The Discovery of the Cosmic Microwave Background Radiation

1
The Discovery of the Cosmic Microwave Background
Radiation

• Robert W. Wilson
• Harvard-Smithsonian Center for Astrophysics

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Talk Outline

• A Brief discussion of Cosmology in the first half
  of the 20th Century.
• Background of the Discovery.
• The discovery of the Cosmic Microwave Background
  Radiation (from my perspective).

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Cosmology in the Early 20th Century

• Prior to the 20th Century cosmology was the study
  of objects in the universe, not the physics of
  the universe as a whole.
• Einstein published General Relativity in 1915
  which established a theoretical framework for
  understanding the universe as a whole.
• There was one cosmological fact The night sky
  is dark (Olber's paradox).
• Einstein ignored it and introduced the
  cosmological constant to allow a solution for
  an infinite static universe . Einstein's biggest
  blunder?
• In 1925 Hubble showed that the Andromeda Nebula
  and other similar nebulae are separate galaxies,
  not part of the Milky Way.
• In 1929 Hubble and Humason formulated the
  redshift-distance law for Galaxies -gt expanding
  or Big Bang universe.

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Hubble Diagram 1929

• Cosmological Principle
• Red shift results from light traveling through
  expanding space.
• Hubble did not sample a large enough volume.
• The Hubble Constant derived from this data is 5x
  too large.
• The Earth was known to be older than the implied
  age of the universe
• Bondi, Gold and Hoyle proposed the steady-state
  theory.

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The Andromeda Nebula (Galaxy)?

• Looking out from the earth, we saw the solar
  system, the Milky Way galaxy and other galaxies,
  mostly in clusters.
• All of these are gravitationally bound and do not
  expand with the universe.
• People were beginning to realize that in galaxies
  and clusters of galaxies, the motions imply more
  mass than is seen Dark Matter.

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Early Bell Labs Communication Satellite Work

• In 1955 John Pierce published a paper in Jet
  Propulsion titled Orbital Radio Relays
• In 1957 Sputnik was launched and then in 1958
  NASA proposed launching the 100 diameter Echo
  balloon made of , 0.0005 aluminized mylar to
  measure the forces in orbit.
• Bell Labs proposed using Echo as a first
  communications satellite.
• The return signal would be weak, so they would
  combine two Bell Labs inventions to form a low
  noise receiving system.
• A traveling wave ruby MASER amplifier (the
  loweswt noise amplifier available)?
• A large horn-reflector antenna
• Echo was launched and used as a microwave relay
  in 1960. Eisenhowers voice was transmitted by
  JPL and received at Crawford Hill
• Telstar was the first active communications
  satellite, launched in 1962

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The 20 foot Horn-Reflector
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My Background

• Sputnik was launched in Oct. 1957 during my first
  semester as a graduate student at Caltech
• I Joined a Radio Astronomy group which was just
  completing the heavy construction for the first
  Owens Valley Interferometer because my interests
  in engineering and physics would both be
  satisfied.
• As a graduate student my one cosmology course was
  taught by Sir Fred Hoyle and philosophically I
  liked the steady-state theory.
• In my thesis I used one of the dishes of the
  interferometer to measure the brightness of the
  part of the Milky Way we could see from the Owens
  Valley at 960 MHz (31 cm wavelength).
• I had no absolute measure of the brightness. We
  are inside the Galaxy, so our line of sight
  passes through some of the Galaxy wherever we
  point.

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Bell Labs 1963

• After finishing my PhD and a one year post-doc I
  took a job at Bell Labs' Crawford Hill Lab in
  1963.
• Crawford Hill people had carried out much of the
  Echo project and been involved in Telstar.
  (Comsat had been created just before I arrived.)?
• Arno Penzias had been hired a year earlier after
  he finished a radio astronomy thesis at Columbia
  with Charlie Townes.
• Why did Bell Labs hire two radio astronomers?
• The attraction to Arno and me was the research
  atmosphere, the generous support and the
  opportunity to use the 20 foot horn-reflector
  with very low noise traveling wave MASER
  amplifiers from Whippany.
• Both Arno and I had used larger antennas for out
  theses, but the 20 foot horn-reflector had
  special properties.

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Our Plans

• Arno and I set out a series of measurements we
  wanted to make using the 20 foot horn-reflector.
  Among them were
• Measure the absolute strength of CasA at 4 Ghz.
  This would be useful for astronomy and satellite
  systems.
• Look for a halo of radiation around the Milky Way
  at 1.42 GHz wavelength and establish a zero level
  for Galactic radiation (fix up my thesis).
• Make a much better search for atomic hydrogen in
  galactic clusters (fix up Arno's thesis).
• By starting with the existing 4 Ghz system, we
  could do the useful measurement of CasA and a
  control measurement for the galactic halo.
• We built the best measuring system we could
• Arno made a liquid helium cooled reference noise
  source.
• I made an accurate switch for comparing the
  antenna to the reference and greatly improved the
  stability of the measuring system.

