MULTIPLE CHOICE. Choose the one alternative that best completes the statement or answers the question.

Transcription

1 Phys333 practice problems for final exam Name MULTIPLE CHOICE. Choose the one alternative that best completes the statement or answers the question. 1) Suppose Hubble's constant H 0 = 65 km/s/mpc. How fast would a galaxy located 500 megaparsecs distant be receding from us? A) 65 Mpc/s B) 32,500 km/s C) 0.65 times the speed of light D) 65 km/s E) 9 km/s 2) What two quantities did Edwin Hubble plot against each other to discover the expansion of the Universe? A) luminosity and temperature B) velocity and distance C) age and distance D) luminosity and distance E) velocity and temperature 1) 2) 3) Why do we call dark matter "dark"? A) It emits no or very little radiation of any wavelength. B) It emits no visible light. C) We cannot detect the type of radiation that it emits. D) It blocks out the light of stars in a galaxy. 3) 4) Why should galaxy collisions have been more common in the past than they are today? A) Galaxies were closer together in the past because the universe was smaller. B) Galaxies were much bigger in the past since they had not contracted completely. C) Galaxy collisions shouldn't have been more common in the past than they are now. D) Galaxies were more active in the past and therefore would have collided with each other more frequently. E) Galaxies attracted each other more strongly in the past because they were more massive; they had not yet turned most of their mass into stars and light. 4) 5) What kinds of objects lie in the disk of our galaxy? A) old K and M stars B) open clusters C) O and B stars D) gas and dust E) all of the above 5) 1

2 6) How is the energy that powers radio galaxies, quasars, and other active galactic nuclei produced? A) by matter-antimatter annihilation near a central black hole B) by matter that has been converted to pure energy being shot out as jets by a central black hole C) by gravity, which converts potential energy of matter falling toward a central black hole into kinetic energy, which is then converted to thermal energy by collisions among the particles of matter D) by nuclear fusion near a central black hole E) by magnetic fields that trap and accelerate charged particles, which then radiate high amounts of energy 6) 7) How do we know that there is much more mass in the halo of our galaxy than in the disk? A) We don't know that there is more mass in the halo; it is only a guess based on theory. B) There are so many globular clusters in the halo that their total mass is greater than the mass of stars in the disk. C) The recent discovery of photinos, combined with theoretical predictions, tells us that there must be a huge mass of photinos in the halo. D) Stars in the outskirts of the Milky Way orbit the galaxy at much higher speeds than we would expect if all the mass were concentrated in the disk. E) Although the question of mass in the halo was long mysterious, we now know it exists because we see so many brown dwarfs in the halo. 7) 8) If there is no dark matter in the Milky Way Galaxy, what is the best alternative explanation for the observations? A) We are not measuring the distances to atomic clouds and stars properly. B) We are not observing all the visible or "bright" matter in the galaxy. C) Our understanding of gravity is not correct for galaxy-size scales. D) We are not attributing enough mass to the visible or "bright" matter. E) We are not measuring the orbital velocities of atomic clouds and stars properly. 8) 9) When does a protostar become a main-sequence star? A) when the protostar assembles from a molecular cloud B) when the rate of hydrogen fusion within the star's core is high enough to maintain gravitational equilibrium C) the instant when hydrogen fusion first begins in the star's core D) when hydrogen fusion is occurring throughout a star's interior E) when a star becomes luminous enough to emit thermal radiation 9) 10) How does the interstellar medium obscure our view of most of the galaxy? A) It reflects most light from far distances of the galaxy away from our line of sight. B) It absorbs visible, ultraviolet, and some infrared light. C) It produces so much visible light that it is opaque and blocks our view of anything beyond it. D) It absorbs all wavelengths of light. E) all of the above 10) 2

3 11) Why might inflation have occurred at the end of the GUT era? A) The universe was too small and needed to grow quickly. B) An enormous amount of energy was released when the strong force froze out from the GUT force. C) Gravity was an extremely weak force at this period in time. D) There wasn't enough matter present to slow down the expansion at that time. E) Large amounts of matter and antimatter annihilated at this time. 11) 12) What is the evidence for an accelerating universe? A) There is far more dark matter than visible matter in the universe. B) The Andromeda Galaxy is moving away from the Milky Way at an ever-increasing speed. C) White-dwarf supernovae are slightly dimmer than expected for a coasting universe. D) White-dwarf supernovae are slightly brighter than expected for a coasting universe. E) White-dwarf supernovae are the same brightness regardless of redshift. 12) 13) What prevents the pressure from increasing as a cloud contracts due to its gravity? A) The pressure is transferred from the center of the cloud to its outer edges where it can dissipate. B) Excess pressure is released in jets of material from the young stars. C) As the cloud becomes denser, gravity becomes stronger and overcomes the pressure buildup. D) Once the cloud reaches a critical density, the pressure becomes degenerate and independent of temperature. E) Thermal energy is converted to radiative energy via molecular collisions and released as photons. 13) 14) Which forces have physicists shown to be the same force under conditions of very high temperature or energy, as confirmed by experiments in particle accelerators? A) the electromagnetic and weak forces B) gravity and the weak force C) gravity and the strong force D) the strong and weak forces E) the strong and electromagnetic forces 14) 15) Why do we think the first generation of stars would be different from stars born today? A) Without heavy elements, the nuclear reactions at the center of the stars would be very different. B) There were no galaxies when the first stars were born. C) Without heavy elements, there was no dust in the clouds and they collapsed faster. D) Without heavy elements, the clouds could not reach as low a temperature as today and had to be more massive to collapse. E) The Universe was much denser when the first stars were born. 15) 16) Approximately how long did the era of nucleosynthesis last? A) 3 years B) second C) second D) 3 minutes E) 3 seconds 16) 3

4 17) Which of the following statements about the cosmic background radiation is not true? A) It had a much higher temperature in the past. B) It appears essentially the same in all directions (it is isotropic). C) It is the result of a mixture of radiation from many independent sources, such as stars and galaxies. D) It was discovered by Penzias and Wilson in the early 1960s. E) It has a temperature of about 3 degrees K above absolute zero. 17) 18) What is Einstein's cosmological constant? A) the size of the cosmological horizon B) the value of the acceleration of the universe C) a repulsive force that counteracts gravity and was introduced to allow for a static universe D) the value that measures the strength of gravity across the universe E) the value of the expansion rate of the universe 18) 19) Why is the era of nucleosynthesis so important in determining the chemical composition of the universe? A) All the elements except hydrogen were produced after the era of nucleosynthesis. B) We can observe spectra from this era to determine what the primordial mix of the elements was at the beginning of the universe. C) We can study the processes that occurred during the era of nucleosynthesis to determine how most of the elements in the universe were created. D) By knowing how much matter was created during the era of nucleosynthesis, we can determine whether the universe is open or closed. E) Except for the small amount of matter produced later by stars, the chemical composition of the universe is the same now as at the end of the era of nucleosynthesis. 19) 20) How many forces operated in the universe during the GUT era? A) two, gravity and the electroweak force B) two, gravity and the GUT force C) one, what we call the "super force" D) three, gravity, the strong force, and the electroweak force E) all of the above forces 20) 4

5 Answer Key Testname: FINAL-MC20 1) B 2) B 3) A 4) A 5) E 6) C 7) D 8) C 9) B 10) B 11) B 12) C 13) E 14) A 15) D 16) D 17) C 18) C 19) E 20) B 5