Transcript Using Galaxy Clusters to Study Structure Evolution

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
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
 2.
 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.
 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?
rm~1/a3
rr~1/a4
where a=characteristic scale size of
universe
Sphere courtesy Wayne Hu
Present
 Present
Time
13 Gyr
0.5 Myr
Extrapolation into the Past
<1 yr
Early
Universe
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 a (plus trace He3, 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.
A Pictorial History of the CMB
Blackbody light
The Observable Universe
Last Scattering Surface
where recombination of
electrons and protons
takes place.
Edge of Observable
Universe- distance light
could have traveled
over age of universe.
Observer
Blackbody light
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 l
– Cooler bodies dimmer, redder, longer l
Planck radiation law
P( )d 
 3 d
4 c
2 2
Stefan-Boltzmann Law
  2k 4  4
P   P()d   2 3 T  T 4

60c 

e kT 1
Blackbody Radiation Cont’d
 =Stefan-Boltzmann
Constant 5.67 x 10-8 Wm2T-4
0.56mW/m2
 300K: 450W/m2
 1000K: 56kW/m2
 104K : 560MW/m2
 10K:
At peak

kT
~3
Cosmic Background Explorer (COBE)
 NASA satellite
 Three
designed to test nature of cosmic background radiation
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+/-0.004 K
Image courtesy COBE homepage.
Imaging the Globe with the COBE DMR
Image of the world
Imaging the Globe with the COBE DMR
Image of the world
Image with COBE angular resolution
Imaging the Globe with the COBE DMR
Image of the world
Image with COBE angular resolution
Image with COBE measurement noise
Imaging the Globe with the COBE DMR
Image of the world
Image with COBE measurement noise
Image with COBE angular resolution
COBE-like image smoothed to reduce noise
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 2.724-2.732 Kelvin
– blue is 2.724 K and red is 2.732 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 2.7279-2.7281 Kelvin
– blue is 2.7279 K and red is 2.7281 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
Amplitude of temperature fluctuations is 30mK +/-3 mK in 10 degree patches.
(1 part in 105)
Image courtesy COBE homepage.
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
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.
For even harmonics of the acoustic wave,
the same initial condition (cooler troughs)
leads to density increase and heating well
before recombination.
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 105 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
Wavelength
Frequency
Waves
Particles
Photons
Light
electron
l
photon
Energy
Photons and electrons
scatter off one another
like billiard balls.
Images from “Imagine the Universe!” site at Goddard Space Flight Center http://imagine.gsfc.nasa.gov/docs/homepage.html
Stellar Spectrum
Simple Model of a Star
Sodium
Magnesium
Calcium
Wavelength l
Observed stellar spectrum. Note
the large number of absorption
lines.
Fusion in the
center of the star
is energy source.
Hot, dense gas cools by
emitting blackbody
radiation. The Sun
emits blackbody radiation with an effective
temperature of 5,500 K.
Atoms in the cooler,
lower density surface gas absorb light
at specific wavelengths, creating
absorption lines.
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.
Infrared image of a man with sunglasses and
a burning match. Black is dim (cold) and
white it bright (hot).
Images from IPAC at the Jet Propulsion Laboratory. The cat image comes courtesy of SE-IR corporation.
Compton Lectures
 Foundations
»1
»2
»3
»4
»5
of the Hot Big Bang Model
“Observing the Expansion of the Universe”
“The Cosmic Microwave Background (CMB)”
“Creation of the Elements in the Early Universe”
“The Dark Night Sky, Causality and Geometry”
“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”