Clicker Question:
What makes up most of the mass (90%)
of the Milky Way Galaxy?
A: hydrogen gas
B: stars
C: dead stars (white dwarfs, neutron stars, and black
holes)
D: we don’t know
Hubble’s Galaxy Classification
Hubble’s “tuning fork” is a convenient way to
remember the galaxy classifications, although it has
no deeper meaning.
Hubble’s Galaxy Classification
A summary of galaxy properties by type
The Distribution of Galaxies in Space
Cepheid variables allow measurement of galaxies
to about 25 Mpc away.
However, most galaxies are farther away then 25
Mpc. New distance measures are needed.
• Tully-Fisher relation correlates a galaxy’s
rotation speed (which can be measured using the
Doppler effect) to its luminosity.
• Type I supernovae all have about the same
luminosity, as the process by which they happen
doesn’t allow for much variation.
Tully-Fisher relation
The rotation of a galaxy results in Doppler
broadening of its spectral lines.
Local Group of Galaxies
Here is the distribution of
galaxies within about 1
Mpc of the Milky Way.
The local group of galaxies
There are three spirals in this group – the Milky Way,
Andromeda, and M33. These and their satellites –
about 45 galaxies in all – form the Local Group.
Such a group of galaxies, held together by its own
gravity, is called a galaxy cluster.
The Virgo Cluster
A nearby galaxy cluster is
the Virgo Cluster; it is
much larger than the Local
Group, containing about
3500 galaxies.
Hubble’s Law: Univeral Recession
Universal recession: All
galaxies (with a couple
of nearby exceptions,
I.E. the local group)
seem to be moving
away from us, with the
redshift of their motion
correlated with their
distance.
Hubble’s Law
These plots show the relation between distance and
recessional velocity for the five galaxies in the
previous figure, and then for a larger sample.
Hubble’s Law
The relationship (slope of the line) is characterized
by Hubble’s constant H0:
recessional velocity = H0 x distance
The value of Hubble’s constant is approximately
70 km/s/Mpc.
Measuring distances using Hubble’s law actually
works better the farther away the object is;
random motions are overwhelmed by the recessional
velocity.
Distance Ladder with Hubble’s Law
This puts the
final step on our
distance ladder.
Dark Matter in the Universe
Other galaxies have rotation curves similar to ours,
allowing measurement of their mass.
Dark Matter in the Universe
Another way to measure
the average mass of
galaxies in a cluster is to
calculate how much mass
is required to keep the cluster gravitationally bound.
Dark Matter in the Universe
Galaxy mass measurements show that galaxies need
between 3 and 10 times more mass than can be
observed to explain their rotation curves.
The discrepancy is even larger in galaxy clusters,
which need 10 to 100 times more mass. The total
needed is more than the sum of the dark matter
associated with each galaxy.
Dark Matter in the Universe
There is evidence for
intracluster superhot
gas (about 10 million
K) throughout
clusters, densest in
the center.
Dark Matter in the Universe
It is believed this gas is primordial – dating from the
very early days of the universe.
There is not nearly enough of it to be the needed dark
matter in galaxy clusters.
Galaxy Collisions
The separation between galaxies is usually not large
compared to the size of the galaxies themselves, and
galactic collisions are frequent.
The “cartwheel” galaxy
on the left appears to be
the result of a head-on
collision with another
galaxy, perhaps one of
those on the right.
Galaxy Collisions
This galaxy collision has led to bursts of star
formation in both galaxies; ultimately they will
probably merge.
Galaxy Collisions
The Antennae galaxies collided fairly recently,
sparking stellar formation. The plot on the right is the
result of a computer simulation of this kind of
collision.
Galaxy Formation and Evolution
Galaxies are believed to have
formed from mergers of smaller
galaxies and star clusters. Image
(c) shows large star clusters found
some 5000 Mpc away. They may
be precursors to a galaxy.
Galaxy Formation and Evolution
This Hubble Deep Field view
shows some extremely distant
galaxies.
The most distant
appear irregular,
supporting the theory
of galaxy formation
by merger.
Galaxy Formation and Evolution
This simulation shows how interaction with a
smaller galaxy could turn a larger one into a spiral.
Galaxy Formation and Evolution
This appears to be an instance of galactic cannibalism
– the large galaxy has three cores.
Black Holes in Galaxies
This galaxy is viewed in
the radio spectrum,
mostly from 21-cm
radiation. Doppler shifts
of emissions from the
core show enormous
speeds very close to a
massive object – a black
hole.
Black Holes in Galaxies
Careful measurements show that the mass of the
central black hole is correlated with the size of the
galactic core.
Black Holes in Galaxies
This figure shows how galaxies may have evolved, from
early irregulars through active galaxies, to the normal
ellipticals and spirals we see today.
Cosmology
The Study of the Universe as a Whole
The Universe on Very Large Scales
Galaxy clusters join in
larger groupings,
called superclusters.
This is a 3-D map of
the superclusters
nearest us; we are part
of the Virgo
Supercluster.
The Universe on Very Large Scales
This plot shows
the locations of
individual
galaxies within
the Virgo
Supercluster.
The Universe on Very Large Scales
This slice of a larger galactic survey shows that, on the
scale of 100–200 Mpc, there is structure in the universe
– filaments, shells and voids?
On larger scales,
things look more uniform.
.
Note: The decreasing
density of galaxies at the
farthest distances is due to
the difficulty of observing
them.
Given no evidence of further structure, assume:
The Cosmological Principle
On the largest scales, the universe is roughly homogeneous (same at all
places) and isotropic (same in all directions). Laws of physics same.
Hubble's Law might suggest that everything is expanding away
from us, putting us at center of expansion. Is this necessarily true?
(assumes
H0 = 65
km/sec/Mpc)
If there is a center, there must be a boundary to define it => a finite
universe. If we were at center, universe would be isotropic (but only
from our location) but not homogeneous:
Finite volume of galaxies
expanding away from us
into...what, empty space?
Us
But if we were not at center, universe would be neither isotropic nor
homogeneous:
Us
So if the CP is correct, there is no center, and no edge to the Universe!
Best evidence for CP comes from Cosmic Microwave Background
Radiation (later).
The Big Bang
All galaxies moving away from each other. If twice as far away from
us, moving twice as fast (Hubble's Law). So, reversing the Hubble
expansion, everything must have been together once. How long ago?
H0 gives rate of expansion. Assume H0 = 75 km / sec / Mpc. So
galaxy at 100 Mpc from us moves away at 7500 km/sec. How long
did it take to move 100 Mpc from us?
distance
time =
velocity
100 Mpc
=
7500 km/sec
=
13 billion years
(Experts note that this time is just
1
).
H0
The faster the expansion (the greater H0), the shorter the time to get to
the present separation.
Big Bang: we assume that at time zero, all separations were infinitely
small. Universe then expanded in all directions. Galaxies formed as
expansion continued.
The Expanding Universe
Using H0  70 km/s/Mpc,
we find that time is about 14 billion years.
Note that Hubble’s law is the same no matter
who is making the measurements
or where they are being made.
The Expanding Universe
If this expansion is extrapolated backward in time, all
galaxies are seen to originate from a single point in an
event called the Big Bang.
So, where was the Big Bang?
It was everywhere!
No matter where in the universe we are, we will
measure the same relation between recessional velocity
and distance, with the same Hubble constant.
But this is not galaxies expanding through a pre-existing, static space.
That would be an explosion with a center and an expanding edge.
If CP is correct, space itself is expanding, and galaxies are taken
along for the ride. There is no center or edge, but the distance
between any two points is increasing.
A raisin bread analogy provides some insight:
But the cake has a center and edge. Easier to imagine having no center or
edge by analogy of universe as a 2-d expanding balloon surface:
Now take this analogy "up one dimension". The Big Bang occurred
everywhere at once, but "everywhere" was a small place.
Imagine a balloon with coins stuck to it. As we blow up the balloon, the
coins all move farther and farther apart. There is, on the surface of the
balloon, no “center” of expansion.
(To understand what it would be like in a 2-d universe, read Flatland by
Edwin Abbott: www.ofcn.org/cyber.serv/resource/bookshelf/flat10 )
If all distances increase, so do wavelengths of photons as they travel and
time goes on.
When we record a photon from a distant source, its wavelength will be
longer. This is like the Doppler Shift, but it is not due to relative motion of
source and receiver. This is correct way to think of redshifts of galaxies.
Proof: The Cosmic Microwave Background Radiation (CMBR)
A prediction of Big Bang theory in 1940's. "Leftover" radiation from
early, hot universe, uniformly filling space (i.e. isotropic,
homogeneous). Predicted to have perfect black-body spectrum.
Photons stretched as they travel and universe expands, but spectrum
always black-body. Wien's Law: temperature decreases as wavelength
of brightest emission increases => was predicted to be ~ 3 K now.
Found in 1964 by Penzias and Wilson. Perfect black-body spectrum at T =
2.735 K. Uniform brightness (and thus temperature) in every direction.
Points are data on the spectrum of the CMBR
from the COBE satellite (1989). Curve is a
black-body spectrum at T=2.735 K.
1% of the
“snow” on a
blank TV
channel is this
radiation!
All-sky map of the CMBR temperature, constant everywhere to one part in
105 ! For blackbody radiation, this means intensity is very constant too
(Stefan’s law).
(WMAP satellite)
Deviations are -0.25 milliKelvin (blue) to +0.25 milliKelvin
(red) from the average of 2.735 Kelvin.
That the CMBR comes to us from every direction is best evidence
that Big Bang happened everywhere in the universe. That the
temperature is so constant in every direction is best evidence for
homogeneity on large scales.
IF the Big Bang happened at one point in 3-d space:
Later, galaxies form and fly apart.
But radiation from Big Bang
streams freely at speed of light!
Wouldn't see it now.
Successes of the Big Bang Theory
1) It explains the expansion of the universe.
2) It predicted the cosmic microwave background radiation, its
uniformity, its current temperature, and its black-body spectrum.
3) It predicted the correct helium abundance (and lack of other
primordial elements).
Misconceptions about the Big Bang
1. “The universe was once small.” The observable universe, which is
finite, was once small. The nature of the entire universe at early times
is not yet understood. It is consistent with being infinite now.
2. “The Big Bang happened at some point in space.” The microwave
background showed that it happened everywhere in the universe.
3. “The universe must be expanding into something.” It is not expanding
into “empty space”. That would imply the Big Bang happened at some
location in space. It is a stretching of space itself.
4. “There must have been something before the Big Bang.”
The Big Bang was a singularity in space and time (like the center
of a black hole). Our laws of physics say the observable universe
had infinitesimally small size, and infinite temperature and
density.
In these conditions, we don’t have a physics theory to describe
the nature of space and time. At the Big Bang, time took on the
meaning that we know it to have.
"Before" is only a relevant concept given our everyday
understanding of time. We must await a better understanding of
the nature of space and time. Such theories are in their infancy.
Shouldn’t be surprising that these concepts are hard to grasp.
So was the heliocentric Solar System 400 years ago.
Cosmic Dynamics and the Geometry of Space
There are three possibilities for the universe in the
far future:
1. It could keep expanding forever.
2. It could expand out for a time and stop
3. It could collapse.
Assuming that the only relevant force is gravity,
which way the universe goes depends on its
density.
Cosmic Dynamics and the Geometry of Space
If the density is low,
the universe will
expand forever.
If it is high, the
universe will
ultimately collapse.
Cosmic Dynamics and the Geometry of Space
If space is homogenous, there are three possibilities
for its overall structure:
1. Closed – this is the geometry that leads to ultimate
collapse
2. Flat – this corresponds to the critical density
3. Open – expands forever
Cosmic Dynamics and the Geometry of Space
In a closed universe,
you can travel in a
straight line and end
up back where you
started.
The Fate of the Cosmos
The answer to this question lies in the actual density
of the universe.
Measurements of luminous matter suggest that the
actual density is only a few percent of the critical
density.
But – we know there must be large amounts of dark
matter.
