What is CMB in Astronomy?

We already know that CMB (Cosmic Microwave Background) is the remnant of the Big Bang fireball, and it is emitted from the last scattering surface before the Universe became neutral (that is, filled with neutral, not ionized, particles). It is also the most perfect black body in the Universe because during the time of the Big Bang fireball, the Universe was so small, enough to mix the temperatures into a single temperature of T ~ 2.73 K.

What is CMB?

The CMB is usually called “the remnant of the Big Bang” or “leftover radiation from the Big Bang” due to the aforementioned definitions of CMB. It was emitted around 400,000 years after the Big Bang. Before the time it was emitted, its temperature was much higher (around 273 million degree Kelvin). During this time as well, atoms were easily broken apart into smaller sub-atomic particles (protons and electrons). These sub-atomic particles caused the scattering of CMB photons, creating its almost isotropic (uniform in all directions) look.

The Last Scattering Surface

380,000 years after the Big Bang, hydrogen was able to form because the Universe was cool enough to do so. The CMB photons, compared to electrons, were not so affected by colliding with hydrogen, so they can now travel in straight lines. This is what we mean by the “last scattering surface”, which implies the last time that CMB photons hit matter (electrons). After that, the Universe has expanded to a large volume already. This also means that when we map the CMB of the Universe, we look back up to 380,000 years after the Big Bang, when the Universe was still opaque to radiation.

CMB Maps

Since CMB allows us to look back to earlier years when the Universe was formed, it helps us learn about the origins of galaxies and other large-scale structures in the Universe. It also helps in measuring certain parameters about the Big Bang theory. The earliest CMB map was made by NASA’s Cosmic Background Explorer (COBE) mission, showing data collected between 1990 and 1992.

In 2003, the Wilkinson Microwave Anisotropy Probe (WMAP) showed a more recent and detailed CMB map. This new CMB map was used by scientists to probe the age of the Universe to be 13.7 billion years old. This means that the CMB light that we see today has traveled over 13 billion years to reach us.

The most precise CMB map was taken in 2013, which came from the European Space Agency’s Planck space telescope. Their results showed a more convincing evidence that dark matter and dark energy, the two important forces that account for the accelerated expansion of the Universe, exist.

Scientists also made use of their results on studying CMB by understanding the so-called “inflation”, the first trillionth second after Big Bang.