The CMB Axis of Evil and the Nature of Randomness

axis of evil planck
Figure 1. Some fluctuations in the cosmic microwave background align around an axis in the sky, called the “axis of evil” and shown in white here. Image due to the Planck collaboration.

This Halloween, Nature News released an article titled Zombie Physics: 6 Baffling Results that Just Won’t Die. It’s a fun article describing several mysteries in physics whose solution sits in a sort of limbo. For fun, I figured, I’d explain some of these mysteries, and give my opinion about possible solutions. And first, I’m going to discuss the CMB Axis of Evil, a strange pattern in the leftover radiation from the Big Bang.

A Much-Too-Short Summary of Cosmic Inflation and the CMB

About 13.8 billion years ago, the universe was extremely hot, so hot that matter couldn’t form at all… it was just a chaotic soup of charged particles. Hot things (and accelerating charges) glow. And this hot soup was glowing incredibly brightly. As time passed, the universe expanded and cooled, but this glow remained, bathing all of time and space in light.

(The reason for why the universe was so hot in the first place depends on whether cosmic inflation is true. Either it’s because the Big Bang just happened or it’s because, after cosmic inflation, a particle called the inflaton dumped all of its energy into creating hot matter.)

Even today, the glow remains, filling the universe. As the universe expanded, the glow dimmed and its light changed colors (due to gravitational redshift), until it became microwaves instead of visible or ultraviolet light. This ubiquitous glow is called the Cosmic Microwave Background, or CMB for short, and if you turn an old analogue TV to an unused channel, some of the static you hear is CMB radiation picked up by your TV antenna.

Since its discovery, the CMB has been one of our most powerful probes of cosmology. It lets us accurately measure how fast the universe is expanding, the relative amounts of normal stuff vs dark energy and dark matter, how the density of matter fluctuated in the early universe, how the Earth is moving relative to the expansion of the universe, and much more.

Some parts of the early universe were more dense and some were less, and this translates to slight, random variation in the color of light in the CMB. And in turn, we can translate this into a temperature. The temperature of the CMB is incredibly consistent across the sky. It’s an almost perfect 2.725 Kelvin. However, there are tiny fluctuations relative to this mean, and these reflect the dynamics of the early universe. Figure 2 shows a map of these fluctuations and I describe how this map is attained in my post on BICEP2.

Figure 2. The measured CMB mapped on a flat surface. (Image due to the Planck collaboration.)

The CMB Axis of Evil

It’s very hard to see in figure 2, but with a little massaging, we can see that many of the fluctuations in the CMB align along a single axis, called the axis of evil, as shown in figure 1. (Formally, the quadrupole and octopole moments of the fluctuations align.) At first glance, is quite strange, because we believe that the fluctuations in the density of the early universe should be randomly distributed in a particular way… and this is exactly the way they are distributed on smaller scales. The mottled look of figure 2 is exactly due to this particular random behaviour of the fluctuations in the CMB.

So what’s going on? There are a couple of possibilities. I’ll go over them and add my opinion (and the scientific consensus or lack thereof).

Errors in Foreground and Modelling

Perhaps the most boring explanation is that we made a mistake when creating the CMB maps like figure 1 and figure 2. As the story of BICEP2 shows, making those maps is very hard. To create them, we have to account for all the other sources of microwave radiation in the universe and carefully remove them from our measurements.

Over time, we’ve gotten incredibly good at this…so good that we can extract all sorts of information about the early universe from the CMB. But that doesn’t mean we’re always right. There could be extra dust in the solar system. Or a confluence of the gravitational pull of distant galaxies on the light of the CMB (called the integrated Sachs-Wolfe effect) could magnify a normal random fluctuation so that it appears significant.

(I am really oversimplifying the integrated Sachs-Wolfe effect here. But that’s a story for another time.)

I think errors in foreground modelling could easily account for the axis of evil.

The Universe is a Doughnut or a Sphere

Imagine an ant living on the surface of a doughnut. The ant is so small that the doughnut appears flat to it. As the ant travels forward, it will eventually return to where it started, no matter what direction it travelled. From our perspective, of course, this is because a doughnut wraps around. But to the ant, this would be quite mysterious! Figure 3 shows the doughnut from both our perspective and the ant’s perspective. This is very similar to how if you travel East on the Earth, you eventually return to your starting place.

travel on a torus
Figure 3. An ant travels on a doughnut. From our perspective (left), the ant returns to where it started because the doughnut wraps around on itself. But from the ant’s perspective (right) it seems to walk in a straight path and eventually return to where it started.

