What exactly is the cosmic microwave background (CMB)?
It turns out that CMB is the radiation emitted when the universe stops being a plasma and turned in to a gas.
So let’s understand now what a plasma is? Plasma is ionized gas a very fast-moving protons and elections. Plasma holds more similarities with gas. As it expands it cools. But the main difference between a plasma gas is that the emission spectrum for plasma is continues. On the other hand gas has both emissions and absorption light, due to the orbitals in which the elections are constrained.
CMB was omitted three hundred thousand years after the Big Bang. Part to that radiation traveled 13.5 billion years just to end its magnificent life absorbs in the detector on the Plank satellite.
300 thousand years after the Big Bang is the time when the transition from plasma to gas happens. For the very first time in the very young age of our cosmos, the light fill in the plasma was finally free to travel across the universe.
Why do we have a transition from plasma to gas in the first place?
The universe has been expanding ever since the Big Bang. As you know both plasma gas decrease the temperature when it expand.
In a plasma a decrease in temperature corresponds to decreased in average energy for the photon. When this average energy
gets below the ionization energy for hydration, the protons and elections are free to combine into atoms and form
neutral gases. And this is when the universe becomes transparent and start emitting the light and move freely.
This light travels billions of years while space is stretching and landed on the earth’s satellite.
Since the space stretches this lights’ wavelength of the light also stretches up to microwave spectrum. Those were first detected accidently by Arno Penzias and Robert Wilson in 1964 using a large radio antenna in New Jersey. For this discovery they awarded the Nobel Prize in Physics in 1978.
In 1989 NASA lunched the Cosmic Background Explorer (COBE), to explore this radiations and understand the beginning of the universe.
NASA second big mission in 2001 was the Wilkinson Microwave Anisotropy Probe (WMAP) to fine tune the details identified in CMB earlier. And then finally in 2009 Planck was satellite mission launched in 2009 to study even greater detail in CMB.
The following picture is one of the main the outcome of all these missions. This is finally the picture of our baby universe.