Download Lecture 9

Modern Astronomy
Stars & Galaxies
Lecture 9
Cosmology
Geraint F. Lewis
University of Sydney 2007
Outline
The Universe as we know it
 Cosmological models
 Observations of the Universe
 The evolution of the Universe
 The distribution of matter
 The future of the Cosmos

The Universe as we know it
www-ed.fnal.gov/projects/exhibits
Fundamental Forces
Strong: 1
EM: 10-2
Weak: 10-5
Gravity: 10-39
Forces: Unification
As the energy of the interactions increases, these force look
more like each other.
 Quantum Electrodynamics: unifies EM & Weak
 Quantum Chromodynamics: EM-Weak & Strong
 Currently trying to add gravity
Superstrings?????
Forces: Unification
hyperphysics.phy-astr.gsu.edu/hbase/astro/unify.html
Cosmological Foundations
General Relativity
In Einstein’s view, gravity is not
really a force!
Importantly, gravity is created not
only by mass, but also by
energy.
Remember, mass is energy E=mc2
Cosmology
Imagine a universe filled
with stationary stars.
If finely balanced, it
remains stationary.
A single star out of place
results in collapse.
Cosmological Constant
This worried Einstein as he thought
the Universe should be static.
He added , the cosmological
constant, which acts as antigravity.
This acts against collapse, but has
negative pressure and appears
unphysical!
Einstein’s Biggest Blunder!
Cosmology
Friedmann & Lemaitre looked at
cosmology within the framework
of General Relativity.
The concluded that, in general,
universes must be expanding or
contracting.
Just how the universe changes with
time depends upon the energies
in the Universe.
When this original work was done,
the only energy considered was
matter!
Evolution
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Too little matter, and the
Universe never slows down
Too much matter, and the
Universe collapses
Just right, the expansion
slows down forever, but never
quite reaches zero
But what does this scale factor
mean? It is the change of
separation between a pair of
objects.
Evolution
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The raisin cake picture
encompasses this picture
As the cake expands, the
distance between raisins
increases
But for this picture to be
accurate, there can be no
edges to the cake!
Where is the centre of the
Universe in this picture?
Geometry

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The ratio of the
circumference to the
diameter of a circle is
=3.141592….
But this is only true in
the flat geometry of
Euclid
What about non-flat
geometry??
Open, flat, closed?
Measure  on the surface
of the Earth
From pole to equator =
10000km
Around the equator =
40000km
 = 40000/(2£10000) = 2
Open, flat, closed?
So, depending upon the density in the Universe
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Open >: Infinitely large & expand forever
Flat = : Infinitely large & expand forever
Closed <: Finite & eventually collapse
Critical density = 1 £ 10-26 kg/m3
= 6 hydrogen atoms / m3
Observing: The redshift
Hubble’s expansion
Hubble found a relation
between distance and
velocity
V = Ho d
where d is the distance to
the galaxy and v is its
velocity.
Hubble’s constant has
been measured to be
Ho = 72 km/s/Mpc
Hubble’s expansion
Imagine the Earth expands
overnight. We might feel squashed
in our beds, but when we awake in
the morning, all the distances would
have increased! What if we lived in
the surface?
What is the redshift?
The redshift is not a Doppler shift.
As the Universe expands, the
wavelength of the radiation is also
stretched.
Mathematically
where z is the redshift and R is the
distance between galaxies.
Which universe is ours?
The cosmological models make
specific predictions on how faint
something should look at a particular
redshift.
This depends upon the content of the
Universe.
Observations of distant supernova
show that we do not live in a universe
containing only matter.
In fact we live in a universe
dominated by Something Else!!!!
Cosmological time dilation

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Cosmological supernovae are like
clocks, they brighten and fade in
a fixed time.
The cosmological equations
predict that we should see the
distant universe run slowly, a
time dilation effect.
This has now been observed,
providing further evidence that
the cosmological models are a
good description of the cosmos.
Putting it all together

