1. Lecture Slides CHAPTER 16: The Evolution of the Universe
Understanding Our Universe
SECOND EDITION
Stacy Palen, Laura Kay, Brad Smith, and George Blumenthal
Prepared by Lisa M. Will, San Diego City College

2. The Evolution of the Universe
Explain the evidence for the Big Bang.
Describe how the conditions of the early universe influenced our modern universe.
Predict the fate of the universe.

3. Big Bang
Because light travels at a finite speed, it takes time for the light from a distant object to reach us.
=> Look-back time
Distant galaxies have a large look-back time.

4. Class Question We have introduced the concept of look-back time.
Which of the following statements is correct?
We see distant galaxies as they are now.
We see distant galaxies as they were in the past.
We see distant galaxies as they will be in the future.
Correct Answer: B

5. Big Bang: Hubble’s Law
Previously, we introduced the Hubble Law, which pointed to an expanding universe.
Galaxies were closer together in the past.
Galaxies will be farther apart in the future.

6. Big Bang: Hubble Time
Recall that the Hubble Constant can be used to determine the age of the universe.
Hubble time: time when separation between galaxies was zero. This is equal to 1/Hubble Constant.
=>Estimate of age of the universe.

7. Big Bang: Age of the Universe
The Hubble time was 13.8 billion years, which means that the universe started 13.8 billion years ago. => Big Bang.
Galaxies are not flying away from each other.
Space itself is stretching or expanding.
Figure 16.3 7

8. Class Question The value of H0 is about 70 km/sec/Mpc. Suppose it
were twice as big. Compared to our current
estimates, what would the age of the universe be?
The same as now.
Younger.
Older.
Correct Answer: B

9. Big Bang: Expansion
The expansion does not affect atoms, stars, or anything else, including laws of physics.
Everything in the universe was once in a tiny volume! The Big Bang happened everywhere.
Figure 16.3 9

10. Big Bang: Redshift
As light comes to us from distant galaxies, the space the light travels through expands, and the light is also stretched out. => Cosmological redshift
Greater distance traveled => greater redshift.

11. Big Bang
If all matter is in a small volume, it means conditions were very hot.
Due to expansion, light redshifted, and temperatures dropped.

12. Big Bang: Prediction
Prediction in the late 40’s by cosmologists (Gamow and Alpher): a blackbody spectrum uniformly redshifted by the expansion of the universe to a temperature of about 5–10 Kelvin.

13. Big Bang: Cosmic Microwave Background (CMB)
A blackbody spectrum with a temperature of about 3 K was discovered in 1965 by Penzias and Wilson. They discovered that no matter how hard they tried to get rid of “noise” it was always there. This led to their Nobel Prize in 1979.
The sky faintly glows in microwaves.
= Cosmic microwave background (CMB) radiation.
Figure 16.6

14. Big Bang: Microwaves
The cosmic microwave background is from the time when the universe was young, hot, and ionized.
At several hundred thousand years, the temperature cooled so protons and electrons could form neutral H atoms.
=> Recombination
Figure 16.7

15. Big Bang: Microwaves (Cont.)
Figure 16.7 (a)

16. Big Bang: Microwaves (Cont.)
Figure 16.7 (b)

17. Amazing data from the Cosmic Background Explorer (COBE)
At this point, light was no longer blocked from its travel by all of the matter.
The light could travel freely and cooled from a temperature of about 3000K to about 2.7 K, as confirmed by satellite data. The cosmic background is now observed at microwave wavelengths (~1mm)

18. Big Bang: CMB Variations
Slight variations in CMB give clues to the rise of structure in the early universe.
Discovery and follow-up observations awarded Nobel Prizes in Physics (for COBE in 2006.
Figure 16.9 18

19. Big Bang: CMB Variations (Cont.)
Figure 16.9 (a) 19

20. Big Bang: CMB Variations (Cont
Big Bang: CMB Variations (Cont.) COBE data, removing Sun and Galaxy’s motion
Figure 16.9 (b) 20

21. Big Bang: CMB Variations (WMAP data)
Figure 16.9 (c) 21

22. Big Bang: CMB Variations (Planck data)
Figure 16.9 (d) 22

23. Fate of the Universe
The fate of the universe depends on how much matter there is in the universe.
Gravity from ordinary and dark matter slows the expansion.
The faster the universe is expanding, the harder it is to stop it and make it contract.
The more mass there is, the more gravity there is to halt the expansion.
Is there enough gravity to halt the expansion? 23

25. Fate of the Universe: Critical Density
Express this amount needed as a ratio to the critical density: mass = ρ/ρc
If mass = 1, density equal the critical density.
Universe slows to a stop at an infinite time. 25

26. Fate of the Universe: Ratio the Actual Density
If mass > 1, the expansion will stop and the universe will contract.
If mass < 1, expansion will go on forever, slowing due to gravity, but never stopping. 26