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The Discovery Measurements

• After assembling our radiometer, our first
  measurement was a big disappointment. The
  antenna was hotter than the reference source and
  it should have been colder. There was an extra
  3.5K antenna temperature which we could not
  explain.
• This was similar to what had been seen before at
  Bell Labs, but we had an accurate measurement
  which could not be dismissed as experimental
  error.
• By this time Dave Hogg and I had measured the
  gain of the 20-foot horn-reflector accurately and
  the most important thing for us to do was to make
  the CasA measurement while that gain was still
  valid.
• We spent 9 months improving our receiver,
  checking that several different ways of
  calibrating it gave consistent answers and
  accurately measuring the strength of several
  radio sources, including Cas A.
• During that time the excess noise remained the
  same and we found no fault in our equipment or
  environment to explain it.

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Identification

• After completing the CasA measurement we started
  cleaning up some parts of the antenna which we
  had not wanted to disturb earlier, including
  removing a pair of pigeons and their droppings.
  This had little effect.
• Then one day Arno called Bernie Burke at MIT ...
• Arno called Bob Dicke at Princeton ...
• Arno and I were happy to have any explanation,
  but neither of us took the cosmology seriously at
  first. Cosmology had never predicted anything.
• We and Dicke's group wrote two separate papers.
  Our measurement might be correct even if the Big
  Bang was not the source.
• We made one final check for antenna problems with
  a transmitter.
• Before the papers were published, there was a
  leak from the ApJ office and Walter Sullivan
  wrote a story about our measurement on the front
  page of the New York Times. That convinced me
  that the world was taking the cosmology seriously.

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Confirmation

• The first confirmation came quickly from an
  unexpected source. From 1939 to 1943 Dunham,
  Adams and McKellar had measured the rotational
  excitation of CN molecules in diffuse
  interstellar clouds from their absorption of star
  light. Herzberg wrote in his standard book on
  the interstellar medium.
• From the intensity ratio of the lines a
  rotational temperature of 2.3 degrees follows,
  which of course has only a very restricted
  meaning.
• The excitation of CN molecules was remembered by
  3 separate groups.
• Burnie Burke told George Field about our
  measurements. George had written a paper while
  an assistant professor at Princeton ...
• Pat Thaddeus asked Nick Wolfe about tests for
  radiation and Nick remembered the CN.
• Shklovsky remembered the CN.
• By the end of the year Wilkinson and Roll had
  made a measurement at 3 cm wavelength which
  agreed with ours.

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Measurements of the CMBR After a Year
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The Theory Was Also Not New

• George Gamow and associates were exploring
  nucleosynthesis in the big bang in the second
  half of the 1940's. They glossed over
  inconvenient facts of nuclear physics.
• In 1949 Alpher and Herman calculated that the
  radiation from the big bang would be 5 K, but
  they also had the nuclear physics wrong.
• In 1953 Alpher, Folin and Herman made the first
  modern analysis of light element formation, but
  did not discuss the radiation.
• Alpher and Herman had inquired about the
  possibility of measuring the radiation
  temperature of the universe, but were told that
  it would be impossible to detect.
• I had read several of Gamow's children's books as
  a boy, but I don't believe that the radiation was
  mentioned and I certainly did not remember it if
  it was.

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Science Does Not Always work the Way the
Textbooks Say 5 Near Misses

• The original CN. Not enough was known about the
  conditions in the diffuse clouds at the time.
• Gamow, Alpher and Herman. In my opinion a
  measurement could have been made in the 1950's.
• Ed Ohm at Bell Labs carefully measured all of the
  components of the satellite receiver he built for
  the 20 foot horn-reflector and measured an excess
  3.1K.
• George Field was on the trail, but discouraged by
  an expert. If he had walked a couple of hundred
  feet across campus to the physics department and
  talked to Dicke, I might not be here today.
• Doroshkevich and Novikov in the last paragraph of
  a 3 page paper on the local radiation density in
  1964 suggested the importance of checking the
  Gamow Theory. They found, but misinterpreted
  Ed Ohm's paper and concluded that there was no
  measurable radiation.

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Prompt Results

• The steady-state theory was put to rest. If the
  spectrum was that of a black body and its source
  still present, it would hide everything behind
  it.
• The steady-state theory was dying anyway.
• 1964 Hoyle and Taylor paper pointing out a
  problem with helium in SS.
• Radio source counts implied an evolution
• Quasars are absent locally
• Steven Hawking showed that the Big Bang is
  possible in general relativity without a
  singularity problem.
• There was an almost immediate acceptance of our
  measurements little resistance to a paradigm
  shift.
• Cosmology gained an observable which could be
  accurately measured.

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From 1965 Until Now

• COBE showed that the spectrom of the CMBR is
  accurately a black body from the radio to the far
  infrared.
• COBE also found the first evidence of variations
  of intensity which seeded the development of
  structure in the universe.
• Cosmology has become a real science.
• A page full of accurate numbers can be derived
  from the CMBR alone.
• Although there are some remarkable fits between
  theory and observation, some large problems
  remain, especially in the very early universe.