Unit 4: Spotlight on the CMB

What is the Cosmic Microwave Background (CMB)? How was it created? How do we observe it now? And what information does it tell us about our universe?

Formation of the CMB

The CMB was formed after the initial distribution of nuclei were formed through BBN (To review these details, refer to the previous module) and is composed of the left-over radiation from the early universe. There were lots of photons zipping around as by-products of atomic fusion and from atomic interactions/decay. After the universe expands and cools for a bit, these photons are able to decouple from the rest of the particles in the universe and evolve separately. These lonely photons are then sent into the universe, their energy decreasing with the expansion of the universe, forever unable to disrupt atomic nuclei. The point at which photons fully decoupled from atomic nuclei is known as the surface of last-scattering. 

We observe these photon survivors today as the Cosmic Microwave Background (CMB). Because these photons were originally created early in the universe, the CMB is often called the 'baby picture' of the universe. These formerly high-energy photons have had their energy reduced by cosmic expansion to measly microwaves. Despite their reduced energy, these photons provide a lot of information about the universe and its evolution. 

To uncover the full picture of how the CMB formed, there are two additional processes that created photons in the early universe that make up a large fraction of the light we see from the early universe. Let's review them here:

Matter - Anti-Matter Annihilation and its Consequences

We briefly covered this concept in An Overview of the Atom, but we will reiterate its importance in the context of the CMB. When matter and its anti-matter collide and annihilate, they will either form two photons or two neutrinos of the same flavor. Generally though, the production of photons is much more likely to occur than the production of neutrinos. 

Remember that this process happened before ionized nuclei formed through BBN. If it didn't, then the anti-matter would still be around and destroy BBN's hard work of creating nuclei. Although the anti-matter is no more, we now have very energetic photons flying through the universe. 

Recombination

First, recall that Big-Bang Nucleosynthesis created just the atomic nuclei that we see in the universe. These are then ionized atoms flying through the universe. Recombination refers to the process through which electrons were captured by the ionized nuclei to form neutral atoms. 

levels of atomic energy

photons can also increase energy levels

How the CMB Looks on the Sky

Now that we have a better idea about how the CMB formed, let's take a look at a map of the CMB found by the Planck 2018 mission.

The largest contaminant spans the middle of this map. There's a lot of dust and star formation which emits light in the microwave spectrum in galaxies. The culprit is none other than the galaxy we live in: the Milky Way. There are also other contaminants to the CMB, like other nearby galaxies and big clouds of dust.

Uniform Temperature

In general, the CMB is very uniform in its temperature. Today, we measure its average temperature to be 2.725 K. The hot and cold spots are temperature fluctuations, changing the measured temperature by at most 0.0005 K in these regions.

The remainder of this unit will focus on the details of how the temperature fluctuations were created and how the distribution of the temperature fluctuations provide information about the Standard Model. When you're ready, go to the next page in this unit: CMB Temperature Fluctuations .