Do you understand the CMBR?
Astronomy is peering out into space to see what’s there, cosmology is weaving together a story to explain what it is, where it came from, and where it’s going.
While all cosmological evidence plays a role in this tapestry, the CMBR may well be at the center when it comes to understanding the deep past and ultimate fate of the universe.
So what the hell is that?
- The oldest light in the universe.
- A picture of the universe going through a phase change.
- A baby picture of the universe, the afterglow of the big bang.
- A picture of the universe at the moment the first atoms formed.
The story of the universe can be summarized simply:
In the beginning, everything used to be quite hot and dense, and for the last 13.8 billion years everything’s been expanding, cooling, and coalescing into small pockets of complexity surround by vast swaths of pretty much nothing.
If you go back far enough, there were no people, no planets, no molecules, no stars…in fact, there weren’t even any elements.
The CMBR, or cosmic microwave background radiation, is a picture of what this looked like- when the only things that existed were giant clouds of hydrogen and helium gas.
It’s a real picture…though it’s taken in the microwave range rather than with visible light.
A phase change of the universe
Just like expanding a volume of gas lowers its temperature (the principle used in just about every modern refrigerator), the expansion of space lowers the temperature of the whole universe.
The CMBR represents a critical point in the universes history- when it cooled enough to go through a phase change.
This change is exactly like the phase changes we’re already familiar with, freezing, boiling, condensing…
Just like lowering the temperature of a gas turns it into a liquid, 380,000 years after the big bang, the universe cooled enough to turn from a plasma into a gas.
— Just as a quick reminder: A plasma is a state of matter that occurs when there is enough energy to strip an atom’s electrons away from its nucleus. —
Before 380,000 years, the universe was so hot that the hydrogen and helium nuclei that had formed within the first few minutes after the big bang couldn’t hold onto any electrons.
The entire universe was a plasma.
Because it was a plasma, the entire universe was opaque- just like a thick fog, you couldn’t see through it. Analogous to how a fog disperses light in every direction preventing you from seeing, in this plasma, photons of light were constantly absorbed and reemitted in random directions, scattering off of protons and electrons before they could travel further than a few centimeters.
When the universe was a temperature of about 3000 K, the hydrogen and helium nuclei could finally hold onto their electrons. This represents the first time that atoms as we usually think of them (a coherent collection of protons, neutrons, and electrons) appeared in the universe.
Finally, the light that had previously been perpetually emitted and absorbed could travel freely.
For the first time, light had decoupled from matter.
The universe had become transparent.
The CMBR is also referred to as the surface of last scattering- it was the last time this light was cast off in random directions.
The light released at this moment has been travelling unimpeded since.
How do we know it’s there?
The light released in this phase change has been traveling through the epic 13.8 billion year journey of cosmic evolution. Through the birth of the first stars, the first supernova explosions and heavy elements that came with them. Through the first planets coalescing and solar systems forming, the development and evolution of life on earth, the birth humanity and the first civilizations, the invention of the internet and space telescopes…
The microwave light collected on the sensors of our space telescopes today are the very same photons that were released at the moment of this transition.
When we point the telescope at every part of the sky and make one big image, this is what we see:
A literal baby picture of the universe.
If you flatten this out into an oval and remove the noise of our galaxy from it, you end up with the image at the beginning of this article.
A brief history
The CMBR was actually predicted in 1948, 16 years before it was detected, but the scientists that actually found it had not even heard of the prediction! It was initially just seen as instrument noise, and they tried every way they could think of (including removing the bird poop from their detector) to eliminate it.
In 1978, 13 years after their accidental discovery, Penzias and Wilson won the nobel prize for it. [s]
The initial measurements were essentially nothing more than white noise on what was expected to be a silent background.
Since its initial discovery, there have been many efforts to probe and better understand this signal with various tools through the years. The most clarity and sensitivity has come from a series of space-based telescopes.
This picture compares our best images of the CMBR over time. With the launch of the James Webb Space Telescope in 2019, we’ll get a clearer picture still.
What’s with those splotches?
The color differences illustrate minute variations in temperature which correspond with tiny variations in density. They’re thought to be caused by quantum fluctuations that were blown up to literally cosmic proportions during the inflationary expansion of the universe.
Though extremely small, these variations are extraordinarily important to the evolution of the cosmos.
These tiny differences are what seeded the first stars and galaxies…they provided the initial gradient of gravitational energy which triggered the collapse of these vast clouds of hydrogen and helium into the first stars.
These tiny fluctuations ultimately give rise to the galaxies, galactic clusters, and superclusters that we see on the largest scales of the cosmos.
How do we know this is the exact right interpretation?
Our understanding of this astounding evidence will almost definitely be enhanced, but the evidence isn’t going away.
The CMBR fits seamlessly alongside the vast number of other observations made probing the nature of the universe. Along with the cosmic redshift, it provides the basis for the cohesive compelling story of how we got to where we are today. Close analysis of the CMBR reinforces our understanding of not only the age of the universe, but it’s size, shape, and makeup as well. But we’ll save that for another article…
New interpretations, explanations and theories are always welcome, but ideas competing to explain the nature of our existence must incorporate and explain this evidence too.
…bonus points if it makes new predictions that can be tested.
Originally published at deepbreadth.io