Eric Adelberger (Washington): Tests of Newton's Inverse-Square Law: Probing the True Geometry of the Universe
It is remarkable that questions about gravitation, the oldest known interaction, are again at the center of physics and that small-scale experiments can address important open issues in fundamental science such as: why is gravity so weak compared to the other interactions? and why is the cosmological constant so small compared to the predictions of quantum mechanics? Modern string theory ideas (new scalar particles and extra dimensions) and other notions hint that Newton's Inverse-Square Law may break down at distances less than 1 mm or even at solar-system scales. I will review some motivations for testing the Inverse-Square Law, and discuss the techniques employed in recent experiments, ranging from ultra cold neutrons and mechanical experiments with torsion balances, etc to laser ranging of the moon.
Philippe Brax (Saclay): Detecting Quintessence in Orbit
I will discuss the physics of the chameleon field when driving the acceleration of the expansion of the Universe.
Raul Jimenez (Penn): Observational Constraints on Scalar Fields
I will review the observational effort to unveil the nature of the scalar field that is driving the expansion of the universe today. I will present current results and constraints on the nature of this field and how these constraints can be made more stringent in the near future (1-2 years). I will argue that we might be able to have a good constraint on it much sooner than we thought.
Stephane Fay (Meudon): Scalar Field Constraints from Homogeneous Cosmology
We will present some necessary conditions on scalar fields allowing to an homogeneous but anisotropic Universe to reach an isotropic state. We will then give the resulting asymptotic behaviour of the metric, the dark energy equation of state, and will talk about the coincidence and some possible degeneracy problems.
Jean-Philippe Uzan (Paris): Variation of the Constants in the Early and Late Universe
Light scalar fields may lead to a variation of the fundamental constants of nature. The recent observational constraints on the time variation of some of these constants will be summarized. I will then discuss what these tests and the tests of the law of gravitation on large scales can tell us about the nature of the dark energy. To finish, the effects of varying couplings in the primordial universe will be discussed.
Jarle Brinchmann (Porto): Constraining the Fine-structure Constant at z ~ 2.5 using Emission Lines
The detection of a change in any physical constant with time would have a fundamental impact on physics. The recent claim of a detection of a change in the fine-structure constant from quasar absorption systems has therefore generated a flurry of theoretical work. The result has been disputed recently by Srianand et al (2004) and the observational situation is therefore unclear. Here we present preliminary results from the first completely independent observational constraints on alpha at z ~ 2. Our study builds on work at lower redshift by Bahcall et al (2004) and uses the [O III] lines at 4959A and 5007A to constrain alpha. We have observed 6 QSOs and radio galaxies with 2 < z < 3. To limit systematics we have used the numerous night sky lines from OH in our spectra to obtain a very good wavelength calibration and the method suffers from almost no other systematic uncertainties. Here we present the survey and the first constraints on alpha(z) from our study.
Graça Rocha (Cambridge): Measuring alpha in the Early Universe
We present a detailed analysis of present and future Cosmic Microwave Background constraints of the value of the fine-structure constant, alpha. We carry out a more detailed analysis of the WMAP data, deriving state-of-the-art constraints on alpha and discussing various other issues, such as the possible hints for the running of the spectral index. We also perform a thorough Fisher Matrix Analysis (including both temperature and polarization, as well as alpha and the optical depth tau), in order to estimate how future CMB experiments will be able to constrain alpha and other cosmological parameters.
Joana Oliveira (Porto): Linearized Bekenstein Varying Alpha Models
We study the simplest class of Bekenstein-type, varying alpha models, in which the two available free functions (potential and gauge kinetic function) are Taylor-expanded up to linear order. Any realistic model of this type reduces to a model in this class for a certain time interval around the present day. Nevertheless, we show that no such model is consistent with all existing observational results. We discuss possible implications of these findings, and in particular clarify the ambiguous statement (often found in the literature) that the Webb results are inconsistent with Oklo.
Nelson Nunes (QMW): Reconstructing the Dark Energy Equation of State with Varying Alpha
I will discuss the possibility of reconstructing the dark energy equation of state from variations in the fine structure constant for a class of models where the quintessence field is non-minimally coupled to the electromagnetic field. It is argued that the sign of the first derivative of the equation of state can be uncovered from the reconstruction, thus providing complementary information to supernova observations.
Claudio Rubano (Napoli): Discussion on Potentials and Tracking Behaviour
Some subtleties on the concept of tracking behaviour are discussed. By means of general exact solutions we show that exponential potentials can perfectly emulate a cosmological constant in the late universe and fit the current data.
