Precision cosmology as a laboratory for particle physics (or, Evidence for a 4th neutrino?) Yvonne Y. Y. Wong RWTH Aachen SpacePart12, CERN November 5 – 7, 2012
Probes of inhomogeneities Distance vs redshift CMB temperature & polarisation Type 1a anisotropies supernovae > 0.2 deg : COBE, WMAP, Planck < 0.2 deg : DASI, CBI, ACBAR, Boomerang, VSA, QuaD, QUIET, BICEP, ACT, SPT, etc. Local Hubble expansion rate (HST calibration of Large-scale SNIa abs. magnitude) matter distribution Galaxy Cluster distribution abundance SDSS, ROSAT/Chandra, Baryon Acoustic CFHTLS, etc. Oscillations feature in the galaxy distribution Cosmic shear Lyman-α forest
The concordance flat ΛCDM model... The simplest model consistent with observations . ● Massless Cosmological constant Neutrinos (3 families) 13.4 billion years ago Composition today (at photon decoupling) Plus flat spatial geometry+initial conditions from single-field inflation
The concordance flat ΛCDM model... The simplest model consistent with observations . ● ν -to- γ energy density ratio fixed by Standard Model physics Massless Photon energy density fixed by CMB Neutrinos temperature & spectrum measurements: (3 families) T CMB = 2.725 ± 0.001K Composition today 13.4 billion years ago (at photon decoupling)
Neutrino energy density (standard picture)... Fixed by weak interactions and Neutrino decoupling at T ~ O(1) MeV. ● the assumption of equilibrium statistics Assuming instantaneous After e + e - annihilation (T ~ 0.2 MeV): decoupling ● T ν = ( 11 ) 1 / 3 4 – Temperature : T γ Photon temperature, number density, & ζ( 3 ) n ν = 6 3 = 3 energy density – Number density per flavour: 2 T ν 11 n γ 4 π 8 ( 11 ) 4 / 3 ρ ν = 7 π 2 4 = 7 4 – Energy density per (massless) 15 T ν ρ γ flavour: 8 3 ρ ν This ratio is put into the ΛCDM model ρ γ ∼ 0.68 by hand and should be tested!
Plan... Evidence for extra neutrino energy density from precision cosmological ● observations What could this extra energy density be? ● What future observations can do ●
1. Evidence for extra neutrinos from precision cosmology...
Evidence for extra neutrinos... Treating the neutrino energy density ● WMAP ACT, SPT as a free parameter, recent ℓ< 1000 ℓ> 1000 observations prefer N eff > 3 at 2σ+. CMB TT Standard value WMAP SDSS (BAO) WMAP WMAP+ACT ACT HST WMAP+ACT+H 0 +BAO Dunkley et al. [Atacama Cosmology Telescope] 2010
Evidence for extra neutrinos... Allowing the neutrino energy density ● WMAP ACT, SPT to be a free parameter, recent ℓ< 1000 ℓ> 1000 observations prefer N eff > 3 at 2σ+. CMB TT Standard value WMAP SDSS (BAO) HST SPT Keisler et al. [South Pole Telescope] 2011
W-7=WMAP-7 Λ CDM + N eff Simplest cosmological model = 2-2.5σ evidence More complex cosmological models Abazajian et al., “Light sterile neutrinos: a white paper”, 2012
How does it work... Main effect of N eff is to delay ● matter-radiation equality . Looks easy to detect... but ● we use the same data to measure at least 6 other parameters: Hubble parameter baryon density (ω b , ω m ,h , A s , n s , τ) optical depth matter density to reionisation primordial fluctuation amplitude & spectral index Figure courtesy of J. Hamann Plenty of degeneracies! ●
What the CMB really probes: equality redshift... Exact degeneracy between the physical matter density ω m and N eff . Ratio of 3rd and 1st peaks sensitive ● to the redshift of matter-radiation 1 + z eq =ω m ω r ≃ ω m 1 equality via the early ISW effect. ω γ 1 + 0.2271 N eff Fixed : z eq Figure courtesy of J. Hamann
What the CMB really probes: sound horizon... Exact degeneracy between ω m and the Hubble parameter h. Peak positions depend on: ● − 2 ) − 1 / 2 (ω m h Flat Λ CDM Sound horizon θ s ∝ θ s = r s 1 da at decoupling ∫ D A Fixed √ ω m h − 2 a − 3 + ( 1 −ω m h − 2 ) Angular distance to the a * z eq , ω b last scattering surface Fixed : z eq , ω b , θ s Fixed : z eq Figure courtesy of J. Hamann
What the CMB really probes: anisotropic stress... Apparent (i.e., not physical) partial However, free-streaming particles ● ● degeneracies with primordial generate anisotropic stress . fluctuation amplitude A s and First real signature of N eff is in the ● spectral index n s . 3rd acoustic peak! Fixed : z eq , ω b , θ s , A s ( l =200) Fixed : z eq , ω b , θ s Figure courtesy of J. Hamann
Measurement of the third acoustic peak (since WMAP 5 years) gives ● lower limit on N eff from WMAP alone. WMAP only +BAO+SN+HST Komatsu et al. [WMAP5] 2008 Upper limit requires combination of WMAP with other observations to ● break the remaining N eff –ω m –h parameter degeneracies. – Pinning down either ω m or h will do!
