Hobson Physics: Concepts & Connections 4e

CHAPTER 11 review questions Einstein’s Gravity: The General Theory of Relativity 1. List two experiments you could do in a spaceship accelerating at 1g through outer space, which might make you think you are at rest on Earth. 2. According to the equivalence principle, what is acceleration equivalent to? 3. In your own words, state the equivalence principle. 4. Give one piece of evidence showing that gravity bends light. 5. As observed in an accelerating reference frame, does a light beam bend? What does this tell us about the effect of gravity on light beams? 6. According to Newton, gravity is a force exerted by material objects on other material objects. What is gravity according to Einstein? The Big Bang 7. About how old is the universe? 8. Describe two different pieces of evidence supporting the big bang. 9. Of what element is the universe mostly made? 10. Following up on the preceding question, what is the second most prevalent element in the universe? 11. Name two elements that were not made in the big bang. Name two that were. The Geometry of the Universe 12. Give an example of a flat two-dimensional space, a curved two-dimensional space, and a two-dimensional space of finite extent. 13. How might we tell from inside our actual three-dimensional space whether space is curved? 14. List the three possible large-scale geometries of the universe, and describe at least one of them. 15. In what fundamental way does the big bang differ from an ordinary explosion? 16. Due to the expansion of the universe, are the galaxies moving through space? Explain. The Shape of the Universe 17. What does the evidence tell us about the overall shape of the universe? 18. List one piece of evidence showing that we live in a flat universe. 19. What is the cosmic microwave background? 20. The big bang emitted lots of high-energy radiation. So why do we detect the big bang radiation primarily as low-energy microwaves? Dark Matter 21. Roughly what percentage of the universe’s total mass is made of ordinary matter? 22. Roughly what percentage of the universe’s total mass is made of dark matter? 23. What is “dark matter”? 24. Why is it called “dark” matter? 25. What led astronomers to hypothesize the existence of dark matter? The Accelerating Universe and Dark Energy 26. Is the universe’s expansion slowing down, speeding up, or maintaining an unchanging speed? 27. Describe the observations that show that the universe is accelerating. 28. What is causing the universe’s expansion to speed up? 29. Roughly what percentage of the universe is made of dark energy? Cosmic Inflation, and a Brief History of the Universe 30. What does the cosmic inflation hypothesis try to explain? 31. According to the cosmic inflation hypothesis, what started the big bang? 32. Why do they call it “inflation”? 33. Since energy is conserved, and since the universe started from only a gasoline tank’s worth of energy, having a mass of only 0.01 milligrams, how can the universe have possibly attained the enormous amount of energy and mass that it has today? 34. Give two pieces of evidence supporting the hypothesis of cosmic inflation. conceptual exercises Einstein’s Gravity: The General Theory of Relativity 1. If you were in a rocket ship in space (far from all planets and stars) accelerating at 2g, how heavy would you feel? 2. In the preceding question, what if your acceleration were, instead, 0.5g? What if you were not accelerating at all? 3. In what way is the general theory of relativity more “general” than the special theory of relativity? 4. Astronauts’ hearts and muscles weaken in space, due to the prolonged weightlessness. How might artificial gravity be installed in a space station to deal with this problem? 5. If you were in a rocket ship in space (far from all planets and stars) accelerating at 2g and you dropped a ball, how would it move as observed by you? 6. In the preceding question, what if your acceleration were instead 0.5g? What if you were not accelerating at all? 7. In the equivalence principle, what is equivalent to what? 8. Why don’t we notice the gravitational bending of light on Earth? 9. Does a high-speed bullet’s path bend more than a light beam bends? Why? 10. A rifle barrel and a laser both point directly toward a target some distance away. General relativity says that the bullet and the light beam both experience the same downward acceleration during their horizontal travel, yet the bullet hits the target well below the laser beam. Explain. The Big Bang 11. Where did your body’s hydrogen nuclei originate? 12. Did your body’s oxygen nuclei originate in the big bang? 13. The big bang has been described as the place where cosmology meets submicroscopic physics. Why? 14. The big bang created just three chemical elements. Why didn’t it create more? 15. Suppose we could instantly reverse the velocities of all the galaxies in the universe. If we then observed distant galaxies, how would they appear? 16. Is there a specific place in the present-day universe where the big bang happened? Explain. The Geometry of The Universe 17. If we consider Earth’s surface to be a two-dimensional “space,” the equator is one “straightest possible line” in this space. Are there other such lines? 18. On Earth’s surface, are the north-south lines of longitude among the straightest possible lines? What about the east-west lines of latitude? 19. If you draw a triangle on the surface of a sphere, the sum of its angles is greater than 180°. What is the largest possible sum? What does the triangle look like? 20. Is there a place in the present-day universe that is the center of the universe? Explain. 21. Are there places in the present-day universe that are at the edge of the universe? Explain. The Shape of the Universe 22. What important event happened at about 400,000 years after the big bang? 23. About 400,000 years after the big bang, the cosmic background radiation was released. How did the universe after this event differ from the universe before this event? 24. Imagine a huge triangle stretching across a large portion of the observable universe. Will the three angles of this triangle add up to the usual 180°, or will they add up to more than, or less than, 180°? 25. If a living observer could have been there to observe the universe only 300,000 years after the big bang, would they have seen anything? Explain. 26. Why couldn’t light travel through the early universe? 27. What does it mean to say that the universe is “flat”? Dark Matter 28. What evidence is there that our Milky Way galaxy might contain “dark” matter? 29. According to current theories, is there dark matter in your room? 30. Why can’t you see the dark matter that is in your room? 31. Since dark matter is invisible, what leads us to think it might exist? 32. Why would the laboratory discovery of dark matter be momentous? 33. Does dark matter interact by means of the gravitational force? How do we know? The Accelerating Universe and Dark Energy 34. Since the universe is accelerating as it expands, is there any doubt among cosmologists that the fate of the universe is to expand forever? Explain. 35. Cosmologists did not expect to find that the universe is accelerating. What did they expect? 36. Why are type 1a supernovas such good markers for determining the rate of expansion of the universe? 37. What is the universe mostly made of? 38. Why do we think there is dark energy? 39. Suppose that a certain galaxy, galaxy X, is so far from our galaxy that the expansion of the universe causes it to move away from our galaxy at half of lightspeed. Does this mean that galaxy X is moving through space at half of lightspeed? Explain. Cosmic Inflation, and a Brief History of the Universe 40. In what ways was the emergence of the four different fundamental forces during the big bang similar to the change of the state of water from liquid to solid? 41. If the theory of cosmic inflation is correct, then what is undoubtedly nature’s single most important example of quantum uncertainties? 42. How could the large-scale structure we see in the universe today have originated from tiny quantum fluctuations? 43. Is the total gravitational energy of an isolated star positive, or negative, or zero? Defend your answer. 44. A huge amount of energy was needed to create all the matter and all the motion that we see in today’s universe. According to the cosmic inflation hypothesis, where did it come from? 45. Cosmic inflation might sound far-fetched, but there is some evidence for it. Describe two pieces of evidence.