Lecture 1: Intro Flashcards
cosmological principle
universe homogeneous & isotropic on scales approx >100 Mpc
big bang model
best description of our universe so far
cos principle tells us to use homogeneity/isotropy. These allow us to
define mass density p as uniform
newtonian gravity tells us a small test particle is subject only to
the effect of the mass at smaller radii
U=
kinetic + potential
T+V
co-moving coordinates - static objects remain
fixed at locations in the x coord system
cosmologists often (sloppily) use
the scale factor (dimensionless) interchangeably with physical distance
we can describe the time evolution of the universe without recourse to
physical dimensions
we can fix a specific area ‘now’ and describe past/future universes
in terms of the scale factor being a multiple/fraction of value ‘now’
freidmann equation
start from energy equation and sub int eh scale factor
k in FE
constant, independent of space and time
tells us of geometry of the universe (curvature)
mathematically, it can be 0, -1 or +1
fluid equation
apply 1st law of thermodynamics to a volume V
differentiate E and V wrt t adn set dS=0 for reversible adiabatic process
from fluid equation: the change in density is the result of the two terms on the RHS, we have
- increase in volume and due to mass conservation, decrease in density
- changes in the internal energy of the fluid as the universe expands
fluid equation - only source of pressure is that
associated with the change in internal energy
a homogeneous universe has no pressure forces because there are no density gradients
fluid equation- if p=0 we have
non-relativistic matter - dust with no change in internal energy
fluid equation - for relativistic matter - radiation- we can have
p≠0 [more generally P=P(p)
