What is the Cosmic Microwave Background?

Hello and welcome to the first installment of “The Interesting Part”, a weekly blog where I look in a little more depth at the Interesting Part (to my mind) of some of the week’s science news.

     For my first week, I thought I’d go a little easy on myself and pick something I already have some knowledge about. It just so happens that a topic fitting this description found its way into three different articles that I came across this week. Firstly, this article from Ars Technica talking about the so-called “cosmological axis of evil” first discussed in a 2005 paper based on WMAP data. My topic also appears in this New Scientist article from last month discussing the “mysteries” of the big bang (warning, requires subscription) and in Annalee Newitz’ piece for Slate, republished in this week’s National Post about a Avi Loeb’s suggestion of a “habitable epoch” in the early universe. The thread that ties these together? The Cosmic Microwave Background or CMB, a faint light that permeates the entire known universe, which originated before galaxies, planets or even stars began to form.

Most articles on the Cosmic Microwave Background describe it as a leftover glow from the Big Bang, or (more accurately) as a leftover glow from the epoch of recombination. While these are true, they don’t give much of an idea of what the CMB is, how it got there or how it can tell us anything about the universe we live in today. In order to get answers to some of these questions we first have to talk about what happens to matter when it gets hot. Very hot. Imagine you are in your kitchen with a pot full of water. You turn on the stove and the water starts to heat up. As it does the bonds between the H2O molecules break and the molecules move apart from one another: The water turns into steam, the steam escapes the pot, and your glasses fog up.


Now, what if we were to capture that steam and continue to heat it? The molecules would be given more and more energy, moving faster and faster, until they had so much energy that the bonds holding them together break. Now we don’t have H20 anymore, just some H, and some O. The water has broken up into its constituent elements. Lets continue in this vein: Pick one of the elements (it doesn’t matter which) and keep heating it. Eventually, the bonds which tie the elementary particles together to make the atoms themselves will be too weak to hold. Now all you have are the protons and neutrons and electrons which once made up the atoms in the elements in the molecules in the pot of water. How are you going to cook your pasta now?


Now, lets go back to about 300,000 years after the Big Bang (still over 300 million years before the first stars formed). Here, all the energy and matter that makes up the present day universe are pressed into a universe 0.1% of its present day size. In other words, its very, very hot. Too hot for the electrons and protons that make up the matter in the universe to form into atoms. Instead, they speed about the universe interacting via gravity and via the electromagnetic force (remember, protons are positively charged and electrons are negatively charged). Note that, as yet, we haven’t mentioned anything about photons (or light at all), which is what the CMB is made up of. How does light enter the picture? All the forces in physics are “mediated” by a particle. This means in order for a force to effect an object, that object has to emit or absorb one or more of these “mediation” particles. For the electromagnetic force, this particle is the photon. (This is why you’ll sometimes hear light referred to as electromagnetic radiation.) Imagine two ice skaters throwing a heavy ball back and forth. As each catches the ball, some of its momentum is transferred to them, and with each catch they move further away. While not a completely accurate description of photon exchange (there’s no way of taking into account like charges attracting, for example), it is a pretty useful way of picturing the process involved.


Going back to our sea of elementary particles in the early universe, this means that as the electrons and protons are interacting with one another, they are creating and absorbing photons. You can imagine a dense soup of electrons, protons and photons zooming about at high speeds, crashing into one another. Light (photons) cannot travel very far before being absorbed by a particle, but it is also constantly being created in interactions, so as photons are absorbed, new ones are emitted: The particles are in equilibrium. The universe, however, is not so stable. Fueled by the energy of the Big Bang, it is expanding with every passing second. The volume available to these particles is getting bigger, but the energy available to them stays the same. The energy is spread out over more space and the temperature starts to drop. As it drops, the particles move more and more slowly, dropping speed until… pop! An electron is moving so slowly it can’t escape the positively-charged pull of a proton. It is captured. Our first atom is formed. All the electrons have approximately the same temperature (and hence speed), so as soon as one is slow enough to be captured, they all are. Another is captured, and another and another. Soon all of our electrons are captured by protons. We have made a sea of hydrogen atoms. This moment in the history of the universe is known (confusingly) as Recombination.


Hydrogen atoms contain a single electron, which is positively charged, and a single proton, which is negatively charged. These two charges are exactly equal, and so they cancel out. We have gone from a storm of charged particles to a calm sea of neutral atoms. What, then, of the photons? Now that the charged particles have paired up, there is nothing to absorb them or deflect them. Suddenly there is nothing in their way, and they can stream across the universe unencumbered. We say that the universe has become “transparent” to photons: they can just pass right through everything. And this is what they did. They passed through the neutral atoms and traveled across the universe as the universe itself expanded and aged until… they reached us.


It is these photons, the ones created by the interactions of early universe electrons and protons and then freed by recombination that we measure as the CMB. They carry information about the universe as it existed just before recombination, and with a little work we can use that information to tell us about our universe then, now, and in the future. Next week, I will talk about exactly what information they carry, how it arises, and what it can tell us.


Thanks for reading,



One thought on “What is the Cosmic Microwave Background?

  1. Pingback: What Can The CMB Tell Us? – Part I | The Interesting Part

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