Roy Maartens (Portsmouth): Brane World Cosmology
Recent developments in string theory suggest that our 4-dimensional universe may be a brane surface embedded in a higher-dimensional spacetime. The matter and radiation fields in our universe are trapped on the brane, but the gravitational field propagates freely through the higher dimensional spacetime. This scenario has been developed into a phenomenology for exploring the cosmological consequences of extra dimensions. There are interesting and exciting new features predicted by these models. In particular, early-universe dynamics (including a possible inflationary period) may be significantly changed. At the same time, high-precision cosmological data are used to place constraints on brane-world models. I will describe the qualitative properties of higher-dimensional gravity and brane-world dynamics. Then I will discuss how cosmological perturbations generated in inflation can be investigated and used to compare brane-world predictions with observations.
Torsten Bringmann (Stockholm): Stability of Homogeneous Extra Dimensions
In order not to be in conflict with observations it is crucial that extra dimensions, if existent, are stable. It is shown that in the context of homogeneous extra dimensions, this can easily be achieved during both (4D) radiation and vacuum energy dominated eras of the cosmological evolution. During matter domination, however, there is no such possibility even for a very general class of stabilization mechanisms. It is furthermore argued that one generically expects time varying extra dimensions during any transition period such as from radiation to matter domination, even for the (assumed) case of stable extra dimensions during the latter.
Stephen Davis (Lausanne): Scalar-Tensor Gravity on a Gauss-Bonnet Brane World
The effective four-dimensional, linearised gravity for a brane world model with higher order curvature terms and a bulk scalar field is analysed. Large and small distance gravitational laws are derived. The model has a single brane embedded in a five-dimensional bulk spacetime, and the scalar field represents the dilaton or a moduli field. The quadratic, Gauss-Bonnet curvature term (and corresponding higher kinetic terms for the scalar) is also included in the bulk action. It is particularly natural to include such terms in a brane world model. Boundary terms and junction conditions for the higher order terms are given. The extra terms allow additional solutions of the field equations, which give better agreement with observational constraints. Brans-Dicke gravity is obtained on the brane. The scalar and tensor perturbations are affected differently by the higher gravity terms, and this provides a way for the scalar modes to be suppressed relative to the tensor ones. Another new (but less useful) feature is the appearance of instabilities for some parameter ranges.
Gilles Esposito-Farese (Paris): Tests of Scalar-Tensor Gravity
The best motivated alternatives to general relativity are scalar-tensor theories, in which the gravitational interaction is mediated by one or several scalar fields together with the usual graviton. The analysis of their various experimental constraints allows us to understand better which features of the models have actually been tested, and to suggest new observations able to discriminate between them. I will review three classes of constraints on such theories, which are qualitatively different from each other: (i) solar-system experiments; (ii) binary-pulsar tests and future detections of gravitational waves from inspiralling binaries; (iii) cosmological observations. While classes (i) and (ii) impose precise bounds respectively on the first and second derivatives of the matter-scalar coupling function, (iii) a priori allows us to reconstruct the full shapes of the functions of the scalar field defining the theory, but obviously with more uncertainties and/or more theoretical hypotheses needed. Simple arguments such as the absence of ghosts (to guarantee the stability of the field theory) nevertheless suffice to rule out a wide class of scalar-tensor models. Some of them can be probed only if one takes simultaneously into account solar-system and cosmological observations.
Francesca Rosati (Padova): Dark Matter Relic Abundance and Scalar-Tensor Dark Energy
Scalar-tensor theories of gravity provide a consistent framework to accommodate an ultra-light quintessence scalar field. While the equivalence principle is respected by construction, deviations from General Relativity and standard cosmology may show up at nucleosynthesis, CMB, and solar system tests of gravity. After imposing all the bounds coming from these observations, we consider the expansion rate of the universe at WIMP decoupling, showing that it can lead to an enhancement of the dark matter relic density up to few orders of magnitude with respect to the standard case. This effect can have an impact on supersymmetric candidates for dark matter.
Anthony Lasenby (Cambridge): Closed Universe Boundary Conditions for Inflation and Predictions for the CMB
A new approach to constructing inflationary models in closed universes is discussed. Conformal embedding of closed-universe models in a de Sitter background suggests a quantisation condition on the available conformal time. This condition implies that the universe is closed at no more than the 10% level. When a massive scalar field is introduced to drive an inflationary phase, this figure is reduced to closure at nearer the 1% level. In order to enforce this constraint on the available conformal time, we need to consider conditions in the universe before the onset of of inflation. A formal series around the initial singularity is constructed, which rests on a pair of dimensionless, scale-invariant parameters. For physically acceptable models we find that both parameters are of order unity, so no fine tuning is required, except in the mass of the scalar field. For typical values of the input parameters we predict the observed values of the cosmological parameters, with a very good fit to the most recent CMB data. The primordial curvature spectrum predicts the low-l fall-off in the CMB power spectrum observed by WMAP. The spectrum also predicts a fall-off in the matter power spectrum at high k, relative to a power law. More general investigations of the primordial power spectrum are also discussed.
Morgan Le Delliou (Montpellier): Quintessence and Non-linear Structure Formation
Dynamical dark energy models coined by the name quintessence solve some problems inherent with the introduction of a cosmological constant (coincidence, fine-tuning, model building, equation of state) to explain the observation of a late cosmic acceleration (SNIa). We study their effects on structure formation to show that precision cosmology measurements can hope to discriminate between those models from the largest scales.
