Big Bang Series Part 3 - Cosmic Microwave Background Radiation
Around 13.7 billion years ago, the universe was born in a massive explosion – the Big Bang. For around 380,000 years, it was searing hot. Particles were moving so fast that they couldn’t even combine to make the atoms we know of today. However, around 380,000 years after the Big Bang, when it cooled to 3000 kelvin (around 5000 F or 2700 C) and the density of the universe had dropped, protons and electrons began to combine into hydrogen atoms. Cosmic microwave background radiation (also known as CMBR) was born.
Radiation and waves
Before we dive into CMBR, let’s talk about what radiation is. Radiation refers to the energy that is emitted from particles. Here, we are going to talk about a specific type of radiation, electromagnetic radiation, which is emitted by charged particles in the form of electromagnetic waves. When particles move fast enough, they can emit electromagnetic waves.
When waves move forward, they also move up and down like a roller coaster. The rate at which they move up and down is measured in frequency, and more frequency equals more energy. Microwaves, which are form of electromagnetic waves, have a low frequency and thus low energy.
Since the universe is expanding, waves traveling through space get stretched. This is called redshifting. Redshifting causes a wave’s frequency to drop. Here is a diagram of what it looks like when a wave gets redshifted.
Cosmic microwave background radiation
So, back to cosmic microwave background radiation (or CMBR for short). Here, I will diagram how CMBR formed.
Part 1: Before 380,000 years after the Big Bang, electrons were free to move everywhere. So when charged particles released electromagnetic waves, they just bounced off stray electrons:
Part 2: However, after 380,000 years after the Big Bang, the universe got colder. This allowed electrons to bind to protons, forming hydrogen atoms. Now, the electromagnetic waves released by charged particles no longer bumped into stray electrons. This allowed them to freely move everywhere:
Part 3: Over the next 13.7 billion years, these electromagnetic waves continued to be released by these charged particles. However, since the universe was expanding, these waves got redshifted (stretched) into microwaves. We can see 13.7 billion year old waves today, and we call them cosmic background radiation, or CMBR. CMBR gives us a good picture of what it was like 13.7 billion years ago and is the strongest evidence we have that the Big Bang occurred:
How to see CMBR
Unfortunately, you need special equipment (infrared scanners) to see them. However, you can hear them right from your TV! A small fraction of the abrasive noise you hear when you turn your TV to a “dead channel” - when the screen becomes all gray and staticky - is actually the background noise of CMBR!
The full-sky image of the temperature fluctuations (shown as color differences) in the cosmic microwave background, made from nine years of WMAP observations. These are the seeds of galaxies, from a time when the universe was under 400,000 years old.
Picture Source: nasa.gov
