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Cosmological Observations—2004

Cosmological Observations—2004. What the data tell us about dark energy and the contents of the universe. DPF 2004, Riverside August 28, 2004. Joe Fowler Princeton University. Current Picture of the Universe. General relativity Homogeneous & isotropic Began with hot big bang

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Cosmological Observations—2004

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  1. Cosmological Observations—2004 What the data tell us about dark energy and the contents of the universe DPF 2004, Riverside August 28, 2004 Joe Fowler Princeton University

  2. Current Picture of the Universe • General relativity • Homogeneous & isotropic • Began with hot big bang • Quantum fluctuations grew during inflation • Galaxies & other structures grew gravitationally from these tiny early fluctuations HST Images

  3. Evidence for a Hot Big Bang • Hubble expansion (recession of distant objects) • Thermal cosmic background radiation • Light element abundances Hubble Ultra Deep Field Released: March 2004

  4. Contents of the Universe • ΛCDM Model • At least 96% of the universe is mysterious! Λ Note that “Λ” here may be a dynamical field a la quintessence, an Einsteinian cosmological constant, or …?

  5. Evolution in an FRW Universe Open (Ω=0.3) Open (Ω=0.3) Flat (matter) Flat (matter) Closed (Ω=5) Closed (Ω=5) ΛCDM • History and fate are determined by proportion of stuff • Express energy densities as Ω, i.e. scaled by the critical density • Today, ρcrit = 5000 eV cm-3 = 6 protons per m3

  6. Roles of Inflation • Solves the “horizon problem” (all visible universe was once in causal contact) • Explains the source of inhomogeneities • Flatness is unstable—but inflation drives towards flatness early on

  7. Matter, Energy and Geometry Current model Accelerating now Decelerating now ΩΛ Matter only Closed Flat Open Ωmatter • Generally only the 2 black lines are considered: flat or matter-only.

  8. Lines of Evidence for Dark Energy ObservationResultInterpretation 1. Distant supernovae ~25% too dim Expansion accelerating 2a. CMB acoustic peak ℓ = 220 Flat universe 2b. Matter distribution Ωm ~ 25% Rules out flat, matter-only 3. CMB + LSS power (fits) All of the above spectra 4. Integrated Sachs-Wolfe Mass at z~0.5 If flat  Λ>0 (a 2 – 3σ) correlated w/ CMB

  9. What is the Dark Energy? Gij – Λgij = 8πG TijCurvature of empty space or Gij = 8πG Tij + ΛgijVacuum energy Vacuum energy opens up more possibilities than curvature. Two key question for observations: • Does Λevolve? • What is its equation of state w ≡ P/ρ? w < -1/3 is required if Λ is repulsive w = -1 is a true cosmological constant

  10. Problems with ΩΛ=0.7 • Why is it not 10120 ? • Why now? Dark energy Matter Radiation ρ/ρcrit log10(a)

  11. Baryon Fraction from Big Bang Nucleosynthesis • Light elements form in first few minutes (D, 3He, 4He, Li) • Ratio of baryon to photon density determines proportions • We know photons (CMB) • Must measure primordial abundance of light elements Tytler et al, 2000 Ωb = 0.041 ± 0.004 (assuming h=0.7) Burles, Nollett & Turner, 2004

  12. Dark Matter Distribution in the Universe • Dark matter clustering drives structure formation on scales larger than galaxies. • Must be “cold” to support the smallest scales observed. R. Cen

  13. Techniques for Studying Matter Distribution Plan: study the evolution of structure by measuring it locally • Number counts of galaxy clusters • Velocity fields of galaxies • Weak gravitational lensing • Galaxy spatial power spectrum • Cold intergalactic gas (Lyman-α forest)

  14. Gravitational Lensing of Background Galaxies Chandra X-ray Observatory Hubble Space Telescope Strong lensing shown here

  15. Weak Lensing • Relies on shear: preferential warping of background galaxies parallel to contours of foreground matter. • A statistical hunt for ellipticity • Shape noise (galaxies have ellipticity ~ 0.3; PSF…) • Shape bias: are some shapes easier to find? Tyson et al 2002

  16. Large Sky Surveys Sloan Digital Sky Survey

  17. Galaxy Power Spectrum Ideally, surveys are flux-limited. 2 degree Field Galaxy Redshift Survey Sloan Digital Sky Survey

