In Search of the Big Bang

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In Search of the Big Bang
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The Hubble Law

• According to the Hubble Law, the space between
  the galaxies is constantly increasing, with
  Velocity H0 D istance

This is not occurring locally the density of
material in the galaxy and the Local Group has
long since caused gravity to reverse the Hubble
expansion. But globally, the universe is
expanding.
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An Age to 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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A Fate to the Universe

• The Hubble Law also implies 3 possible fates for
  the universe
• The universe will expand forever (an unbound or
  open universe)
• Gravity will eventually reverse the expansion and
  cause the universe to collapse into a Big
  Crunch (a bound or closed universe)

• The universe is precisely balanced between open
  and closed (a marginally bound or flat universe)

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The Shape of the Universe

• According to Einstein, mass bends space. This
  means that the universe has a shape. This shape
  is related to the density of the universe.

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The Age and Fate 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, and the universe
  would expand forever.
• 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. For a flat universe, the age is 2/3 of
  1/ H0.
• The faster the universe is expanding (i.e., the
  larger the value of H0), the more matter there
  must be to close the universe.
• H0 is therefore key to knowing the age and fate
  of the universe! And note H0 V / D, and
  velocities are easy to measure via the Doppler
  shift! All you is the distances to galaxies!

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The Distances to Galaxies

• In general, galaxies are too far away to observe
  RR Lyrae or main sequence stars. You need a
  brighter standard candle!

Recall the Instability Strip. Pop II (low mass)
objects arent the only type of star to wander
through the strip after igniting helium. High
mass (Pop I) stars can also enter the strip.
These stars are called Cepheid variables.
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The Cepheids of the Large Magellanic Cloud

• Cepheid variables can be 100 times brighter than
  RR Lyr stars, but they do not all have the
  same brightness. They are difficult to measure
  in the Milky Way due to dust, but many Cepheids
  exist in the Large Magellanic Cloud, our nearest
  (non-dwarf) galaxy.

The LMC is close enough so that we can identify
its RR Lyrae stars. We therefore know its
distance.
l L / r2
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The Cepheid Period-Luminosity Relation

• In 1912, Henrietta Leavitt showed that LMC
  Cepheids had a range of brightness (some
  extremely luminous, some faint). But the
  brighter the Cepheid, the longer it took to
  pulsate. This Period-Luminosity relation makes
  Cepheids a standard candle.

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The Distance Ladder
Cepheids
RR Lyrae Stars
Spectroscopic Parallax
Trigonometric Parallax
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Cepheid Distances

• Using Cepheids as a standard candle, one can
  obtain the distances to galaxies as far away as
  20 Mpc.

But this is still not far enough away. Peculiar
velocities are still too important. We need a
brighter standard candle!
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The Tully-Fisher Relation

• According to Newton, the rotation speed of a
  galaxy depends on its mass, and the greater the
  mass, the brighter the galaxy.

If we can translate mass into absolute
luminosity, we can have a standard candle that is
as bright as a galaxy. And we can do this by
calibrating the relationship using galaxies whose
distances are known from Cepheids.
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Type Ia Supernovae

• When an accreting 1.4 M? white dwarf goes over
  the Chandrasekhar limit, it becomes a Type Ia
  supernova. SN Ia can be seen across the
  universe.

We can determine exactly how bright SN Ia are by
measuring their brightness in galaxies with known
Cepheid distances.
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The Distance Ladder
Hubble Law
T-F Relation
SN Ia
Cepheids
RR Lyrae Stars
Spectroscopic Parallax
Trigonometric Parallax
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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
• 13 billion years, if we live in an empty universe
• 9 billion years, if we live in a flat universe

But the stars in globular clusters are at least
13 billion years old. Did we do something wrong
?
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Telescopes as Time Machine

• 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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Galaxies at High Redshift
Some of these galaxies date from a time when the
universe was only 10 of its present age!
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Galaxies at High Redshift
In the deepest 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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The Microwave Background

• 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 History of Light

• 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
• 1998 Blackbody spectrum measured by the COBE
  satellite

Prediction of Big Bang confirmed!
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The All-Sky 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.

Blue is cooler (moving away) red is hotter
(moving toward)
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The All-Sky Microwave Background

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

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The All-Sky 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 All-Sky Microwave Background

• From the equation of state, slightly higher
  temperatures means slightly 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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The All-Sky Microwave Background

• Over time, the very small density fluctuations of
  the early universe have been amplified many times
  by gravity. The galaxies and clusters we see
  today grew from the very small fluctuations in
  the microwave background.

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The All-Sky Microwave Background

• The hot gas of the early universe cools and, with
  the aid of gravity, gets turned into galaxies and
  clusters of galaxies.

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Formation of Structure
Over time, the very small density fluctuations of
the early universe have been amplified many times
by gravity.
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The Shape of the Universe

• The microwave background fluctuations also allow
  us to determine the shape of the universe. (The
  method is complicated it has to do with how far
  apart the positive (and negative) areas appear on
  the sky. Theory tells us how far they should be,
  and we can observe how far apart they are.)

We observe that the Universe is Flat!
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The Deceleration of the Universe

• The age of the universe depends on both its
  expansion rate (the Hubble Constant) and its
  density. Determining density is hard, since
  most of the mass is invisible. But over time,
  gravity has slowed down the expansion rate. By
  looking into the past, we can see how the
  universe has decelerated.

HUBBLE DIAGRAM
Closed
Closed
Flat
Flat
Empty
Empty
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Measuring the Deceleration
Type Ia supernovae can be used as standard
candles to look across the universe and measure
the deceleration via a Hubble Diagram. This was
done in 1998. The answer is
The universal expansion is not slowing down at
all due to gravity. In fact, the expansion is
speeding up!!!
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The Accelerating Universe!!!

The universe is not slowing down at all. In
fact, its speeding up!!! We live in an
accelerating universe! Its as if theres
another force pushing the universe apart a
Cosmological Constant!!!
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The Accelerating Universe!!!

Whatever this force is, we think that it is
growing stronger as the universe evolves. The
more empty space in the universe, the greater the
acceleration as if the vacuum of space has
pressure!