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Robert W. Wilson Harvard-Smithsonian Center for Astrophysics. The Discovery of the Cosmic Microwave Background Radiation . Talk Outline. A Brief discussion of Cosmology in the first half of the 20 th Century. Background of the Discovery.
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Robert W. Wilson Harvard-Smithsonian Center for Astrophysics The Discovery of the Cosmic Microwave Background Radiation
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).
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 -> expanding or Big Bang universe.
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.
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.
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. Eisenhower’s voice was transmitted by JPL and received at Crawford Hill • Telstar was the first active communications satellite, launched in 1962
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.
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.
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). • Bystarting 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.
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.
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.
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.
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.
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.
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.
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.