  18. Galaxy Power Spectrum Systematics Redshift distortion (due to peculiar velocity) Galaxy “bias” Tegmark et al 2003 astro-ph/0310725 Seljak et al 2004 astro-ph/0406594

  19. The Lyman-α Forest Quasar Clouds containing Neutral Hydrogen • Absorption by H atoms in bulk IGM (λ=121 nm). • Test ΛCDM at unique range of z and small size. Hubble Space Telescope Keck HIRES Figure: Bill Keel

  20. Matter Power Spectrum • Many techniques covering over 4 decades of size. Max Tegmark + SDSS λ, not k

  21. Power Spectrum Results • Completely consistent with ΛCDM model • Dark Energy • Consistent with pure cosmological constant • Inflation • Simplest possible scenario • Primordial slope n=0.98 ± 0.02 • Tensor/Scalar ratio r < 0.36 (95% CL) • Neutrinos • Massive  reduce structure on small scales • 3 ~degenerate families: Σ m < 0.42 eV • 3 massless + 1 (LSND): m < 0.79 eV ruling out LSND solutions at 2σ Max Tegmark + SDSS Seljak et al, 2004

  22. Cosmic Microwave Background • As universe cooled below 3000 K, became transparent. • Most thermal photons last scattered then (at z=1089). • CMB is the most distant light we’ll ever be able to see. • Probes the initial conditions for structure formation.

  23. CMB Basic Facts • Thermal blackbody at T=2.725±0.003 K • Emitted at T~3000 K • Isotropic to ~30 x 10-6 2.731 FIRAS spectrum Residuals 2.721 Fixsen et al 1996

  24. Wilkinson Microwave Anisotropy Probe • Twin telescopes facing 140o apart. • Always measuring differences of Temp. • Amplifiers kept at 90 K without refrigeration. Once the “Princeton Isotropy Experiment” = “PIE in the sky” NASA/WMAP Team

  25. WMAP Goal Map entire mm-wave sky • 5 frequencies • 35 μK noise per 0.3° square pixel • 0.5% absolute calibration Tegmark & Efstathiou

  26. WMAP Radiometers Each “differencing assembly” measures ΔT in analog. Both signals go through all amplifiers! Pospiezalski, NRAO Other figures: NASA/WMAP Team

  27. WMAP Mission Profile • Launched June 30, 2001 • 3 months to L2 (1,500,000 km distant) • Survey for 2—5 yrs At L2, WMAP can keep the sun, moon, and earth behind it at all times. All figures: NASA/WMAP Team

  28. WMAP Sky Maps in 5 Frequencies 22 GHz 30 GHz 40 GHz +200 μK Lowest frequency (galactic electrons) Highest (some dust) -200 μK 90 GHz 60 GHz All figures: NASA/WMAP Team

  29. WMAP CMB-Only Map NASA/WMAP Team Internal linear combination map

  30. Temperature Power Spectrum • Spherical harmonic power spectrum—a radical compression of the map for cosmological purposes. NASA/WMAP Team

  31. Acoustic Peaks • Peaks correspond to a well-understood physical size (145 Mpc): they are “standard rulers.” • Peak at ℓ=220 indicates no global curvature from z=0 to z=1089. • Ratio of peaks #1/#2 constrains baryon density.

  32. Temperature-Polarization (TE) Cross-power • Cross-power spectrum sensitive to ionization resulting from early hot stars. • Data at ℓ>20 fit the cosmology dictated by the TT power spectrum. • Only DASI has detected polarization anisotropy (EE) as of August 28, 2004.

  33. WMAP Interpretation Extremely strong support for: • Hot big bang model • Existence of baryons, dark matter, and dark energy (4/23/73 ratio) • Gaussian primordial fluctuations + inflation

  34. WMAP Surprises NASA/WMAP Team • The first stars ignited much earlier than thought: 200 Myr (1.5% of current age). • Very low quadrupole • How can WMAP tell? • Early stars massive • Massive stars hot (UV) • UV ionizes nearby gas • Ionized atoms polarize CMB • Polarization correlates with T

  35. WMAP Results by the numbers • Age of the universe: 13,700,000,000 years (± 1.5%) • Age when stars first shone: 200,000,000 years • Age at last scattering: 379,000 years (z=1089±1) • Expansion rate (Hubble constant): 71 km s-1 Mpc-1 (± 5%) • Flatness: Ωt = 1.02±0.02 • Optical depth to last scattering τ = 0.17±0.04 • Apparent fate of the universe: Expand forever (?) • These figures include constraints from, for example, 2dF galaxy redshift survey and Supernovae Ia.

