Astronomy & Astrophysics Chapter 17 Homework If you are pressed for time or do not want to devote

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214 Bennett, Donahue, Schneider, Voit

Chapter 17. The Birth of the Universe

This chapter concentrates on the Big Bang model of

presents the evidence in favor of it.

Teaching Notes (by Section)

For the previous edition, our decision to discuss the birth of the universe (the hot

Big Bang model) and the cosmic microwave background (CMB) prior to more

detailed discussions about dark matter (and its role in galaxy formation) and dark

energy was inspired in part by the conclusive nature of the high-precision, high-

accuracy measurements of the Wilkinson Microwave Anisotropy Probe (WMAP)

and the Planck mission. These CMB studies provide our best constraints on the

Section 17.1 The Big Bang Theory

This section outlines the scientific history of the universe according to the Big

Bang theory.

If you are pressed for time or do not want to devote time to deeper

cosmological issues, this section may be the only one you want to cover.

Instructor Guide for The Essential Cosmic Perspective, Eighth Edition 215

Section 17.2 Evidence for the Big Bang

In this section, we present the two strongest pieces of evidence in favor of the Big

Bang theory: the cosmic background radiation and the helium abundance of the

universe.

The relationship between the microwave background fluctuations seen by

COBE and the underlying mass distribution is less direct than it might seem at

first glance. In perturbations larger than the cosmological horizon, matter

The moment at which microwave background photons begin to stream

freely across the universe is often said to be simultaneous with the

Section 17.3 The Big Bang and Inflation

This section presents three well-known problems with the original Big Bang

model the origin of structure, the smoothness problem, and the flatness problem

and then discusses how an early episode of inflation might solve all three.

We pay little attention to the steady state theory of the universe in this

216 Bennett, Donahue, Schneider, Voit

Density perturbations in the early universe must all have had roughly the

same amplitude when they were first encompassed by the cosmological

Section 17.4 Observing the Big Bang for Yourself

In this section, the last on cosmology and the universe, we want to leave the

students looking up at the sky and thinking for themselves, so we pose the

f Olbers

paradox and its implications for the evolution of the universe.

has either a finite number of stars, or it changes in time in some way that

Answers/Discussion Points for Think About It/See It for Yourself

Questions

The Think About It and See It for Yourself questions are not numbered in the

book, so we list them in the order in which they appear, keyed by section number.

Section 17.2

(p. 453) A variety of stars and galaxies spanning a wide range of redshifts

would produce thermal radiation at many different temperatures, quite

(p. 455) The Big Bang fused 25% of the normal matter in the universe into

Instructor Guide for The Essential Cosmic Perspective, Eighth Edition 217

Section 17.4

(p. 460, SIFY) A typical night sky near a city may have as few as a dozen

Solutions to End-of-Chapter Problems (Chapter 17)

Visual Skills Check

Review Questions

1. The Big Bang theory is the scientific theory of

moments. The theory is based on the idea that the universe began as an

2. An antimatter particle has the same mass as its particle equivalent, but

other properties such as charge are opposite, so an anti-proton has negative

3. The four forces that operate in the universe at low energies are gravity,

electromagnetism, the strong nuclear force, and the weak nuclear force. There

4. The universe has gone through several major eras over its history. These

eras are as follows.

The Planck era: The time before 10 43

The particle era: This era started with the separation of the electromagnetic

and the weak forces and ended when the universe became cool enough that

particle production ceased. At the end of the particle era, all of the quarks

in the universe combined into protons and neutrons.

The era of nucleosynthesis: During this era, the temperature of the

5. Our current theories cannot describe the universe during the Planck era

6. Grand unified theories are those that predict the merger of the strong and the

electroweak forces. According to these theories, only two forces operated

7. Inflation refers to the dramatic expansion of the universe that brought it

8. We think that there must have been a slight imbalance of matter and

antimatter in the early universe because some matter has survived

9. The era of nucleosynthesis lasted about 5 minutes, until the universe cooled

down enough to end fusion. This era was important to the present universe

10. When we observe the cosmic microwave background, we are seeing the

universe as it was when it was 380,000 years old. The photons we see have

11. The cosmic microwave background was discovered accidentally by two

physicists working for Bell Labs. They were calibrating a sensitive microwave

antenna and found that they could not get rid of some of the noise despite their

12. The Big Bang theory, along with the observed current temperature of the

13. The three questions left unanswered by the Big Bang theory before inflation

were as follows:

Where did structure come from? We need to explain the origin of the

density enhancement that led to the formation of galaxies and larger

220 Bennett, Donahue, Schneider, Voit

Why is the universe so uniform? We need to explain why the universe

has the same density in regions separated by vast distances. Inflation

explains this because those regions were in contact right before the

14. We can test inflation by looking at the fluctuations in the cosmic microwave

15. Olbers erse and the number of stars in the

universe were infinite, then in every direction we looked, every line of sight

Does It Make Sense?

