6. Measuring Cosmological Parameters Flashcards
What are standard candles?
Objets to compare the Luminosities and redshifts in order to measure the luminosity distance d_L
- Requires very high luminosities ( 10^9 solar) to get to z = 1
What are the two types of standard candles?
Cepheids and Type I a supernovae
What are cepheids?
Supergiant stars with average luminosities between 400-40000 solar luminosities
Why does the average luminosity of a cephied vary?
Due to pulsations
How can the luminosity of the cepheid be measured?
The period of the pulsations is correlated with the luminosity
- Measure flux to get d_L
- After 30 Mpc, need larger luminosities
When do we get a Type I a supernova?
When we have accretion from a donor onto a WD
- Above 1.4 solar masses, the gravity overcomes the electron degeneracy which supports the star and it collapsese
What is the rough luminosity of a Type I supernova?
3-5 x10^9 solar luminosities
What is the main cause behind the Type I supernovae being so luminous?
The increased density causes runaway fusion
- Lots of energy released in a very short space of time as the star collapses
Describe what luminosity relationship T I a supernovae and cepheids have
Cepheids - period luminosity relationship
T I a - shape luminosity relationship (Phillip’s relation)
Describe what the Phillip’s relation looks like on a graph
The supernovae that rise and fall in flux very rapidly are less luminous than the ones which are much brighter
- Negative parabolas skewed to the positive side stacked on top of each other
How many samples of standard candles do we need to measure expansion and acceleration?
Enough to have measurements to z ~ 1
When collecting standard candles for T I a, what was observed, and what conclusion was drawn?
They appeared fainter (more distant) than they should have in a flat matter universe
- Was deduced to be due to accelerating expansion
