1. Different stuff dumped and/or the WD itself is different in the past => the resulting SNe could be different
So distant SNeIa could be different from nearby ones in terms of brightness!

2. What about dust?
Normal dust scatters blue light more than red: why sunsets are red
Look at color of the SNe Ia
They have the same color =>
no ordinary dust (what we see in our galaxy)
What if weird dust that only dims SNe Ia?
Further away, more dust in the way => more distant SNe would appear more dimmer but not redder

3. So we could be fooled, but
There is no direct evidence for this dust

4. Summary:
The two “worries” (evolution and dust) regarding SNe Ia
=>We need to make lots more observations and cross check with other methods (CMB and Clusters) =>

5. How often does a SN go off?
About 1/100 years per galaxy => need to look at lots of galaxies
Took 10 years to get 40 measurements Want 10 times more to “nail things down”=> you need to be very young or get clever! (look up )

8. Next Up: Clusters of Galaxies

9. Core of Coma Cluster, Enlarged
7 million lt yrs
Coma Cluster

10. A cluster of galaxies is the largest self-gravitating system known in the Universe
A cluster contains about 1000 galaxies that “swarm” around a gravitational center at 1000 km/sec!
The diameters are = 3 million light years
This background is part of the Coma cluster
These are galaxies
This is a background cluster at 1+z = 1.5!

11. How to we measure mass? But why?
We know how many clusters there are per unit volume
Measure of the mass of clusters
lower limit to Wm (galaxies outside clusters are negligible)
Also use evolution of clusters with time to derive Wm

12. Now, how do we measure the mass?
Galaxy motions
Hot gas atmosphere
Gravitational lens
First method like measuring the mass of the earth by the motion of the Moon
Second method like using the temperature and mass of the gas atoms in the Earth’s atmosphere to derive mass of Earth

13. First Method:
Or, centrifugal force = gravitational force
Assume we have circular motion, then mv2/R = GMm/R2
If M >> m this works
Little m’s cancel
measure v and R, then derive M

14. Replace the M by the mass of the entire Cluster
Measure the velocity of galaxies
Assume orbits are circular

15. 3 billion years for an orbit =>
Look at many galaxies
To derive average circular motion
Derive and average orbit =>
Derive about 1015 solar masses for cluster!
But in galaxies total see about 3 x 1013
=> missing mass, dark matter