As I briefly alluded to a couple of posts ago, in my series on the Big Bang and inflationary cosmology, the discovery of the Cosmic Microwave Background (CMB) played a decisive role in building support for the theory of a Big Bang and in undermining the Steady State model.
Tonight, I'd like to launch a three-part posting on the CMB. The remainder of the present piece will constitute Part I, focusing on the early theorizing about the CMB. In the coming weeks, Part II will focus on the accidental discovery of the CMB, and Part III on the more recent, high precision measurement of it.
According to Simon Singh's (2004) book, Big Bang, CMB theorizing goes back to Ralph Alpher and Robert Herman in 1948, with Alpher's mentor George Gamow also being associated. (The famous Alpher, Bethe, Gamow paper, with the name of Hans Bethe inserted primarily to make the authorship sound like the Greek letters alpha, beta, and gamma, dealt with a different issue, nucleosynthesis.)
What Alpher and Herman did was formulate ideas about the interactions of temperature, light, and states of matter (particularly one known as plasma) in the early universe. The following passages from Singh convey the main points of Alpher and Herman's theorizing:
They realised that as the universe expanded, its energy would become spread through a greater volume, so the universe and the plasma within it would steadily cool. The two young physicists deduced that there would be a critical moment when the temperature would become too cool for a plasma to exist, at which point the electrons would latch onto the nuclei and form stable, neutral atoms of hydrogen and helium. The transition from plasma to atoms happens at roughly 3,000 [degree Celsius] for hydrogen and helium, and the duo estimated that it would take 300,000 years or so for the universe to cool to this temperature. This event is generally known as recombination... (p. 330).
If the Big Bang model was correct, and if Alpher and Herman had got their physics right, then the light that was present at the moment of recombination should still be beaming its way around the universe today... In other words, the light that was released at the end of the plasma epoch should currently exist as a fossil. This light would be a legacy of the Big Bang (p. 332).
One further refinement involved accounting for the stretching of light waves in an expanding universe. "Alpher and Herman confidently predicted that the stretched Big Bang light should now have a wavelength... invisible to the human eye,... located in the so-called microwave region of the spectrum" (Singh, p. 333).
Hence the name Cosmic Microwave Background.
Brian Greene, in his 2004 book The Fabric of the Cosmos, presents an even more vivid account of how one can imagine the CMB:
If your eyes could see light whose wavelength is much longer than that of orange or red, you would not only be able to see the interior of your microwave oven burst into activity when you push the start button, but you would also see a faint and nearly uniform glow spread throughout what the rest of us perceive as a dark night sky (p. 226).
Given the importance of the CMB prediction for testing the theory of a Big Bang, and what seems like an intrinsically interesting idea in its own right, one would think that Alpher and Herman's suggestion would have sparked immediate attempts to detect the CMB. But, as Singh notes, "Unfortunately, Alpher and Herman were completely ignored. Nobody made any serious effort to search for their proposed CMB radiation" (p. 333).
How the CMB was ultimately discovered will be the topic of my next entry...