- 2841 - CMB - Cosmic Background Radiation - First discovered in 1965 by Penzias and Wilson who were doing experiments with radio antennas for Bell Telephone labs. The theory of its existence was first proposed by astronomer George Gamow in 1948, but they did not have any radio receivers that could listen for those frequencies back then.
--------------------------- 2841 - CMB - Cosmic Background Radiation
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- In 2007, I attended a lecture at Stanford University about the “Cosmic Background Radiation“. At that time astronomers believed it to be compelling evidence for the Big Bang. The radiation has been studied for decades.
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- The radiation left the Big Bang as Gamma Rays and has stretched with the expansion of the Universe to where the wavelengths are in the millimeter to centimeter wavelength today, which is the microwave frequency range.
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- A plot of this radiation looks exactly like a Blackbody Curve. All Blackbody Curves look the same shape they just are plotted over different wavelengths and energy densities.
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- The Cosmic Background has wavelengths on the x-axis from 0.5 to 0.05 centimeters. On the y-axis the Flux ranges from 100 to 400 megajansky/sterdian. The peak of the curve occurs at 0.11 centimeters which is equivalent to a temperature of 2.7325 +- 0.017 Kelvin.
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- The radiation left 370,000 years after the Big Bang at 3,000 Kelvin ( before that it was trapped in the hot ionized plasma) . The Universe has expanded by a factor of 1000 and therefore the temperature has cooled by a factor of 1000, to about 3 Kelvin.
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- The amazing thing about studying astronomy is that the farther you can see the further back in time it is. When you look back 13,000,000,000 years there are no galaxies. They have not formed yet. That is as far back as you can see with visible light.
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- This distance corresponds to a redshift of 6. A redshift is the ratio of how much the wavelength stretches with the expanding Universe divided by the original wavelength.
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------------------------ ( z = w - wo / wo). ( Where wo is the original wavelength)
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- The redshift of the microwave background is: z = 1100. That allows us to see back in time to 370,000 years after the Big Bang. Actually it did not happen all at once. The radiation actually started 115,000 years after and ended 487,000 years after the Big Bang according to the calculations. This was the period when ionized hydrogen captured electrons and became the neutral hydrogen atom, thus freeing up the photons to escape into the Universe and become the Cosmic Background..
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- Astronomers have measured the background radiation to a millionth of a degree. It spans the frequency range from 60 to 600 gigahertz, from 0.5 to 0.05 centimeters wavelength.
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- Most of the radiation in the entire Universe today is this background radiation. It is only 2.73 Kelvin but it fills all observable space. And, that is a very big area. This radiation is 5*10^-5 of the total density of the Universe.
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- In electron volts the energy of the radiation is 0.25 eV. This is much less than the 13.6 eV for the ionization energy of hydrogen. 13.6 eV is how much energy is required to free the electron from the hydrogen atom.
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- When you look at the sky you see blue because that is the frequency, wavelength, or temperature of the atmospheric radiation. That is the frequency of light that your eyes detect.
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- However, if you eyes could detect the entire electromagnetic spectrum you could see the microwave background radiation coming from all directions in space, day or night. It fills everything. It is equally bright in all directions. It is a sea of Blackbody radiation.
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- The Cosmic Background Radiation was first discovered in 1965 by Penzias and Wilson who were doing experiments with radio antennas for Bell Telephone labs. The theory of its existence was first proposed by astronomer George Gamow in 1948, but they did not have any radio receivers that could listen for those frequencies back then.
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- Today we have such accurate instruments that operate above the atmosphere the Cosmic Background radiation has become the most precisely measured Blackbody spectrum in nature.
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- A Blackbody Curve is a plot of energy density versus wavelength. The Cosmic Background peak energy density is 400 megajansky/steradian, occurring at 1.1*10^-3 meters wavelength.
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- The Curve follows a very complex equation but the peak is simply the temperature of 2.9*10^-3 meter-Kelvin divided by the wavelength.
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------------------------- T = 2.93*10^-3 m*K / 1.1*10^-3 m = 2.725 Kelvin.
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------------------------- The frequency at the peak is 270 Ghz.
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- A megajansky is a unit of radiant flux density = 10^-20 watts / m^2 / hertz. A steradian is a unit of a solid angle subtending a center of a sphere of radius to a portion of the surface area that is radius^2.
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- From this data astronomers have concluded the age of the Universe to be 13.7 billion years. That the first generation stars came to light 200,000,000 after the Big Bang. That the Universe is made up of 73% Dark Energy and 23% Dark Matter, and only 4% ordinary matter that we are made of.
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- Because the same temperature is seen in all directions we conclude that the Universe is in thermal equilibrium. This could best be explained with Alan Guth’s “Cosmic Inflation Theory“.
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- When you look at the sky you see a perfectly smooth, solid, single radiation of blue light. At 500 nanometers blue light wavelength. Your eye cannot resolve any variations it just gets a blur of blue.
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- However, if your eye had a million times better resolution it would see hot and cold spots, ripples, due to water vapor, dust, turbulence, etc. The same thing happened with the Cosmic Background radiation. When their resolution was from 0 to 4 degrees all they saw was a perfectly smooth, solid, radiation at 2.73 Kelvin.
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- But, in 1997 their resolution had improved to detect 2.721 to 2.729 Kelvin and a pattern of hot and cold regions appeared over a difference of 0.0002 Kelvin at a frequency of 70 gigahertz. These hot and cold regions are thought to have originated as quantum uncertainty fluctuations when the Universe was the size of an atom. Today, they are the galaxy clusters of denser, hotter matter in the otherwise rarified ,colder space.
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- Further study of the Cosmic Background radiation hopes to detect the polarization of this radiation. The physics of how photons scatter off free electrons is called the “Thomson Scattering” effect and it induces polarization.
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- Astronomers hope to learn more about the evolution of the Universe with polarization data. They also want to learn more by detecting gravitational waves and by detecting neutrinos.
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- Photons have been great, but we need more data. This stuff is all theory. Be skeptical. Per ponderous theories require per ponderous observational data to support them.
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- From the book “Principles of Physical Cosmology“, P.J.E. Peebles, 1976
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- September 24, 2020 757 2841
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