Chapter 20
Cosmology
Hubble Ultra Deep Field
Galaxies and Cosmology
• A galaxy’s age, its
distance, and the age
of the universe are all
closely related
• Galaxies formed
when the universe
was young and have
aged along with the
universe
Parallax
Measure the
distances to
nearby stars
Star clusters
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Brightness =
Luminosity
4π (distance)2
Properties you can directly observe and measure:
• Brightness
• Change in brightness over time
• Color
• Rotation speed
A standard candle is an object whose luminosity we
can determine without measuring its distance
Cepheid variable stars
are very luminous
standard candles
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White-dwarf
supernovae
all have
same peak
luminosity:
standard
candles
Can be seen up to 10 billion light years away!
Tully-Fisher
Relation
Entire galaxies
can also be
used as
standard
candles: faster
rotation =
greater total
luminosity
Giant ellipticals:
if you’ve seen one,
you’ve seen them all…
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Homework assignment
Hubble measured the distance to nearby galaxies using
Cepheid variables as standard candles (1927, Mt Wilson Obs)
Hubble found that the spectral features of virtually all
galaxies are redshifted  They’re all moving away from us
Hubble found that the further away a galaxy is, the
faster it is receding from us!
Slope = y / x
=
velocity
distance
=
1
time
Time = age of the
universe!
Hubble’s Law:
velocity = H0 x distance
Distances of
farthest
galaxies are
now
measured
from their
redshifts!!
A balloon’s surface expands but has no center or edge
Cosmological Principle
The universe looks about the same no matter
where you are within it
• Matter is evenly distributed on very large scales
in the universe
• No center & no edges
• Not proved but consistent with all observations
and predictions of the Big bang theory
Distances between
faraway galaxies
changes because
the space between
them expands!
distance?
Think of lookback
time rather than
distance
Redshift is NOT
the Doppler shift!
Expansion stretches photon wavelengths causing a
cosmological redshift directly related to lookback time
observations
show us very
distant
galaxies as
they appeared
a long time
ago
(Old light
from young
galaxies)
Galaxies of different ages look different from one another
Collisions play an important role in galaxy evolution
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Collisions were much more common when U. was young,
because galaxies were closer together
Many of the galaxies we see at great distances (when U.
was young) look violently disturbed
Giant elliptical
galaxies at the
centers of
clusters seem
to have
consumed a
number of
smaller
galaxies
Collisions
may explain
why giant
elliptical
galaxies tend
to be found
where
galaxies are
closer
together
Quasars are the
most luminous
galaxies
• The highly redshifted spectra of quasars indicate large
distances
• Redshift --> distance --> luminosities of some quasars are
>1012 LSun
• Variability shows that all this energy comes from region
smaller than solar system: active nucleus with
supermassive black hole!!
Galaxies
around
quasars often
appear
disturbed by
collisions
Dark Matter, Dark Energy, and
the Fate of the Universe
Mass within Sun’s
orbit:
1011 MSun
Observable stars
and gas clouds:
~few 109 MSun
Dark matter and dark energy
Dark Matter: An undetected form of mass that emits little or
no photons, but we know it must exist because we observe
the effects of its gravity
Dark Energy: An unknown form of energy that is causing
the universe to expand faster over time
What is the Universe made of?
• “Normal” Matter:
~ 4.4%
– Normal Matter inside stars:
– Normal Matter outside stars:
• Dark Matter:
• Dark Energy
~ 0.6%
~ 3.8%
~ 25%
~ 71%
Rotation curve for the solar system
12
orbital speed
10
8
6
Series
4
2
0
0
10
20
30
distance from sun
40
50
Spiral galaxies all tend to have flat rotation curves
indicating large amounts of dark matter
The visible
portion of a
galaxy lies
deep in the
heart of a
large halo of
dark matter
measure the
velocities of
galaxies in a
cluster from
their Doppler
shifts
Mass is 50 x
larger than
the mass in
stars!
Clusters contain
large amounts hot
gas: emits x rays
Temperature of hot
gas tells us cluster
mass:
85% dark matter
13% hot gas
2% stars
Gravitational lensing of background galaxies also tells us
the mass
What is dark matter made of?
• Ordinary Dark Matter (MACHOS)
– Massive Compact Halo Objects:
dead or failed stars in halos of galaxies
• Extraordinary Dark Matter (WIMPS)
– Weakly Interacting Massive Particles:
mysterious neutrino-like particles
Two Basic Options
• Ordinary Dark Matter (MACHOS)
– Massive Compact Halo Objects:
dead or failed stars in halos of galaxies
• Extraordinary Dark Matter (WIMPS)
– Weakly Interacting Massive Particles:
mysterious neutrino-like particles
The
Best
Bet
MACHOs do not
cause enough
lensing events to
explain all the
dark matter
Why Believe in WIMPs?
• There’s not enough ordinary matter
• WIMPs could be left over from Big Bang
• Models involving WIMPs explain how galaxy
formation works

Gravity of dark matter is what caused protogalactic clouds
to contract early in time
WIMPs don’t
contract to
center because
they don’t emit
photons, so
they can not
radiate away
their orbital
energy
Maps of galaxy positions reveal extremely large
structures: superclusters and voids
WIMP models agree better with observations
Fate of
universe
depends
on the
amount
of dark
matter
Lots of
dark matter
Critical
density of
matter
Not enough
dark matter
Amount of dark matter
is ~25% of the critical
density suggesting fate
is eternal expansion
Not enough
dark matter
But expansion
appears to be
speeding up!
Dark
Energy?
Not enough
dark matter
Brightness of distant white-dwarf supernovae tells us how
much universe has expanded since they exploded
Accelerating universe is best fit to supernova data