PHYSICS 1040 - ELEMENTARY ASTRONOMY - HOMEWORK #14
1. A _______________ (quasi-stellar object) is a star-like object that emits huge amounts of energy from a very small region of space. Observations with the Hubble Space Telescope have shown that a _______________ is the extremely active nucleus of a very distant galaxy. The typical _______________ time for these objects is about 10 billion years, so these energetic galactic nuclei must be very old/young (circle one). The are probably powered by a _______________ black hole at their center.
2. Olbers' paradox asks, "Why is the sky dark at night?" If the universe is infinite in extent and has existed forever, then you should see a star in any direction you look in the sky. This means that the night sky should be ___________ instead of dark! The solution to Olbers' paradox is that the universe has not existed forever; it was created about _____ billion years ago in the __________ ___________. The light from any object more than _____ billion light years away has not had time to reach us, and so the sky looks __________ at night!
3. The scale below shows the distance from the Earth (E) to two galaxies G
1 and G2. On the second scale, draw where the two galaxies will be by the time the universe has expanded to twice its present size.
During this expansion, galaxy G1 has moved a distance of _____ Mpc and galaxy G2 has moved _____ Mpc. This means that galaxy G2, which is twice as far away from the Earth as galaxy G1, is moving away __________ as fast as galaxy G1.
4. The Big Bang was not an explosion of matter into empty space, like the explosion of a bomb. Instead, the Big Bang was an explosion of both __________ and __________, and the resulting expansion of the universe is still going on today. Imagine baking a loaf of raisin bread. As the bread rises in the oven, all of the raisins move away from each other, just as all of the galaxies in the universe are moving __________ from each other. But the raisins are not moving through the dough; the dough is expanding with the raisins. In the same way, galaxies are not moving through space. Instead, they are being carried along with the expanding space in a motion called the __________ __________. The actual motion of a galaxy through space is a departure from the __________ __________, called the galaxy=s _______________ motion.
5. The _______________ _______________ describes the effect of __________ energy on the universe. __________ energy fills all of space, causing a repulsive force between any two points in space. For two points that are close together, the effect is too small to notice. However, if the two points are twice as far apart, the repulsive force between them is __________ as great. In this way __________ energy is causing the expansion of the universe to _______________. In fact, within 1015 years, even the nearest galaxies will disappear from view as the accelerating expansion of the universe carries all galaxies away from us faster than the light from those galaxies can approach us!
6. There are three possibilities for the overall geometry of space. Suppose two laser beams start out moving parallel to each other.
a. In a universe where space has __________ curvature, the laser beams will always remain parallel to each other. This is called a __________ universe.
b. In a universe where space has _______________ curvature, the laser beams will gradually move toward each other until the beams cross. This is called a __________ universe.
c. In a universe where space has _______________ curvature, the laser beams will gradually move away from each other. This is called an __________ universe.
7. The _______________ _______________ is the value of the average density of the universe that is required for the geometry of the universe to be flat. If H0 = 71 km/sec/Mpc (million parsecs), then this value is about 9.4 x 10-30 g/cm3 (equivalent to roughly six hydrogen atoms for every cubic meter of space). This value of the _______________ _______________ is compared with the sum of three measured densities to determine the overall geometry of the universe:
a. the average density of __________ in the universe;
b. the average density of __________ matter in the universe; and
c. the average density of __________ energy (divided by c2) in the universe.
The sum of these three densities is the average density of the universe. The average density divided by the critical density is represented by the Greek letter Aomega@ (Ω):
There are three possibilities for the value of Ω.
Ω = 1: the average density of the universe is equal to the critical density, and the universe is __________d.
e.
Ω > 1: the average density of the universe is greater than the critical density, and the universe is __________f.
Ω < 1: the average density of the universe is less than the critical density, and the universe is __________
8. The _____________ _______________ _______________ (CMB) is blackbody radiation that is a remnant of the __________ __________, the fiery explosion of space at the beginning of time that gave birth to a hot, dense universe about _____ billion years ago. The universe has been expanding and cooling off ever since, and is now at a temperature of _____ K. According to Wien's Law (see Question #4 on Homework #3), any object of this temperature emits most of its energy in the _______________ part of the electromagnetic spectrum.
9. The overall geometry of the universe can be determined by observing the characteristic size of hot and cold spots in the cosmic microwave background. If our universe is flat, then the images of the cosmic microwave background will be dominated by hot and cold spots of around 1 degree in size (above center). If, on the other hand, the universe is closed or open, then the bending of light by the curvature of space will distort the images. If the universe is closed so parallel lines converge, then the images will be magnified by this curvature, and hot and cold spots will appear larger than 1 degree on the sky (above left). Conversely, if the universe is open so parallel lines diverge, then hot and cold spots will appear smaller than 1 degree on the sky (above right). A comparison of these computer simulations with the actual observation of the cosmic microwave background (top) indicates that space is very nearly __________, so
Ω = 1.
10. In February 2003 the results of a new high-resolution survey of the cosmic microwave background were announced. The results from the Wilkinson Microwave Anisotropy Probe (WMAP) showed that:
a. the universe is _____ billion years old (accurate to 1 percent), and that the universe is composed of
b. _____ percent dark energy,
c. _____ percent dark matter,
d. and only _____ percent atoms.
The WMAP value of the Hubble constant is
e. _____ km/sec/Mpc (accurate to 5 percent).
These values indicate that the universe will expand forever.