Breaking degeneracies with the CMB damping tail... ACT since 2010 SPT since 2011 Multipoles l > 1000. ● Also Planck Probe photon diffusion scale: ● θ d θ s = r d 1 / 4 ∝ω m r s Sound horizon from acoustic Fixed by WMAP : peaks z eq , ω b , θ s , A s ( l =200) Hou, Keisler, Knox et al. 2011 CMB acoustic peaks (WMAP) & damping tail (ACT, SPT) together breaks the N eff –ω m –h degeneracy and measures N eff > 3 at 1.5σ.
The role of non-CMB observations... Adding BAO & local Hubble expansion rate measurements reduces the ● N eff –ω m –h degeneracy and pushes the detection significance to 2σ+ . BAO = baryon acoustic oscillations; ● sensitive to ω m and h. Baryon acoustic oscillations in the matter power spectrum Percival et al. 2010
2. Is it really an extra neutrino?
Particle content of the concordance ΛCDM model... Observables Galaxies, Nuclei/ Cold Hydrogen electrons dark matter clouds, etc. Spacetime EM interaction metric Cosmic Massless microwave Photons neutrinos background
Particle content of the concordance ΛCDM model... Non-interacting Interacting Observables (no vel. dispersion) Non-relativistic Galaxies, Nuclei/ Cold Hydrogen electrons dark matter clouds, etc. Extra neutrinos = extra relativistic, Spacetime non-interacting stuff EM interaction metric (no requirement on the quantum Relativistic numbers or statistics) Cosmic Massless microwave Photons neutrinos background
“Extra neutrinos” don't have to be real neutrinos... Any particle species whose production is associated with some thermal ● process and that decoupled while relativistic at relatively late times [T< O(100) MeV] will behave (more or less) like a neutrino as far as cosmological observations are concerned. Neutrino temperature ∑ i ρ ν ,i +ρ X = N eff ( 4 ) 2 7 π per definition 15 T ν 8 T ν = ( 11 ) 1 / 3 4 Three SM neutrinos T γ ( 0 ) =( 3.046 +Δ N eff )ρ ν Other light stuff Corrections due to non-instantaneous decoupling, finite temperature effects in the EM plasma, and flavour oscillations
Axion number density today Example 1: Hot QCD axions... Relative number density (relative to 1 ν ) Peccei-Quinn scale f a < 10 8 GeV. ● Axion decoupling occurs after QCD ● phase transition (T < 150 MeV). – Dominant processes: Axion temperature today π+π ↔π+ a a + N ↔ N +π (relative to ν temperature) Relative temperature Can contribute up to ● Δ N eff < 0.57 Hannestad, Mirizzi & Raffelt 2005 Pseudoscalar = 1 dof, Bose-Einstein statistics; Axions are a little colder than neutrinos m a [eV] 7 5 f a / GeV = 6 × 10 6 × 10
Sun γ a γ virtual e, Ze Aune et al. [CAST] 2011 CERN Axion Solar Telescope (CAST) L = 9.26 m B = 9 T (Decomissioned LHC test magnet) The parameter region for hot axions is probed also by CAST which looks for axions from the sun. 6 × 10 8 6 × 10 7 6 × 10 6 f a [ GeV ]
Not coupled to the Z Example 2: light sterile neutrinos... Short-baseline Two-flavour oscillations: reactors Baseline Mass splitting ν e disappearance 2 L 2 ( 4 E ) Δ m 2 2 θ sin Prob α→β = sin Prob α→ α = 1 − Prob α→β Mention et al. 2011 Energy Each anomaly can be LSND individually explained in ν e appearance terms of active-sterile oscillations with: MiniBooNE 2 2 Δ m SBL ∼ 1 → 10eV ν e appearance sin 2 2 θ SBL ∼ 10 − 3
Experimentally preferred Δ m 2 and ● mixing favour the production and thermalisation of sterile neutrinos in the early universe via ν α ↔ν s oscillations + ν α scattering. m s > m α Δ N eff ∼ 1 → Can easily produce → Sterile states have the same temperature as the SM neutrinos. m s < m α Hannestad, Tamborra & Tram 2012
Can we tell which particle it is? In the completely general case , no. ● But, if the particle is a thermal relic and has a mass so that it becomes a ● nonrelativistic particle species today, then in principle – Particle mass – Temperature – Quantum statistics (Fermi-Dirac or Bose-Einstein) can be determined from a combination of cosmological observations. – Not possible with current observations
3. Coming up next...
Planck and N eff ... WMAP Planck (expected) If N eff is as large as 4, it ● Acoustic peaks will be settled almost and damping tail immediately by Planck (launched May 14, 2009; public data release early 2013). 1σ sensitivities Hamann, Hannestad, Lesgourgues, Rampf & Y 3 W 2010
Recommend
More recommend
