Astronomy Rough Notes – Big Bang
BRING: ppt, polarizers, light bulb
DISCLAIMER: These notes do NOT cover everything you need to know. You may need to look up some item
or concept online or in a text. Test questions are not exact copies of the OBJECTIVES but if you know the
OBJECTIVES thoroughly, you should do well on the exams.
HANDOUTS: None
OBJECTIVES:
Briefly describe how scientists think the universe evolved since its birth. What is that theory called?
List four pieces of evidence for that theory (Hubble’s work, cosmic abundances of H and He, CMB, Planck
results).
Explain how Hubble’s work suggests that theory.
Explain how the 3K background radiation (CMB) supports that theory.
Explain why the radiation from the Big Bang appears cool.
How old is our universe? How old is our solar system?
Give a brief time line for the formation of the universe (BB, first stars, our galaxy, our solar system).
What was the significance of the Planck results?
What is the composition of the universe?
OPENING:
Do not trust your intuition. Use it BUT measure and test.
Three hallmarks of science
(Show an illusion here.)
REFERENCE:
Good source on how we know the age of the universe (from AAS and ASP)
http://education.aas.org/publications/AncientUniverseWeb.pdf
More notes including matter density etc
http://bifrost.astr.cwru.edu/academic/luckastr202/Lectures/17_Cosmology.ppt#256,1,Cosmology
Good resource
http://map.gsfc.nasa.gov/universe/bb_concepts.html
CLASS:
Universe springs into existence about 13.7 billion years ago
Expanded and cooled ever since
Called the Big Bang
Rough scenario/timeline to follow
Cosmic Calendar (Carl Sagan)
Compress time since Big Bang into one calendar year
http://home.pacific.net.hk/~paulchui/cosmic.html
http://visav.phys.uvic.ca/~babul/AstroCourses/P303/BB-slide.htm
Big Bang model timeline/predictions
*Time = 0 to 10-43 second: our physics can’t handle – conditions too extreme – called the Planck time
*Rapid inflation (see Alan Guth, horizon problem, flatness problem)
By 10-5 second (10 millionths): universe filled with gamma ray photons
proton - antiproton production/annihilation
sea of energy/particles flickering back and forth
Universe continues expanding and cooling
By 0.0001 second: no more proton-antiproton pair production
slight excess of matter over antimatter
still producing electron-positron
By 4 seconds: protons, neutrons, electrons of today’s universe produced
By 3 minutes: He produced
*By 30 minutes: Universe cooled so that nuclear production stops
75% H, 25% He
*By 380 000 years: Universe cooled to about 3000 K, becomes transparent
*By 200 million years: first stars form, make heavier elements for next generation of stars…
*First galaxies appear soon after first stars, likely influenced by dark matter
*1-14 billion years: Small galaxies merge to form larger ones. Milky Way takes shape.
*By 10 billion years, our solar system takes shape.
Note: The Big Bang makes specific testable predictions from 10-43s on. It does not say anything about how it all
started.
Evidence that Big Bang model is correct (Not all the evidence but some critical ones and some of the easier to
follow pieces)
0. Olber’s Paradox
Why is the sky dark at night?
Suppose the universe is infinite (infinitely old),
Then every line of sight would lead you to a star.
So the sky should always look bright, even at night.
Resolution: The universe is not infinite (nor infinitely old).
Therefore the universe had a beginning.
1. Hubble’s work – see last lecture
1a. All galaxies have red shifts (Hubble)
 All galaxies moving apart
 Universe is expanding
 Earlier small beginning
1b. More distant galaxies (viewed earlier in time) move faster (Hubble’s Law)
 Rapid expansion earlier, slowed by gravity
 Big Bang
Note: Space-time itself expands pulling galaxies along with it.
2. Predictions from Big Bang model (see above time line)
a) Example: Relative abundance of H to He
Prediction: H and He fused from protons and neutrons in the first few minutes
And 75% H to 25% He (by mass)
Now see 75% H, 25% He
Later stars enriched with heavier elements made in first few generations of stars
b) Example: First galaxies
Prediction: First galaxies form soon after first stars
(as of Nov 2010, earliest galaxy seen (See http://www.wired.com/wiredscience/2010/10/most-distant-galaxy/)
(as of Apr 2011, see http://hubblesite.org/newscenter/archive/releases/2011/12/ )
3. Radiation from Big Bang
Cosmic Microwave Background (CMB)
Found by Penzias and Wilson in 1963
2.73 K background radiation (3 K)
http://www.bell-labs.com/project/feature/archives/cosmology/
Why cool temperature from hot beginning?
Universe initially very hot (gamma rays?). But early Universe also very dense so radiation could not propagate.
After 380 000 years, Universe cooled to ~3000K (IR?) and matter and radiation decouple. Subsequently,
Universe expands so these IR waves stretched to longer λ as radio/micro waves. Look like they came from a
black body at 2.73K.
Various names for this:
Cosmic Microwave Background (CMB), Microwave Background, 3K background radiation, etc
(For more see Cosmological Principle, Isotropy, Homogeneity)
4. Planck results (See
http://www.esa.int/Our_Activities/Space_Science/Planck/Planck_reveals_an_almost_perfect_Universe )
[Not for exam but Max Planck was a German physicist who help originate Quantum Mechanics.]
[Not for exam but the European Space Agency has a satellite in orbit called Planck that studies the CMB.]
CMB is very smooth radiation but today’s universe is lumpy (stars, galaxies, voids…)
Planck satellite results:
Detects very fine temperature variations in CMB
Distinguishes between various BB models
Sets date of BB at 13.8 billion years ago
Radiation decouples from matter 380 000 years after BB
First stars form 200-400 million years after BB
Composition of universe*
~5% (really 4.9%) “normal” matter like H, He, stars, galaxies (often called baryonic matter)
~1/4 (really 26.8%) dark matter
~2/3 (really 68.3%) dark energy (more later)
5. Galaxies change with time
Grow from small to large and
Older stars and galaxies have more heavy elements than younger ones
See Hubble deep field images
http://antwrp.gsfc.nasa.gov/apod/ap030625.html
http://antwrp.gsfc.nasa.gov/apod/ap020901.html
6. Polarization of CMB (cosmic background radiation)
a. See Science News, 28 Sep 2002, vol. 162, p195
Or http://www-news.uchicago.edu/releases/photos/polarization/index.html
Demo on polarization
b. Recently, astronomers at the University of Minnesota and elsewhere looked carefully at the
polarization of the CMB and claim that they see evidence for “inflation” that we will discuss in the next
lecture.
7. Computer models that include dark matter give results that look like today’s “bubble-like” universe
http://www.psc.edu/research/cosmology/psc_gc3.html
http://archive.ncsa.uiuc.edu/Cyberia/Cosmos/CosmosCompHome.html
Modified 6 January 2016