Answers to Example Questions for the Final Exam


1. False. Globular clusters are the old metal poor clusters that are found throughout the halo of the galaxy. They were used by Shapley to pinpoint the center of the galaxy, because he assumed they were in a spherical distribution centered on the center of the galaxy.

5. c. Although the exact distance is not well know, it is in the range 8-10 kpc (24-30 kly). It is certainly not any of the other values which are way off.

3. False. Remember, dust in the disk of our galaxy prevents us from seeing very far in the optical or ultraviolet spectral regions. So, despite Hubble being very useful in many areas of astronomy, it is no better than any other telescope at trying to see through the dust --- indeed the galaxy nucleus is very well hidden in the optical. Of course, infra red or radio telescopes CAN see to the center because the dust is less opaque to these longer electromagnetic waves.

4. c. Recall that orbits respond only to matter inside the orbit (the pull from matter outside, if spherically distributed, cancels itself out and adds nothing). Remember also that ALL matter contributes to the gravity --- not just stars.

5. False. Remember, it is simple unionized single hydrogen atoms that produce 21 cm wavelength radio emission, when the electron flips its spin direction. The hydrogen involved in fusion to make helium is ionized and has no simple bound electron. 21 cm emission is very useful for studying the galaxy since it passes through dust which blocks light

6. False. This is truely a hopeless statement. The more accepted picture of spiral arm creation is either (a) self-propagating star formation (to make patcy flocculent spirals) or (b) the density wave picture, in which the orbits of stars create a spiral pattern of enhanced density which leads to enhanced star formation (to make the so called grand design spirals).

7. a. Certainly, c is backwards (the bulge gets smaller along the spiral sequence), and there is no indication that the sizes of galaxies are themselves linked to spiral form.

8. False. Recall that stars are so tiny compared to their separation that the chance of any two colliding during a galaxy collision is quite small.

9. d. This is another example of the missing, or dark, matter. Although the mass in clusters includes the cluster atmosphere, which is about equal to the mass in stars in galaxies, there appears to be much more mass present which is not obviously visible. This is the dark matter, and may comprise 90% of the mass of the cluster.

10. The thin arcs are distorted images of very distant background galaxies, lensed by the foreground cluster. The mass of the cluster bends light and so the cluster can act like a lens, making (albeit rather distorted) images of more distant galaxies. Since the cluster is roughly spherical, the distorted images take on the shape of arc segments roughly concentric with the cluster center. The arcs allow astronomers to measure the strength of the lens, and hence the mass of the cluster. From this they discover that the mass of the cluster is dominated by dark matter.

11. 160 Mpc. Since V = Ho x d, then d = V/Ho = 12000/75 = 160 Mpc.

12. 50 km/s/Mpc (units are important here). From question 11, Ho = V/d

13. a. Since the Hubble expansion is such that galaxy redshifts are proportional to their distance, then it turns out that every galaxy, no matter where it is, sees the same expansion law.

14. 16.5 Gyr. For Ho = 60 km/s/Mpc, we need to ask how long it takes a galaxy to travel 1 Mpc moving at 60 km/s. Now, 1 Mpc is 106 x 3.08 x 1013 km = 3.08 x 1019 km. So this takes 3.08 x 1019/60 = 5.13 x 1017 seconds = 16.5 x 109 years (recall, 1 year = 31 million seconds).

15. False. Galaxies seen at large distances are also seen at large lookback times, and hence they appear YOUNGER (not older) than nearby galaxies.

16. b. At recombination, the temperature was about 3000 K, and this only seems to be 3K now due to the large redshift (or, alternatively stated, the photons have been stretched by the expansion of space). The other answers, a,c,d are all true.

17. The answer is similar to the previous question, and there is more than one way to look at this. As the universe expands, it cools. During the first 380,000 years it cools from extremely high temperatures down to about 3000K, whereupon the electons and protons combine to form atoms --- thus when the universe turns transparent and the photons no longer interact with the matter, the photons have a 3000K black body spectral distribution. But the universe continues to expand, and the photons get stretched by the expansion which increases their wavelength, giving a lower temperature black body spectrum. After an expansion factor of 1000, the temperature has dropped from 3000K to 3K, and we see these stretched photons all around us as the microwave background. Alternatively, we look so far away and so far back in time, to when the universe was 3000K hot, that the redshift is so great, that the optical glow from this heat is redshifted into the microwave part of the spectrum. Both explanations are correct and are different ways of looking at the same thing.

18. d. The critical density of the universe (about 9.5 x 10-27 kg/m3; or about 4 H atoms per cubic meter) is just sufficient to give a flat geometry to the space part of spacetime.

19. c. A negative curved geometry implies the universe is open with infinite future (density less than critical).

20. False. For any decelerating universe, the inverse Hubble constant (sometimes called the Hubble time) is LARGER than the true age of the universe, since the deceleration has slowed the recession, so if we naively use the current speed we overestimate the time it has taken to get to its current size.

21. One way to measure the geometry of space is to find a giant triangle which spans the universe and measure the sum of its internal angles -- do they equal 180 deg or not ? One such triangle has a base at the microwave background and two long sides with us at the vertex. We know the length of the long sides (our distance to the microwave background). The feature which makes the triangle's base is the wavelength of sound waves, the longest of which are simply the speed of sound times the age of the universe, which comes to 230,000 light years. Depending on the geometr of space, this length will appear larger or smaller on the sky. The fact that it is found to be about 1 degree shows us that the geometry is very close to being flat. Apparently, we live in a universe who's density is very close to the critical density.

22. c. The nucleosynthesis era spanned from a few seconds to about 3 minutes after the Big Bang, as the temperature dropped from about 3 billion degrees to 300 million degrees.

23. Strong, Weak, Electromagnetic, and Gravity. No, at very early times they were all thought to be a single force. As the temperature dropped, first Gravity then the Strong force separated out, then finally the Electroweak force split into the separate weak and electromagnetic forces. All this occurred well before the first microsecond.

20. False. Because matter and antimatter annihilate on contact to produce energetic gamma rays, we would see these gamma rays if any known objects were made of antimatter. Even intergalactic space is sufficiently dense that an antimatter galaxy moving through intergalactic space made of matter would produce visible gamma rays. We see no such gamma rays, and conclude that all of the visible universe is matter and not antimatter. It is presently thought that the origin of this matter/antimatter asymmetry in the universe started shortly after the big bang close to the inflationary era, when an important particle, the X boson, decayed with a slight preference to matter rather than antimatter. When, later, the matter and antimatter annihilated as the universe cooled, there was a slight excess of matter left over (about 1 part in a billion), and that is the matter which makes up all the current universe.