The Expansion of the Universe

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The Expansion of the Universe

• Paul J. Thomas
• Department of Physics and Astronomy
• UW - Eau Claire

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Introduction

• All galaxies, except a few local ones, are
  receding from us.
• The more distant the galaxy, the greater the
  recession speed.
• The expansion apparently began in the Big Bang
• Will the universe continue expanding or start to
  contract someday?

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Lookback Time

• We can see galaxies billions of light years away.
• Therefore we see these galaxies as they were
  billions of years ago!
• Looking out into space is like looking back in
  time!
• The universe was probably different in the past.

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Topics of Discussion

• Hubbles Law
• The Big Bang Model
• The Cosmic Microwave Background (CMB)
• Dark Matter and the density of the universe
• The long-term future of the universe

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Expansion of the Universe

• Slipher discovered that virtually every spiral
  galaxy has a red-shifted spectrum (1912).
• Distance and recession speed are connected by
  Hubbles Law
• V H0 r,
• where V recession speed (km/s),
  
• r distance (Mpc)
  
• H0 Hubbles Constant (km/s/Mpc).

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Hubbles Law

• We can measure V from the redshift of light from
  the galaxy.
• We can determine r by the use of standard candles
  (e.g. Cepheid variables).
• Plug these numbers into Hubbles Law
• V H0 r,
• and we get a value for H0.

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Standard Candles

• We cant use parallax over galactic distances, as
  they are too great
• If we knew the specific brightness of a star, we
  could use the inverse square law to calculate its
  distance
• We use variable stars, such as Cepheid variables

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Cepheid Variables

• These are giant stars (103 - 105 L?)
• They cannot balance pressure and temperature
  well, and go through phases of expansion and
  contraction
• We see these phases as regular cycles of
  brightening and dimming (1 - 60 days)
• The greater the luminosity of a variable, the
  longer the period

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HST views Cepheid Variables in M100
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Values for Hubbles Constant

• Current best estimate of H0
• 710.04 km/s/Mpc (WMAP spacecraft)

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George-Henri Lemaître (1894-1966)
Fred Hoyle (1915-2001)
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Penzias and Wilson

• The discovery of the cosmic microwave background
  by Penzias and Wilson transformed cosmology from
  being the realm of a handful of astronomers to a
  'respectable' branch of physics almost
  overnight.
• -Michael Turner,
• University of Chicago

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Cosmic Background Radiation (CBR)

• Black Body radiation that is isotropic (all
  directions at same strength)
• Peak energy at 1 mm wavelength, corresponding to
  2.7 K
• This is the remnant energy from the Big Bang

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WMAP
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The Big Bang Model

• The Universe began in an episode of high
  temperature and density about 13 billion years
  ago.
• Matter, energy and physical laws came into being
  at that time.

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The Big Bang

• The Big Bang was not an explosion of matter and
  energy in pre-existing space.
• Space and time came to be during the Big Bang.
• Physical laws came into being then, too. (They
  did vary in the very earliest stages).

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Four Fundamental Forces

• The Strong Nuclear Force
• Binds the nucleus of atoms together. Limited
  range 10-15 m.
• The Electromagnetic Force
• Acts on charges and magnetic objects. Unlimited
  range.

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Four Fundamental Forces

• The Weak Nuclear Force
• Binds neutrons and similar particles together.
  Limited range 10-16 m.
• Gravitation
• Attracts massive objects to each other. Unlimited
  range.

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Unifying the Forces

• In our most powerful particle accelerators, we
  are able to collide protons and electrons at very
  high speeds (99.999999999 of the speed of
  light).
• This produces conditions similar to those just
  after the Big Bang.

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Unifying the Forces

• In our particle accelerators, we see
  electromagnetic weak nuclear force ? combined
  electroweak force.
• We think the other forces would combine at higher
  energies.
• So, during the Big Bang, one original force
  split to become the four forces we know.

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Particle Accelerator
Fermilab Accelerator (Proton Synchrotron),
Batavia, Illinois
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Particle Accelerator
Main accelerator ring, Fermilab
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Events of the Big Bang

• Time Temperature (K) Event
• 0 ? All forces unified
• 10-43 s 1032 Gravity separates from
• other forces
• 10-35 s 1027 Strong nuclear force
• separates inflation
• period

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Events of the Big Bang

• Time Temperature (K) Event
• 10-12 s 1015 Weak, electromagnetic
• forces separate
• 10-6 s 1013 Quarks combine to form
• protons, neutrons
• 3 min 109 He forms from H

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Events of the Big Bang

• Time Temperature (K) Event
• 106 y 105 Stable atoms form
• universe becomes
• transparent to light
• 108 y 103 First galaxies form
• 4.5 1017 s 2.7 Life on Earth
• (Now)

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The Age of the Universe

• H0 has units of 1/time
• H0 distance/(time distance)
• 1/H0 is the Hubble Time, tH.
• This is the time since the Big Bang.
• H071 km/sec/Mpc ? tH 13.7 billion years

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Will the Universe Recollapse?

• Required density for Universe to recollapse 4.5
  10-30 g/cm3 critical density.
• Observed density of luminous material 2
  10-31 g/cm3.
• But there may be 20 this amount in dark matter.

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Curvature of the Universe

• The curvature of the universe as a whole is
  determined by its mass density, ?.
• A universe with a mass density greater than the
  critical value, ? gt 1, will be a spherical closed
  universe.
• A universe with a mass density ? lt 1will be an
  open, hyperbolic universe.

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Spherical Universe

• ? gt 1.
• Universe is closed, spherical geometry.
• Angles of large triangles will sum to gt 1800.
• Universe will eventually contract into big
  crunch.

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Hyperbolic Universe

• ? lt 1.
• Universe is open, hyperbolic geometry.
• Angles of large triangles will sum to lt 1800.
• Universe will expand forever.

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Flat Universe

• ? 1.
• Universe is flat, Euclidean geometry.
• Angles of large triangles will sum to 1800.
• Universe will expand forever, at an
  ever-decreasing rate.

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WMAP
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What is the observed value of ??

• Based on luminous matter, ?LM 0.05.
• Including dark matter, ?TOT 1.0.
• So it is very important to accurately determine
  how much dark matter there is!
• The current consensus is that ? 1, so we live
  in an flat universe.
• Dark energy, with ?DE0.65, seems to be
  increasing the expansion rate.

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Type Ia Supernova - 10 billion ly
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