Cosmology AS7009, 2009 Outline Introduction to the CMBR Lecture 7 History of CMBR research Support for the Big Bang model Properties of the CMBR Temperature The dipole anisotropy Small -scale temperature fluctuations Origin of the CMBR Recombination Decoupling Last scattering surface Small -scale temperature fluctuations Cosmological information Covers chapter 9 in Ryden
”for their discovery of the blackbody 2006 Nobel prize in Physics form and anisotropy of the cosmic microwave background radiation ”
John C. Mather George F. Smoot NASA Goddard Space University of California Flight Center Berkeley, CA, USA Greenbelt, MD, USA
Cosmic Microwave Background Radiation (CMBR) - Quick Facts - History of CMBR research I
Comes from all directions in the sky Black body spectrum with: ≈ T0 2.73 K Peak wavelength ≈ 2 mm Nobel prize to Close to isotropic , except for: Smoot and Mather Large -scale doppler (dipole ) anisotropy for their work with COBE due to our motion with respect to the CMBR Small -scale temperature fluctuations due to density fluctuations at z ≈ 1100
1 History of CMBR research II History of CMBR research III
1934: First prediction of the existence of the 1992: COBE satellite ≈ CMBR Close to perfect BB, with T ≈2.73 K Large -scale dipole Tolman : Expanding Universe should be filled by Small -scale temperature fluctuations (~10 -5 K) thermal radiation its hot past Late 90s: MAXIMA & BOOMERanG balloons 1948: First prediction of the current CMBR 1948: First prediction of the current CMBR Small -scale temperature and polarization variations temperature 2003 - now : WMAP satellite ≈ Gamow , Alpher & Herman: T 0 5 K Full -sky maps of polarization and small -scale temperature 1965: CMBR discovered by Wilson & Penzias variations ≈ 2009 ( launched May 14): Planck satellite Temperature measured to be T 0 3.5 K Superior polarization measurement Planck -scale physics ???
Hot Why is there a CMB? Cold, dark
Early Universe (t<240 000 yr): Hot → Galaxies Baryons ionized form Universe opaque to photons e Photon -baryon plasma stanc Now or di Time Cosmic expansion → Here/now Universe neutral at t~240 000 yr Universe transparent to photons
Photon trajectories from the Early Support for the Big Bang model Universe
Expansion of the Universe The primordial abundances of light Now elements The age consensus Neutral d Ionize The CMBR fog) (cosmic The temperature of the plasma was about 3000 K when the CMBR was emitted. Cosmic expansion → Energy loss due to redshift → T ≈ 2.73 K
2 The CMBR as support for the Big Bang model I The CMBR as support for the Big Bang model II
Existence of the CMBR : Temperature of the CMBR: Richard Tolman (1934): Expanding Universe T0 = 2.73 K fits Big Bang model should be filled with thermal radiation from hot (but note: the a priori prediction was not this precise)
past Big bang model predicts: T (z) = (1+z) T 0 CMBR ≈ ”Afterglow of the Big Bang” Confirmed by measurements up to z ≈ 3 Difficult to understand in Steady State-type Small-scale temperature anisotropies: cosmologies Results in cosmological parameter values consistent with other methods
Properties of the CMBR I: Properties of the CMBR II: Spectral shape and temperature The Dipole Anisotropy
Doppler shift due to our movement (dominated by the motion of the Sun around the Milky Way and of the Local Group Temperature average over all directions:
Suggestion for literature exercise: Properties of the CMBR III: Strange CMBR anisotropies Small -scale temperature fluctuations
Overall CMBR, including Doppler dipole
CMBR with dipole subtracted
CMBR with Milky Way and nearby structure subtracted → Small-scale temperature fluctuations (RMS 10 -5 K) The ”Axis of evil ” & the cold spot : Very important for cosmological model fitting! Signatures of non -standard cosmology ?
3 Origin of the CMBR: Origin of the CMBR: Radiation -matter equality Important Concepts Matter Radiation -matter equality Radiation Density Photon decoupling Matter- Dark energy domination Recombination Radiation- domination Last scattering surface The Sachs -Wolfe effect Time Acoustic peaks Radiation-matter equality happened at z ≈ 3570, T ≈ 9730 K, t ≈ 47 000 yrs
Origin of the CMBR: Origin of the CMBR: Decoupling I Decoupling II During radiation-domination, and during a short period Rate of scattering interactions for this process: in the matter-dominated era, photons kept the atoms ionized c Γ = = n σ c Thomson scattering : λ e e γ + e- → γ + e- This process freezes out when: Mean free path of photons: Γ < H λ = 1 n σ This leads to decoupling of photons from the baryonic plasma e e → Baryons and photons evolve separately
Origin of the CMBR: Origin of the CMBR: Recombination Last Scattering Surface At around the same time, the expansion of the Universe Last scattering surface causes the energy of the photons to drop below 13.6 eV → Hydrogen starts (re)combining and the Universe goes from ionized to neutral, which speeds up the decoupling Now Recombination happened at z ≈ 1370, T ≈ 3740 K, t ≈ 240 000 yrs Neutral nized Photon decoupling happened at Io mic fog) z ≈ 1100, T ≈ 3000 K, t ≈ 350 000 yrs (cos CMBR photons reach us from a fog-like ’wall’. This last scattering surface is located at z ≈ 1100, T ≈ 3000 K, t ≈ 350 000 yrs
4 Origin of the CMBR: The Sachs -Wolfe effect Small -scale temperature fluctuations Density fluctuations present at the time of last scattering are evident as spatial temperature fluctuations in the CMBR Φ l Recall : θ = Underdense Gravitational redshift dA Gravitational blueshift region
In the benchmark model , the horizon distance at zCMBR Average corresponds to θ ≈1° H density θ θ Potential energy On scales > H: Primordial CDM density fluctuations θ θ On scales < H: Acoustic oscillations in the photon - baryon fluid Overdense region
The late/ integrated Sachs -Wolfe effect (or Rees -Sciama effect ) The Angular Power Spectrum I The gravitational red/blueshift of CMBR photons due to structure along the line of sight towards the last scattering surface. When studying CMBR temperature fluctuations Static potential well → Blueshift climbing in, redshift climbin out (no net effect) as a function of angular scale, one usually plots: But net redshifts/blueshifts will happen if the potential well gets shallower/deeper while crossing! (ll + )1 2/1 ∆ = C T T 2π l Is a huge, expanding void where : along the line of sight the reason for the CMBR l is the multipole (note : high l means small θ ) ’cold spot’? δT C is the angular correlatio n function of l T
The Angular Power Spectrum II Cosmological Information I WMAP 5-year data and model fit Ω = − Ω + Ω ⇒ k 1 ( M Λ ) K) µ ( Ω = 0 ⇒ Flat
T k ∆
The positions of the CMBR peaks are very sensitive to the geometry. The observed positions 3rd 2nd 1st peak indicate that our Universe is very close to flat! Temperature fluctuations
Primordial Acoustic oscillations
5 Cosmological Information II
Mild degeneracy: The amplitude ratios of the first three peaks are sensitive to the baryon density
Cosmological Information III Cosmological Information IV
Mild degeneracy: The amplitude ratios of Example of strong the first three peaks degeneracy: are also sensitive Hubble constant and Ω to the CDM Λ variations density mimic each other (if other parameters are held fixed)
Cosmological Information V Cosmological information VI
Example of strong Benchmark model degeneracy: Ω Ω Hubble constant and M=0.3, Λ=0.7 Ω variations -1 -1 Λ H0=72 km s Mpc mimic each other (if other parameters are held fixed)
6