Homework 6 Name: Due Date: June 9, 2008 1. Where in the universe does the general expansion occur? A) everywhere in the universe, including our local space upon Earth, the solar system, our galaxy and the space between galaxies B) only in galaxy-sized and larger structures, such as clusters of galaxies and the voids between them C) only in the space that separates clusters of galaxies D) only within individual galaxies, while the space between them and between clusters of galaxies remains static 2. Recent results from very bright supernovae in very distant galaxies seem to indicate that the expansion of the universe A) is accelerating (speeding up). B) is decelerating (slowing down). C) has now stopped and the universe will shortly begin to contract again toward a recollapse. D) is continuing at a constant rate and has done so since just after the Big Bang. 3. Which of the following statements correctly describes the universe for the entire first 380,000 years of its life? A) It was opaque to radiation. B) All of the fundamental forces of nature were unified into one force. C) It was a sea of gradually decreasing nuclear reactions. D) It was filled with free quarks (not confined inside neutrons or protons). 4. The universe began in the Big Bang. When did the first stars and galaxies begin to appear? A) not until about 3.2 billion years later B) immediately, as they were a direct product of the Big bang C) about 380,000 years later D) about 400 million years later 5. What significant event occurred 380,000 years after the start of the Big Bang? A) Electrons and nuclei combined to form neutral atoms. B) Quarks became confined in nuclei. C) The production of helium ceased. D) All of the galaxies we see today formed. 6. The degree of flatness of the universe, which determines whether we live in an open or a closed universe, has been determined recently by measuring A) the extent of the bending of light from distant galaxies, the so-called lensing. B) the average density of matter compared to the average density of radiation energy. C) the typical size of the hot spots in the structure of the cosmic microwave background. D) the hemispheric asymmetry in the temperature of the cosmic microwave background radiation. Page 1
7. We believe the cosmic singularity which began the universe occurred 13.7 billion years ago. This means that A) the present distance (comoving radial distance) to the most distant objects whose light reaches us is more than 13.7 billion light years. B) the universe is 13.7 billion light years in radius. C) the present distance (comoving radial distance) to the most distant objects whose light reaches us is 13.7 billion light years. D) all objects we can see are now less than 13.7 billion light years distant. 8. In cosmology, to what does the phrase critical density refer? A) the density of the universe above which the universe is bounded and below which it is unbounded B) the density of the universe below which the universe is bounded and above which it is unbounded C) the density of the universe above which matter is ionized and the universe is opaque D) the density of the universe below which the universe will stop expanding 9. What is it that keeps localized regions of space, such as things upon Earth, planetary systems, star clusters, and whole galaxies, from participating in the general expansion of the universe? A) the centrifugal force produced by their motion around a massive central object (e.g., the Sun, supermassive black holes, etc.) B) their locations in places where irregularities in the chaotic Big Bang explosion permitted matter to condense C) the mutual gravity between objects in these systems D) the powerful and all-pervading gravity from the central supermassive black holes of galaxies, which holds everything in place within the galaxies 10. Within which time frame from the initial Big Bang is it felt that the present laws of physics completely fail us, because time and space as we know them did not exist? A) t = 0 to 10 43 s, the Planck time, after which gravity froze out of the universe B) t = 0 to 1 second, during which time photons interchanged freely with electron-positron pairs C) t = 0 to 10 6 years, over which radiation dominated the universe D) t = 0 to 10 35 s, after which the strong nuclear force froze out of the universe 11. Is the universe really expanding? If so, how do we describe the expansion? A) An infinitely small but infinitely dense clump of matter exploded, sending the galaxies (or superclusters of galaxies) hurtling out through space. B) It is not expanding it is our local space that is getting smaller as we fall into a supermassive black hole, making the universe seem bigger and bigger. C) The energy from all the stars is heating the universe, making it expand like a gas that is heated. D) Space itself is expanding, carrying the galaxies (or superclusters of galaxies) with it. Page 2
