ASTRO/PHYS 1040: Elementary Astronomy, Dr. Sohl, Homework Chapters 13-14
Due at start of class
score: ___/41 Name:
Name:
KEY
Name:
Part 1. Definitions and vocabulary.
Match the following words with the definitions below.
antimatter
accelerating universe
Big Bang theory
Big Crunch
closed universe
coasting universe
cosmic microwave background (CMB)
critical universe
dark energy
dark matter
electroweak era
era of atoms
era of galaxies
era of nuclei
era of nucleosynthesis
flat universe
gravitational lens
GUT era
inflation
large-scale structures
MACHOs
open universe
particle era
Planck era
recollapsing universe
rotation curve
WIMPs
1. An electron is a particle of matter, whereas a positron is a particle of ____.
1. antimatter
2. Current theories don’t work this early in the Universe’s history, when all forces were one. 2. Planck era
3. The Universe cools a bit; gravity condenses out from the Grand Unified Force at this time. 3. GUT era
4. The Universe cools some more and the strong force is condensed out during this time.
4. electroweak era
5. During this time photons were spontaneously producing protons and antiprotons.
5. particle era
6. After most antimatter was destroyed, some remaining protons formed nuclei at this time.
6. era of nucleosynthesis
7. During this era the cooling Universe’s atoms were still too hot to stay together.
7. era of nuclei
8. Electrons stayed with protons, atoms became stable, and photons moved easily at this time.8. era of atoms
9. At about 1 billion years old the Universe cooled enough that large structures formed.
9. era of galaxies
10. At the start of the era of atoms, photons could move freely. We see this today as the ___. 10. CMB
11. At 10-38 seconds, this event probably occurred, eventually leading to galaxy formation.
11. inflation
12. We cannot see the majority of the Universe. But we measure its gravity. This is called __. 12. dark matter
13. A plot of measurements of orbital velocity vs. distance from an object.
13. rotation curve
14. Gravity from a galaxy cluster can bend light around itself, distorting objects behind it.
14. gravitational lens
15. Dark matter might be big objects like faint stars and brown dwarfs.
15. MACHOs
16. Dark matter might be subatomic particles that don’t interact strongly with anything.
16. WIMPs
17. On huge scales the Universe is not random but has vast walls and voids of galaxies.
17. large-scale structures
18. Recent discoveries indicate that this is overpowering gravity and speeding up expansion. 18. dark energy
19. With no dark energy and enough gravitational mass, the recollapsing universe will end as 19. a Big Crunch
20. A recollapsing universe is also known as a __.
20. closed universe
21. Without dark energy and with too little mass to recollapse, our universe will be a __.
21. coasting universe
22. Our best current evidence indicates that there is a repulsive dark energy making us an __. 22. accelerating universe
Part 2. Questions (19 points total, 2 points each, except problem 5. Write clearly, if I cannot read it, it is wrong.)
1 Name the four fundamental forces of nature known today.
i _____gravity_________________________
ii ______electromagnetic __________
iii ____strong force ____________________
iv ____weak force________________
2. The Universe started out very hot but has cooled. What is the current average temperature of the Universe? 2. __2.73 K _
3. What happens when a proton collides with an antiproton?
They annihilate themselves into pure energy by converting into two photons.
4. What kinds of particles were present during the era of particles?
Approximately equal amounts of matter and antimatter in the form of protons and antiprotons. There were also a large
number of quarks.
5. (3 points) Evidence seems to indicate that the Universe underwent a brief period of enormous inflation. What three major
things does this help to explain?
1. The density enhancements that led to galaxy formation.
2. The smoothness of the cosmic microwave background.
3. The flat geometry of the observable universe.
6. The flat part of the galaxy’s rotation curve tells us that stars in the outskirts of our galaxy are doing something that we
can’t explain without adding in dark matter. What are these stars doing?
Outer stars are orbiting around the galaxy just as fast as stars are revolving that are closer to the center of the galaxy.
7. We cannot see dark matter. How do we infer that it exists?
We observe it gravitational effects on matter that we can see. That is, we see it’s gravitational influence on visible matter.
8. What is the role of dark matter in structure formation?
The gravity from dark matter was strong enough to stop the expansion of the universe in large regions and to pull gas
together into protogalactic clouds. This also appears to be responsible for arranging galaxies into large-scale structures such
as gigantic chains, walls and sheets that surround vast voids.
9. What is the evidence for dark energy?
Observations of distant supernovae indicate that the universe was expanding slower than we expected and that the expansion
seems to be accelerating. We know not what this force is that is causing the universe to speed up. It takes energy to cause a
force and we cannot attribute this energy to anything that we can observe (i.e., visible, light-emitting objects) and so we call
it “dark energy.”