cepheids
- stars which brighten and dim periodically
- tight relation between period of oscillation and absolute luminosity
Ladder step 4: Nearby galaxies
Measuring cepheids period and observed brightness gives us distance
Ladder step 1: our solar system
- Radar ranging, i.e. sending a signal and seeing how long it takes to return
- d = 1/2 * (time-lag * Velocity)
Ladder step 2: nearby stars
parallax - d(parsecs)=1/angle(arcseconds)
only in our galaxy
Ladder step 3: distant stars
- Main sequence fit idea
- Current limit: Nearest galaxies [Need to identify individual stars within star clusters]
Ladder step 5: distant galaxies
Rotation rate of galaxy and observed brightness gives
us distance
What did Edwin Hubble show?
distance and velocity are linked
“The farther away a galaxy is, the faster it is moving away from us.” i.e universe expansion
Hubbles law
v = H × d
v - recession velocity (Doppler Shift) (km/sec)
H - hubbles constant (km/sec/Mpc)
d - distance (Mpc)
Redshift gives us?
distance
Star clusters
- Distribution not uniform in space
- stars and galaxies preferentially found near other stars
- Galaxy clustering induced by the gravitational attraction
Void size
grows overtime as matter is pulled away from underdense regions
Galaxy cluster mass
increases over time
Why don’t galaxies themselves expand?
- Gravity
- only the space between them is moving
Centre of the universe
- no galaxy is at the “center”;
- no galaxy at the edge
- Every galaxy sees every other galaxy moving away
How is light affected by universe expansion?
- light wave is being stretched as it travels through expanding universe
- cosmological redshift
Why does hubbles law produce an age of the universe that is slightly off?
assumed constant expansion
Lookback time
- how long light has taken to get to us today
- used as distance in an expanding universe requires time aswell
the big stretch
The fabric of space BETWEEN galaxies
is STRETCHING, carrying the galaxies
away from one another
universe growth and acceleration
-universe expansion is acclelerating
-but Size can grow with no acceleration (constant velocity
expansion)
What could change the expansion rate of the universe?
- Attractive force = deceleration Gravity pulls mass together
- Repulsive force = acceleration Something pushes spacetime apart, speeds expansion
Naive expectation is that the expansion is slowing down: WHY?
Gravity
Instead of decelerating, why is the universe is accelerating?
Dark Energy counteracts gravity
Until what point was universe expansion decelerating?
5 billion years ago
primary constituent of the Universe
dark energy (70%), dark matter (25%), ordinary matter (5%)