12. The future of the overall universe, in terms of its ultimate evolution and whether it will expand forever or eventually contract again, is determined by which of its parameters? A) the present volume of the universe B) the intensity of cosmic microwave background radiation C) the average density of matter within it D) the temperature of the gas within it 13. How do we know that matter in the early universe was extremely smooth (i.e., not really lumpy)? A) because quasars are spread almost completely uniformly around the sky B) because the expansion of the universe is almost completely isotropic C) because at the present time galaxies are spread almost completely uniformly through the universe D) because the cosmic background radiation is almost completely isotropic 14. The cosmological redshift of the light from very distant galaxies is caused by A) absorption of blue light by interstellar dust between us and the galaxy, so that only the red wavelengths reach us. B) the Doppler shift, in which the photon's wavelength is stretched as the photon is being emitted by the galaxy's motion away from us through space. C) the expansion of space, stretching the photon's wavelength while the photon is traveling toward us. D) the rotation of the universe around its center (faster at greater distances from us). 15. What does Hubble's law give for the age of the universe? (H 0 = Hubble's constant, v is the recessional velocity of objects in the universe, and d = distance to objects in Mpc.) A) age = d/h 0 B) age = v/h 0 C) age = 1/H 0 D) age = H 0 16. What causes cosmological redshift of photons that reach us from distant galaxies? A) The photons were emitted from the galaxies much earlier in time when the overall temperature of matter was much lower. Hence, the observed photons are redder, the farther away from Earth that they were produced. B) The photons have traveled across space that has been expanding and their wavelengths have expanded with it, becoming redder. C) The photons were emitted by objects that were moving rapidly away from us, and thereby have been reddened by the Doppler effect. D) The photons have moved from high gravitational field regions toward lower fields, thereby becoming reddened. Page 3
17. Measurement of structure in the cosmic microwave background radiation has recently indicated that we live in a flat universe between a closed and an open universe and yet the measured density of detected matter and radiation is only 20 40% of the critical density required for a flat universe. In what form is the other 60 80% of the matter likely to be? A) dark energy, emitting no radiation and generating no detectable gravitational effects B) antimatter, which generates a negative gravitational effect and emits radiation only if it meets matter and is annihilated C) neutrinos, which have very little rest mass and are very difficult to detect, but are very abundant D) large numbers of small primordial black holes, whose gravitational effects are spread throughout the universe and which emit no radiation 18. The one physical force that extends farthest in our universe, and is not canceled out by other effects, is A) the weak nuclear force. B) the gravitational force. C) the electromagnetic force. D) the strong nuclear force. 19. What is the cosmic light horizon? A) It is the distance from which light can travel to us over the finite age of the universe, representing a viewing distance limit for us upon Earth. B) It is the distance beyond which we cannot see because of absorbing matter in the universe. C) It is the distance at which (because we see back in time as we look out into space) galaxies are just being formed. D) It is the maximum distance to which our own radio and television signals will have traveled through the universe since radio was invented. 20. When did the universe cool to a temperature of about 3 K? A) one second after the Big Bang, when electron-positron pair production ceased B) three minutes after the start of the Big Bang, when primordial nuclear reactions ended C) very recently D) 380,000 years after the Big Bang, when the universe became transparent to radiation 21. The resolution of Olber's paradox (i.e., the reason why the sky is dark at night) is that A) the light from very distant stars is bent out of our line of sight by the gravitational fields of nearby galaxies. B) the light from stars beyond a certain, very large distance is completely absorbed by matter between us and the star. C) matter cannot have traveled farther than light has traveled during the age of the universe, so there ARE NO stars beyond a certain distance from us. D) we cannot see those stars that are farther away from us than the distance that light has traveled since the beginning of the universe. Page 4
22) What is the Tully-Fisher relation? a) The longer the period of a galaxy's variable stars, the more luminous it is. b) The redder a galaxy's color, the further away it is. c) The smaller a galaxy appears, the further away it is. d) The faster a spiral galaxy's rotation speed, the more luminous it is. e) The further away a galaxy is, the faster its recession velocity. 23) What do scientists mean by the critical density of the universe? a) the minimum density that a universe needs in order to form galaxies b) the minimum density that a universe needs in order to form stars c) the minimum density that a universe needs in order to create hydrogen d) the precise density marking the dividing line between a universe that has enough mass to contract again and a universe that will continue to expand forever e) the average density of the space between galaxies 24) I observe a galaxy that is 100 million light years away: what do I see? a) the light from the galaxy as it is today, but it is blueshifted b) the light from the galaxy as it was 100 million years ago and it is redshifted c) the light from the galaxy as it is today, but it is redshifted d) the light from the galaxy as it was 100 million years ago and it it blueshifted e) Nothing: the galaxy lies beyond the cosmological horizon. Page 5