Cosmic Microwave Background

No matter where in space we point our telescopes we see the same thing. A background radiation that appears to be the same in all directions. Like rain on a garden house – no matter which window you look through you see the same drops in all directions.

Is This Poo Or The Big Bang?

This background radiation was discovered in 1964 by two astronomers, Robert W. Wilson and Arno A. Penzias, by accident. They wanted to observe the Milky Way’s halo, and while preparing the telescope they measured a “noise signal” in all their observations. In an attempt to remove the signal, they had to locate the source. Maybe it was radiation from a large nearby city (New York) or maybe it was from the large Van Allen belts around Earth? At some point they even thought it was pigeon poo that was stuck to the antenna.

All of these attempts to remove the signal failed, because the source of this mysterious background radiation was actually the Big Bang. It was the afterglow from the creation of the Universe almost 14 billion years ago that Wilson and Penzias had detected. 14 years later, they would receive the Nobel prize for their groundbreaking discovery. It was not only the start of modern cosmology, it was evidence that they universe wasn’t a steady state, but instead the universe had a beginning and now we could directly see the remnants from this incredible event.

The cosmic microwave background measurement, WMAP, from NASA. It shoes a uniform distribution of radiation or temperature across the sky. This image shows a tiny temperature difference of ± 200 microKelvin, which eventually grew to become galaxies. But how can we see this radiation so far back in time?
Credit: NASA / WMAP Science Team.

But, if Big Bang happened 14 billion years ago, how can we detect it today?

A Big Bang Theory

This afterglow from Big Bang is called the CMB – Cosmic Microwave Background. Cosmic because it’s on the large scale in space, the cosmos, Microwave because the radiation has a wavelength of a bit more than 1 mm and by definition we call these waves microwaves and Background because it’s everywhere in space. The CMB is also present around us right now.

I want to start by making it clear that this radiation is not a “footprint” or some still-standing structure in the universe that we can see because it now is forever statically printed into space-time fabric.

The CMB is made up by electromagnetic waves that travel freely at the speed of light and have done so since only very shortly (precisely: 370,000 years) after the Big Bang when neutral atoms were formed and the universe became transparent. Light could travel freely for the first time ever, and it did! And it continued to do so for 14 billion years until today.

Now, our telescopes can capture these photons, but how? If the photons have traveled uninterruptedly in a straight line for 14 billion years, how can we detect them today? After all, we need the photons to travel into out telescopes – it doesn’t work the other way around. We can’t send a telescope to the edge of the universe and pick up the signals that are roaming around out there.

The Universe Is Infinite And The Big Bang Happened Everywhere

In order to properly understand the CMB, we need to properly understand the Big Bang. At least as a concept, we can skip the math. We also need to understand that the universe is infinite. Infinity is very difficult to comprehend, and add to that how we just established that the universe had a beginning, the Big Bang, then it will be even more confusing. But fear not, we will go through this mess together. Ready?

What we normally call the universe is just a small part of the entire infinite universe know as the observable universe. The observable universe grows over time. Currently, the radius of the observable universe is 46 billion light-years. This corresponds to the distance light has traveled freely since the Big Bang 14 billion years ago. These two numbers are not the same, because the infinite universe continuously expands. Illustration: Astronomicca.

When we talk about the universe, we often mean “the observable universe”. That is the part within the circle. This circle grows bigger over time (read here how the universe grows) and hence our observable universe increases as light from distant galaxies reach us (read here how light travels from distant galaxies). This also means that when we go back in time, the observable universe was smaller. The sphere within which we could see stuff was smaller. The detectable horizon was closer. If we go back long enough, all the way to the Big Bang, we will see that the observable universe was merely a tiny dot. And this is where the misunderstanding usually happens. Because the Big Bang was not something that happened in one place. It did not happen in the center of our tiny little dot. It happened everywhere at the same time.

We often imagine the Big Bang to be some event that happened in one place. Illustration: Astronomicca.

In the illustration below to the left many Big Bangs are shown to happen at the same time. But it wasn’t just many Big Bangs. It was Big Bangs everywhere. So to be completely accurate, the illustration should probably look something like the right illustration:

Big Bang happened everywhere in the infinite universe. Illustration: Astronomicca.

A big uniform monochromatic illustration (right image) is not very explanatory of anything so in the spirit of understanding, it’s okay to simplify (left image).

So, with Big Bang happening everywhere, it means that the light we now call CMB was emitted from everywhere in the universe. Let’s for a moment pretend you could be located anywhere in the infinite universe, and let’s pretend you were located somewhere far away from Earth. An expanding spherical observable universe would then form around you, just like it does around us here on Earth. There is nothing special about our location in the universe. It might look like we are sitting in the center of some big round balloon full of stars and galaxies, but we’re not. It’s just what it looks like to us when we point our telescopes towards space.

The reason why CMB reaches our telescopes is because we are receiving “Big Bang radiation” from every single point on our observable-universe-sphere.

The CMB shows the light that was emitted at “all the Big Bangs” that happened on the border of our sphere which is located 46 billion light-years from us. It therefore fits perfectly that this light reaches us now, 14 billion years later. It is magnificient that the CMB is the same in all directions! This indicates that all of these points earlier were located the same place which allows them to carry the same information today. Illustration: Astronomicca.

That light was emitted at a distance that corresponds to reaching us after traveling freely for 14 billion years. Read that sentence a few times, because it contains the information you need to properly understand why we see CMB in all directions.

Why The CMB Proves Big Bang

The CMB is the most compelling evidence we have for the Big Bang theory. It is, in other words, the most convincing evidence we have for the universe having a fixed beginning instead of being static, i.e. not having a beginning but just being (whatever that means).

The reason for this is shown in the illustrations above. The fact that we see the same radiation, everywhere, in all directions is very strong evidence that all of this radiation was sent out from the same place. It’s like walking into a room where everybody, no matter which direction you look in, is wearing yellow clothes and pink shoes. It would be an unfathomable coincidence if everywhere people just suddenly decided to dress exactly the same at the same time. Not one single person is wearing black pants. Not one single person chose to wear blue sandals. If everybody wears the same clothes it’s likely because they all got their clothes from the same place in a coordinated process.

The same goes for the CMB. We can not find anything in the background radiation that differs from the rest (the differences we see in the CMB are at the level of 0.0001 K, which is vanishingly little). There is no indication that some parts of the radiation originated differently than the rest. Instead, we see the same radiation in all directions everywhere in the universe. And this is likely because it all, at some point 14 billion years ago, originated from the same event, the Big Bang.

I think that is so nuts. A singularity stopped being a singularity – and here you are reading my post online. Thank you for making it this far. If you liked it, please consider joining the world’s perhaps smallest space community on Patreon.


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