Cosmology
• The study of the structure
and evolution of the
Universe as a whole.
• Seeks to answer questions
such as:
The Hubble Deep Field
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How big is the Universe?
What shape is it?
How old is it?
How did it form?
What will happen to it in
the future?
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Distribution of Galaxies
A map of one million galaxies
• Galaxies appear to be
distributed throughout the
Universe
• Galaxies clump together
into clusters and
superclusters.
• Distant galaxies appear to
be moving away from us.
• We know this from the
redshifts in the spectrum
of galaxies.
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The Big Bang
A plot of the distance of
galaxies versus their
recessional velocity.
The slope of the line is
Hubble’s constant.
• Since the Universe is expanding it
must have been smaller in the past.
• At some point in the past the entire
Universe must have been packed into
some very small size.
• In 1927, Abbé George Lemaitre, a
Belgian cosmologist and priest
proposed that the Universe began as a
“Primeval Atom” and estimated its
age.
• To estimate the age you need to know
the expansion rate – in other words
Hubble’s Constant, H.
• H is 71 +/- 4 km/sec/Mpc giving an
age to the Universe of 13.7 +/- 0.2
Billion years.
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The Expansion of the Universe
• The motion of galaxies away from
us is due to the expansion of the
Universe.
• Like raisins in a loaf of bread,
when baked, all move away from
one another, so too are all the
galaxies moving apart.
• The raisins (galaxies) are not
moving. It is the bread (space)
itself that carries the raisins
(galaxies) away.
• Space itself is expanding and
simply carrying the galaxies with
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it.
Olber’s Paradox / Size of
Observable Universe
• Q: If the universe is infinite in extent, why isn’t the night
sky uniformly bright in all directions from the light from
all galaxies?
• Answers
– The light from distant (rapidly receding) galaxies is
redshifted to wavelengths we can’t see with our eyes
– If (since) stars did not exist at the very beginning of the
universe, we receive no starlight beyond the distance to
the oldest stars (the forest of stars is limited)
– The light from galaxies outside our cosmic horizon has
not reached us
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Cosmic Horizon
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Composition of the Early Universe
• As the Universe cooled Hydrogen
nuclei began to form.
• Conditions were still hot enough
for fusion reactions to occur
forming Helium.
• Nearly all elements heavier than
Helium however were formed later
by stars.
• We notice that the oldest stars have
much less heavy elements than the
Sun. This is further evidence for
the Big Bang theory.
• After neutral atoms formed the
radiation that filled the Universe
was allowed to freely propagate.
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Cosmic Nucleosynthesis
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A particle physicist’s proof of the
Big Bang
Curves show the relative fraction of
primeval elements compared with
the proton-neutron density for the
present day.
The bottom line is that the universe
was 24% Helium after the Big
Bang; the remaining 3% (presentday abundance of Helium is 27%)
was made in the core of stars.
This has been verified by
comparing the amount of Helium in
old stars with that of newer stars
(made from fusion-processed gas
ejected by supernovae).
A prediction of this theory is that
we should see a cosmic microwave
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background in all directions in
space
Cosmic Microwave Background
• Universe was so dense initially that any emitted light was
absorbed on-the-spot (opaque)
• At some point the universe expanded and cooled enough
so that the radiation due to the temperature of the matter
(recall Wien’s law) could propagate and reach us today
• The universe became transparent at about 380,000 years
after the Big Bang, when it was about 3000K
• Since the universe is expanding that radiation is redshifted.
The spectrum of the radiation today has a characteristic
temperature of 2.7K (in the microwave region of the
electro-magnetic spectrum
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Cosmic Microwave Background
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Surface of Last Scattering
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The First Few Moments
• Initially the Universe was incredibly hot.
• Matter and energy were freely transforming into one another
through Einstein’s Mass-Energy relation.
• The mass formed were in matter anti-matter pairs. Matter and
anti-matter destroy one another when they come into contact but
just enough matter existed to survive.
• This remaining matter is what fills the Universe.
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Big Bang Timeline
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Geometry of the Universe
•Closed/finite: sum of
triangle angles is more than
180 degrees
•Open/infinite: sum of
angles is less than 180
degrees
•Flat: sum of angles is 180
degrees
•How to measure a large
enough triangle in the
universe?
•Instead look at size of
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CMB fluctuations
The Universe is Flat
WMAP
observations
show
fluctuations on
a scale
corresponding
to theoretical
model
fluctuations for
a flat universe
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The Accelerating Universe
Observations of distant Type Ia
supernovae suggest that the expansion
of the universe is accelerating.
A consequence of this
observation is that the universe
may continue expanding
indefinitely.
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Critical Density
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Dark Energy
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The Fate of the Universe
An HST picture showing a nearby
galaxy and dozens of more distant
galaxies.
• The Universe may expand
forever (open) or it may
collapse back in on itself
(closed).
• Which occurs depends on the
amount of mass in the
Universe.
• With enough mass, the mutual
gravitational attraction will
slow the expansion and cause it
to collapse.
• Astronomers do not see enough
mass to cause this collapse to
occur.
• There might be enough “Dark
Matter” however.
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