Post on 30-Aug-2018
The Cosmic Distance Ladder
Each successive “rung”
must be calibrated against
the preceding “rungs”
RR Lyraes
&
Edwin Hubble &
the Great Debate
1920s—astronomers were
divided in 2 camps: those
who thought “spiral
nebulae” were in the Milky
Way, & those who thought
they were separate galaxies
Hubble resolved the debate
in 1924, using Cepheid
variables to prove M31 was
a separate galaxy
Hubble's Redshift Observations
Nearby galaxies with resolved stars: Hubble derived
distances using Cepheid variables
Distant galaxies, where stars are unresolved: Hubble used H & K spectral lines
What
does it
mean?
Cosmological Principle: our place
in the universe is not special.
If this is true, then…
1. The universe is expanding.
2. Therefore the universe used to
be smaller.
3. At some time t = 0, the
universe may have had no
physically meaningful size
(singularity)
Implications
Continued
The universe’s age is
finite (no word on its size)
The beginning of its
expansion is called “The
Big Bang” (a derisive
term that stuck)
The observable universe,
extrapolated forward
from the observed rates
of expansion, would
appear to be ~93 billion
LY across if light traveled
infinitely fast
Observable universe ≠
entire universe
Brief History of the Universe
IN 1998, WE FOUND THAT SINCE THE UNIVERSE WAS ~9 BILLION
YEARS OLD, ITS EXPANSION RATE HAS BEEN INCREASING.
DARK ENERGY WAS PUT FORTH AS AN EXPLANATION
Redshift (z) Formula:
Velocity in terms of Redshift:
Use c = 3 x 105 km/s (*Conversion factors don't
affect sig-figs)
Distance
m = Apparent magnitude of the galaxy (measured)
M = Absolute magnitude (assume -22 [exact])
d is in units of parsecs
Reminder (see also
Appendix A):
Hubble's Law (written 2 ways):
H0 has weird units: km/s/Mpc –
must convert d to Mpc!
Average all your H0 values &
compute the % discrepancy using
the following theoretical value:
You will also get an H0 value from the slope of the best-fit
line on your graph.
Compute the % discrepancy between the H0 from your
graph & the theoretical value above.
67.8 ± 0.8