1. Hubble’s Law Our goals for learning What is Hubble’s Law?
How do distance measurements tell us the age of the universe?
How does the universe’s expansion affect our distance measurements?

2. We measure speeds with the Doppler shift.
All galaxies except the nearest have a redshift
i.e. They’re all moving away from us
Fig. 14.3 2

3. By measuring distances to galaxies, Hubble found that redshift and distance are related in a special way

4. the further away a galaxy is, the faster it is travelling
Hubble’s Law: velocity = H0 x distance

5. Redshift of a galaxy tells us its distance through Hubble’s Law:
velocity H0

6. Distances of farthest galaxies are measured from redshifts
This is the furthest distance-finding technique on the 'Cosmological Distance Ladder'
Some techniques use standard candles and some do not.

7. How do distance measurements tell us the age of the universe?

8. The expansion rate appears to be the same everywhere in space
The universe has no center and no edge (as far as we can tell)‏

9. One example of something that expands but has no center or edge is the surface of a balloon
All observers see the same view.
All see other galaxies moving away, with the ones farther away moving more quickly.
The universe is expanding uniformly.

10. Cosmological Principle
The universe looks about the same no matter where you are within it
Matter is evenly distributed on very large scales in the universe
No center & no edges
Not proved but consistent with all observations to date

11. Hubble’s constant tells us age of universe because it relates velocities and distances of all galaxies
Age =
~ 1 / H0
Distance
Velocity

12. Calculating 1 / H0 gives an age for the universe of 13
Calculating 1 / H0 gives an age for the universe of billion years old!

13. What have we learned? What is Hubble’s Law? velocity = H0 x distance
The faster a galaxy is moving away from us, the greater its distance:
velocity = H0 x distance
How do distance measurements tell us the age of the universe?
The measurements let us calculate the expansion rate of the universe, which lets us calculate how long the universe took to expand.

14. Thought Question What do we mean by the expansion of the universe?
A. Galaxies are moving apart through space.
B. Spacetime itself is expanding.
C. Everything is expanding, including Earth, our bodies, etc.

15. Evidence for the Big Bang
Our goals for learning
How do we observe the radiation left over from the Big Bang?
How do the abundances of elements support the Big Bang theory?

16. How do we observe the radiation left over from the Big Bang?

17. The cosmic microwave background – the radiation left over from the Big Bang – was detected by Penzias & Wilson in 1965

18. Background radiation from Big Bang has been freely streaming across universe since atoms formed at temperature ~ 3,000 K: visible/IR

19. Background has perfect thermal radiation spectrum at temperature 2
Background has perfect thermal radiation spectrum at temperature 2.73 K
Expansion of universe has redshifted thermal radiation from that time to ~1000 times longer wavelength: microwaves

20. WMAP gives us detailed baby pictures of structure in the universe

21. How do the abundances of elements support the Big Bang theory?

22. Protons and neutrons combined to make long-lasting helium nuclei when universe was ~ 3 minutes old

23. Big Bang theory prediction: 75% H, 25% He (by mass)‏
Matches observations of nearly primordial gases

24. What have we learned?
How do we observe the radiation left over from the Big Bang?
Radiation left over from the Big Bang is now in the form of microwaves—the cosmic microwave background—which we can observe with a radio telescope.
How do the abundances of elements support the Big Bang theory?
Observations of helium and other light elements agree with the predictions for fusion in the Big Bang theory

25. Thought Question How do we know the universe is expanding?
A. the big bang theory
B. the motion of the Andromeda Galaxy
C. the redshift measured in many galaxies
D. inflation

26. Thought Question A. UV B. Visible C. Far Infrared D. Microwaves
At which wavelength is the Universe the brightest?
A. UV
B. Visible
C. Far Infrared
D. Microwaves

27. What is the history of the universe according to the Big Bang theory?

29. Planck Era
Before Planck time (~10-43 sec)‏
No theory of quantum gravity

30. Four known forces
in universe:
Strong Force
Electromagnetism
Weak Force
Gravity

31. We think at moment of the big bang the 4 forces were unified at super high temperatures.
At the first instant the forces separated out.

32. First gravity from the other 3 sub-atomic forces.
Then the strong force
Then finally electromagnetism separates from the weak force.

33. Inflation
As the forces separate, the fabric of the universe expands out like a balloon
This extreme rapid expansion is called “inflation”, a critical moment in the Big Bang
Occurs after gravity separation, but before electromagnetism separation

34. Particle Era
The inflating universe starts to cool, energy starts being stored as matter
E = mc2

35. Photons converted into both matter and anti-matter as pairs of particles and antiparticles
E = mc2
Early universe was full of particles and radiation because of its high temperature

36. Particle Era
Amounts of matter and antimatter nearly equal
(Roughly 1 extra proton for every 109 proton-antiproton pairs!)‏

37. NOTE –everything so far:
Force separation,
Inflation,
Fundamental matter formation,
has taken place within 1 second of the universe’s existence!

38. Era of Nucleo-synthesis
Nuclei begin to fuse
Protons and neutrons will start to stick together
(takes ~ 3 minutes)‏

39. Era of Nuclei
Helium nuclei form
Universe has become too cool to blast helium apart

40. Era of Atoms
Atoms form at age ~ 380,000 years
Background radiation released

41. So far every photon of light released from a particle has been absorbed by another particle.
Now the universe is sufficiently large (and the particles sufficiently clumped) that light escapes absorption and roams the universe.
This is the Cosmic Background Radiation

42. Era of Galaxies
Galaxies form at age ~ 1 billion years

43. Thought Question
How does the motion of the photons change after the universe has cooled to below K and atoms form?
A. They move more slowly.
B. They can move further because there are less things to bump into
C. They move more quickly
D. They can't move very far without bumping into things.

44. What have we learned? What were conditions like in the early universe?
The early universe was so hot and so dense that radiation was constantly producing particle- antiparticle pairs and vice versa
What is the history of the universe according to the Big Bang theory?
As the universe cooled, particle production stopped, leaving matter instead of antimatter
Fusion turned remaining neutrons into helium
Radiation traveled freely after formation of atoms