What if our universe was like the doughnut, but in three dimensions? So if you start going in a direction, say towards Andromeda, and keep going for as long as possible, billions of light years, you would eventually get back to where you started (ignoring of course that the universe is expanding and thus the distance you would have to travel would increase faster than you could travel it).

What if, perhaps we see the same things on both sides of the axis of evil because they are literally the same things and the universe has wrapped around on itself? In the original paper discussing the axis of evil, the authors discuss this very possibility. It’s a nice idea, but it can actually be tested by trying to match images of stars and galaxies (and fluctuations in the cosmic microwave background) on opposite sides of the sky to see if they look the same. The results, however, are not favourable. So no one takes this idea very seriously… even though it’s very clever.

Cosmic Variance

This one takes a bit of explanation. So bear with me. First, let’s talk about something called a posteriori statistics.

A Posterioiri Statistics

Imagine a teacher breaks her students into two groups. She tells one group to flip a coin ten times and record the result as a sequence of heads or tails. The group might record, for example,


which would correspond to a string of four tails, then a string of four heads, then one head, and one tail. She tells the other group of students to make up ten coin flips, but try to do so in a way that looks random. The two collections the students return are:




And, masterfully, the teacher immediately picks out the truly random sequence.  Which one is it? How does she do it? The second sequence, TTHHHHTHTH, which looks very structured, is the random one.

The human mind is very good at picking out patterns, and attributes a cause to every pattern it sees. But random numbers, very naturally, randomly in fact, appear to make patterns, even though the pattern doesn’t mean anything. It’s just random noise. The teacher takes advantage of this. She knows her students will avoid creating a sequence that looks too structured, because they don’t think random numbers look like that. But random numbers can easily look like that.

Of course, the probability that precisely the second sequence would emerge is less than one percent. But the emergence of some sequence that looks vaguely like the second sequence is vastly more likely.  You can think of this like finding a cool looking cloud, or Jesus in your morning toast. You see the cool looking cloud and you think “Wow! A cloud that looks like an airplane! What are the odds?” But you should be thinking “Wow! A cloud that looks like an airplane! The odds of me finding a cloud that looks like something interesting are quite high because there are a lot of clouds and a lot of things I think are interesting.”

This sort of thinking is called a posteriori statistics. And in general, it causes mistaken analysis.

The CMB Axis of Evil

So what does this have to do with the CMB? Well, people who study the CMB are well aware of the danger of a posteriori statistics, so they try to avoid thinking in this way. One way to avoid this sort of thinking is to make many many measurements. If you have a huge number of sequences of coin flips, on average, the randomness (or lack thereof) will become manifest.

And this is indeed what we do for most of the cosmic microwave background. The fluctuations on small scales, which give figures 1 and 2 their mottled texture, are numerous and we can do many statistics on them by looking at different areas of the sky.

But the axis of evil is different. It covers almost the whole sky. And we only have one sky to make measurements of! So it’s not possible to do good statistics. The fact that we have only one universe to measure, which we believe emerged from random processes, and that we can’t do statistics on a whole ensemble of universes is called cosmic variance.

And cosmic variance interferes with our ability to avoid a posteriori statistics. It lets us fool ourselves into believing that the way our universe turned out is special, when there may in fact be a multitude of equally probable ways our universe could have been. And it is entirely possible that the axis of evil is one such “fluke.”

It is possible, in principle, to reduce the effects of cosmic variance. If we could move to another position in the universe, we would be able to see a different portion of the CMB (because the light that could have reached us since the CMB was created would come from a different place in the universe). In 1997, Kamionkowski and Loeb suggested using the emissions of distant dust to extrapolate what the CMB looks like to that dust. In principle, it would be possible, but very very hard, to use this trick to test whether or not the axis of evil comes from cosmic variance.

As you may have guessed from the amount of time I devoted to the explanation, I find cosmic variance to be a very compelling cause of the axis of evil.

The Most Likely Story, In My Opinion

So… what do I think is the cause of the axis of evil? The following is my opinion and not rigorous science. But it went something like this. Due to random fluctuations in the way the universe could have been, something that looks like the axis of evil formed in the CMB, but much less significant. This would be the cosmic variance explanation. To this day, the “axis of evil” remains statistically insignificant. But, because our models of cosmic microwave sources and filters look like in the universe and in our solar system are flawed, and because we don’t take the integrated Sachs-Wolfe effect into account, the axis of evil appears much bigger to us than it actually is.

So in my mind the axis is caused both by imperfect experiments and analysis and by the human need to find patterns in everything.


