The Planck Mission was a European Space Agency (ESA) mission (2009-2013) with significant participation from NASA. The analysis of the data was completed in 2015. The Planck space observatory mapped the Cosmic Microwave Background (CMB).
If you set an analogue TV between channels it is estimated that about 1% of the signal that it receives ("static" or white noise) is due to the Cosmic Microwave Background, a low level microwave radiation that is present everywhere in the universe. The CMB is very bright at millimetre-wavelengths.
(image from wikipedia)
They were awarded the Nobel prize for their discovery in 1978
Here is a video excerpt from the 2004 BBC film "Hawking". It is the interview of the two scientists just before the Nobel award ceremony.
Excerpts from the video:
"On the 10th December 1978, Arno Penzias and Robert Wilson received the Nobel Prize for their 1965 discovery of the leftover heat from the Big Bang explosion."
"It was beautiful, I came out of the woods and there it was; a perfect piece of equipment, the antenna, a great big horn more like a gramophone horn lying on its side, the size of a house. In the middle of a field, on a hill, 20 miles from New York City. Picking up the hiss. Pointing at the stars."
(Reporter) "What was it for, the twenty feet horn?"
"We wanted to measure noise from the outer edges of the Milky Way. Arno build a cold load."
(Reporter) "What is that?"
"5 Imperial gallons of liquid helium. You know how much helium that is?"
(Note: the reporter is British and this is a UK measurement unit)
"A hell of a quantity of helium. An unbelievably accurate reference against which you can measure the noise you see."
"Bob made a great switch to connect the receiver alternately to the antenna and the cold load reference."
"Noise is heat. The higher the heat, the more intense the noise."
"You may want to ask if we got noise. We got a lot of noise, which means a lot of heat, far more than what the Milky Way could have given us."
"The heat we were receiving should have been two degrees lower than the cold load, lower than the reference. But it was hotter, three degrees hotter. Hotter than what the Milky Way could produce. Hotter than the sum of all of galaxies beyond."
"So we figured, it had to be something closer to home."
"There was some high altitude bomb testing back in the 50s. Maybe it’s the leftover radiation. But it would diminish over time and what we were getting was totally constant."
"White dielectric material. Lots of it, inside the antenna horn. Had to be it. Had to be the cause of the hiss."
(Reporter) "White’s dialectric material?"
"Pigeon shit, all over the horn."
"We cleared out all the dielectric material. On our hands and knees in our labcoats in the horn scrapping the white dielectric material, the white stuff."
"The hiss was still there, the pigeon stuff was not the hiss."
Another hypothesis is made. The surrounding environment noise, the noise of the city produces the hiss. The horn was 20 miles from New York. The scientists say:
"Maybe it’s New York! Maybe the hiss is New York!"
"We figured that if there is any city in the world to give you three degrees of hot radio noise it must be the Big Apple! My family alone could probably make this hiss."
"We pointed the antenna to New York City. All that energy spread out across the northern horizon walking from subway rails, hum from power lines, the radio amplifier at Kennedy airport spewing out radio noise by a kilowatt."
"That was not it. It gave us a reasonable amount but not enough heat."
"What is it? It is the sound of the beginning of time. The leftover heat from the Big Bang. The three degrees that hasn’t cooled yet."
"This god damn beautiful hiss, it connects … (a moving story is told, that of a Jewish family which escapes from Germany to go to America to survive and live the American dream)."
"It is everywhere. It is all around us. It is 15 billion years old."
"And we found it. That is our discovery."
"We have to go get the prize Arno."
The Universe consisted initially of a mixture (soup) of electrons, protons and neutrinos; photons were trapped in the mix. In the beginning, the density of the universe was very high: as all the above were concentrated in a small space, (1) photons could not travel big distances and as light could not be propagated, the universe was dark of “foggy"; (2) electrons and protons, which normally attract each other, could not combine and form simple atoms, because photons would collide with them and would disassociate them.
Following the Big Bang, the universe would start to expand and photons could travel long distances. Light started its cosmic journey. We see it today in the form of the cosmic microwave background. As the photon inhibition was relieved, electrons and protons could form simple atoms. The simplest atom consisting of one proton and one electron would be the hydrogen atom.
However the journey of the photons has a bumpy road!
This is explained in the section "Why it is lumpy?" at this link.
The Planck ToolKit Introduction
A very nice picture when enlarged: