Origins of the CMB

Decoupling of Matter
The first light radiated after decoupling is now known as the CMB.

During the first 380,000 years after the Big Bang, the universe was so hot that all matter existed as plasma. During this time, photons could not travel undisturbed through the plasma because they interacted constantly with the charged electrons and baryons, in a phenomenon known as Thompson Scattering. As a result, the universe was opaque.

As the universe expanded and cooled, electrons began to bind to nuclei, forming atoms. The introduction of neutral matter allowed light to pass freely without scattering. This separation of light and matter is known as decoupling. The light first radiated from this process is what we now see as the Cosmic Microwave Background. Similarly, in the video below, the precipitate in a solution of magnesium hydroxide scatters light from a flashlight, making it opaque to radiation.

[2.5a] Movie: The Last Scattering | Download

Why is the CMB so Cold?

Redshift scale of the CMB
Light from the CMB is redshifted as the universe expands, cooling it over time.

The CMB is a perfect example of redshift. Originally, CMB photons had much shorter wavelengths with high associated energy, corresponding to a temperature of about 3,000 K (nearly 5,000° F). As the universe expanded, the light was stretched into longer and less energetic wavelengths.

By the time the light reaches us, 14 billion years later, we observe it as low-energy microwaves at a frigid 2.7 K (-450° F). This is why CMB is so cold now.

The expansion of space cools down the CMB.

What do the Colors on the CMB Map Represent?

Although the temperature of the CMB is almost completely uniform at 2.7 K, there are very tiny variations, or anisotropies, in the temperature on the order of 10-5 K. The anisotropies appear on the map as cooler blue and warmer red patches. But what do these minute fluctuations mean?

Map of the CMB created from data gathered by the Wilkinson Microwave Anisotropy Probe (WMAP).

These anisotropies in the temperature map correspond to areas of varying density fluctuations in the early universe. Eventually, gravity would draw the high-density fluctuations into even denser and more pronounced ones. After billions of years, these little ripples in the early universe evolved, through gravitational attraction, into the planets, stars, galaxies, and clusters of galaxies that we see today.

Down the Rabbit Hole [2.5b] Down the Rabbit Hole: Acoustic Oscillations

Why are Maps of the CMB Shaped like Ovals?

CMB v. Earth
The spherical map of the CMB translates to an oval in the same way a globe translates to a familiar oval map when flattened.

The CMB is shaped like an oval for the same reason that many maps of the world are ovals. You can't take a sphere and make it flat without tearing it, because a sphere is fatter in the middle than at the top and bottom.

To see why this is true, peel an orange and try to flatten it. The only way you can accomplish this is by tearing the peel, or distorting it. Instead of "tearing" the map of the CMB, it is depicted as an oval, which is the shape with the least angular distortion of the original sphere.

Down the Rabbit Hole [2.5c] Down the Rabbit Hole: Imaging the CMB

The Predictive Power of the CMB

The Cosmic Background Explorer, COBE
In 1992, physicists used the orbiting COBE satellite to make the first detailed measurements of the CMB anisotropy.

The CMB is one of the strongest pieces of evidence for the Big Bang model. The theory makes highly accurate predictions about the size and types of anisotropies in the CMB as well as its nearly perfect blackbody spectrum, all of which have been verified by experiment and observation. The discovery of the CMB in the 1960s marked the end for several competing cosmological models including the Steady State Theory.

Classroom Cosmology [2.5d] Classroom Cosmology: Understanding the CMB

With the information attained from the CMB, we can begin to understand the formation of the structure and matter of the universe.

Down the Rabbit Hole [2.5e] Down the Rabbit Hole: Black Body Radiation

Cosmic Conundrums [2.5f] Cosmic Conundrums: Cosmic Microwave Background