I owe a huge thanks to my friend and colleague, Ryan Westernacher-Schneider, who told me this story last spring and compiled a summary and list of references. Ryan basically wrote this blog post. I just paraphrased and summarized his words.

Further Reading

I’m not the first science writer to cover this material. Both Ethan Siegal and Brian Koberlein have great articles on it. Check them out:

  • This is Brian Koberlein’s article.
  • This is Ethan Siegal’s.

For those of you interested in reading about the axis of evil in more depth. Here are a few resources.

  • This is the first paper to discuss the axis of evil. It also discusses the possibility that the universe is a doughnut.
  • This paper coined the term “axis of evil.”
  • This paper discusses the possibility of solar-system dust producing the axis of evil.
  • This paper discusses the integrated Sachs-Wolfe effect and how it enhances the axis of evil.
  • This paper proposes a way of reducing cosmic variance.
  • This is the collected published results by the Planck collaboration, which analyses all aspects of the CMB in great depth.

Related Reading

If you enjoyed this post, you might enjoy my other posts on cosmology. I wrote a two-part series on the BICEP2 experiment:

I have three-part series on the early universe:

I have a fun article that describes the cosmic microwave background as the surface of an inside-out star:

30 thoughts on “The CMB Axis of Evil and the Nature of Randomness

  1. It might be a distorted shape as a result from highs field and inflation. Resulting in the image that looks like some symmetrical distortion. If we ever found an ice crystal that was extreme huge. Then on a huge scale it be symmetric but on a small scale it would not mirror anything. Because it was the symmetrical seed that started it. But over time it can distort when it grows big. Something like that is what we see

    1. Ah, you’re proposing that inflation went on long enough that the axis of evil was a much smaller in physical size. Yes, that could be. I’d put that under cosmic variance since, if we could observe multiple universes, or a bigger portion of the sky, we might see the effect disappear.

        1. Well, not necessarily… the minimum we need is 60 e-foldings.That could have taken as few as 10^{-31} seconds.

          But that means the universe grew by a factor of e^60.

          Anyway, what you’re saying is essentially the same as “cosmic variance.”

  2. There’s a great quote attributed to Richard Feynman about a posteriori statistics:

    “You know, the most amazing thing happened to me tonight… I saw a car with the license plate ARW 357. Can you imagine? Of all the millions of license plates in the state, what was the chance that I would see that particular one tonight? Amazing!”

    1. Thanks for reading!

      I’m not talking about the dipole anisotropy… The dipole anisotropy is due to our motion with respect to the CMB frame. I’m talking about the quadrupole and octopole anisotropies…

      And my conclusion is that the pattern isn’t significant, just like the Wikipedia article. But since people keep on talking about it, it’s worth describing in more detail, which is what I did.

  3. Interesting that you skipped why it is called the axis of evil. So why are you even writing about it? Are the implications of the axis of evil are so offensive you can’t even mention them? You prefer to try to explain it away because it offends your religion? Interesting and kind of pathetic

      1. I think Bruce is a geocentrist. This small but vocal group has taken the CMB Axis of Evil as evidence that the Earth is the center of the universe (because of its rough alignment with the Solar System). Your “religion” then would probably be cosmology or methodological naturalism or observable reality or some other such nonsense ;).

        More specifically, I guess the idea is that God made the universe anisotropic as some sort of message or something. I’m not sure about the details. It doesn’t really make any sense.

    1. It’s a tongue-in-cheek reference to George W. Bush’s speech labeling Iran, Iraq, and North Korea “the axis of evil,” which was politically relevant at the time this astrophysical phenomenon was discovered. The CMB preferred axis is considered troubling (and thus “evil”) to astronomers because it appears to contradict important assumptions about the universe, including the Copernican Principle. This principle states that our position and orientation in the universe are not “special” in a certain sense. If it is false, then many of our measurements are inherently unreliable, because they could be unrepresentative of the universe at large.

  4. If the universe is expanding, then doesn’t that mean that we and everything else are expanding along with it at the same rate? If this is correct, why can we measure it? You’d have to have something to measure against to see the expansion. Since we are measuring from inside the universe wouldn’t that be impossible? Lol sorry if I’m misunderstanding. 🙂

    1. We aren’t expanding, the Solar system isn’t expanding, the galaxy may be, but not at a rate that we can measure. Even the closer galaxies aren’t moving, relative to us, at a rate that can be measured. It is only the colour shift of distant galaxies that can be measured. Hot hydrogen emits a known spectrum of light, a spectrum which moves towards red or violet depending on whether it is moving closer or farther from us (or us from it depending on your POV).

