Monday, June 1, 2009

Introduction to the Cosmic Microwave Background

The first series of posts contained one argument for the existence of dark matter. The response from my mother among others was tentatively positive, although most comments seemed to agree that I was perhaps going a bit too fast with the math and trying to pack too much in (my beloved sister has weighed in with a somewhat more negative opinion for which I thank her with all the fraternal feeling I can muster). I take the point that this blog may need more romance and less dry insistence, and I will attempt to respond accordingly.

Therefore, my next topic will be another argument for the existence of dark matter, and in my opinion one of the cooler phenomena in physics (I understand that my stating something is "cool" is not necessarily sufficient evidence, but I will try to explain) - the Cosmic Microwave Background or CMB for short (another good name, by the way).

In very broad strokes, the CMB is an echo or an image of the universe as it was 13 billion years ago (when it was only four hundred thousand years old - relative to the human lifespan, it's like we have a baby picture from when the universe was 1 day old). Much as archaeologists can learn about prehistoric epochs from fossils (or mosquitos trapped in amber) and geologists can infer the climate from ice cores that have been frozen for thousands of years, physicists can discover information about the contemporary contents and future evolution of the universe by studying the CMB.

So what is the CMB? It's a sea of light streaming across the universe in all directions that was produced 13 billion years ago and has not touched anything since that time. This light isn't visible to us, because its wavelength (remember these posts) is in the microwave band (i.e. too long to be visible by our eyes, but with enough intensity [thankfully not present in the actual CMB or else we'd all be in trouble], perfect for heating up instant hot chocolate [too quaint?]). It's always there though, and like a photograph, each individual photon contains an image of the universe shortly after the big bang.

The illustration (click for a bigger view) shows the history of the universe from the Big Bang to the present. The CMB is produced at the green and blue ellipse during the very early universe and detected in the present by the satellite labeled "WMAP."

I'll stop there for now, but hopefully the reader will want to know more. I'll probably refer to two web sites a great deal in the coming posts. The best existing CMB experiment is the Wilkinson Microwave Anisotropy Probe, or WMAP, and they have a great resource at from which I've taken the illustration. The second web site is where I learned most of what I'll be talking about, the homepage of Professor Wayne Hu of the University of Chicago. He's done a great job explaining all the details and implications of the CMB in simple terms, and I hope to do half as good a job.


  1. How can you be sure the CMB emerged/started 13 billion years ago, as opposed to, say 14 billion? How can you be so precise? Also, how do you KNOW there was a Big Bang? Does everybody know except me? I love the idea of the CMB, a new expression entirely in my vocabulary.

  2. You are right to question the precision used in the 13 billion year number. The number most commonly published is 13.7 plus or minus 5%. In casual conversation, cosmologists simply truncate the number although mathematically it would be more correct to round it up to 14.

    The major obersvation evidence for the big bang is that the observed relative abundances of three primordial elements, Hydrogen, Helium and Lithium strongly agrees with the theoretical values predicted for nuclear genisis in a universe undergoing rapid expansion.

    I am especially impressed that you asked the How-We-Know-That question because most people are far too trusting. For example, most people will tell you that the Earth goes around the Sun but if you ask them what is the observational evidence to gainsay the simple eyeball evidence and common use speech of the phrase, sun-set, they are at a loss. After all, we never speak of the horizon rising, which is more correct. When science becomes a matter of blind-faith then it is no better than superstition.

  3. The CMB project sounds great and really interesting I think that it will be really helpful to many people who work in that.