27. Fate of the Universe: Mass of Dark Matter
Ordinary stars and galaxies: only  = 0.02.
Dark matter in and between galaxies:  = 0.3. 27

28. Fate of the Universe
At the end of the 20th century, we thought we understood the expansion.
1990s: We were surprised to discover the expansion is speeding up!
The vertical axis is the separation between between 2 points in space 28

29. Fate of the Universe: Expansion of the Universe
If the expansion were slowing, then the rate of expansion at earlier times should be larger than now.
We see evidence that the expansion was instead smaller in the past, and larger now! 29

30. Fate of the Universe: Accelerating Expansion
The expansion is accelerating, not slowing down.
This indicates that there is a type of energy in the universe that pushes on space, acting against gravity. 30

31. Fate of the Universe: Dark Energy
This energy is called dark energy.
This dark energy is associated with the cosmological constant, .
Changes possible outcomes since it is more difficult for gravity to reverse the expansion. 31

32. Fate of the Universe: Recent Estimates
 is about 0.7
mass is about 0.3.
Sum total is 1, which means the universe is “flat.”
Universe is accelerating and will expand forever.
Figure (c) 32

33. Class Question If the observed density of the universe is greater
than the critical density, what is the fate of the
universe?
The universe will exist forever as it is right now.
The universe will expand forever.
The universe will collapse back on itself.
Correct Answer: C

34. Early Universe
Pairs of particles and antiparticles were created and annihilated in the early universe.
Particle type is dependent on temperature and energy => Higher energy allows for greater particle mass.
Figure 16.13 34

35. Early Universe (Cont.)
Figure (a) 35

36. Early Universe (Cont.)
Figure (b) 36

37. Early Universe: Electron-Positron Pairs
The universe originally was full of photons, particles, and anti-particles.
As the universe cooled, the particles and antiparticles annihilated each other, somehow leaving slightly more particles than antiparticles.
Figure 16.13 37

38. Early Universe: The Standard Model
There are four forces that govern the universe: gravity, electromagnetic, weak nuclear, and strong nuclear.
Each came into being at a different time very early in the history of the universe.
QED describes the electromagnetic force.
Electroweak describes QED + weak.
Standard model describes the combination of the ideas in QCD (strong) and electroweak. 38

39. Early Universe: Theory of Everything
In the early universe, strong force + electroweak force = grand unified theory (GUT).
Predicts proton decay.
GUT + gravity = theory of everything.
Some GUTS include extra “compact” dimensions! 39

40. History of the Universe40

41. Early Universe: The Cooling Universe
As the universe cooled after the Big Bang:
Established physical forces.
Photons, electrons, positrons.
Matter-antimatter annihilation, leaving many photons and a little normal matter. 41

42. Early Universe: The Cooling Universe (Cont.)
As the universe cooled after the Big Bang:
Matter and photons cooled.
Atomic nuclei formed (Big Bang nucleosynthesis).
Recombination: atoms, galaxies, stars. 42

43. Inflation: The Flatness and Horizon Problem
Case for the Big Bang is very compelling, but there are some puzzles to consider.
The flatness problem: The combination mass +  is very close to 1. Why?
The horizon problem: The CMB is almost exactly the same temperature in all directions although these regions would have lost contact with each other long ago. 43

44. Inflation: Early, Rapid Expansion
One explanation: inflation—a very rapid expansion at extremely early times.
Smooths and flattens the early universe.
The inflationary Big Bang is the current most accepted model for the origin of the Universe. 44

45. Inflation: Multiverse
There could be other universes almost identical to ours.
=> multiverse.
An infinite universe could contain an infinite number of disjointed, observable universes. 45

46. Chapter Summary
The universe started with a Big Bang, which occurred nearly 14 billion years ago, and has been expanding ever since.
The universe has no center and no edge, although the observable universe has limits.
Observations of the cosmic microwave background support the Big Bang Theory.
Current estimates predict that the universe is both expanding and accelerating, and will expand forever. 46

47. Astronomy in Action
Observable vs. Actual Universe
Click the image to launch the Astronomy in Action Video
(Requires an active Internet connection) 47

48. Astronomy in Action
Infinity and the Number Line
Click the image to launch the Astronomy in Action Video
(Requires an active Internet connection) 48

49. Astronomy in Action
Expanding Balloon Universe
Click the image to launch the Astronomy in Action Video
(Requires an active Internet connection) 49

50. This concludes the Lecture slides for
Understanding Our Universe
SECOND EDITION
Stacy Palen, Laura Kay, Brad Smith, and George Blumenthal
Prepared by Lisa M. Will, San Diego City College
This concludes the Lecture slides for
CHAPTER 16: The Evolution of the Universe
wwnpag.es/uou2 50