1. The Cosmic Microwave Background: A Love Story
11/28/2017
Dr. Andrew Rivers
Northwestern University

2. Big Bang Test 1: Redshift-Distance Relationship
Hubble Law Expansion
Image Credit: Ernest Norcia
Observed redshifts of galaxies due to an expansion of the universe.
Image Credit: Freedman et al. (2001)
Expansion rate (Hubble Constant) + distance gives origin time.

3. Big Bang Test 1b: Age of Universe Matches Oldest Stars
HR Diagram Turn-off Pt.
Image Credit: NASA and H. Richer
Oldest Stars in M4 (Milky Way Globular Cluster).
Stars in cluster form together.
Result: Ages of oldest star clusters from turn-off consistent with Big Bang expansion age from Hubble constant.
Brightest stars turn off first, fainter later.
Location of turn-off point=star cluster age

4. Looking out in space is equivalent to looking back in time
Looking out in space is equivalent to looking back in time. Is it possible to look far enough out in space to observe the Big Bang itself?
The Andromeda galaxy is 2.3 Million Light-years away. We see it as it was 2.3 Myr in the past.
Most distant galaxy in the universe? 13.3 Billion light years away. Light from 420 million years after Big Bang.

5. A Mysterious Observation
Penzias & Wilson were studying Milky Way Radio emission.
Calibrated signal in microwave spectrum.
Could not account for “noise” observed.
The 1978 Nobel Prize in Physics winning Bell Telephone Labs scientists Robert Wilson & Arno Penzias in front of the antenna which helped them discover cosmic microwave background radiation. Retrieved from One Minute Astronomer (
  Image Credit: Ted Thai/Time Life Pictures/Getty Images)
Robert Wilson and Arno Penzias and the Bell Antenna they used to discover the microwave background.

6. Observed Characteristics of Noise
Independent of the direction they pointed the telescope.
Corresponded to an “antenna temperature” of 3.5 K
Not due to atmosphere (would be greater if pointed toward horizon)
Did not vary with time of day or year!
Not from Milky Way or Solar System
Cosmic?
About 1% of the fuzz between analog TV channels was the noise observed by Penzias & Wilson

7. Foundation 1: Big Bang Theory: A Hot Past
Big Bang universe was more dense and hotter in the past.
The Universe has cooled from this hot dense state.

8. Foundation 2: Photon-Matter Interactions+ - - - + + + - + - -
Opaque (a fog) +
Photons of light interact strongly with free (ionized)electrons and protons + + - + - - + +
Transparent - -
Photons of light rarely interact with stable atoms (paired electrons & protons)

9. Prediction of the Big Bang: Formation of Stable Atoms
Early Universe: At high enough temperatures, photons would ionize atoms.
Early Universe: soup of free protons, electrons and photons (and a few Helium nuclei).
Look-Back Time
Later Universe: Universe cools enough for stable atoms to form (de-ionization).

10. As the universe expands it cools
Eventually the temperature drops enough so that protons can bind with electrons  electron capture
Stable, neutral Hydrogen and Helium atoms

11. Observing the Opaque to Transparent Boundary
Looking out in space=looking back in time.
We see through the universe until the last-scattering-surface boundary.
De-ionization
Look-Back Time
Plasma
Opaque
Transparent!
When we look at the CMB signal we are looking back through % of the Universe’s history.
Last Scattering Surface
Analogy: the sun becomes transparent at the photosphere
What does the universe look like 380,000 years after the Big Bang?

12. What would we see? Doppler shift
Radiation released at ~3000K (all released at same Temp)
Predict: Blackbody Radiation
Doppler shift
Remember red shift: the radiation is travelling through the universe to us, while the universe is expanding.
Predict: Light extremely redshifted compared to release.
Image Credit: David Koerner, Northern Arizona University

13. Observed Spectrum of CMB: A near perfect Blackbody
Blackbody spectra are produced by one-temperature, opaque and non-reflecting objects.
Peak of blackbody indicates the temperature of the universe today.
Near perfect Blackbody observed by COBE
What will happen to this Blackbody curve as the universe continues to expand?
CMB Peak

14. Cosmic Microwave Background Distribution in Space
What does the universe look like 380,000 years after the Big Bang?
Cosmic Background Explorer (COBE)
A baby picture of the universe, as seen by COBE
Everything that will ever be is built on this. Is something hiding?

