Hmm what is this weird noise source?

Here is the deal. At your job in the 60's at Bell labs, your boss gives you a new device that you need to understand everything about. The device is the new Holmdel Horn Antenna which is a detector used to measure a very faint signal that bounces off echo ballons. What you do for the next few years is "characterize your detector", just a fancy way of saying that you study the way the detector works in excruciating detail.

You begin by turning on your detector and studying how much power actually hits your detector. Next, you ask the question where does this power come from? You run a series of tests to understand the signal on your detector. You find that sometimes the power readings are too high or too low and you ask yourself why. You find that some are due to the rising and the setting of the sun. You find that some power fluctuations are due to how hot the actual detector is. You find teeny power fluctuations due to the changes in the seasons throughout the year. And this is what you spend your time doing hunting for noise sources. Power fluctuations due to anything other than the signal you are looking for are considered noise sources.

This was the exact job of Arno Penzias and Robert Wilson. They spent their time in the early 1960's running tests on their detector to identify noise sources. This detector operates in the radio regime of the electromagnetic spectrum (more on that later). That means their device is sensitive to long wavelength light.

The mystery arises when Penzias and Wilson can not identify why their detector is reading too much power. They go back to their calculations over and over again and can not figure out WHY this is there. They are stumped. They think and think and think about where this extra power could be coming from. They figured out it wasn't the earth, it wasn't the sun, they did research to find if it was in our galaxy and didn't think so. So what do you do?

Call a friend and ask for help.

Wilson & Penzias

Bernie Burke

Penzias and Wilson got and the phone with a friend of theirs at MIT named Bernie Burke. They asked him if he knew of any extra-galactic radio sources that may be contributing to their signal. Burke replied by saying that he read a pre-print article by Jim Peebles discussing that there may be left over radiation from the early universe. This could potentially be what you are seeing. Why don't you give Robert Dicke from Princeton a call? He has made a prediction about where we should see this signal and is gathering a team to go detect it.

Robert Dicke

Penzias and Wilson got excited because this could be a very important discovery. At the time, there was a HUGE debate in the scientific community about whether the universe was expanding or staying still. There was no experimental evidence to tell you one way or another.


Following their friend's advice, Penzias gave Dicke a call and asked about what he thought of their findings. Dicke sent over Peebles' article for a read. After a few phone exchanges, Robert Dicke, Jim Peebles and Dave Wilkinson made their way over from Princeton to the detector in New Jersey.

Jim Peebles

After visiting the site, both groups spent some time working out whether or not this detection was THE detection they were looking for. They decided to publish separate papers in the same Physical Revue Letters edition. Peebles, Dicke and Wilkinson's paper proposed that if the universe had a very hot beginning and expanded over time, then left over light will have cooled into the radio regime where we could detect it. Penzias and Wilson's paper discussed their apparatus and the methods along with their measurement. They reference Peebles, Dicke and Wilkinson as having the proper interpretation of their result.

Dave Wilkinson

So let's just stop for a minute and recap. You went to work to characterize a detector that is looking for faint radio signal bouncing off echo balloons. During the process of trying to understand all of your noise sources, you run into a puzzle you can't solve. You ask a friend for their opinion on what it could be. They happened to read a paper that offers a suggestion on what how to interpret what you see. And by you picking up the phone to confirm this, you establish a relationship with some of the greatest scientists of all time. Before you know it, you and your co-worker have just stumbled across arguably the most important experimental evidence for cosmology. Your detection is how we have confirmation that we live in a universe that began from a hot big bang.

That is pretty cool.

image credits : wikipedia

just how epic is the CMB measurement?

We have all seen this picture. We know what it is called. We can say where it comes from. We know the discovery story.

But I asked the question: "Just what is the big deal with the cosmic microwave background?" 

Furthermore, How did this serendipitous story of discovery lead to the most significant cosmological experiment to date? I personally felt that there were some critical steps left out of the story that did not allow me to appreciate just how important this measurement is and how the process of discovery *actually* happened. I am going to spend a few posts talking about it. Why? Because it is cool.

The CMB in a nutshell : 

• CMB = Cosmic Microwave Background
• The cosmic microwave background is a measurement that the temperature of the entire sky is 2.73 Kelvin = -270.3 Celsius = -454.54 Fahrenheit
• The picture above is a map of the very,very tiny temperature fluctuations in the sky as measured by the WMAP instrument. The colors follow our human intuition of how colors related to different temperatures.* The warmer and cooler colors correspond to warmer and cooler temperatures fluctuations. 
• In the very early universe, photons of light could not escape the opaque environment. When the universe expanded and cooled, the environment became transparent enough that photons of light were able to escape. These photons from the very early universe IS the radiation we measure in the microwave regime. (Don't worry, I will provide a much longer explanation of this later!)
• The discovery of the CMB was made by two scientists working at Bell Labs in NJ. The discovery of the excess radiation was made by Penzias and Wilson. The interpretation of what this excess radiation actually is was provided by Peebles, Dicke, and Wilkinson. 
• What parameters did the CMB give us? (not a complete list)
• Age of the universe
• Amount of baryons in the universe (all the stuff we are made of and can see!)
• Amount of dark matter
• Amount of dark energy
• The geometry of the universe
• Man that is a lot! What else do you need? More on these parameters too!

I am going to spend a few posts talking about the discovery story, where the CMB photons come from, how to interpret data, and ALL of the cosmological parameters that we get out of the CMB.

Stay tuned!

*This statement of our "human intuition of how colors relate to temperature" is not a good physical way of categorizing a color to temperature relationship. In reality, bluer colors are actually much much hotter than the redder colors. Looking at a flame can confirm this for you. What color is the center of the flame? Is the center of the flame hotter or cooler than the outside of the flame?