The Fate of the Cosmos
However, the best estimates for the amount of dark
matter needed to bind galaxies in clusters, and to
explain gravitational lensing, still only bring the
observed density up to about 0.3 times the critical
density, and it seems very unlikely that there could be
enough dark matter to make the density critical.
The Fate of the Cosmos
Type I supernovae can be used to measure the behavior
of distant galaxies.
If the expansion of the universe is decelerating, as it
would if gravity were the only force acting, the farthest
galaxies had a more rapid recessional speed in the past,
and will appear as though they were receding faster
than Hubble’s law would predict.
The Expansion of the Universe Seems to be Accelerating
Type I supernovae: from ones in nearby
galaxies, know luminosity. In distant
galaxies, determine apparent brightness.
Thus determine distance.
Works for more than 3000 Mpc.
From redshifts, they are not expanding as
quickly from each other as galaxies are now.
Taking this into account, best age estimate of
Universe is 13.8 Gyrs.
Redshift (fractional shift in
wavelength of spectral lines)
The gravity of matter should retard the expansion. But a new distance
indicator shows that the expansion rate was slower in the past!
=> Expansion rate is increasing
=> The measured acceleration implies that there is more “dark
energy” than the energy contained in matter.
H0 was
smaller in
past (i.e.
for distant
galaxies)
The Cosmological Constant, 
Introduced by Einstein in 1917 to balance gravitational attraction and
create static Universe (turned out to be wrong!). Can think of  as
repulsive force that exists even in a vacuum. But accelerating universe
indicates there is a . Also often called "dark energy". We have little idea
of its physical nature.
The measured acceleration
implies that there is more
“dark energy” than the
energy contained in matter.
The Fate of the Cosmos
This figure shows
our current
understanding of
the composition of the
universe. Only
4% is normal matter!
Dark energy will
dominate more & more
Universe will expand
forever
The Fate of the Cosmos
This figure shows the history of the universe as we
presently understand it – beginning with the big bang,
followed by rapid inflation; slowing of the expansion due
to gravity and finally accelerating expansion due to dark
energy.
The Early Universe
The First Matter
At the earliest moments, the universe is thought to have been
dominated by high-energy, high-temperature radiation. Photons had
enough energy to form particle-antiparticle pairs. Why? E=mc2.
pair
production
annihilation
At time < 0.0001 sec, and T > 1013 K, gamma rays could form protonantiproton pairs.
At time < 15 sec, and T > 6 x 109 K, electron-positron pairs could form.
Annihilation occurred at same rate as formation, so particles coming in
and out of existence all the time.
As T dropped, pair production ceased, only annihilation. A tiny
imbalance (1 in 109) of matter over antimatter led to a matter universe
(cause of imbalance not clear, but other such imbalances are known to
occur).
Primordial Nucleosynthesis
Hot and dense universe => fusion reactions.
At time 100-1000 sec (T = 109 - 3 x 108 K), helium formed.
Stopped when universe too cool. Predicted end result: 75% hydrogen,
25% helium.
Oldest stars' atmospheres (unaffected by stellar nucleosynthesis)
confirm Big Bang prediction of 25% helium.
The Early Universe
The total energy of the universe consists of both
radiation and matter.
As the universe
cooled, it went from
being radiationdominated to being
matter-dominated.
Dark energy becomes
more important as
the universe expands.
Calendar History of the Universe (Carl Sagan)
http://www.youtube.com/watch?v=Ln8UwPd1z20
The Early Universe
Inflation
A problem with microwave background:
Microwave background
reaches us from all
directions.
Temperature of background in opposite directions nearly identical. Yet
even light hasn't had time to travel from A to B (only A to Earth), so A
can know nothing about conditions at B, and vice versa. So why are A
and B almost identical? This is “horizon problem”.
Solution: Inflation. Theories of the early universe
predict that it went through a phase of rapid expansion.
Separation
between two
points (m)
If true, would imply that points that are too far apart now were
once much closer, and had time to communicate with each other
and equalize their temperatures.
Inflation also predicts universe has flat geometry:
Microwave background observations seem to suggest that this is true.