Cosmology
Key Points
The speed of light is finite, so the farther away something is, the earlier in the history of the universe we are seeing it.
At really great distances, you are looking at such early times that nothing has had a chance to form and you see nothing.
The observations from the greatest distance to the closest observations act like a time machine so that we can see a cumulative history of the universe.
Distance measures:
- Radar ranging: works in the solar system (10-4 light years)
- parallax: works for nearby stars (102 light years)
- Sketchier methods (but it’s all we have!) that depend upon standard luminosities to which we can compare the apparent magnitude to find distance:
- Main sequence fitting: works out to the Magellenic clouds (105 light years)
- Cepheid variable stars: Used for nearby galaxies where we can resolve individual stars (107 light years)
- Tully-Fisher Relationship: works to 1010 light years (the farthest we can see); measures rotational velocities to tell us about brightness
- Type Ia supernovae
Cosmological redshift (as shown in the Hubble Diagram--velocity vs. distance) tells us that things are moving away from us and the universe is expanding uniformly in all directions.
Cosmological redshift differs from normal Doppler shift in that it is the space expanding rather than a particular object moving away from us.
The Hubble constant tells us the velocity of the expansion of the universe.
Expansion of the universe means that the universe must have been much smaller in the past, so small that it would have been a singularity.
Question
Marcel uses main sequence fitting to measure the distance to galaxy Greg. He finds that the distance is 1010 light years. Edwin Hubble III (the original Hubble’s great-great-grandson) says that Marcel’s distance is wrong.
- What is Hubble’s basis for doubting Marcel’s measurement?
- What is a better way to measure the distance for something 1010 light years away?