  5. I think : That everything is energy. Energy in certain forms, exhibits the property that we call gravity. As for this axis, I suggest that before the big bang, where was a formless, sizeless accumulation of pure energy. This accumulation was spinning, then spontaneously fizzed, producing countless bubbles of pure energy. Inside these bubbles was, and still is, a void in spacetime, a true Black Hole, but enveloped in energy. I call these bubbles “Femptons”. Femptoms have a slight tendency to cluster, then exhibit mass. These clusters collect and disperse, flitting backwards and forwards between being just energy, in minute quantities, so widely dispersed that we can’t detect them (Dark Energy) and Matter, again so scattered that we can’t detect individuals, just the overall effect (Dark Matter)

  6. Perhaps the answer is staring you right in the face and thar our place in (outter?) space IS special. Designed by the designer; Created by the Creator. Put smack dab in the middle to show His handiwork. So what do I think? God made Earth, then us in His image. I’ve always had a problem with the swirling gas model anyway. What happens when a pizza maker takes his ball of dough and spins it? It becomes a disc; the Earth couldnt coalesce into a ball while continuing to spin on and on. The centrifuge has remained on in the standard model making sphere making next to impossible. In a lot of ways, science lives in opposite world. Thanks

    1. And I KNOW that the basis of your argument is nothing more than a work of fiction. Possibly a guide for morality, though much of the older part encourages acts of violence against the innocent, or even where they are not innocent, that the suggested violence far outweighs the offense. There is zero evidence that there is a God, though I actually believe that there is. However, my belief is that humans are not really that important, certainly not important enough for God to have created us in his own image, or even interested in us as individuals. IMO, humans are a most despicable species, hell bent on destroying what God did give us with our overconsumption and contempt for all life. There is very little in the Bible that can be proven. There are a few incidents which fit with historical events, but nothing that would, in the scientific world, be considered proof. I don’t consider belief in God to, in any way, go against science, rather, I see God in everything around me, the wonders of the microscopic world as well as the macroscopic. I suggest that God MAY have created DNA to get life started, and that he MAY have given it a nudge now and them, but life DID evolve, humans DID develop for ape-like animals in Africa, and we MAY not even be the most intelligent species around. Religion has, since it’s first inception, been used and abused to control other people.
      As far as a centrifuge is concerned, the mass of the World, proportionate with the spin can, and gravity does, keep the Earth very close to a sphere, the diameter of which was calculated quite accurately, by Aratosthenes about 250 BC, and you can easily replicate his experiment and calculations yourself. Similarly, using only a fairly low cost telescope, you can calculate the orbits of our moon, and the other planets, just as Galileo did. He was able to show the Catholic church that the Earth isn’t the centre of our solar system, never mind the centre of the universe. They were even more entrenched than you, but probably considerably smarter.

      1. …except for the fact that all the evidence and arguments Galileo presented at his trial modern science confirms are all false. LOL!

          1. Learn your history, Galileo presented over 15 arguments for the Earth in motion including the tides! Lol! They all were taken as no proof and he was eventually condemned and rightly so! None of his arguments are true today as well! Lol!

            And the church NEVER accepted any of his arguments! As a matter of fact …the Church condemned the notion of the earth in motion and the sun being in the center of the universe and static as a FORMAL heresy in 1616 and 1633.

            Geocentrism is a dogma in the church as a result of the formal declaration!

            St. Ballermine at the very end addressed Galileo to “Give me proof & then we’ll take another look at the scriptures.”

            To this day there is no categorical proof whatsoever that proves the earth is in motion. However all laboratory experiments instead have shown a static earth.

          2. Laboratory experiments ? All you need is a pendulum to show that the Earth is spinning. The planets don’t remain the same distance from Earth, but they do (almost) from the sun. Some have elliptical orbits, and even those are influenced by the other planets. The church, that being the Roman church DOES recognise the heliocentricity of our solar system. Indeed, they facilitate research which depends on it. Read and inwardly digest:

  7. Since every physical thing in the universe seems to be in a state of rotation, l suspect that in some sense, the whole visible universe is also rotating, not easy to visualise in three dimensions, and what is seen in the CBM is an artifact of that rotation.

    1. Everything rotating: It is be highly unlikely that ANYTHING would be perfectly still. Basic physics says that galaxies will eventually come into a planar form.

      1. Tell that to the CMB which has it’s quadrupole and octupole aligned with the Earth’s Ecliptic and the dipole with the Earth’s equator! 😱 Equinox!

What do you think?