Lecture 32 Flashcards
What is the current Hubble constant?
70 km/s/Mpc
What is the best way to think of the expansion of the universe?
It is best to think of the expansion as a big stretch with the fabric of space between the galaxies stretching and carrying the galaxies away from one another
Why do galaxies themselves not expand?
They are held together by gravity
How does overlaying dots show that we are not at the centre of the universe?
If the dots are overlayed over other dots every galaxy appears to be moving away from every other galaxy
What happens to light in the expanding universe?
Cosmological redshift occurs as the the light experiences the stretch of the expanding universe
What is redshift?
How much longer a light’s wavelength is now compared to when it was emitted
What is the redshift formula?
1+ z= λnow/λthen
Why is Andromeda blueshifted?
Andromeda is blueshifted as it is falling towards us (v=-300 km/s). This is due to the two galaxies overcoming Hubble expansion so that space is not expanding locally.
Why is the Virgo Cluster redshifted even though the Milky Way is falling towards it?
The gravity of the Virgo Cluster is pulling the Milky Way towards it against the expansion of spacetime. The Milky Way will never reach Virgo as space is expanding faster than the galaxy falling.
What are the two types of redshift?
- peculiar redshift
- cosmological redshift
What is peculiar redshift?
Doppler shift from galaxy motion (gravity dominated on the scale of galaxy groups and clusters (few Mpc)).
What is cosmological redshift?
Redshift that is caused by expanding space (Hubble Law v=Hxd).
Hubble expansion dominates between objects on scales larger than a few Mpc.
What is the dimension of Ho?
1/time
What is the formula for the age of the universe (“Hubble Time”)?
age of universe= 1/Ho
How does distance work in a stationary universe?
If galaxies were 400 Mly apart when the light left, they would be 400 Mly apart when light arrived. Therefore, the light travelled 400 Mly.