  36. CMB Future: Secondary Anisotropies Cosmic Microwave Background Study structure as it forms Clusters “heat” the CMB (SZ Effect) Early stars Massive clusters distort CMB maps CMB seen now has passed through all these objects! Primary CMB Ionization effects Grav lensing of CMBCluster surveys 0.4 Myr ~200 Myr 3000— 13,700 Myr 1000-5000 Myr now

  37. CMB Future: Polarization from Gravity Waves Polarization B modes are “handed” and not produced by scalar perturbations. A strong signature of inflation. But at what level?... DASI collaboration, 2002 Hu & Dodelson, 2002 E modes B modes Wayne Hu

  38. Matter Distribution Imprinted on CMB Cosmic Microwave Background The “Late-time integrated Sachs-Wolfe effect” • CMB blue shifts entering large overdensities. • In matter-only universe, red shift on exit cancels this out. • In a Λ-dominated universe, expansion outweighs clustering. • Higher T correlates with high mass density.

  39. Several 2 to 3σ ISW Detections Boughn & Crittenden, 2004 • In a flat universe, any ISW implies dark energy. X-ray catalog / CMB angular correlation function • Need a tracer of mass. • WMAP + • SDSS (red) 2.0σ • NVSS (radio) 2.2σ • HEAO-A1 (X-ray) 2.5σ • Combined analysis of last 2 yields • (1.13±0.35) x Λ CDM prediction. • ISW alone rejects @ 3σ an allowed WMAP solution with no Λ and high matter content. Xray x CMB data Λ CDM Model 1σ, 2σ range of null MC

  40. Hubble’s Diagram and the Expanding Universe • Uniform expansion  v=Hod • But the next order is interesting! Trace the dynamics of the expanding universe. • Requires an extremely bright light standard: Supernovae “Distance modulus” Δm = 5  factor of 10 in luminosity distance

  41. Type Ia Supernovae • Type I = deficient in Hydrogen; Ia have Si+ absorption • Requires “real time” data analysis • Can now find SN Ia on demand and pre-schedule the follow-up spectroscopy 3 HST discoveries before / after SN2002hp ( ~ 2 months) HST-ACS

  42. Type Ia Supernovae as Standard Candles • Model is an accreting white dwarf, passing the Chandrasekhar limit • Actually, a 1-parameter family in: • Peak brightness • Rate of decline • Color Can reduce dispersion 3x N.B.: evolution slows by (1+z) 1 month

  43. Evidence for Recent Accelerated Expansion • Hubble diagram curvature consistent with universe that’s accelerating now • Effect is only a ~25% dimming of SNe around z=0.5 • Possible confounding effects: • Evolution • Extinction (by very gray, homogeneous dust) • No evidence for either, but we must be very sure.

  44. Evidence for Earlier Deceleration • 16 new SN from HST at typical z~1 • As expected from ΛCDM, dimming trend reverses! • Strongly suggests not evolution or dust Jerk Riess et al 2004

  45. Supernovae Interpretation • SN Hubble diagram constrains (ΩΛ-1.4Ωm) • If flat universe, then Ωm=0.29±0.04

  46. Cosmic Concordance • The model may be crazy, but everything is consistent (so far): • Flat universe • Dark energy (~70%) • Still need non-baryonic DM (and not neutrinos) (Pre-2004)

  47. Probing Inflation (and is it correct?) • CMB degenerate in: • n the primordial perturbation spectral index • τ the optical depth through reionized universe • r the ratio of scalar to tensor fluctuations (the upper limit 0.35 is already approaching what some simple models predict) • Large scale structure surveys and E-mode CMB polarization can help break these. • Detect the B-modes of CMB polarization (next decade?) • B-modes would rule out ekpyrotic (cyclic) scenarios WMAP should soon tell us how hard this will be (foregrounds)

  48. Probing Dark Energy Require more studies covering the z<2 range: • Supernovae—need dedicated, wide-field, fast camera • Cluster counts—need a distance-independent probe (S-Z effect surveys coming online in 2-3 years)

  49. Summary The leadingΛCDM model (dark energy + cold dark matter) is consistent with all the data!

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