The Cosmic Microwave Background Based partly on slides Joe Mohr ( University of Chicago)

History of the Universe: superluminal inflation, particle plasma, atomic plasma, recombination, structure formation

Outline 1. Introduction »Relic radiation »Penzias and Wilson at Bell Labs 2. Blackbody radiation »Electromagnetic spectrum »Lamps, stars and people »Effects of expansion 3. COBE and WMAP »Nature of the bkgd radiation »Uniformity of background »Detecting our motion »Seeds of structure formation 4. Review

Implications of an Expanding Universe Reactions to an expanding universe »Gamow predicts (1940’s) hot, dense early phase »Novikov predicts (1962) relic radiation from hot, dense phase »Dicke was interested in finding a radiation background Arno Penzias and Robert Wilson »Study radio emission at 7cm »Bell Labs in New Jersey »Discover background in 1965 –temperature is 3 Kelvin –isotropic »Dicke explained significance Penzias and Wilson with radio horn

Effects of Expansion on Light As the universe expands, light wavelengths stretch with space. Photons gravitationally red shifted or simply stretched with the expanding space. Sphere courtesy Wayne Hu Temperature is directly proportional to wavelength. The effective temperature of a blackbody spectrum decreases as the wavelength stretches. Galaxy velocities: Doppler shifts or universal expansion?  m ~1/a 3  r ~1/a 4 where a=characteristic scale size of universe

Extrapolation into the Past Present day »Universe cold (3K) with low matter density »one hydrogen atom per 10 cubic meters »400 million CMB photons per cubic meter »CMB photons and matter rarely interact - transparent »typical matter in form of atoms and molecules Recombination or last scattering surface »universe hot (3,000K) and a billion times denser »photon energy high enough to ionise atoms and molecules »plasma of e -, p + and  (plus trace He 3, deuterium, Li and Be) »CMB photons coupled to matter through collisions Pre-recombination »universe even hotter and denser »CMB photons coupled to matter through collisions »Early universe hot enough for pair creation, neutrino opacity and many particle processes. Time Early Universe Present 13 Gyr 0.5 Myr <1 yr

A Pictorial History of the CMB Observer Last Scattering Surface where recombination of electrons and protons takes place. Edge of Observable Universe- distance light could have traveled over age of universe. Blackbody light The Observable Universe Blackbody light emitted in the surface of last scattering travels in all directions. We only see that portion which happens to set off in a direction that leads it into one of our detectors.

Blackbody Radiation Every opaque object emits blackbody radiation Blackbody spectrum »Continuous spectrum, depends only on temperature –Hotter bodies brighter, bluer, shorter –Cooler bodies dimmer, redder, longer Stefan-Boltzmann Law Planck radiation law

Blackbody Radiation Cont’d  =Stefan-Boltzmann Constant 5.67 x Wm 2 T -4 10K: 0.56mW/m 2 300K: 450W/m K: 56kW/m K : 560MW/m 2 At peak

Cosmic Background Explorer (COBE) NASA satellite designed to test nature of cosmic background radiation Three instruments »FIRAS- Far Infrared Absolute Spectrophotometer –measure CMB spectrum »DMR- Differential Microwave Radiometers –measure variations in temperature on the sky »DIRBE- Diffuse Infrared Background Experiment Image courtesy COBE homepage.

FIRAS Spectrum of CMB Theoretical blackbody spectrum 34 observations over-plotted largest deviation 0.03% T=2.728+/ K Image courtesy COBE homepage.

Imaging the Globe with the COBE DMR Image of the world Images courtesy E. Bunn

Imaging the Globe with the COBE DMR Image of the world Image with COBE angular resolution Images courtesy E. Bunn

Imaging the Globe with the COBE DMR Image of the world Image with COBE angular resolution Image with COBE measurement noise Images courtesy E. Bunn

Imaging the Globe with the COBE DMR Image of the world Image with COBE angular resolution Image with COBE measurement noise COBE-like image smoothed to reduce noise Images courtesy E. Bunn

COBE DMR Image The sky temperature with range from 0-4 Kelvin Microwave background is very uniform at ~3 Kelvin Image courtesy COBE homepage.

