Georges Lemaître’s theory of an expanding Universe, which has become known as the Big Bang, was supported by Hubble’s observations. The expanding Universe idea was challenged by influential scientists who believed the Universe was both infinite (and therefore not expanding) and steady state (unchanging). Supporters of the Big Bang idea needed to find other evidence that could confirm their model was correct.
The cosmic microwave background radiation (CMBR) is radiation left over from the big bang. When the universe was very young, only 380,000 years old, just as space became transparent to light, electromagnetic energy would have propagated through space for the very first time. At this stage in its development, the temperature of the Universe would have been about 3000K. Nowadays, the temperature of space has fallen to approximately 2.7 K (that’s 2.7 K above absolute zero!) and, using Wien’s Law, we can confirm that the peak wavelength of the electromagnetic radiation is so long that the background radiation lies in the microwave portion of the em spectrum.
The CMBR was first detected in 1964 by Richard Woodrow Wilson and Arno Allan Penzias, who worked at Bell Laboratories in the USA. They were building a radio wave detector when they found a source of noise that seemed to come from every direction. That the noise came from every direction ruled out a specific star or galaxy. They were also able to rule out the urban environment but…
“then they found droppings of pigeons nesting in the antenna. They cleaned out the mess and tried removing the birds and discouraging them from roosting, but they kept flying back. “To get rid of them, we finally found the most humane thing was to get a shot gun…and at very close range [we] just killed them instantly. It’s not something I’m happy about, but that seemed like the only way out of our dilemma,” said Penzias. “And so the pigeons left with a smaller bang, but the noise remained, coming from every direction.” source: NPR.org
Here are Wilson and Penzias next to their detector. From the size of the detector, you can see why pigeons might find it a good place to stay.
Wilson and Penzias shared the 1978 Nobel Prize for Physics for their discovery of the CMBR.
The speaker at 5:52 is James Peebles, who was awarded the 2019 Nobel Prize for Physics.
While the CMBR is evidence in support of the Big Bang model, measurements of this radiation have suggested it is very smoothly distributed throughout the Universe. Too smooth for lumpy bits of matter such as stars and galaxies to form. Clearly we have stars and galaxies, so we need to understand how this can happen when the CMBR is so uniform. It turns out we needed better detectors to see the tiny variations within the CMBR.
Three different satellite missions have been used to map the CMBR across the visible Universe. The first of these was COBE (1989-1993).
The second mission was the Wilkinson Microwave Anisotropiy Probe or WMAP (2001-2010), but many of the images we see of the mapped CMBR came from the third satellite provided by the European Space Agency mission called Planck. (2009-2012). Planck used liquid helium at a temperature of 4K to cool it’s detector for maximum sensitivity. Unfortunately, its helium supply only lasted for three years but it produced some incredibly detailed measurements in that time.
Grab some popcorn and watch these videos about the cosmic microwave background.
Professor Copland’s talk (below) is good but don’t go beyond 10 minutes!