15. Big Bang Test 2: Cosmic Microwave Background
Observation
Big Bang Theory: expanding, cooling universe goes from opaque to transparent when atoms form
Near perfect Blackbody
Looking out into the universe and back in time, we can see the last scattering surface (opaque boundary).
Doppler shift: observed radiation will be highly redshifted because of universe expansion.
Smooth distribution= isotropic & homogeneous universe

16. Subtract the average to see if there is hidden structure (small variations from the mean).
The Dipole Anisotropy due to the motion of the Earth with respect to the CMB
What happens when we subtract out the Dipole? Is anything else hiding?

17. Subtract the Dipole
The Milky Way has some emission in microwaves. This emission is small compared to the CMB, but shows up here.

18. Spots on the Microwave Background: the universe is not entirely smooth!
Recall:
Average, dipole and MW subtracted
Variations in temperature are small: 1/10000 th of a degree.
COBE reveals small variations in the background: seeds of structure

19. The self-construction of the universe
Small temperature variations in CMB imply slight variations in density: seeds of structure formation.
Today’s universe
Early universe
Galaxies/clusters
CMB
Gravity acts on initial perturbations building structures. Dark matter (HDM, CDM varieties) drives structure formation

20. A (nearly) Perfect Universe: Observations of WMAP & Planck
The L2 point is now a common place to place deep-space telescopes. Kepler’s third law would suggest that an object further from the sun would orbit slower and therefore the telescope would slowly fall behind the Earth. However, the Earth also has a gravitational pull. At this particular distance the telescope is simultaneously in orbit around the Sun and the Earth. The combined velocity of these two motions give a telescope at this position a velocity that matches the Earth’s. Therefore the telescope tracks along with the Earth as it orbits the Sun.
Artist Rendition of WMAP Satellite moving to L2 point

21. WMAP & Planck: Precisely Measure Cosmological Parameters
By actually measuring features of the CMB, we can determine history & fate of the universe
Parameter 1: Hubble Constant
Comparing results from COBE and higher res. WMAP
Spots seen by three telescopes

22. Where do the spots come from?
Quantum mechanical energy fluctuations during inflationary period.
These fluctuations pulse through the universe as sound waves.
When the universe become transparent sound waves are frozen in (light and matter are decoupled)

23. The Observed Universe of fluctuations Is composed of these peices
Image Credit: Clem Pryke, University of Chicago
The relative strength (power) of these fluctuations is determined by the cosmic parameters of our universe.
Image Credit: Max Tegmark, MIT

24. WMAP Satellite Full-sky map of CMB fluctuations
Largest spots on the CMB are from larger pulsations that only oscillated once before the universe became transparent.
WMAP Satellite Full-sky map of CMB fluctuations (

25. WMAP Power spectrum distribution of fluctuations in the CMB.
Sound wave resonances in the early universe
WMAP Power spectrum distribution of fluctuations in the CMB.

26. Observation: not all spots are equally common
Prediction: Oscillations of right frequency so that they reach a maximum compression at time of transparency “resonate”
Universe as musical instrument: fundamental and higher harmonics.
Relative height of peaks reveals cosmic parameters.
The full moon is about ½ of a degree so these spots are twice as large.
Image Credit: ESA and Planck collaboration
Planck Power spectrum distribution of fluctuations in the CMB.

27. The recipe of the universe
The Answer: From the Cosmic Microwave Background (WMAP), we can intuit the percentages of normal matter, dark matter and dark energy.

28. The New Recipe
The Planck universe indicates slightly more normal and dark matter than previous measurements and a slightly older universe (13.8 Billion years).