Introduction to Cosmology

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The Origin of the Universe
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Today

• News story Do the most massive stars explode?
• Structure in the Universe (continued from
  Tuesday)
• The Origins of the Universe
• Coming Up
• Homework 9  Due tomorrow at 9am
• Telescopes Project SZ Tonight next week

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Structure in the Universe Distances Motions of
Galaxies
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Mapping Galaxies in the Universe
Galaxies are not randomly distributed. They are
in clusters and filaments separated by huge empty
voids.
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Mapping Galaxies in the Universe
Galaxies are not randomly distributed. They are
in clusters and filaments separated by huge empty
voids.
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Mapping Galaxies in the Universe
Galaxies are not randomly distributed. They are
in clusters and filaments separated by huge empty
voids.
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Mapping Galaxies in the Universe
Galaxies are not randomly distributed. They are
in clusters and filaments separated by huge empty
voids.
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Galaxies on the Surfaces of Bubbles
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The Size of the Universe

• If you were to make a Universe, would you give it
  a finite size, or make it infinite?

If you make it finite
In a finite Universe, gravity eventually takes
over and causes a big collapse.
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The Size of the Universe

• If you were to make a Universe, would you give it
  a finite size, or make it infinite?

If you make it infinite
In an infinite Universe, we would see the surface
of a star at every point in the sky, so the night
sky would be bright! This is Olbers paradox.
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The Cosmological Constant

• In 1918, Einstein realized the difficulty with a
  finite universe, and the impossibility of an
  infinite universe. So to keep the universe from
  collapsing, he postulated the existence of a
  Cosmological Constant (i.e., an extra
  anti-gravity term to counteract attraction).
  This is represented by ?.

12
The Dynamic Universe

• But the Hubble Law solves both the problem of
  universe collapse and Olbers paradox.
• Since the galaxies are moving away from each
  other, gravity will not necessarily cause the
  universe to collapse. So a finite universe is
  possible.
• The larger the distance, the larger the velocity.
  Galaxies at the other end of the universe have
  their light Doppler shifted out of the optical,
  explaining why the night sky is dark. An
  infinite universe is possible.

Einsteins reaction The Cosmological Constant
was my greatest blunder.
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The Cosmological Principle

• Since we are not at the center of our Solar
  System, our Galaxy, or our Local Group of
  galaxies, it is exceedingly likely that were
  also not at the center of the universe. We
  therefore adopt the cosmological principle, which
  states that the universe (on average) must look
  the same to everyone, no matter where he/she/it
  is. In other words,
• The Universe is homogeneous (i.e., smooth)
• The Universe is isotropic (no special direction)
• Then why should the galaxies all be moving away
  from us?

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Summary the distance ladder
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Summary Structure in the Universe

• On the largest scales of cosmic structure,
  Galaxies have an ordered distribution, as if on
  the surfaces of bubbles
• A finite, motionless Universe would collapse
• An infinite, unchanging Universe would produce a
  bright night-time sky (Olbers paradox)
• Hubble Law solves both dilemmas and allows for
  either to be possible, in principle
• In an expanding universe, Hubble Law is seen by
  everyone

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The Birth of the Universe The Big Bang
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Hubble Expansion and the Big Bang

• The fact that more distant galaxies are moving
  away from us more rapidly indicates that the
  universe is expanding. This implies that the
  Universe was born in a huge explosion, or Big
  Bang.

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The Balloon Analogy

• It is as if all galaxies exist on the surface of
  a balloon. The space between all the galaxies is
  constantly increasing.

The galaxies are not flying apart into the
Universe. The Universe itself (space-time) is
expanding. The galaxies are riding along as the
fabric of space expands.
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The Balloon Analogy

• The expansion of the Universe also causes light
  to get stretched to longer wavelengths, that is,
  redshifted.