16. According to the Big Bang theory, the early universe had nearly equal

amounts of matter and antimatter. This statement makes sense. According

17. According to the Big Bang theory, the cosmic microwave background was

created when energetic photons ionized the neutral hydrogen atoms that

18. Observed characteristics of the cosmic microwave background can be

explained by assuming that it comes from individual stars and galaxies.

19. According to the Big Bang theory, most of the helium in the universe was

created by nuclear fusion in the cores of stars. This statement does not make

20. According to the hypothesis of inflation, large-scale structure in the

universe may have originated as tiny quantum fluctuations. This statement

21.

most likely arose by chance. This statement does not make sense. Inflation

22. Inflation is a nice idea, but there are no known ways to test whether it really

happened. This statement does not make sense. The inflation model makes

23. In the distant past, the cosmic microwave background consisted primarily of

infrared light. This statement makes sense. In the distant past, the cosmic

24. The main reason the night sky is dark is that stars are so far away. This

25. Patterns in the cosmic microwave background tell us about conditions in the

early universe that ultimately led to galaxy formation. This statement makes

Quick Quiz

222 Bennett, Donahue, Schneider, Voit

31. a. the uniformity of the cosmic microwave background

Process of Science

36. Examples of unanswered questions and ways to test them are many, so here

are a few: What triggered an inflation period? How big is the universe

beyond our horizon? (These could be tested by further CMB studies, for

37. Kepler might have imagined that there was a giant sphere hiding the rest

of the infinite heavens from us. Scientists today would test this hypothesis

using the principles of thermodynamics, which say that such a sphere would

Group Work Exercise

38. In this task, the students are asked to consider hypothetical observations that

may contradict the Big Bang theory, and decide whether each observation

(1) could be explained with the existing Big Bang theory, (2) could be

explained with a revision to the Big Bang theory, or (3) would force us to

abandon the Big Bang theory. It might help to set the rules of the discussion.

For example, is the Big Bang theory defined by current cosmological

Instructor Guide for The Essential Cosmic Perspective, Eighth Edition 223

Short Answer/Essay Questions

39. The ideal story will begin with the genesis of a proton as a partner in a

40. This is a creative writing project that should be constrained by science. It

41. Particle accelerators can create new particles from the energy of high-speed

collisions. They are an example of E = mc2 in action. In the early universe,

42. The student is asked to justify a wager of up to $100 on a bet that we

understand the universe at 1 minute. A similar question to the instructor is quite

useful: How would the instructor bet his or her $100 on the theory being correct

at various epochs of the early universe? This chapter is often uncomfortable for

43. We cannot ever detect the early universe because it is hidden to our gaze,

44. According to Figure 13.19, the elements with atomic numbers between

those of helium and carbon, lithium, boron, and beryllium are at a local

45. Following are some of the features of the universe that are satisfactorily

explained by the Big Bang plus inflation (and that would have to be

explained by alternative models): the cosmic microwave background, the

224 Bennett, Donahue, Schneider, Voit

Quantitative Problems

46. To find out how much matter and antimatter would be needed to supply the

only about 40 times this.

47. When the distances between galaxies are twice as large as they are today,

the peak wavelength of photons will also be twice as large as it is today.

48. If the largest bumps on a 1-meter table were at a scale of 1 part in 100,000,

their sizes would be

49. a. The apparent brightness formula is

b. -years, we need to

Instructor Guide for The Essential Cosmic Perspective, Eighth Edition 225

c. The number of stars we would need at 10 billion light-years to make the

d. We estimated the number of stars in the observable universe in Chapter

1 to be around 1022, so there would only have to be many times more

stars than there are in the observable universe to make the night sky as

Discussion Questions

50. This question and Question 51 are more opinion questions about the ability

51. What are the strengths and failures of the current Big Bang theory? We have

spent much time discussing the successes, and here we can highlight some

of its