D-branes. These branes are cosmologically produced in a generic fashion by the Kibble mechanism. From an entropic point of view, the formation of lower-dimensional branes is preferred and D3-brane-worlds are exponentially more likely to form than higher-dimensional branes. Virtually any brane configuration can be created from such phase transitions by adjusting the tachyon profile. A lower bound on the number defects produced is: one D-brane per Hubble volume.

The Dilaton and Modified Gravity

We consider the dilaton in the strong string-coupling limit and elaborate on the original idea of Damour and Polyakov whereby the dilaton coupling to matter is minimized and vanishes at a finite value of the dilaton field. Combining this type of coupling with an exponential potential, the effective potential of the dilaton becomes matter density dependent. We study the background cosmology, showing that the dilaton can play the role of dark energy. We also analyze the constraints imposed by the absence of violation of the equivalence principle. Imposing these constraints and assuming that the dilaton plays the role of dark energy, we consider the consequences of the dilaton on large scale structures and, in particular, the behavior of the slip functions and the growth index at low redshift.

Unified description of screened modified gravity.

We consider modified gravity models driven by a scalar field whose effects are screened in high density regions due to the presence of nonlinearities in its interaction potential and/or its coupling to matter. Our approach covers chameleon, f(R) gravity, dilaton and symmetron models and allows a unified description of all these theories. We find that the dynamics of modified gravity are entirely captured by the time variation of the scalar field mass and its coupling to matter evaluated at the cosmological minimum of its effective potential, where the scalar field has sat since an epoch prior to big bang nucleosynthesis. This new parametrization of modified gravity allows one to reconstruct the potential and coupling to matter and therefore to analyze the full dynamics of the models, from the scale dependent growth of structures at the linear level to nonlinear effects requiring N-body simulations. This procedure is illustrated with explicit examples of reconstruction for chameleon, dilaton, f(R) and symmetron models.

N=1 Supersymmetric Cosmic Strings

Abstract We investigate the microphysics of supersymmetric cosmic strings. In particular we focus on the vortices admitted by N = 1 supersymmetric abelian Higgs models. We give the vortex solutions and demonstrate that the two simplest supersymmetric cosmic string models admit fermionic superconductivity. Further, by using supersymmetry transformations, we show how to solve for the fermion zero-modes giving rise to string superconductivity in terms of the background string fields. We note that this constrains all supersymmetric grand unified theories with abelian strings.

Cosmic vortons and particle physics constraints.

We investigate the cosmological consequences of particle physics theories that admit stable loops of superconducting cosmic string - {\it vortons}. General symmetry breaking schemes are considered, in which strings are formed at one energy scale and subsequently become superconducting in a secondary phase transition at what may be a considerably lower energy scale. We estimate the abundances of the ensuing vortons, and thereby derive constraints on the relevant particle physics models from cosmological observations. These constraints significantly restrict the category of admissible Grand Unified theories, but are quite compatible with recently proposed effects whereby superconducting strings may have been formed close to the electroweak phase transition.

Is the radion a chameleon

The chameleon mechanism is a way to give an effective mass to a light scalar field via field self-interaction and interaction with matter. We study this mechanism in models in which the couplings are field dependent and find that the properties are very different from the case of constant couplings. The consequences of a runaway potential for the radion field in brane world scenarios and whether the radion can play the role of dark energy is investigated. The cosmological evolution during the inflationary epoch, the radiation and the matter dominated epochs are discussed as is the compatibility of the radion field with local tests of gravity.

Primordial spectrum of gauge fields from inflation

Abstract We show that conformal invariance of gauge fields is naturally broken in inflation, having as a consequence amplification of gauge fields. The resulting spectrum of the field strength is approximately B l ∝l −1 , where l is the relevant coherence scale. One realisation of our scenario is scalar electrodynamics with a scalar whose mass is large enough to evade observational constraints — the obvious candidates being supersymmetric partners of the standard-model fermions. Our mechanism also leads naturally to amplification of the standard-model Z -boson field due to its coupling to the electroweak Higgs field. At preheating, the spectrum of the Z field is transferred to the hypercharge field, which remains frozen in the plasma and is converted into a magnetic field at the electroweak phase transition. With a reasonable model of field evolution one obtains a magnetic field strength of the order of 10 −29 Gauss on a scale of 100 pc, the size of the largest turbulent eddy in a virialised galaxy. Resonant amplification in preheating can lead to primordial fields as large as 10 −24 Gauss, consistent with the seed field required for the galactic dynamo mechanism.