4. Cosmic Dynamics Flashcards
What is the co-moving coordinate?
R_s(t) = a(t) r_s
- Comoving radius of sphere r_s
- Comoving distance bewteen galaxies is fixed
What is the time derivative of the comoving coordinate, and why?
0 as the comoving distance between galaxies is fixed
What can we conclude if the time derivative of the scale factor is < or > 0?
< 0: Contracting sphere
> 0: Expanding sphere
How do open and closed universes evolve over time for a matter dominated universe?
(graph of a(t) vs t)
Open universes expand forever
Closed universes initially expand but then contract in a big crunch
What does the relativistic FR tell us about the curvature over time?
It cannot change
- If it can be measured we know the curvature
How does pressure effect the expansion of the universe?
It slows down expansion
Name particles that have a positive gas pressure
Photons, neutrinos and gases of baryons
- Slow expansion of Universe
From the acceleration equation, when will we have accelerating expansion?
If the energy density is > 0 and if P < -1/3 energy density
What is ω?
The constant relating the pressure to the energy density
What values of ω are of interest?
ω = 0: Pressureless. Universe contains matter
ω = 1/3: Universe contains radiation
ω < -1/3: Provides positive acceleration (observed)
What is the cosmological constant Λ?
When ω = -1 and P = - energy density
How are the density parameters related for a flat universe?
Ω_r + Ω_m + Ω_Λ = 1
Ω = density parameter = Energy density / critical Energy density