COBE DMR Image: 1,000X Zoom The sky temperature with range from Kelvin –blue is K and red is K Dipole pattern in temperature indicates motion –Doppler Effect at level of ~0.005 K –Solar system is traveling at ~400 km/s with respect to CMB Image courtesy COBE homepage.

COBE DMR Image: 25,000X Zoom The sky temperature ranging from Kelvin –blue is K and red is K Dipole variation from Solar system motion removed Red emission along equator is galactic emission Other fluctuations are likely cosmic in origin Image courtesy COBE homepage.

COBE DMR Image: Galaxy and Dipole Removed Image courtesy COBE homepage. Amplitude of temperature fluctuations is 30  K +/-3  K in 10 degree patches. (1 part in 10 5 )

WMAP reduced in resolution to COBE

WMAP All Sky Image 2002 galaxy removed

WMAP half sky image and examples of fluctuations on varying scales

The Angular Power Spectrum of the CMB

1999 Image Analysis: theory and experiment

Analysis of CMB Images Angular Power Spectrum

Gravitational Enhancement Before recombination dark matter fluctuations with scale size matching the fundamental acoustic wave cause increased clumping of baryons and photons. Photons from the troughs are red shifted. By the time of recombination the excess density regions have been heated enough that the phase is reversed and the temperature fluctuations are 3 times enhanced.

Second Harmonic Gravitational Suppression For even harmonics of the acoustic wave, the same initial condition (cooler troughs) leads to density increase and heating well before recombination. Because of the shorter scale size there is enough time for pressure (blue arrows) to act to oppose gravity (white arrows), thus suppressing the second peak.

Summary Microwave background observed »Penzias and Wilson at Bell Labs in 1965 with sensitive radio telescope »NASA Cosmic Background Explorer (COBE) satellite in early 1990’s »NASA WMAP Microwave Anisotropy Probe 2002 »CMB photons have travelled 13 billion years to reach us Nature of cosmic background radiation »precise blackbody spectrum with temperature of 2.725K »highly uniform temperature –small dipole: evidence for our motion at ~400 km/s –anisotropies: 1 part in 10 5 if you examine 10 degree patches of sky -image analysis consistent with detailed cosmological model involving acoustic oscillations in early universe Universe hot and dense enough to behave as blackbody in past » Fluctuations over non-causally connected regions implies inflation »Fluctuations over causally connected regions allows determination of mass density, dark matter and dark energy »See Wayne Hu Sciama lecture, animations and Sci Am Feb 2004

The Electromagnetic Spectrum Images from “Imagine the Universe!” site at Goddard Space Flight Center Light Waves Particles Photons electron photon Energy Wavelength Frequency Photons and electrons scatter off one another like billiard balls.

Stellar Spectrum Simple Model of a Star Fusion in the center of the star is energy source. Hot, dense gas cools by emitting blackbody radiation. The Sun emits blackbody rad- iation with an effective temperature of 5,500 K. Atoms in the cooler, lower density sur- face gas absorb light at specific wave- lengths, creating absorption lines. Observed stellar spectrum. Note the large number of absorption lines. Magnesium Sodium Calcium Wavelength  Intensity

Infrared Emission from Living Things Infrared image of a cat. Orange is brighter (and warmer) and blue is dimmer (and cooler). Note the warm eyes and cold nose. Images from IPAC at the Jet Propulsion Laboratory. The cat image comes courtesy of SE-IR corporation. Infrared image of a man with sunglasses and a burning match. Black is dim (cold) and white it bright (hot).

Compton Lectures Foundations of the Hot Big Bang Model »1“Observing the Expansion of the Universe” »2“The Cosmic Microwave Background (CMB)” »3“Creation of the Elements in the Early Universe” »4“The Dark Night Sky, Causality and Geometry” »5“A Timeline for the Universe” Current Topics in Observational Cosmology »6“Mapping the Large Scale Structures in the Nearby Universe” »7“Observing the Seeds of Structure Formation in the CMB” »8“Detecting Dark Matter with the Chandra X-ray Satellite” »9“Measuring the Size and Geometry of the Universe” »10“Using Shadows in the CMB to Map the Edge of the Universe”