So the redshifts that we measure for galaxies can
be thought of as resulting from the expansion of
space itself, over the billions of years that the
light has been traveling to us.
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Testing the Big Bang Hypothesis

• Under the Big Bang hypothesis, the Universe was
  very different in the past. Can we prove this?
  Yes!
• Light travels at a finite speed the light we see
  today started out long ago. The farther away the
  object, the further back in time we observe.
  (And remember, the greater the distance, the
  greater the redshift.)

With big telescopes or telescopes in space, we
can look for high-redshift galaxies and look back
in time.
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Testing the Big Bang Hypothesis
In the deepest HST images, the high redshift
galaxies appear bluer, and more irregular than
galaxies in the nearby universe. Many high
redshift galaxies are interacting.
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Testing the Big Bang Hypothesis

• Suppose we were to look further back in time, to
  when the universe was only 100,000 years old.
  At that time
• The universe was very dense and under great
  pressure.
• According to the equation of state, high pressure
  means high temperature.
• According to the blackbody law, high temperature
  means light was produced.
• Since this was a long time ago, if we were to
  observe it, the light would be redshifted into
  the microwave region of the spectrum.
• Since the entire universe was glowing, this light
  should come from all over the sky.

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The Microwave Background

• The light from the Big Bang should now appear as
  emission from a blackbody at 3 degrees above
  absolute zero.

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Prediction vs. Observation

• 1948 3 degree blackbody emission from the entire
  universe predicted by George Gamow
• 1965 3 degree blackbody emission found by Arno
  Penzias and Robert Wilson (Bell Labs, NJ)
• 1998 Blackbody spectrum measured by the COBE
  satellite

Prediction of Big Bang confirmed!
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The Microwave Background

• Because the earth is moving through space, the
  microwave background should be redshifted in one
  part of the sky, and blueshifted in another part
  of the sky.

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The Microwave Background

• When the earths motion is removed, the
  distribution of microwaves on the sky becomes
  more uniform.

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The Microwave Background

• When emission from cold gas in the Milky Way is
  removed, the remaining distribution becomes very
  (but not perfectly) smooth.

The fluctuations are only a few parts in 10,000!
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The Microwave Background

• Higher temperatures means higher densities and
  pressures. The red areas are over-dense by a
  factor of 1.00004.

From these primordial density fluctuations come
todays galaxies and clusters.
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Helium in the Universe

• If the universe began as a high density soup of
  protons and neutrons, some of those particles
  must have undergone fusion.

In the Big Bang, about 1 of every 10 hydrogen
atoms should have been changed to helium. Thats
almost exactly the helium abundance we observe
for the universe.
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History of the Universe

• The Big Bang occurred 13.7 billion years ago.
  Since then
• 10-43 seconds ???? Grand unification
• 10-35 seconds quarks form gravity begins to
  exist
• 10-12 seconds particles form and annihilate
• 0.00001 seconds protons, neutrons form
• 3 minutes fusion of hydrogen to helium ends
• 100,000 years release of the microwave
  background

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The Age of the Universe

• The Hubble Law implies the universe began with a
  Big Bang, which started the galaxies flying
  apart. It also implies a finite age to the
  universe. This age depends on two things
• The expansion rate of the universe. (How fast
  are the galaxies flying apart?)
• The density of the universe. (How much is
  gravity slowing down the expansion?)

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The Age of the Universe

• If there were no mass (i.e., no gravity) in the
  universe, the Hubble expansion would proceed at a
  constant speed. The age of the universe would
  then just be given by 1 / H0.

In a real universe with mass, gravity must have
(over time) slowed the Hubble expansion. In the
past, the galaxies must have been moving apart
faster. The age must therefore be less than
1/H0. If you can measure H0, you can estimate the
age of the universe.
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The Age of the Universe

• Our current measurements give a value of the
  Hubble Constant of H0 72 ? 8 km/s/Mpc. This
  implies an age for the universe of lt13 billion
  years.

But the stars in globular clusters are at least
13 billion years old. How can some stars be
older than the universe? Did we do something
wrong ?
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The Fate of the Universe

• Will the Universe expand forever?
• Or will it stop expanding and collapse (a Big
  Crunch)?