This is the second part in what I had originally seen as a two part series on what may be the neatest tools in astronomy’s tool belt for indirectly examining the stuff of the universe. I say originally thought, because as I sit here writing, I’m thinking this is going to evolve into three parts. In this entry I want to address where is CMB came from and how it tells us where we’re going. (image credit: NASA / WMAP Science Team)
Pick up pretty much any astronomy text, look up Cosmic Microwave Background, and you’ll find something along the lines of: “The Cosmic Microwave Background is a relic of the moment the universe cooled enough for recombination to take place. Prior to that moment the universe was opaque to radiation. Today we see this left over radiation as a 2.725 K degree microwave background radiation.” The book will then go onto explain how the CMB was detected.
Did any of that make sense to you? I know it didn’t make sense to me the first dozen or so times I read it over the years. Let me see if I can make sense of this scientific obstruction for you.
After the Big Bang the universe was hot. Really hot. Dante didn’t have enough levels in hell kind of hot. So hot everything was pure energy. Over time, the universe expanded, and just like the gas expanding as it comes out of a can of compressed air, the expanding universe cooled. By the end of the First Three Minutes (Link goes to book on this by Steven Weinberg) atomic nuclei had solidified, and the universe was a mix of electrons, light, and nuclei. These three things formed a plasma; the same type of stuff that fills fluorescent bulbs and neon lights. In a fully ionized plasma, atoms have no electrons at all. The electrons have such a high energy that they can’t bond to the nuclei! As the electrons and protons interact with one another, they exchange energy, and sometimes they give off photons in the process. They also interact with already existing photons, absorbing them and re-emitting them in new directions and at new colors. In the extremely dense, high-energy plasma of the early universe, the photons were being constantly emitted and re-absorbed, and an individual photon couldn’t travel any appreciable distance (even on the atomic scale!) without being absorbed. It’s this “Can’t get anywhere without being absorbed” part that makes the universe opaque.
Opaque can be taken a couple different ways. My bathroom mirror is opaque because photons can’t go through it (they just bounce off at visible wavelengths). A neon “Open” sign is also opaque, but in this cause it’s because a photon can’t get through it without interacting with the gas and being absorbed (and potentially re-emitted).
So, from about minute 3 to year 380,000 the photons, electrons, and nuclei formed an interacting soup. Then one day, at one specific moment, the universe cooled enough that the electrons and and nuclei could bond. Heat is just a form of energy, and as the universe cooled, the individual electrons lost energy and could eventually bond with atomic nuclei.
For reasons that have never made sense to me, the moment the electrons bonded with the nuclei is called recombination. This was, in fact, the first time the electrons formed lasting bonds with nuclei, so it was the moment of initial combination. Someday I hope to find someone who can explain the name to me who was around when they gave recombination the name recombination. Until then, I’m just going to nod and smile.
As the electrons bonded with the nuclei, they had a bunch of different energies. Some of the electrons were bouncing around fairly slowly, and when they went from free ranging to bound, they gave off a low energy photon. Others were zippying along fairly fast with a high energy, and when they bonded with a nuclei, they gave off high energy photons. Most of the electrons were somewhere in between. If one were to make a plot of the number of photons as a function of color (and this has been done by way more than one astronomer), that plotting person would get a shape called a black body curve. Hot objects, in idealized situations, give off light in a distribution that always has the same shape, but changes in color with temperature. (Check out this neat website for an applet you can play with.)
So, at the moment of combination, which we call recombination, photons flew away from the newly formed atoms with a blackbody distribution of colors. Every single bit of space emitted photons in every direction. This means that no matter where in the universe you are, you will see CMB photons. It also means, as time goes on, we’re going to continue to see CMB photons, but they’ll be photons that formed farther away from us than the ones we’re seeing today. It is as though we are seeing the universe clear out a sphere of stuff for us to look at. Five billion years ago, the CMB originated from a significantly closer point. Had we been able to watch the universe for the past 5 billion years, we might have seen the CMB move away from a spot where the first stars blink appeared to on. Then, we might have seen new galaxies form in that same piece of space that was the furthest visible piece of space when our Sun formed.
It is sad that on human time scales we can’t actually perceive our visible horizon moving away from us.
We will never be able to see beyond the CMB. It is a wall of light that blocks everything that happened before it was formed. The wall is always moving away, but it will always be there.
But that’s okay. It actually does a lot of good, and helps us understand a lot of different things about our universe. I’m going to go into to all the cosmology that we get from the CMB tomorrow, but before I post this post, I’m going to go into one more thing: How the CMB helps us see where we are going.
So, here we are on the planet Earth. As we look at the CMB, we can see in our observations Doppler shifts that come from our motion around the Sun. We can also see our motion around the Milky Way. We can also see our galaxy’s motion in the local group. And, we can also see our local group’s motion as it gets pulled (and not pulled) by the gravity of stuff (and lack of stuff) around us. (A story on voids not pulling can be found in this post).
So, it is because of the Doppler shifts observed in the light of the CMB that we are able to see exactly how we are moving relative to the Universe as a whole. The CMB is uniformly coming from a bit farther away every moment, but its color and brightness doesn’t change in any perceptible way (and given the overall size of the universe, it’s moment-to-moment change in distance doesn’t matter either). This makes the CMB a constant light of constant color that we can use to get amazingly precise Doppler measurements of our own velocity. (image credit: DMR, COBE, NASA. See this link for good explaination)
In essence, the CMB is acting like the beam in a police officer’s radar gun, and all we have to do is build the detector to to catch the beam.
So, while it is really hard (and in practicality impossible) to know exactly where we are in the greater, not fully visible to us, universe, the CMB at least let’s us very accurately measure what direction we are traveling it. And that lets us know where to look to find what is gravitationally pulling (or failing to pull) on our little system as it travels through the vastness of the cosmos.