Backreaction in a statistically homogeneous and isotropic universe

Cosmology I & IIFall 2012Cosmology 20121Cosmology I & IICosmology I: 4.9.-21.10.Cosmology II: 29.10.-16.12.

http://theory.physics.helsinki.fi/~cosmology

Lectures in A315, Mon & Tue 14.15-16.00Syksy Rsnen, C326, syksy.rasanen at iki.fiExercises in A315, Fri 12.15-14.00, starting 14.9.Sami Nurmi, sami.nurmi at helsinki.fiExercises appear on the website on Monday, and are due the following MondayExercises form 25% of the score, the exam 75%Cosmology 20122Cosmology IIntroductionBasics of general relativityFriedmann-Robertson-Walker (FRW) modelsThermal history of the universeBig Bang nucleosynthesis (BBN)Dark matter

Cosmology 20123Cosmology IIInflationCosmological perturbation theoryStructure formationCosmic microwave background (CMB)Cosmology 20124Observations: basicsElectromagnetic radiationRadio wavesMicrowavesIRVisible lightUVX-RaysGamma raysMassive particlesCosmic rays (protons, antiprotons, heavy ions, electrons, antielectrons)NeutrinosGravity waves?Composition of the solar systemCosmology 20125Observations in practiceMotion of galaxiesDistribution of galaxies (large scale structure)Abundances of light elementsCosmic microwave backgroundLuminosities of distant supernovaeNumber counts of galaxy clustersDeformation of galaxy images (cosmic shear)...Cosmology 20126Cosmology 2012

Laws of physicsGeneral relativityQuantum field theoryAtomic physics, nuclear physics, the Standard Model of particle physicsStatistical physics and thermodynamicsCosmology 20128The Standard ModelMatter particles

Quarks and leptons(3 families)

Gauge bosons

Photon: EM interactionGluons (8): strong interactionW+, W-, Z: weak interaction

Higgs boson

Gives masses to W, Z and fermions

Cosmology 20129Homogeneity and isotropy: observations

http://map.gsfc.nasa.gov/media/101080/index.htmlCosmology 201210Homogeneity and isotropy: observationshttp://sci.esa.int/science-e/www/object/index.cfm?fobjectid=47333

Cosmology 201211Homogeneity and isotropy: observationsarXiv:astro-ph/0604561, Nature 440:1137.2006

Cosmology 201212

Homogeneity and isotropy: observationsCosmology 201213Homogeneity and isotropy:theoryThe observed statistical homogeneity and isotropy motivates theory with exact H&IThe Friedmann-Robertson-Walker modelThe expansion of the universe is described by the scale factor a(t)Extrapolating the known laws of physics we find that 14 billion years agoa 0, , T Cosmology 201214The Big BangThe early universe wasHotDenseRapidly expandingH&I and thermal equilibrium easy to calculateHigh T high energy quantum field theoryCosmology 201215Timeline of the universet( E-2)E

13-14 Gyr10-3 eVthe present day

10 Gyr10-3 eVexpansion accelerates (dark energy)400 Myr10-2 eVreionisation40 Myr10-110-2 eVfirst structures form400 000 yr0.1 eVlight and baryonic matter separate;atoms and the CMB form50 000 yr1 eVmatter overtakes radiation3-30 min0.1 MeVBig Bang Nucleosynthesis1 s1 MeVneutrino decoupling

10-5 s100 MeVQCD phase transition (?)10-11 s100 GeVelectroweak phase transition (?)10-1310-36 s1031016 GeV baryogenesis?10-1310-36 s1031016 GeVinflation?10-1310-42 s1031019 GeV quantum gravity?16Structure formationCMB shows the initial conditionsThe early universe is exactly homogeneous except for small perturbations of 10-5Seeds of structure

Gravity is attractive fluctuations grow into galaxies, clusters of galaxies, filaments, walls and voids, which form the large-scale structure of the universeCosmology 201217

Cosmology 201218

Cosmology 201219

Cosmology 201220Structure formationOrigin of fluctuations: inflationA period of acceleration in the early universeQuantum fluctuations are stretched by the fast expansion and frozen in place

Growth of fluctuationsDue to ordinary gravityDepends on the initial state plus the matter compositionBaryonic matter is too smoothly distributed at last scatteringCosmology 201221Dark matterLuminous matter: stars, gas (plasma), dustLarge-scale structure, CMB anisotropies, motions of stars in galaxies, galaxies and gas in clusters, gravitational lensing, BBN, ... there is invisible matterBaryonic matter: cold and hot gas, brown dwarfsHowever, the majority of matter (about 80%) is non-baryonic, either cold dark matter (CDM) or warm dark matter (WDM, m > 10 keV)Neutralinos, technicolor dark matter, right-handed neutrinos, ...Cosmology 201222Dark energyExactly homogeneous and isotropic models with baryonic and dark matter dont quite agree with the observationsMeasured distances are longer by a factor of about 1.5-2.0 and the expansion is faster than predicted by a factor of 1.2-2.2.Three possibilities:1) There is matter with negative pressure which makes the universe expand faster (dark energy)2) General relativity does not hold (modified gravity)3) The homogeneous and isotropic approximation is not good enoughCosmology 201223Dark energyDark energy is the preferred optionDark energyhas large negative pressureis smoothly distributedhas an energy density about three times that of baryonic plus dark matterThe most natural candidate is vacuum energyCosmology 201224for the discovery of the accelerating expansion of the Universe through observations of distant supernovaePhysics Nobel prize 2011

Saul PerlmutterBrian P. SchmidtAdam G. RiessCosmology 2012 dark energy [...] is an enigma, perhaps the greatest in physics today