Dark Matter
•
•
•
•
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The Universe is flat
The Universe is 13.7 billion yrs old
Interacts mainly via gravity
Not stars, gas, rocks etc
Not large black holes
Possibly an elementary particle
Dark Energy
•
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•
•
•
Not dark matter!
Looks like Einstein’s 
Must be exotic: strings, defects
Is beginning to dominate
Accelerates the expansion
Accelerated expansion
Running backwards
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Running the Universe backwards, we see
there was a point where the scale factor was
zero.
This marks the Big Bang, or start of the
Universe.
Currently, science cannot answer what caused
this event.
But science can describe the universe in detail
from 10-43 seconds after the event to the
present day!
At the beginning
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The smaller the scale factor, the
hotter the universe was. The very
early universe was very hot!
The energy from inflation
became particles and radiation
10-6 secs, protons and neutrons
formed
1 sec, protons & neutrons join to
make deuterium and helium
3 mins, cooking ceased with the
universe 75% hydrogen and
25% helium
300,000 yrs, the Universe is cool
enough for electrons to join
atoms
There should be a background of
radiation left over from this
event!!
Background Radiation
In 1941, Herzberg saw that
molecules in interstellar space
were too energetic.
He concluded that they must be
bathed in a radiation of
temperature 2.3K.
“a rotational temperature of 2.3k
follows, which has of course only a
very restricted meaning”
Penzias & Wilson confirmed this in
1965 (and won the Nobel in 1978)
Cosmic microwave background
The universe is bathed in cool radiation
left over from its energetic start.
Some of the snow on tv is this CMB
Two important predictions
 CMB was hotter in the past
 CMB should not be completely smooth
Cosmic microwave background
Relativistic cosmology predicts that the
temperature of the CMB should scale with the
size of the universe.
Astronomers have looked at distant molecules in
the early universe and have mentioned
temperature of the CMB, finding it to be hotter
in the past, in agreement with the
cosmological model.
www.eso.org/outreach/press-rel/pr-2000/pr-27-00.html
The CMB
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While the CMB has a
mean temperature of
2.7K, there should be
small (10-5) variations
superimposed on it.
These variations
reflect the distribution
of matter in the very
early Universe.
Anisotropy
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Inflation predicts a
very specific pattern
on the CMB (red line)
Data from COBE and
now WMAP have
revealed this pattern
matches the theory!
In the beginning…
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… the Universe was very smooth
Inflation results in very small
ripples in the matter at the level
of 1 part in 10000 (as seen in
the CMB)
Gravity causes material to flow
into denser regions, forming the
large scale structure of galaxy
groups, clusters and
superclusters
Again need a Universe in a
computer
The growth of large scale structure
http://virgo.dur.ac.uk & http://www.nbody.net
Large Scale Structure
Sloan galaxy survey (www.sdss.org)
Surveys of the Universe reveal galaxies
to lie on the foamy structure predicted
from inflation!
Present: 14£109 yrs
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Stars are metal rich
More metals made in supernova
New stars have less and less
hydrogen as they become
polluted with metals
Mass is getting locked up in
white dwarfs, neutron stars and
black holes!
Future: 16£109 yrs
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The Milky Way and Andromeda
collide and form a single large
elliptical
Future: 17£109 yrs
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The Sun dies
Future: 7£1011 yrs
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With the accelerated expansion,
galaxies beyond the Local Group
fade from view.
The sky outside the Local Group
will be black!
Future: 1013 yrs
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Nuclear fuel is exhausted
Small red stars finally burn out
All star light begins to fade
All potential raw material for star
formation is locked away in
stellar remnants
Future: 1016 yrs
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Stellar interactions finally eject
all planets
Future: 1028 yrs
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Galaxies finally dissolve, with
90% of stars ejected into
intergalactic space
The remaining stars spiral into
the central black hole
Future: 1032 yrs
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Protons decay and matter
dissolves into radiation and
electrons.
Future: 1067 ! 10100 yrs
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Stellar and supermassive black
holes evaporate via Hawking
radiation
Nothing left by a sea of ever
cooling radiation
The universe is now a cold, dark,
lifeless place, a little bit like
Canberra.
See you next week!