Download Part II, page 129 (instructions on page 127)

Part II, page 129 (instructions on page 127)
• Using Figure 2, fill in the blank entries in the lower right of Table 1: you’re measuring how far galaxies A and B moved between images II and III, divided by the time elapsed between images II and III.
• Then, using the bottom row of Table 1 (which you’ve just filled in), in Figure 3 plot the velocities of galaxies A and B as a function of their distance.
• Then answer questions 8D, 9, 10 (pages 129­130).
What is Hubble’s Law?
Distances of farthest galaxies are measured from redshifts
The spectral features of virtually all galaxies are redshifted ⇒ They’re all moving away from us
Hubble’s Law: velocity = H0 x distance
Two possible explanations for Hubble’s Law.
If we assume there is nothing special about the Earth, we can figure out which one is correct.
The universe has 3 space dimensions but no center and no edges (though there are limits to how far we can see). The universe expands in the time dimension.
Surface of a balloon expands but has no center or edge
A balloon universe has two space dimensions but no center or edge in space; it expands in the time dimension (animation)
If our universe had 2 space dimensions (not 3) plus 1 time dimension, it could be like the surface of a balloon that expands with time. The balloon's surface has 2 dimensions in which you can move, but no center or edge. What does the center of the balloon represent?
1. The physical center of the universe
2. The beginning of time
3. The end of time
How do distance measurements tell us the age of the universe?
Your friend leaves your house. She later calls you on her cell phone, saying that she’s been driving at 120 km an hour directly away from you the whole time and is now 120 km away. How long has she been gone?
1. 1 minute
2. 60 minutes
3. 120 minutes
You observe a galaxy moving away from you at 0.1 light­years per year, and it is now 1.25 billion light­years away from you. How long has it taken to get there?
1. 8 million years
2. 12.5 million years
3. 8 billion years
4. 12.5 billion years
Hubble’s constant tells us age of universe because it relates velocities and distances of all galaxies
Age = Distance
Velocity
~ 1 / H0
Hubble’s Law: velocity = H0 x distance
The distances between
widely separated galaxies will change while light travels between them.
distance?
lookback time
distance?
The distances between
widely separated galaxies will change while light travels between them.
Astronomers often think in terms of lookback time rather than distance.
The most distant galaxies we've found so far have a lookback time of about 13 billion years. That is, the light we see from them today took 13 billion years to reach us.
How far away are those galaxies today?
1. Less than 13 billion light years away
2. 13 billion light years away
3. More than 13 billion light years away
lookback time
distance?
The distances between
widely separated galaxies will change while light travels between them.
Astronomers often think in terms of lookback time rather than distance.
Cosmological Horizon
Maximum lookback time of 14 billion years limits how far we can see (everything we can see is our observable universe)
Expansion of the universe stretches photon wavelengths causing a cosmological redshift related to lookback time
What have we learned?
• How do distance measurements tell us the age of the universe?
• Combining distance measurements with velocity measurements tells us Hubble’s constant, and the inverse of Hubble’s constant tells us how long it would have taken the universe to reach its present size if the expansion rate had never changed. Based on Hubble’s constant and estimates of how it has changed with time, we now estimate the age of the universe at about 14 billion years, which restricts our view of the universe to lookback times smaller than that age. Activity 37, pages 131­133
• Remember: 1 parsec (pc) = 3.26 light­years, the distance light travels in 3.26 years
• 1 megaparsec (Mpc) = 1 million parsecs = 3.26 million light­years, the distance light travels in 3.26 million years