Konstantinos DimopoulosLancaster University

Hot Big Bang and Cosmic InflationHot Early Universe: CMBOn large scales: Universe = UniformStructure: smooth over 100 Mpc: Universe m Fractal

Hot Big Bang and Cosmic InflationCosmological Principle: The Universe is Homogeneous and IsotropicHorizon Problem: Uniformity over causally disconnected regionsCosmic Inflation: Brief period of superluminal expansion of spaceInflation produces correlations over superhorizon distances by expanding an initially causally connected region to size larger than the observable UniverseThe CMB appears correlated on superhorizon scales (in thermal equilibrium at preferred reference frame)Incompatible with Finite Age

Hot Big Bang and Cosmic InflationInflation imposes the Cosmological PrincipleInflation + Quantum VacuumC. Principle = no galaxies!Where do they come from?Quantum fluctuations (virtual particles) of light fields exit the Horizon

The Inflationary ParadigmA flat direction is requiredThe Universe undergoes inflation when dominated by the potential density of a scalar field (called the inflaton field)

Solving the Flatness ProblemInflation enlarges the radius of curvature to scales much larger than the HorizonFlatness Problem: The Universe appears to be spatially flat despite the fact that flatness is unstable

The end of InflationReheating must occur before BBN

Particle Production during InflationSemi-classical method for scalar fiedsVacuum boundary condition:

Particle Production during InflationHawking temperature

Particle Production during InflationCurvature Perturbation: Scaleinvariance

The Inflaton HypothesisTight constraint Fine tuningThe field responsible for the curvature perturbation is the same field which drives the dynamics of inflation

The Curvaton HypothesisCurvaton = not ad hocDuring inflation the curvatons conribution to the density is negligible The curvaton is a light fieldRealistic candidates include RH-sneutrino, orthogonal axion, MSSM flat direction

The curvaton mechanismAfter unfreezing the curvaton oscillates around its VEVCoherent curvaton oscillations correspond to pressureless matter which dominates the Universe imposing its own curvature perturbation

Scalar vs Vector FieldsScalar fields employed to address many open issues: inflationary paradigm, dark energy (quintessence) baryogenesis (Affleck-Dine)

Scalar fields are ubiquitous in theories beyond the standard model such as Supersymmetry (scalar parteners) or string theory (moduli)However, no scalar field has ever been observed Designing models using unobserved scalar fields undermines their predictability and falsifiability, despite the recent precision dataThe latest theoretical developments (string landscape) offer too much freedom for model-buildingCan we do Cosmology without scalar fields?Some topics are OK:Baryogenesis, Dark Matter , Dark Energy (CDM)Inflationary expansion without scalar fields is also possible:However, to date, no mechanism for the generation of the curvature/density perturbation without a scalar field exists

Why not Vector Fields?Basic Problem: the generatation of a large-scale anisotropy is in conflict with CMB observationsHowever, An oscillating massive vector field can avoid excessive large-scale anisotropyAlso, some weak large-scale anisotropy might be present in the CMB (Axis of Evil):Inflation homogenizes Vector FieldsTo affect / generate the curvature perturbation a Vector Field needs to (nearly) dominate the UniverseHomogeneous Vector Field = in general anisotropic

Massive Abelian Vector FieldTo retain isotropy the vector field must not drive inflationVector Inflation [Golovnev et al. (2008)] uses 100s of vector fields

Vector CurvatonPressureless and IsotropicVector field can be curvaton if safe domination of Universe is possibleVector field domination can occur without introducing significant anisotropy. The curvature perturbation is imposed at domination

Particle Production of Vector FieldsConformal Invariance: vector field does not couple to metric (virtual particles not pulled outside Horizon during inflation)Breakdown of conformality of massless vector field is necessary

Particle Production of Vector FieldsCases A&B: vector curvaton = subdominant: statistical anisotropy only

Non-minimally coupled Vector Curvaton

Non-minimally coupled Vector CurvatonLongitudinal component:The vector curvaton can be the cause of statistical anisotropysaturates observational bound

Statistical Anisotropy and non-GaussianityNon Gaussianity in vector curvaton scenario:Non-Gaussianity = correlated with statistical anisotropy:Smoking gun

ConclusionsA vector field can contribute to the curvature perturbationIn this case, the vector field undergoes rapid harmonic oscillations during which it acts as a pressureless isotropic fluidHence, when the oscillating vector field dominates, it introduces negligible anisotropy (Axis of Evil?) The challenge is to obtain candidates in theories beyond the standard model, which can play the role of the vector curvatonPhysical Review D 74 (2006) 083502 : hep-ph/0607229Physical Review D 76 (2007) 063506 : 0705.3334 [hep-ph]Journal of High Energy Physics 07 (2008) 119 : 0803.3041 [hep-th]If particle production is isotropic then the vector curvaton can alone generate the curvature perturbation in the UniverseIf particle production is anisotropic then the vector curvaton can give rise to statistical anisotropy, potentially observable by PlanckCorrelation of statistical anisotropy and non-Gaussianity in the CMB is the smoking gun for the vector curvaton scenarioarXiv:0806.4680 [hep-ph]arXiv:0809.1055 [astro-ph]