David Wiltshire (Canterbury): Stable Gravastars - An Alternative to Black Holes?
The gravastar picture developed by Mazur and Mottola is one of a very small number of serious challenges to our usual conception of a black hole. In the gravastar picture there is effectively a phase transition at near where the event horizon would have been expected to form, and the interior of what would have been the black hole is replaced by a segment of de Sitter space. While Mazur and Mottola were able to argue for the thermodynamic stability of their configuration, the question of dynamic stability against spherically symmetric perturbations of the matter or gravity fields remains somewhat obscure. In recent work [Class. Quantum Grav. 21 (2004) 1135] Matt Visser and I have constructed a model that shares the key features of the Mazur-Mottola scenario, and which is sufficiently simple for a full dynamical analysis. We find that there are some physically reasonable equations of state for the transition layer that lead to stability.
Pedro Gonzalez Diaz (Madrid): The Cosmic Phantom Field
We review the properties of the cosmic phantom field in different dark energy models, including the emergence of the Big Rip in the future, the axionic nature of the phantom stuff, its quantum thermodynamic properties and accretion onto black holes and wormholes.
David Wands (Portsmouth): Inflation and the Origin of Large-scale Structure
I will review how quantum fluctuations of scalar fields during inflation are related to large-scale perturbations in the density of radiation and matter after inflation. In simple single field models of inflation this leads to a Gaussian distribution of adiabatic density perturbations, but alternative models such as the curvaton scenario or modulated reheating can yield non-Gaussian or non-adiabatic signature.
Katherine Mack (Princeton): Phenomenological Classification of Inflationary Potentials
We present results from a phenomenological approach to the classification of inflationary potentials. Using the slow-roll parameters to generate a population of inflationary models, we will examine the attractor solutions in the phase space of the observables. This framework allows us to investigate the relationship between the change in the scalar field over the course of inflation (delta-phi) and the tensor-scalar ratio (r) for these models.
José Pedro Mimoso (Lisbon): Self-similar Dynamics after Warm Inflation
We analyze the dynamics of warm inflation models with general viscous effects. We classify the situations yielding asymptotic self similar behaviour.
Paul Shellard (Cambridge): Evolution and Cosmological Consequences of Cosmic Strings
This lecture will provide an accessible introduction to cosmic strings and their cosmological consequences, while also discussing interdisciplinary applications to vortices in a condensed matter context. Analytic and numerical modelling of a string network will be described, along with recent applications to brane models in higher dimensions. The cosmological consequences of cosmic strings will be reviewed, emphasizing the latest constraints from the cosmic microwave sky observations.
Sujata Gupta (Portsmouth): Non-Gaussianity in the CMB
The bispectrum calculated from the CMB data cannot distinguish between many well-posed structure formation models, although many of them do predict some small measure of non-Gaussianity. I shall discuss further tests of non-Gaussianity their ramifications for three structure formation models.
Fernando Quevedo (Cambridge): Inflation from String Theory
We will review the recent attempts to derive cosmological inflation from string theory. Emphasis will be given to the issues of D brane inflation, tachyon condensation, moduli fixing and de Sitter space from string theory.
Laura Allen (Portsmouth): Cosmological Perturbations Through a Simple Bounce
We present a detailed study of a simple scalar field model that yields non-singular cosmological solutions. We study both the qualitative dynamics of the homogeneous and isotropic background and the evolution of inhomogeneous linear perturbations. We calculate the spectrum of perturbations generated on super-Hubble scales during the collapse phase from initial vacuum fluctuations on small scales and then evolve these numerically through the bounce. We show that a power-law collapse phase with scale factor proportional to $(-t)^{2/3}$ can yield a scale-invariant spectrum of adiabatic scalar perturbations in the expanding phase, though the amplitude of tensor perturbations places important constraints on the allowed initial conditions.
Paulo Moniz (Covilha): Brane-world States from a Generalized Chaplygin Gas
In this work we consider a FRW model with a generalized Chaplygin gas content, which we investigate within a twofold purpose. On the one hand, we present a quantum cosmological analysis, with the aim of extracting physical consequences concerning the generalized Chaplygin gas parameters. Three physically admissible cases are possible. In particular, we identify a minisuperspace configuration with two Lorentzian sectors, separated by a classically forbidden region. The Hartle-Hawking and Vilenkin wave functions are obtained, together with the corresponding transition amplitudes. It is found that, for sensible, initial conditions, the parameters characterizing the generalized Chaplygin gas have a discrete spectrum. On the other hand, we propose and explore a phenomenological association between some brane-world scenarios and a FRW minisuperspace cosmology with a generalized Chaplygin gas. This permits to consider that, if going beyond the Hamilton-Jacobi equation (i.e, the WBK approximation) the brane tension may also have a discrete spectrum of allowed values.