# ASTR 123

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Astronomers believe that nearly all galaxies are moving away from us because:
The space in between the galaxies is expanding
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Current estimates place the value of H0 near 22km/s/Mly giving 14 billion years as the age of the universe. If H0 were in fact 44km/s/Mly, the approximate age of the universe would be:
Half as much
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Suppose Hubble's constant is H0 = 22km/s/Mly. Then we would expect a galaxy located 10 million light years away to be moving away from us at a speed of about:
220km/s
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An astronomer measures the velocities of two galaxies, galaxy A and galaxy B. He finds that galaxy A is moving at twice as fast away from us as galaxy B. Assuming these galaxies are moving because the expansion of space, which statement is true?
Galaxy A is twice as far as galaxy B.
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Suppose we lived in a Universe that was shrinking rather then expanding. If you measured the velocities of many galaxies, what would you find?
All galaxies are moving toward you.
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From our vantage point in the Milky Way, we see distant galaxies all moving away from us, and more distant ones moving faster. Suppose you lived in a distant galaxy, what would you observe?
The same rule that we observe: all galaxies moving away from you, and more distant ones moving the fastest.
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One reason Hubble's constant isn't exactly know today is :
The exact distance of a galaxy is hard to measure.
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Why do you think we observe a dark space between the galaxies?
The universe has an infinite age (about 14 billion years).
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Which of the following statements best describes what astronomers mean when they say that the universe is expanding?
The average distance between galaxies is increasing with time.
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Note that an observer located at the Local Raisin would see Raisins 1, 2, and 3 all move away from her during the video. What would an observer located at Raisin 2 see?
Raisin 1 and Raisin 3 both move away from her.
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The table in the video shows you the speeds of Raisins 1, 2, and 3 as measured from the Local Raisin. Suppose instead that you measured speeds as seen from Raisin 2. An observer at Raisin 2 would measure __________.
Local Raisin speed = 4.0 cm/hr; Raisin 1 speed = 2.0 cm/hr; Raisin 3 speed = 2.0 cm/hr
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The following statements describe ways in which the analogy might apply to the real universe. Which statements are correct?
--> An observer at any raisin sees more distant raisins moving away faster, just as an observer in any galaxy sees more distant galaxies moving away faster. --> The raisins stay roughly the same size as the cake expands, just as galaxies stay roughly the same size as the universe expands. --> The average distance increases with time both between raisins in the cake and between galaxies in the universe.
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Based on what you've learned from the raisin cake analogy, which two properties of distant galaxies do astronomers have to measure to show that we live in an expanding universe?
their distances and speeds
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Today, the evidence that we live in an expanding universe is extremely strong because astronomers have measured the motions of millions of galaxies. Nevertheless, in science, we must always remain open to the possibility that some future observation could call even our most strongly supported theories into question. Which of the following hypothetical observations would not be consistent with what we expect in an expanding universe?
You discover an extremely distant galaxy that is moving toward us.
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The following figures give the approximate distances of five galaxies from Earth. Rank the galaxies based on the speed with which each should be moving away from Earth due to the expansion of the universe, from fastest to slowest.
(Fastest) -5 billion light-years -2 billion light-years -800 million light-years -230 million light-years -70 million light-years (Slowest)
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The following figures give the approximate speeds at which five galaxies are moving away from Earth due to the expansion of the universe. Rank the galaxies based on their distance from Earth, from farthest to closest.
(Farthest) -130,000 km/s -45,000 km/s -18,730 km/s -5,264 km/s -1,577 km/s (Closest)
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The following figures give the approximate speeds at which five galaxies are moving away from Earth due to the expansion of the universe. Rank the galaxies based on the amount of redshift that would be observed in each galaxy's spectrum, from largest to smallest.
(Largest redshift) -130,000 km/s -45,000 km/s -18,730 km/s -5,264 km/s -1,577 km/s (Smallest redshift)
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Hubble's law is a relationship between galaxy speeds and galaxy distances. This relationship can be shown as a line on a graph of speed versus distance. Each of the following four graphs shows a possible relationship expressing Hubble's law. Rank the graphs based on their predictions of the speed, from fastest to slowest, for a galaxy located 400 million light-years away from Earth.
(Fastest) -Speed away from Earth (km/s) 30000, Distance from Earth (millions of light-years) 400 -Speed away from Earth (km/s) 30000, Distance from Earth (millions of light-years) 600 -Speed away from Earth (km/s) 20000, Distance from Earth (millions of light-years) 1000 -Speed away from Earth (km/s) 15000, Distance from Earth (millions of light-years) 1200 (Slowest)
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The following four graphs are the same as those from Part D, with each showing a possible relationship for Hubble's law. Rank the graphs based on the prediction they each would make for the current age of the universe, from oldest to youngest.
Oldest universe) -Speed away from Earth (km/s) 15000, Distance from Earth (millions of light-years) 1200 -Speed away from Earth (km/s) 20000, Distance from Earth (millions of light-years) 1000 -Speed away from Earth (km/s) 30000, Distance from Earth (millions of light-years) 600 -Speed away from Earth (km/s) 30000, Distance from Earth (millions of light-years) 400 (Youngest universe)
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When we observe a distant galaxy whose photons have traveled for 10 billion years before reaching Earth, we are seeing that galaxy as it was when the universe was
4 billion years old
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I observe a galaxy that is 100 million light years away. Which of the following do I see?
the light from the galaxy as it was 100 million years ago, and it is redshifted.
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The horizontal axis of the graph measures the time since the Big Bang in seconds. The extreme left of the horizontal axis, where it meets the vertical axis, is labeled 10−45 seconds (that is a decimal point followed by 44 zeroes and then a one). If we move along two tick marks to the right on the horizontal axis, it is labeled 10−35 seconds. By what factor does the time increase from one tick mark on the horizontal axis to the next tick mark toward the right?
10^5
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The vertical axis of the graph measures the temperature of the universe in kelvin. If we move up one tick mark to the next, by what factor does the temperature increase?
10^2
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In basic terms, what does the graph show?
The universe is getting cooler with time.
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To get your bearings on the horizontal time axis of this plot, it is useful to calculate the age of the universe in seconds today. If you multiply the age of the universe in years by the number of seconds in a year, approximately what is the current age of the universe in seconds?
4x10^17
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Suppose you want to know what the temperature of the universe was 1 billion years ago. Where along the horizontal axis should you look?
almost all the way to the far right
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What was the approximate temperature of the universe when the universe was just 1 second old?
10^10
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From Part C, you know that in basic terms, the graph shows that the temperature of the universe decreases with time. In more specific terms, this graph shows that the temperature of the universe __________.
fell very rapidly when the universe was young, but is changing very gradually today.
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What happens to gas when it freely expands?
It gets less dense and cools.
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The four fundamental forces that operate in the universe today are ________.
strong force, weak force, electromagnetic force, gravity
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The electromagnetic force is stronger than gravity. Why is it over-powered by gravity on large scales?
Most objects are electrically neutral.
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According to the Big Bang theory, why do we live in a universe that is made of almost entirely of matter rather than antimatter?
During the first 0.001 second after the Big Bang, particles and antiparticles were made in almost but not perfectly equal numbers. Everything annihilated except the very slight excess of matter particles.
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What kind of particle does the Large Hadron Collider accelerate?
Protons
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The Large Hadronic Collider set a new world record in December, 2009. What was the record for?
The highest ever kinetic energy of a proton colliding in a human-made particle accelerator
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What famous (or infamous) United States collider, set to be built in Texas, was cancelled in 1993?