The Hidden Background of the Cosmos

The unforeseen prehistoric secret of the Cosmos, revealed

Image: Horn Antenna at Bell Labs, New Jersey, the site of the discovery of the Cosmic Microwave Background Radiation. (Credits: Wikimedia Commons)

There exists a static noise throughout outer space, just like the radio static noise from your television. The origin of this noise is related not only to our origin but also to that of the Universe itself!

Discovered as an “accidental discovery”, this noise is what is known as 'Cosmic Microwave Background Radiation’. What is it and why is it such a big deal?

A review on Radiation

• Radiations consist of electric and magnetic fields oscillating perpendicular to each other as well as perpendicular to the direction of propagation of radiation.

credit:https://commons.wikimedia.org/wiki/File:EM-Wave.gif

• An ideal body that is heated emits radiation of all wavelengths is called a ‘Black Body Radiation’.
• These electromagnetic radiations being emitted have different intensities at different wavelengths which trace out in a graph like the one shown below.

Credits: Wikimedia Commons (https://en.wikipedia.org/wiki/File:Wiens_law.svg)

• This graph is known as the ‘Black Body Curve' or ‘Thermal Spectral Graph’. It is known as a ‘Thermal Spectrum' because of the random motion of the particles present inside a system that is in turn, dependent on its temperature. So, everything that has temperature also emits electromagnetic radiation that can be traced out in a thermal spectrum.
• Now, things start getting interesting at a temperature of 2.7K. The thermal spectrum traces out to be an exact copy of CMB (Cosmic Microwave Background).

So, what is Cosmic Microwave Background?

Cosmic Microwave Background is electromagnetic radiation, which in layman’s terms, is residual heat energy left from the Big Bang.

To explain how this CMB is related to the Big Bang, we have to travel to the beginning of, well, everything. Let's travel back to almost 3.8 billion years when the universe was nothing bigger than a quantum particle, known as the 'Singularity’ of “infinite” density. This universe (its space-time, to be exact) then began to expand (it is still under expansion, and that too, is accelerating.)

For the next 380,000 years, the universe was a bowl of blazing hot soup mixed with elementary particles like electrons, protons, and photons. However, at such high temperatures, electrons and protons were restricted to combine into atoms. This universe was hence present in the form of plasma (because of a very high temperature) and emitted electromagnetic radiation like a Black Body. However, due to high density, these radiations could not travel huge distances before colliding with another free electron and being deviated from their path, so it was as if the whole universe was just a strong beam of orangish light being snuffed out by a fog.

Credit: European Space Agency

It was after these 380,000 'Dark Years' that the formation of the Cosmic Microwave Background took place. At 3000K, the universe slowly began to cool down, matter started condensing to form planets, galaxies, and electrons and protons could combine to form atoms. Now, due to the lack of free electrons throughout the universe, these electromagnetic radiations could now travel long distances before being deflected. This radiation now persists throughout outer space and is known as the Cosmic Microwave Background.

So, why is outer space black instead of orange? This is because of the expansion of spacetime. As discussed above, outer space is under an accelerated expansion, so logically, light traveling through it is also expanding (let me reiterate this, Hubble Expansion is the expansion of space-time itself, so everything inside it also expands with it), which in reality means the wavelength of light is being ‘stretched’ to longer wavelengths, this process is known as Cosmological Redshift. So the earlier orange/red space is now black because of the “stretching” of the wavelength of radiation of orange color to that of greater wavelengths that lie beyond the visible ranges.

Credit: David Butler (https://howfarawayisit.com/wp-content/uploads/2020/11/The-Cosmos-2020.pdf)

But why does it actually matter? This is because it helps us in predicting the origin of the universe by approximately calculating for how long a radiation signature has been traveling before reaching the earth. Thus, studying this is essential for us to trace the history of the origin of our universe that eventually roots back to our own origins.